Chiari-like Malformation (CM) and Syringomyelia (SM) in the Cavalier King Charles Spaniel

Chiari-like malformation (CM) is believed to play a major role in the cause of syringomyelia (SM) in cavalier King Charles spaniels. While some forms of SM are known to have other causes, this article focuses primarily upon its relationship with CM.

CM is a complex skull and craniocervical junction malformation associated with a short skull, that is common in some brachycephalic toy breed dogs and especially the cavalier (CKCS). The brain is overcrowded in the skull, and there is also overcrowding of the spinal cord in the upper neck vertebrae. In the CKCS, this situation is compounded due to the cavalier having a comparatively large brain. The cavalier appears to have a brain more appropriate for a bigger dog, about the size as that of a Labrador retriever.

This disproportion causes the brain, particularly the cerebellum, to squeeze through the foramen magnum – the hole at the back of the skull, in the occipital bone – partially blocking the flow of cerebrospinal fluid (CSF) down the spinal cord. This both causes pain and the creation of fluid which collects in pockets in the spinal cord, which is what SM is. CM can cause irreversible damage to the spinal cord, resulting in additional pain and other neurological disorders.

SM is an extremely serious condition in which on or more of these "syrinxes" or "syringes", develop within the spinal cord near the brain. It is also known as "neck scratcher's disease", because one of its common signs is scratching in the air near the neck. "Syringomyelia" is Latin for "cavity within the spinal cord".

SM is rare in most breeds but has become very widespread in cavalier King Charles spaniels, the Brussels Griffon (Griffon Bruxellois), and Chihuahuas. The number of diagnosed cases in cavaliers has increased dramatically since 2000. Researchers estimate that more than 95% of cavaliers have CM and over 50% may have SM. The severity and extent of syringomyelia also appear to get worse in each succeeding generation of cavaliers. It is worldwide in scope and not limited to any country, breeding line, or kennel, and experts report that it is believed to be inherited in the cavalier. More ...

Symptoms

CM/SM seldom can be detected in young puppies, as symptoms of it usually are not evident before the age of six months or years later.

Dogs diagnosed with CM and SM may have no outward symptoms at all. If CM-affected dogs do have symptoms, they indicate pain (CM-P). The most common ones are: (a) vocalization (barking, whining, moaning) particularly when being picked up under the chest or when changing position; (b) head scratching or head rubbing; (c) reduced activity, such as a reluctance to climb stairs or jump; (d) behavioral changes, such as becoming timid, anxious, or aggressive; and/or (e) touch aversion.

SM-affected dogs may be asymptomatic if the syrinx is small and does not interfere with the spinal cord. Larger syrinxes -- those having a diameter of 4 mm or more -- can damage the spinal cord and cause symptoms such as phantom scratching, scoliosis, and weakness in the limbs.

Pain is the most important clinical sign of CM. Symptoms may vary widely among different dogs, but the earliest sign often is that the dog feels a hypersensitivity in its neck area, causing in some an uncontrollable urge to scratch at its neck and shoulders. Then usually follows severe pain around its head, neck, and shoulders, causing it yelp or scream. Click here or the YouTube logo to see videos of cavaliers with CM/SM symptoms. As the disease progresses, it destroys portions of the cavalier's spinal cord, and is so painful that the affected dog may contort its neck and even sleep and eat only with its head held high. The dog's legs may become progressively weaker, so that walking becomes increasingly difficult. Some dogs deteriorate to the point of paralysis. More ...

Diagnosis

The only accurate way of confirming diagnosis of the disease is through the use of magnetic resonance imaging (MRI) scanning, which can be an extremely costly procedure. The MRI allows the veterinary neurologist to study the spine for the presence of any abnormality which might obstruct the flow of the cerebrospinal fluid. Accurate MRI results require that the dog be anesthetized. Clinic charges for MRI examinations of canines have been known to vary from a rare discounted rate of $600.00 to over $2,000.00.

The names and locations of veterinary neurologists who are board certified by the American College of Veterinary Internal Medicine (ACVIM) are on our Neurologists webpage.

Another disorder common to cavaliers and with symptoms similar to CM/SM is Primary Secretory Otitis Media (PSOM), which is a highly viscous mucus plug which fills the middle ear and causes the tympanic membrane to bulge. Because the pain and other sensations in the head and neck areas, resulting from PSOM, are so similar to symptoms due to SM, the possibility that the cavalier has PSOM and not SM should be determined before diagnosing SM. More ...

Treatment

Treatment options for CM/SM are very limited. But first of all, it is important to distinguish SM with symptoms from SM without symptoms. As a general rule, CM/SM without symptoms (asymptomatic) should not be treated with drugs.

Anticonvulsants, such as gabapentin (Neurontin, Gabarone), have been successful in some more severe cases. Pregabalin (Lyrica, Accord, Alzain, Lecaent, Milpharm, Prekind, Rewisca, Sandoz, Zentiva), amitriptyline (Elavil, Tryptizol, Laroxyl, Sarotex), and oral opioids (pethidine or methadone) are alternatives. Methylsulfonylmethane (MSM) is recommended by some veterinary neurologists as a dietary supplement.

Drugs which reduce the production of cerebrospinal fluid, including proton pump inhibitors such as omeprazole (Prilosec), and the diuretic, furosemide (Lasix, Diuride, Frudix, Frusemide), and spironolactone (Aldactone), may be useful, but clinical data on their use and effectiveness is lacking. Carbonic anhydrase inhibitors, such as acetazolamide (Diamox) also serve to decrease the flow of cerebrospinal fluid, but their adverse side effects of abdominal pain, lethargy, and weakness limit long term use.

Before the disease progresses to its severe form, the use of cortisteroids, such as prednisolone, or non-steroidal anti-inflammatory drugs (NSAIDs, such as Rimadyl and Metacam) may relieve the symptoms but not the deterioration. Cortisteroids have serious side effects, such as weight, gait, and skin changes, and harmful suppression of the immune system. Long term use of these drugs is not advised. As a general rule, they should be reserved for a last resort, although some neurologists will start initial treatment of symptomatic dogs with a combination of an anticonvulsants, such as gabapentin, and a none-inflammatory dose of prednisolone.

Surgery to allow the cerebrospinal fluid to flow normally may be necessary to reduce the pain and deterioration. However, such surgeries are technically difficult and should be performed only by specialists. In some cases a shunt is installed. Although surgery often is successful, it is very expensive, and many dogs either have a recurrence of the disease or still show signs of pain and scratching. The most frequent reason for recurrence reportedly is the development of post-operative scar tissue. At least one neurologist has been inserting titanium mesh, in an effort to prevent such scar tissue from building up. More ...

Breeders' Responsibilities

CM/SM has a tendency to be more severe in each subsequent generation, and with an earlier onset. Breeders should follow the SM Breeding Protocol. The aim of the breeding protocol is to reduce the incidence of symptomatic syringomyelia in the cavalier breed, and not to create litters of puppies guaranteed not to have SM. The chance of producing an affected dog cannot be predicted without knowing the inheritance.

What You Can Do

• Donate to the Cavalier Health Fund.

• Send MRI scans of cavaliers 5 years old or older and which do not have SM, along with MRIs of those dogs' family members, to Dr. Clare Rusbridge at neurovet@virginmedia.com.

• Ease your dog's symptoms by using a comfortable harness instead of a collar and leash. One of the best harnesses for cavaliers with CM/SM symptoms is the BRILLIANT K9 "Lucy Small" harness. It is easy to put on and easy to take off. Watch the videos: "Opening the harness" and "Walking the dog with the harness".

Chiari-like malformation (CM) is believed to play a major role in the cause of syringomyelia (SM) in cavalier King Charles spaniels. While some forms of SM are known to have other causes, this article focuses primarily upon its relationship with CM.

CM is a complex skull and craniocervical junction malformation associated with a short (brachycephalic) skull, that is common in some brachycephalic toy breed dogs and especially the cavalier King Charles spaniel (CKCS). The skull is too small for the brain and there is also overcrowding of the spinal cord in the upper neck vertebrae. In the CKCS, this situation is compounded due to the cavalier having a comparatively large brain. The cavalier appears to have a brain more appropriate for a bigger dog, about the size as that of a Labrador retriever.

This disproportion causes the brain, particularly the cerebellum, to squeeze through the foramen magnum – the hole at the back of the skull, in the occipital bone – partially blocking the flow of cerebrospinal fluid (CSF) down the spinal cord. This both causes pain and the creation of fluid which collects in pockets in the spinal cord, which is what SM is. CM can cause irreversible damage to the spinal cord, resulting in additional pain and other neurological disorders.

CM is an inherited disorder which is rare in most breeds but reportedly has become very widespread in cavalier King Charles spaniels (CKCS) and the Brussels Griffon (Griffon Bruxellois) and Chihuahuas. Some researchers estimate that as many as 95% of CKCSs may have Chiari-like malformation (CM or CLM), the skull bone malformation believed to be a part of the cause of syringomyelia, and that more than 50% of cavaliers may have SM.* It is worldwide in scope and not limited to any country, breeding line, or kennel, and experts report that it is inherited in the cavalier King Charles spaniel. CM is so widespread in the cavalier that it may be an inherent part of the CKCS's breed standard.

CM may first appear at any age, although many dogs (up to 45%) will develop first signs ofCM before their first brithday, and approximately 40% first signs between ages one and four years. As many as 15% will develop signs as of middle-age (between ages six and eight years.

CM can be progressive, in the sense that over a period of several months, the length of the cerebellar herniation can increase significantly. However, the severity of CM in a dog does not predict the presence of syringomyelia in that dog. Other factors may influence the development of a syrinx.

See Karen Kennedy's* Understanding Canine Chiari Malformation and Syrningomyelia for diagrams of the occipital bone and foramen magnum.

These four terms -- (1) Chiari-like malformation (CM or CLM), (2) Canine Chiari, (3) Caudal occipital malformation syndrome (COMS), and (4) Occipital hypoplasia (OH) -- have been used to identify the malformation believed to play a role in the cause of syringomyelia. Although they technically mean different things, they often are used interchangeably. Some neurologists prefer one term over the others. However, researchers meeting at the International Conference on Syringomyelia at the Royal Veterinary College in London in November 2006 agreed upon the use of Chiari-like malformation (CM or CLM) to describe the malformation found in the Cavalier and to a lesser extent in a few other brachycephalic* breeds.

More recently, Canine Chiari (CC) has been used to shorten and better describe the name of the disorder. Because prior to the November 2006 London conference, CM and OH and COMS all were used to describe the same malformation, they all are used interchangeably in this article.

The term, Caudal Occipital Malformation Syndrome (COMS) had been used, particularly by some specialists in the United States, to describe the disorder. Some diehard neurologists persist in using this term when referring to Chiari-like malformation in cavaliers. The authors of a 2012 German article insist that:

"... [T]he Chiari-like malformation in the Cavalier King Charles spaniel is characterized by indentation of the occipital (bone) with cerebellar herniation and is more correctly termed caudal occipital malformation syndrome."

Occipital hypoplasia (OH) has been used to describe the displacement of the cerebellum into the area of the foramen magnum and a kinking of the medulla and an indentation of the cerebellum. "Hypoplasia" is a medical term defined as underdevelopment or incomplete development, and so, "occipital hypoplasia" in this instance means an underdeveloped or incompletely developed occipital bone, which is part of the back of the skull. However, at the November 2006 London conference, this term was rejected because there is no proof yet that the condition is related to a hypoplastic occipital bone. The actual disorder is believed to be caused either by an unusually small occipital bone or a confining membrane within the occipital bone, resulting in the cavity in the skull containing the cerebellum to be too small to fully contain it, leading to overcrowding of the caudal fossa and obstruction of the neural structures, including the incomplete closure or development of the neural tube through which flows the cerebrospinal fluid (CSF).

In a January 2009 article, researchers concluded that: "While several factors are associated with neurologic signs [of SM], occipital hypoplasia appears to be the most important factor."

However, in a June 2012 article, German researchers compared the volumes of occipital bones of cavaliers with and without syringomyelia and of French bulldogs. They did not find a reduced volume of the occipital bone of CKCSs, compared to the bulldogs. They concluded: "These results do not support occipital hypoplasia as a cause for syringomyelia development, challenging the paraxial mesoderm insufficiency theory."

Occipital hypoplasia is to be distinguished from occipital dysplasia, which is an incomplete ossification of the supraoccipital bone, causing a widening of the foramen magnum. The more brachycephalic is the shape of the dog's skull, the more likely there will be occipital dysplasia. The cavalier is a brachycephalic breed, and therefore a combination of both occipital hypoplasia and occipital dysplasia can occur in the CKCS. In a 2008 German study, the researchers recommend that cavaliers be screened for both occipital hypoplasia and occipital dysplasia.

In a December 2018 article, a team of Romanian and German researchers used computed tomography (CT) to diagnose CM, SM, and occiptial hypoplasia in a 21-month-old female cavalier King Charles spaniel. CT showed a typical brachycephalic head conformation, shortened facial bones, and a dome shaped calvarium. The supraoccipital bone was short and stunted, and the foramen magnum appeared enlarged, with part of the cerebellar vermis protruding. Evidence of a syrinx was observed in the spinal cord at C2. Changes of the occipital bone showed occipital hypoplasia with incomplete formation of the bone (See Fig. 6, below).

Chiari-like malformation (CM or CLM) has had a variety of definitions over the years as more is learned about its likely causes.

The most recent definition -- in this July 2018 article -- attempts to include in a general fashion all of the distortions found in the skulls of CM-affected dogs:

"CM might be described as any distortion of the skull and craniocervical junction which compromises the neural parenchyma and cerebrospinal fluid circulation causing pain and/or SM."

In this other July 2018 article, it was proposed that the disorder might be better described as a brachycephalic obstructive cerebrospinal (CSF) channel syndrome (BOCCS) with similarities to brachycephalic obstructive airway syndrome (BOAS).

Chiari-like malformation is named after a similar condition in humans, discovered by Dr. Hans Chiari. Researchers estimate that up to 95% of CKCSs may have CM. The back half of the cavalier's skull typically may be too small to accommodate all of the brain’s cerebellum, which may also be too large, and so it squeezes through the foramen magnum – the hole at the back of the skull, in the occipital bone – partially blocking the flow of cerebrospinal fluid (CSF) down the spinal cord. This is called cerebellar herniation. The variable pressure created by the abnormal flow of CSF is believed to create the SM cavities – called syrinxes – in the spinal cord. The cavalier appears to have a brain more appropriate for a bigger dog, about the size as that of a Labrador retriever.

Previously to 2010, CM was defined as "decreased caudal fossa volume with caudal descent of the cerebellum, and often the brainstem, into or though the foramen magnum." The 2010 definition was "a condition characterized by a mismatch in size between the brain (too big) and the skull (too small). There is not enough room for the brain and the back part (cerebellum and medulla) is pushed out the foramen magnum."

In an October 2014 article, UK researchers found three different definitions of CM:

1. Indentation: "Indentation of the caudal aspect of the cerebellum —- defined as a concave, rather than flattened or convex, caudal border of the cerebellum."

2. Impaction: "Impaction of the cerebellar vermis into the foramen magnum -— defined as deformation of the shape of caudo-ventral vermis into a point such that the angle between lines drawn along the caudal and ventral borders of the cerebellum meet at an acute, rather than an obtuse, angle. This definition was considered analogous to descent into the foramen magnum that has been used previously." (In photo of a CKCS at right, black arrow points to malformation of the caudal fossa of the occipital bone with visualization of the vermis.)

3. Herniation: "Herniation of the cerebellar vermis through the foramen magnum -— defined as extension of the cerebellar vermis caudal to a line drawn between the ventral aspect of the supraoccipital bone (opisthion) and the caudal border of the basioccipital bone (basion)."

They concluded that only the "herniation" definition distinguishes CM-dogs because "there is a high prevalence of cerebellar indentation and impaction in the normal canine population, suggesting they are unreliable as defining factors for CM."

However, even prior to that change, in a February 2014 article, a neurology team studying the Griffon Bruxellois (Brussels Griffons) recommended a redefinition of CM, explaining:

"This study supports the view that CM is a multifactorial condition that includes the shortening of the entire basicranium, loss of convexity of the supraoccipital bone, invagination of the cerebellum under the occipital lobes and possibly by increased proximity of the atlas to the occiput. As a compensatory change, there is increased height of the rostral cranial cavity and lengthening of the dorsal cranial vault. Overcrowding in the caudal cranial fossa and the craniocervical junction is a defining feature. The study provides the basis of a quantitative assessment of CM which might identify risk of syringomyelia and suggests that CM should be redefined so that account is taken of the overcrowding of the entire cranial fossa and craniocervical junction with reorganization of the brain."

In a March 2016 study of Griffons Bruxellois, the definition was tweaked again, and this time, much more complexly worded, as follows:

"a more global cranium and craniocervical junction abnormally characterized by insufficiency of the supra and basioccipital bones with compensatory rostral cranium doming, shortening of the skull base and increased proximity of the cervical vertebrae to the occiput resulting in overcrowding of the neural parenchyma in the caudal fossa."

In a November 2019 article, a team of UK researchers reviewed the medical records of 66 cavaliers, 40 of which had syringomyelia (SM) and the other 26 did not; 55 had Chiari-like malformation (CM) and 11 did not. The dogs were grouped by (1) control group of 11 with no Chiari-like malformation (CM-N); (2) CM pain group (CM-P) of 15 dogs; (3) clinical SM group (SM-S) of 40 dogs. SM-S dogs included those with outward symptoms of SM (variable phantom scratching, scoliosis, etc.) and a syrinx of at least 4 mm. The researchers divided their study into two sub-sets, the first examined head features related to the dogs' soft palates, and the other examined features related to their hard palates; both sub-sets also included review of the dogs' features related to forebrain flattening and olfactory bulb rotation. The olfactory bulb is a bulb of neural tissue within the dog’s fore-brain. Their work included comparing the shape of the "stop" of each dog, which is the degree of the angle where the nose and skull meet, and the indentation between the eyes at that point. A "gentle stop" has the least angular shape and a "pronounced stop" has the sharpest angle.

They found (see figure 5 below):

• CM-N dogs (no CM) had the least brachycephalic head, a gentle stop with the greatest upper jaw area between the hard palate and the frontal bone, and the longest soft palate length.

• CM-P dogs (painful CM) had the least distance between the hard palate and cranium, a pronounced stop, and a displaced olfactory bulb.

• CM-S dogs (large syrinx) had the most reduced middle craial bone area and shortest distance between the connection of the hard and soft palates with the base of the cranium.

They conclude that dogs with CM-P had the shortest muzzle lengths, and that "a reduced distance between the hard palate and the frontal bone was particularly associated with CM-P." Dr. Clare Rusbridge, one of the researchers, explained:

"Dogs with clinically relevant CM/SM are more likely to have brachycephalic features of the rostral skull flattening with reduction of nasal tissue and a well-defined stop. This evidence not only enhances our understanding of the disease and 'at risk' head conformation but could also impact on the assessment of MRI and disease diagnosis. It suggests the whole skull should be analyzed and not just the hindbrain currently required in prebreeding screening. This information has implications not only for breeders and pet owners but also for the veterinary profession to raise awareness about the welfare aspects of breeding. Furthermore, an increased risk for SM and painful CM might not be confined to brachycephalic breeds but other miniaturized purebreeds and hybrids that have gained in popularity as pets."

Co-researcher Dr. Susan P. Knowler explained:

"This study suggests that the whole skull, rather than just the hindbrain, should be analysed in diagnostic tests. It also impacts on how we should interpret MRI from affected dogs and the choices we make when we breed predisposed dogs and develop breeding recommendations. ... The brachycephalic features that can be seen from outside is a head that has flattening at the front with reduction of nasal tissue and a well-defined stop."

In a September 2019 article, a team of UK neurology researchers used "machine-learning" to identify biomarkers which distinguish between cavaliers with and without pain due to Chiari-like malformation (CM-P) and also those with and without syringomyelia (SM). Thirty-two CKCSs were included in the study, of which 10 had pain due to CM, 11 had symptomatic SM (SM-S), and 11 controls which had neither CM-P nor SM. "Machine-learning" is a process of a computer not explicitly programmed by people, which looks for patterns and data, then analyzes that information and draws conclusions and makes predictions from that gathered information. In this case, the machine looked for morphological changes in the dogs, which may not be apparent to human observers, thereby removing potential bias or blindness that may be produced by a hypothesis driven expert observer approach. The machine learning approach was to understand neuromorphological change and to identify image-based biomarkers in dogs with CM-P and and symptomatic SM (SM-S). Upon comparing dogs with CM-P or SM-S to the control group, candidate biomarkers were identified in specific regions of the brain for CM-P and for SM-S, particularly between the presphenoid bone and area between the soft palate and the tongue, which they concluded indicates both conditions being strongly related to changes within that area. This is a very preliminary study aimed at developing these biomarkers into a clinical diagnostic test.

In an April 2019 abstract, a team of UK neurology researchers (Eleonore Dumas [right], Susan Penny Knowler, Felicity Stringer, Clare Rusbridge) examined MRI scans of 66 cavalier King Charles spaniels, including 11 without either syringomyelia (SM) or painful Chiari-like malformation (CM-P), 15 with only CM-P, and 40 with clinically severe CM/SM. They report finding that the SM-affected CKCSs "had a more ventral orientation of the olfactory bulbs and shorter distance between basicranium and hard palate." They concluded that cavaliers with symptomatic CM/SM are more likely to have brachycephalic snouts and "'midface' hypoplasia similar to craniosyostosis Crouzan syndrome."

In a January 2019 article by a team of UK and Swedish researchers (Susan Penelope Knowler, Lena Gillstedt, Thomas J Mitchell, Jelena Jovanovik, Holger Andreas Volk, Clare Rusbridge), 13 cavalier King Charles spaniels were examined by UK breed judges, using a checklist, to determine if the risk of Chiari-like malformation (CM) and syringomyelia (SM) could be identified by visual assessment of head shape. The results showed a positive correlation between the judges' evaluations and the risk of CM/SM sufficient to warrant a larger study of the breed. Figure 5 (below) shows the most extreme range of checklist scores. The researchers concluded:

"This prospective investigation demonstrated that it was possible to compare subjective evaluation of head conformation with objective measurements and revealed a significant correlation between the subjective visual evaluation of head conformation and an objective evaluation of dorsoventral doming using photographs. However, this pilot investigation demonstrated that individual adjudicators can vary in their interpretation of the CKCS breed type and also suggests that measuring the cephalic index or rostrocaudal doming alone is not a reliable indicator of brachycephaly but should be taken together with a visual evaluation and take account of other features, such as those on the checklist and the size of the dog."

In a July 2018 article, Dr. Susan (Penny) Knowler (and Gabriel L. Galea, Clare Rusbridge) reviews over 20 years of neurological research into the conditions of Chiari-like malformation (CM) and syringomyelia (SM) primarily in cavalier King Charles spaniels (CKSC) and a few other brachcyphalic breeds. She covers the key morphocenetic processes involved in CM/SM, including (a) anatomical abnormalities, (b) brachycephaly, (c)cranio-cervical junction abnormalities, (e) embryology (fetal development), (f) the brain and ventricles, (g) the skull, and (h) genetics of CM. She provides a current defintion of CM as:

"a malformation of the skull and craniocervical junction which compromises the neural parenchyma to cause pain and/or disrupt CSF circulation which can result in SM."

In a January 2017 article, the UK researchers further pursued their analysis of CM resulting from an overall disorder of the conformation of the CKCS brain and skull. They stated:

"Thus, CM is not just a reduction in the cranial base and caudal fossa. The `ellipticity' of the brain provides a quantitative value to compare the natural oval shape of the Control cohort to the more global brachycephalic CM pain and two SM cases. The reduced size and rotation of the olfactory bulb, together with the clival angle (cranial base angulation between the ethmoidal plane and the clival plane), is associated with a shortened muzzle and increased stop and a `face' that tilts up like a human. ... The morphing movie (S1 Movie) highlights the dynamic changes of the skull conformation and brain parenchyma associated with progressive brachycephaly and airorhynchy, shortening of the basicranium and supraoccipital bones and the proximity and angulation of the atlas and dens."

Describing cavaliers specifically, the researchers stated:

"Ten of the fourteen significant variables were found in the CKCS with one, line a-c [see red line a-c in the diagram below], unique to the breed. Line a-c indicates the proximity of the sphenooccipital synchondrosis to the atlas bone. This study confirms the findings of others that the CKCS with SM have a reduced caudal fossa size a presumed consequence of early closure of the spheno-occipital and possibly other cranial sutures. Compared to other breeds including the GB, the CKCS has considerably greater incidence of cerebellar deformation by the supra-occipital bone and vermis herniation. These findings and the coexistence of occipital dysplasia and hypoplasia suggest that the CKCS may have additional predisposing risk factors for SM compared to the other breeds."

In an April 2016 abstract, UK neurological researchers found evidence that CM may be the result of an overall disorder of the conformation of the CKCS brain and skull. They examined MRI scans of the skulls of 70 cavaliers, divided into four categories: SM with phantom scratching (15 CKCSs); clinical SM (e.g. pain) but no phantom scratching (17 CKCSs); behavioral signs of pain with CM but no SM (25 CKCSs); and CKCS with no SM and no behavioral signs of pain or scratching (13 dogs -- “CKCS control”). They also had an “other-breed-control” group of 19 dogs (including 5 brachycephalic -- short-muzzled), with normal brain sizes.

They hypothesized that there may be insufficient room within the skull for the forebrain, and that may contribute to backward displacement and overcrowding of the hindbrain. They focused upon the forebrain's olfactory bulb (OB -- also called olfactory lobe), which is at the lower front of the forebrain and directly behind the olfactory receptor cells in the dog’s nose. The more brachycephalic (short-muzzled) the dog, the more the OB tends to be lower and the more the frontal lobe tends to be flattened against the front of the skull. (Compare the normal location of the canine forebrain in the diagram at the left, with the flattened frontal lobe and the lower olfactory lobe of a CM/SM-affected cavalier, at the right.)

They found that the more severe the CM/SM condition of the cavaliers in the study, the smaller the mean size of the OB, and that there was a significant difference between the cavaliers in the four CM/SM groups and the dogs in the other-breed-control group. They also noticed a trend towards more ventrally (lower) orientated OB with increasing CM/SM severity. They concluded:

“This study suggests that CM should be considered a more global brain and skull conformational disorder with features of extreme brachycephaly including smaller more ventrally orientated OB; however, further work is required and the measurement technique has been refined for future studies. We recommend that future studies into MRI conformation of CM and SM uses rigorous phenotyping based on clinical signs and age.”

In an April 2015 article on the subject of the kinking (or elevation) of the medulla, researchers examined 36 cavaliers (33 having Chiari-like malformation and 26 having syringomyelia) and reported finding that higher elevation of kinking of the medulla related to neurological clinical signs of CM/SM. They also found that brainstem position measurements at the caudodorsal-most border of the fourth ventricle (called the "obex position") were associated with both the presence and severity of syringomyelia. An obex position measurement of ≤3.5 was sensitive (79%) and highly specific (90%) for the presence of syringomyelia.

(The photo at right from the April 2015 article shows how the position of the brainstem was evaluated by measuring the distance between the obex (the caudodorsal-most border of the fourth ventricle) and a line drawn parallel to the foramen magnum. This was termed the "obex measurement". )

There is not yet a consensus among veterinary investigators as to how to measure the cavalier's occipital bone to determine what should be the shape of the cerebellum within a "normal" CKCS's occipital bone. Dr. Clare Rusbridge, BVMS, MRCVS, PhD, DipECVN (right), of the Stone Lion Veterinary Centre in London, England, a leading investigator into SM, has described the three "classic features" of Chiari-like malformation as: (1) loss of the normal round shape of the cerebellum, which can appear to be indented by the occipital bone; (2) displacement of the cerebellum into and through the foramen magnum, i.e. herniation; and (3) kinking of the medulla. 2009 and 2010 UK studies in which Dr. Rusbridge later participated (discussed below) suggest that caudal fossa volume may also play a role in CM.

In a 2006 study conducted by Dr. Natasha J. Olby and Dr. Sofia Cerda-Gonzalez, both board certified veterinary neurologists, and others at North Carolina State University's College of Veterinary Medicine's Department of Clinical Sciences and the IAMS Pet Imaging Center in Raleigh, NC., they have concluded that the incidence of caudal fossa and cervical spinal abnormalities is high in Cavaliers, and that the pathogenesis of syringomyelia is multi-factorial rather than due to a single malformation.

In a 2009 Scottish study led by Dr. Jacques Penderis, of 70 cavaliers and 80 dogs of other breeds, the researchers found that "all [of the] CKCSs had abnormalities in occipital bone shape. ... CKCSs had a shallower caudal cranial fossa and abnormalities of the occipital bone, compared with those of mesaticephalic dogs. These changes were more severe in CKCSs with syringomyelia."

However, in a January 2009 article, Drs. Sofia Cerda-Gonzalez, Natasha J. Olby, Susan McCullough, Anthony P. Pease, Richard Broadstone, and Jason A. Osborne failed to find the same association when comparing the caudal fossa of CKCS with and without syringomyelia using three-dimensional measurement methods.

Research journal articles published in 2009 and 2010 point to evidence that cavaliers' hind-skull volumes are not different from other small breeds, particularly those with short muzzles, and that the percentage of the volume of the caudal fossa -- the hind-skull cavity -- to the volume of the total cranial cavity, did not differ significantly between those CKCSs with and without SM.

However, these studies also found that the volume of hindbrain within the hind-skull was significantly greater for young -- 2-years and younger -- cavaliers with SM than older dogs -- 5 years and older -- without SM. They also found that increased hindbrain volume in CKCSs with SM, compared to that of the hind-skull, was directly correlated with the size of the dogs' syrinxes.

The first of these investigations was a 2009 German study of 40 cavaliers and 25 dogs of other brachycephalic breeds. The researchers found that: (1) "All CKCSs had cranial characteristics consistent with CLM"; and (2) "There were no significant differences between CKCSs and brachycephalic dogs with respect to the ... volumes of the CF [caudal fossa*] ...". They concluded: "Results of this study suggested that descent of the cerebellum into the foramen magnum and the presence of syringohydromyelia in CKCSs are not necessarily associated with a volume reduction in the CF of the skull."

Similarly, in a 2009 UK study comparing the cerebral cranium volumes of the CKCS with those of other small breeds and the Labrador retriever, Hannah Cross and Drs. Rusbridge and Rodolfo Cappello found that cavaliers do not have a proportionately smaller caudal fossa compared to other small breeds, but that the CKCS's brain is comparatively large.

In that 2009 UK study, the researchers stated:

"When compared with Labradors, CKCS had proportionately the same volume of parenchyma [hindbrain] in their caudal fossa [skull], hence there is a mismatch of volumes with too much parenchyma in a too small caudal fossa causing overcrowding. ... Other small breeds of dogs had a proportionately smaller volume of parenchyma in their caudal fossa which can explain why, despite having a similar sized caudal fossa to CKCS, they do not experience overcrowding. It is hypothesised that through the miniaturisation process of other small dogs, both the cranium and brain are proportionately smaller but in CKCS only the cranium has reduced in volume, hence why there is a higher incidence of CM in CKCS than other small breeds.

"Cavalier King Charles spaniels also had a greater percentage of their cranial fossa filled with parenchyma (cranial fossa parenchyma percentage) compared with small breeds and Labradors which had a similar percentage. Overcrowding in CKCS might therefore occur due to a mismatch in volumes in both the caudal fossa and cranial fossa of the skull, suggesting the cranial fossa is also involved in the pathophysiology of CM."

They conclude:

"The results support mesoderm * insufficiency or craniosynostosis ** as the pathogenesis of Chiari-like malformation (CM) in CKCS. It presents evidence for overcrowding of the caudal fossa due to a mismatch of brain parenchyma and fossa volumes as to why CKCS and not other small dogs are affected."

This suggests both a possible genetic cause of the displacement of the cerebellum through the foramen magnum, as well as evidence that the cavalier's skull may not be too small, but that its hindbrain is too large, hence the "mismatch".

To the contrary, however, in a 2009 Scottish study led by Dr. Jacques Penderis, of 70 cavaliers and 80 dogs of other breeds, the researchers found that "all [of the] CKCSs had abnormalities in occipital bone shape. ... CKCSs had a shallower caudal cranial fossa and abnormalities of the occipital bone, compared with those of mesaticephalic dogs. These changes were more severe in CKCSs with syringomyelia."

In a 2010 UK study report in the Journal of Small Animal Practice (JSAP), Colin J. Driver (right), Dr. Clare Rusbridge, et al. reiterated findings that the variations in the dimensions of the cavaliers' posterior [caudal] cranial fossa* may not be associated with syringomyelia, since cavaliers do not have a proportionately smaller caudal cranial fossa compared to other small breeds. See, also, an abstract of that study presented before the European College of Veterinary Neurology (ECVN).

The JSAP 2010 study researchers found that a cavalier with a higher volume of hindbrain within the skull is more likely to have SM, and the greater the volume of hindbrain, the larger the syrinx. They also found a direct relationship between between the dimensions of the brain ventricles ("ventriculomegaly" -- see below) and the size of the syrinx.

In addition, the 2010 JSAP research suggested that there may be a "failure of communication" between the paraxial mesoderm* and the cranial somites** with the closing neural tube*** in the embryo, resulting in loss of coordination between the growth of the skull and the hindbrain. When functioning properly, the growth of the mesoderm supports and helps to facilitate the closure process of the neural tube. They concluded that overgrowth of the cerebellum in the embryo may cause the mis-match, because cavaliers have proportionately more hindbrain volume than other small breed dogs. They stated: "Early growth plate closure may result in CM because despite the dynamic nature of osseous tissue, it would be unable to accommodate the developing brain."

Then, later in 2010, the authors of the 2010 UK JSAP report presented an abstract before the 2010 congress of the British Small Animal Veterinary Association (BSAVA), in which they re-affirmed that, while SM occurs in cavaliers which have CM, it is the mis-match between the volumes of the hindbrain and the hind-skull which is believed to actually lead to SM, if not be the cause of SM. In that abstract, the authors go on to conclude that the more marked volume mis-matches they found between the hindbrain and the skull, the more severe the SM which affected the young dogs -- under 2 years of age -- in the study.

In a December 2010 UK study, led by Colin Driver, the researchers' results were consistent with the previous findings that ventriculomegaly and a small but significant increase in caudal fossa parenchyma are associated with syringomyelia. Further, this December 2010 study also found that the volume of the skulls of CKCS under 2 years of age and SM-affected were significantly smaller than the skull volumes of cavaliers over 5 years of age and SM-clear.

The UK studies in 2009 and 2010 suggest that a disproportionately large hind portion of the brain may be a necessary element of SM in the breed. These 2009 and 2010 research reports explain why CM has been re-defined as "a condition characterized by a mismatch in size between the brain (too big) and the skull (too small). There is not enough room for the brain and the back part (cerebellum and medulla) is pushed out the foramen magnum."

In a June 2011 study, which included Drs. Rusbridge, Driver, and McGonnell, they reported that twelve CM-affected cavaliers' foramen magnums and the length of cerebellar herniation "increased significantly" between MRI scans 9.5 months apart. they concluded:

"This work could suggest that overcrowding of the caudal cranial fossa in conjunction with the movements of cerebrospinal fluid and cerebellar tissue secondary to pulse pressures created during the cardiac cycle causes pressures on the occipital bone. This leads to a resorption of the bone and therefore an increase in caudal cranial fossa and foramen magnum size allowing cerebellar herniation length to increase."

In an April 2012 study by Thomas A. Shaw, Imelda M. McGonnell, Colin J. Driver, Clare Rusbridge, and Holger A. Volk, they concluded that:

"the CKCS has a relatively larger cerebellum [in purple at right] than small breed dogs and Labradors and there is an association between increased cerebellar volume and SM in CKCS. In contrast to small breed dogs and Labradors, CKCS exhibit correlation between increased cerebellar volume and cerebellar crowding within the caudal CCF, suggesting that CCF growth in CKCS is not keeping pace with the growth of the cerebellum.

"These findings support the hypothesis that it is a multifactorial disease process governed by increased cerebellar volume and failure of the CCF to reach a commensurate size."

They also found:

(a) "CKCS under the age of 2 with SM have an increased cerebellar volume when compared to CKCS over the age of 5 without SM. This supports hypothesis that increased cerebellar volume in CKCS is associated with syringomyelia. Previous volumetric studies in CKCS have shown that there is an association between SM and CCF parenchyma volume, but this is the first time that cerebellar volume has been linked to SM. The cerebellum to brain volume ratio is consistent between normal dogs and has been shown to decrease with cerebellar degenerative disorders, but it has never been shown to be increased in size in a canine neurological disorder."

(b) "The degree of cerebellar crowding in the caudal CCF is correlated with increased volume of the cerebellum in CKCS, and this is not seen in small breed dogs or Labradors."

(c) "The degree of crowding may determine the degree of foramen magnum obstruction, and in turn the tendency for syrinxes to form. Cerebellar volume is potentially a key factor in determining the degree of obstruction and interference in normal CSF flow through the foramen magnum, which disposes dogs to the subsequent development of SM."

(d) "In CKCS an increase in relative cerebellar volume is correlated with an increase in cerebellar crowding in the caudal CCF. It should be noted that small breed dogs and Labradors do not show the same relationship. We infer from this result that during cranial development in Labradors and small breed dogs, a compensatory mechanism maintains the relationship between cerebellar volume and CCF dimensions, and this mechanism is defective in CKCS."

(e) "We also found in CKCS that cerebellar crowding in the caudal CCF is more sensitive to changes in relative cerebellar volume than cerebellar crowding in the rostral CCF, which is consistent with the theory that increased cerebellar volume results in the cerebellum shifting caudally and causes obliteration of dead space in the caudal CCF. This also causes herniation of the cerebellum through the foramen magnum (i.e. CM)."

(f) "In this study, we find that in CKCS, unlike small breed dogs or Labradors, there is a positive correlation between the volume of the cerebellum and degree of crowding in the caudal CCF, which suggests that CM may be due to CCF development not keeping pace with growth of the cerebellum. This supports the idea that CM/SM in CKCS may in fact be multifactorial and an abnormal development process affecting the CCF may be acting as a disease modifier."

(g) "Impaired CCF development may be caused by a failure of communication between one or more of these progenitors and the developing neural tube (specifically, rhombomere 1, which gives rise to the cerebellum). Alternatively, it could simply be explained by premature closure of growth plates between the bones of the CCF."

(h) "It has also been noted on post-mortem examination of CKCS and other small breed dogs that the supraoccipital bone overlying the cerebellar vermis is remarkably thin and sometimes eroded so that the foramen magnum is enlarged dorsally, which could indicate that there has been substantial bone resorbtion. Work is needed to elucidate the mechanisms of occipital growth in dogs to determine the extent to which an osteoresorbtive process can mitigate an enlarged cerebellum in CKCS and in other breeds."

However, in a June 2012 article, German researchers Martin J. Schmidt, Martin Kramer, and Nele Ondreka compared the volumes of occipital bones of cavaliers with and without syringomyelia and of French bulldogs. They did not find a reduced volume of the occipital bone of CKCSs, compared to the bulldogs. They concluded:

"These results do not support occipital hypoplasia as a cause for syringomyelia development, challenging the paraxial mesoderm insufficiency theory. This also suggests that the term Chiari-like malformation, a term derived from human studies, is not appropriate in the Cavalier King Charles spaniel."

The authors of this 2012 German article seemed mired in the pre-2010 definition of Chiari-like malformation. They state:

"... [T]he Chiari-like malformation in the Cavalier King Charles spaniel is characterized by indentation of the occipital (bone) with cerebellar herniation and is more correctly termed caudal occipital malformation syndrome."

They also appear to be unduly dismissive of the studies beginning in 2009 which found that the cavalier's cerebellum is relatively larger than that in other breeds. The authors of the 2012 German article did not include cerebellum size in their study, and their comment about the 2009-2012 reports simply is:

"Results of studies proposing a mismatch between cerebellar and caudal cranial fossa volume in this breed and in comparison to other breeds were controversial. In some studies, there was a mismatch between caudal fossa parenchyma and caudal fossa volume in dogs with syringomyelia and overcrowding was proposed as a cause of syringomyelia development. In most studies, however, no difference was found between caudal fossa volume in Cavalier King Charles spaniels with and without syringomyelia, although this was not universal." (Emphasis added.)

In a February 2013 report, UK researchers T. A. Shaw, I. M. McGonnell, C. J. Driver, C. Rusbridge, and H. A. Volk compared MRI scans of 45 CKCSs, 38 dogs of other small breeds, and 26 Labrador retrievers, and concluded:

"The data support the hypothesis that CM/SM in CKCS is a multifactorial disease process governed by the effects of increased hindbrain volume and impaired occipital bone development. The present authors recently reported that CM/SM is linked to increased cerebellar volume (Shaw and others 2012). In view of this, the aetiopathogenesis of CM/SM may equivocally be mediated by conditions independently affecting the developing occipital bones and cerebellum, or by dysregulation of a signalling mechanism coordinating the growth of the developing hindbrain and occipital skull."

In a June 2013 report, UK and German neurology researchers Joe Fenn, Martin J. Schmidt, Harriet Simpson, Colin J. Driver, and Holger A. Volk, having compared 22 cavaliers with SM and 12 without SM, found that in CKCSs with SM the percentage of space taken by venous sinuses in the brain is significantly lower than the volume occupied by the brain's parenchyma. Venous sinuses are a network of channels in the brain, which receive blood from the brains veins and also receive cerebrospinal fluid (CSF) and empty blood into the jugular vein. The report concludes that: "These results support a role for reduced venous drainage and parenchymal ‘overcrowding’ of the CCF [caudal cranial fossa] in the pathophysiology of SM."

In a 2013 doctorate dissertation, German Dr. Melanie Klinger studied the cranial base growth plates of 58 cavaliers and 24 other brachycephalic dogs and 67 mesocephalic dogs for the their first 18 months. She found that:

"In the CKCS the growth plate closure occurred about the 5th month of life. The second group which was composed of the brachycephalic participants of the study followed next. Finally the synchondrosis sphenooccipitalis ossificated in representatives of mesocephalic breeds around the 13.5th month."

She concluded:

"The results confirm the assumption that the premature ossification of the sphenooccipital synchondrosis is the cause of the reduced skull length for brachycephalic breeds. ... With regard to the pathogenesis of the CM the present results support the exceptional position which the CKCS possesses among the brachycephalic breeds."

See also this June 2013 article.

The severity of CM in a dog does not predict the presence of syringomyelia in that dog. Therefore, other factors are believed to influence the development of a syrinx, including atlanto-occipital overlapping (AOO) .

Ongoing research into genetic correlations between CM and SM seeks to determine whether different genes may control the expression of SM and CM. If so, it may be possible to select breeding stock which has been diagnosed to have CM but may not be expected to produce offspring with SM genes.

The atlas is the spinal vertebra (C1) closest to the skull. It sits next to the foramen magnum, the hole in the occipital bone. The "atlanto-occipital joint" is the connection between the atlas and the occipital bone, and is stabilized by ligaments. "Atlanto-occipital overlapping" (AOO) is characterized by a decreased distance between the atlas and the occipital bone. In some cases, the dorsal arch of the atlas may actually protrude into the foramen magnum. See the image below (courtesy of: www.wikispaces.com).

In a February 2014 article, a leading neurology team studying the Griffon Bruxellois observed:

"In other words a developmental anomaly resulting in a Chiari malformation may also be associated with abnormalities of the atlas, axis and dens. In the dog, the most important craniovertebral junction abnormality associated with CM is atlanto-occipital overlapping, which has been reported as similar to basilar invagination in humans."

In a January 2016 article, Cornell University neuroglogists examined the MRIs of 271 dogs, measuring the proximity of the atlas to the foramen magnum. They found a close association (higher than previously reported) between atlanto-occipital overlapping (AOO) and small breed dogs, including cavalier King Charles spaniels, affected with clinical signs of syringomyelia (SM).

See also these related articles: May 2009; October 2009; January 2010; April 2013; November 2015; May 2016; and June 2016.

See Pain due to CM under Symptoms below ...

Syringomyelia (SM) is a condition of the development of fluid-filled cavities in the spinal cord, which is believed by researchers to be due to abnormal flow of cerebrospinal fluid (CSF) between the brain and the spinal cord through the foramen magnum at the base of the skull. "Syringomyelia" is Latin for "cavity within the spinal cord".

SM was first identified by veterinary neurologists in the 1990s, while classic symptoms, such as air scratching, had been reported anecdotally in the 1980s.

The severity and extent of syringomyelia also appear to get worse in each succeeding generation of cavaliers. Other breeds known to be affected to a lesser extent include the Affenpinscher,Bichon Frisé, Boston terrier, bull terrier, French bulldog, Havanese, King Charles spaniel (the English toy spaniel), Maltese terrier, miniature dachshunds, miniature and toy poodles, Papillon, Pomeranian, Pugs, Shih Tzu, Staffordshire bull terrier, and the Yorkshire terrier. See SM in Other Breeds, below, for links to Internet articles about syringomyelia in some of these breeds.

Courtesy of The Dog Channel

Syringomyelia is also known as syrinx and hydromyelia, and occasionally mis-identified as Arnold Chiari malformation. "Syringomyelia" is Latin for "cavity within the spinal cord".

Technically, hydromyelia is a dilatation of the central canal within the spinal cord, and syringomyelia is the cavitation of the spinal cord parenchyma. Combined, they are referred to either as syringo-hydromyelia (SHM) or hydro-syringomyelia. The disease is referred to generally as syringomyelia and SM herein. This condition is similar, but not identical, to Arnold Chiari Type I Syndrome in humans.

Syringomyelia also may be described as syringomyelia secondary to Chiari-like malformation (CM or CLM) -- also called canine Chiari. CM is also referred to as occipital hypoplasia (OH) or caudal occipital malformation syndrome (COMS). The full relationship between CM and the development of SM is not fully understood. The combination of CM and SM usually is abbreviated as CM/SM.

Syringomyelia (SM) is defined as "a condition that results in the development of fluid-containing cavities within the parenchyma of the spinal cord. as a consequence of abnormal cerebrospinal fluid movement." (November 2006 International Conference on Syringomyelia).

Cerebrospinal fluid normally flows back and forth between the brain and spinal cord with each heart beat. As the heart pumps blood to the brain, the CSF flows from the brain through the hole called the foramen magnum to the spinal cord, to accommodate the increased volume of incoming blood.

Syringomyelia is believed to result when the cerebrospinal fluid is prevented from circulating normally between the brain and spinal cord, due to a narrowing or blockage of the CSF flow at the foramen magnum, thereby forcing the CSF at a higher than normal pressure into the spinal cord. The pressure difference causes the spinal cord to distend or pull apart, creating a cavity called a syrinx, and squeezing fluid either from blood vessels and other tissues or CSF into the cavity. (See above a magnetic resonance imaging [MRI] scan of a cavalier's brain and spinal cord, with the arrow pointing to a syrinx [the elongated white area] within the spinal cord. In the images of a cavalier below, the red arrow points from a syrinx to a cross-section of the spine at that point. This image is Figure 3 from this November 2018 article.)

However, in a September 2015 abstract before the ESVN-ECVN, UK researchers created a computer model of the spinal cord, subarachnoid space (SAS), dura mater, and the epidural space of a cavalier King Charles spaniel affected with CM/SM. They performed exaggerated movement of the spinal cord during the cardiac cycle, seeking to confirm a theory that abnormities in the circulation of the cerebrospinal fluid (CSF) generate pressures that drive the fluid into the cord. Instead, they found that CSF pressure gradients are unlikely to cause fluid movement into the cord, sufficient to generate syrinxes. They concluded:

"On the other hand, although the shear stress in the cord is low, its location and cyclic nature indicates the possibility that this may be the factor that generates the initial tissue damage, which eventually leads to the formation of syrinxes."

Syringomyelia is an extremely serious, progressively worsening spinal disease which is rare in most breeds but is becoming very widespread in cavalier King Charles spaniels of all bloodlines. In May 2005, Dr. Rusbridge and Susan P. (Penny) Knowler, BSc (Hons), who have been studying the disease in several hundreds of cavaliers, reported that a conservative estimate is that at least 50% of cavalier King Charles spaniels have a degree of Chiari-like malformation, although not all are so severely affected as to have syringomyelia. In February 2010, Dr. Georgina Child, board certified veterinary neurologist in Australia, reported that of 60 asymptomatic cavaliers scanned as potential breeding stock, 50% had SM syrinxes. In a September 2010 report of 804 cavaliers, Mrs. Knowler and others estimated that "the lifetime risk of developing syringomyelia in the study population was estimated to be 55%."

In a 2011 study of 49 cavaliers diagnosed with SM, Dr. Rusbridge and others found that "total syrinx size was positively correlated with age" of the dogs. In a June 2011 study of 555 cavaliers without any symptoms of syringomyelia, 25% of the one year old dogs had SM and 70% of the dogs aged 6 years and older had SM.

R ole of the ventricle system (vetricular dilatation - ventriculomegaly)

The brain's ventricle system consists of four cavities which are connected with the spinal cord's central canal. The four ventricles are known as the two lateral ventricles, the third ventricle, and the fourth ventricle. The ventricles are the source of CSF and are the brain's respository of CSF.

Some neurologists are including in their examination reports an analysis of whether the ventricles are dilated, and if so, a measurement of the amount of their dilatation. The medical term for dilated lateral ventricles is ventriculomegaly. In a December 2010 UK study, also led by Colin Driver, the researchers' results were consistent with the previous findings that ventriculomegaly and a small but significant increase in caudal fossa parenchyma are associated with syringomyelia.

In an October 2016 abstract, German researchers compared the perfusion of blood in the periventricular white matter of 23 cavalier King Charles spaniels with ventriculomegaly compared to control dogs consisting of 10 healthy Beagles. They found that cerebral blood flow and volume were significantly lower in the cavaliers. They concluded that the dogs with ventriculomegaly may have a form of normal pressure hydrocephalus (NPH).

In an August 2017 article, Dr. Schmidt and his team studied the ventricle system of 42 cavaliers -- 32 CKSCs with ventriculomegaly and 10 control CKCSs. They used "dynamic susceptibility contrast perfusion" magnetic resonance imaging (DSC-PMRI), which allows them to quantify the volume of blood passing through the brain tissue. They found that cerebral blood flow (CBF) is reduced in the periventricular white matter of CKCSs with ventriculomegaly, which makes some increase of intraventricular pressure likely. They stated that these findings make it plausible that ventriculomegaly may be a form of internal hydrocephalus. They observed that when intraventricular pressure is increased, a higher cerebrospinal fluid (CSF) volume is forced from the ventricles at a higher velocity.

Below is a comparison between a canine brain with normal lateral cerebral ventricles (A) and one with enlarged lateral ventricles (B). From a May 2015 study led by Dr. Martin J. Schmidt.

There are other forms of syringomyelia in canines. Spinal dysraphism or spinal dysplasia is a genetic disorder in which puppies normally under the age of three months display a bunny hopping gait and wide-based stance and scoliosis, due to the spinal cord not developing completely in the womb. Dalmatians, English setters, golden retrievers, rottweilers, visla, and Weimaraners have been identified with this disorder. Also, SM may be caused by tumors, cysts, or trauma. Neither of those are discussed here.

Also, benign, small syrinxes are a common incidental finding on MRI examinations. Therefore, clinical

correlation is important in order to determine if the syrinx is associated with the Chiari-like malformation and/or symptoms.

The following paragraphs discuss typical symptoms (clinical and behavioral signs) displayed by dogs diagnosed with Chiari-like malformation (CM) and/or syringomyelia (SM). CM and SM each cause different symptoms. Most signs of CM are due to pain. (See "Pain Due To CM", below.) SM also can cause pain, but usually only late in the progression or in the most severe cases. (See "Pain Due to SM", below.) The most common symptom of SM is phantom scratching.

Several of these signs are so common that they may be for other reasons and unrelated to CM or SM at all. For example, prior studies have shown that approximately 25% of dogs that display clinical signs of SM are not found to have a syrinx on MRI scans. Scroll down to "Other disorders with similar symptoms" for a discussion of likely other disorders causing similar signs.

Symptoms of CM and SM may vary widely among different dogs, but the most common sign of SM (and not of CM) often is that the dog feels a sensitivity in its neck area, causing in some an uncontrollable urge to scratch at its neck and shoulders excessively, particularly when walking or during other forms of exercise, and usually without making skin contact. This is called phantom scratching. It is believed that it is not a response to pain. See this November 2017 article.

Phantom scratching had been believed to be due to an increase in the pressure of the flow of cerebrospinal fluid through the central canal from the brain down the spinal column, causing the central canal to expand and press against the nerves of the spinal column and creating a pins-and-needle-like tingling or a burning-type pain, and other strange sensations (called dysaesthesia or paresthesia), which prompt the dog to scratch.

However, in an April 2016 abstract, UK researchers found that phantom scratching is associated with a very wide syrinx -- 4 mm. wide or greater in cavaliers -- that extends to the superficial dorsal horn (SDH) in the C3-C6 spinal segments. The study found that phantom scratching is associated with a large dorso-lateral syrinx that extends to the SDH in the C3-C6 spinal segments (C2-C5 vertebrae). The study did not find an association to damage of other areas of cervical spinal cord. They suggested that phantom scratching is due to damage to projection neurons in lamina I of the superficial dorsal horn (SDH) with resulting reduced descending inhibition to the lumbosacral scratching CPG [central pattern generator -- neural circuits controlling a stereotyped sequence of muscle contractions]. ... They concluded that if a dog has an SM syrinx extending to the SDH then it is at risk for phantom scratching. In a November 2017 article -- an extended publication of the April 2016 abstract -- the same investigators concluded:

"SM associated phantom scratching appears associated with MRI findings of a large syrinx extending into the mid cervical SDH. We hypothesise that damage in this region might influence the lumbosacral scratching central pattern generator (CPG). If a scratching SM affected dog does not have a large dorsolateral cervical syrinx with SDH involvement then alternative explanations for scratching should be investigated."

This article suggests that only dogs with both CM and SM will phantom scratch, and that dogs with only CM will not. It also suggests that the action is very similar to fictive scratching which occurs in animals with severed spinal cords.

In a November 2017 article, a team of researchers at NC State University and Cornell University prepared and studied the results of a questionnaire answered by owners of 50 cavalier King Charles spaniels, 20 with only Chiari-like malformation (CM) and 30 with both CM and syringomyelia (SM). Of the 50 dogs, 33 were symptomatic and 17 were not. The most common presenting sign was phantom scratching, occurring in 32 dogs, and the most common sign of pain was crying out when being lifted, occurring in 11 dogs. The researchers found that owner-reported findings were not significantly associated with presence or severity of SM or neurologic examination findings. See Figure 1 (right), a chart of the symptoms reported by the owners of the 33 symptomatic dogs.

Ten of the 20 CM-only dogs reportedly displayed "classic" signs of neuropathic pain. The researchers concluded:

"The conclusion that SM causes pain in CKCS is complicated by the finding that dogs with CM but no SM can show classic signs of neuropathic pain, as illustrated by 10 dogs in our study. ... To conclude, the full range of signs reported by owners of CKCS includes a variety of manifestations of pain, with phantom scratching as the most commonly reported sign followed by crying out when being lifted. Owner reporting of pain and scratch frequency and severity captured by the ChiMPS-T correlates with the owner-reported surface area affected by these signs in their dogs. Neither the scores nor the surface area reported correlated with the presence or severity of SM, highlighting uncertainty on the source of pain in these dogs."



In a July 2019 article, a team of UK neurology researchers studied the records of 130 cavalier King Charles spaniels diagnosed with CM and some also with SM to determine which symptoms related to CM and to SM and to syrinx diameter, in order to use the data in future studies of diagnosis, treatment, and genetics of CM/SM. Dogs were grouped based upon whether they had no SM (Group 1) up to having a syrinx greater than 4 mm (Group 4).

Signs found NOT RELATED to syrinx presence or size:



• Vocalization (65.4%) (except being picked up under the sternum (breastbone), which was more common among dogs with no or mild SM).

• Spinal pain -- 54.6%

• Reduced activity -- 37.7%

• Reluctance to jump or to climb stairs --35.4%

• Aversion to touch or grooming -- 30.0% (ears, head, and neck region -- 25.4%)

• Change in emotional state (more timid, anxious, withdrawn, or aggressive) -- 28.5%

• Disrupted sleep -- 22.3%



Signs found TO BE RELATED to syrinx presence or size:



• Phantom scratching -- 67% of Group 4 dogs and none in other groups

• Scratching or rubbing of the head or ears -- 28% but less common in Group 4 dogs

• Scoliosis -- 27% of Group 4 dogs and none in other groups

• Postural defects -- 15% of Group 4 dogs and none in other groups

• Weakness -- 39% of Group 4 dogs and none in other groups

The researchers stated that their findings suggest that "phantom scratching is highly unlikely with small syrinxes." They also found that among the dogs in their study, PSOM (primary secretory otitis media) is common in symptomatic CM-affected dogs and in SM-affected dogs, thereby raising the possibility of confusing the cause of some signs and behaviors, since PSOM-affected dogs tend to engage in ear-rubbing. They concluded:

"The study further suggests that SM-specific signs are phantom scratching, scoliosis, and sensory and motor signs that can be related to spinal cord damage by the syrinx and are associated with large syringes (transverse width ≥4 mm). Non-SM-specific signs include vocalization (described as without obvious trigger, when shifting position when recumbent and when being lifted under the sternum to a height), spinal pain, head and ear rubbing or scratching, aversion to touch, refusal or difficulty jumping or doing stairs, exercise intolerance/reduced activity, sleep disruption, or behavioral change described as becoming more anxious, timid, aggressive, or withdrawn. These non-SM-specific signs could reflect CM-P [CM associated pain]."

The implication from this study is, therefore, that the major signs of pain expressed by CM/SM cavaliers definitely are not due to having a syrinx and probably are due to having Chiari-like malformation, but the authors cannot definitely attribute the painful expressions solely to CM.

Some dogs perform facial or head rubbing or spontaneous vocalizations. Click here or the YouTube logo (right) to see videos of cavaliers with SM symptoms. Videos of are available under Related Links below.

Symptoms, signs, and behaviors which may be due to CM or SM include:

• Phantom scratching (SM), discussed above

• Expressions of pain (CM), discussed below

• Temderness at the neck, shoulders, and/or head (CM)

• Head shaking (CM or SM)

• Lip licking (CM)

• Feet licking or chewing -- excessive (SM)

• Head rubbing -- side to side on the floor (CM)

• Reduced physical activity (CM or SM)

• Behavioral change to more timid, anxious, or aggressive (CM)

• Touch aversion (CM)

• Inability to lower head to eat or drink (CM or SM)

• Weakness of the hind limbs (SM)

• Stiffness of the limbs (SM)

• Restlessness -- inability to settle down (CM)

Many symptoms of CM and/or SM, such as scratching, are so ordinary (when not excessive or compulsive) that they could be attributed to any of several common causes, including flea bites or allergies. Others, such as limping or lack of muscle coordination also could be confused with injuries or other disorders.

Scoliosis (abnormal curving of the spine) is another physically apparent symptom. (See photo of a young cavalier with scoliosis, below.) An outward deviation of an eye, called exotropia also is common among dogs affected with CM.

SM and CM very seldom can be detected in young puppies, as symptoms usually are not evident before the age of six months or even many years later. There is no way to know in advance of the symptoms whether a dog is normal or is a syringomyelia carrier which does not develop the disease but can pass it on to its offspring.

The condition causes damage to the spinal cord and usually results in symptoms of hypersensitivity, intense pain, and leg dysfunction. The primary symptoms may vary widely, and in some cases, a cavalier may even have SM without displaying any outward symptoms at all. Some cavaliers diagnosed with SM lack any clinical signs. It also is possible that a dog with Chiari-like malformation (CM) does not have syringomyelia (the syrinx in the spinal cord), but still may have symptoms of SM due to the CM obstructing the flow of cerebrospinal fluid (CSF). This also is attributed to a direct compression of the medulla oblongata, which is involved in the modulation of pain.

Ease the symptoms by using a comfortable harness instead of a collar and leash. One of the best harnesses for cavaliers with CM/SM symptoms is the BRILLIANT K9 "Lucy Small" harness, available on Amazon. It is easy to put on and easy to take off. Watch the videos: "Opening the harness" and "Walking the dog with the harness".

Dogs with CM but not SM can experience discomfort, including pain. Such pain is evidenced by vocalization (barking, whining, moaning) particularly when being picked up under the chest, when shifting position while lying down, head and ear rubbing or scratching, refusal or difficulty jumping or climbing stairs, unwillingness to exercise, and being withdrawn, with the signs being consistent over months and with other sources of pain being eliminated. This is attributed to a direct compression of the medulla oblongata, which is involved in the modulation of pain.

In a November 2018 article, Drs. Rusbridge and Knowler summarize how to diagnose pain due to CM as follows:

"Diagnosis of CM-pain is made by appropriate clinical signs in addition to MRI brain findings of a brachycephaly with rostrotentorial crowding including rostral flattening, olfactory bulb reduction and rotation, increased height of the cranium with reduction of the functional caudotentorial space and hindbrain herniation. There may also be changes suggesting raised intracranial pressure such as loss of sulci definition with ventriculomegaly. The cisterna magna is reduced."

In an October 2012 study by UK researchers of 48 cavaliers, nine of which had only CM and the rest also had SM, neuropathic pain progressed in 75% of the dogs over a mean average period of 39 months. The researchers noted that it is not fully understood how CM/SM causes neuropathic pain, and they did not make any such finding. However, their report confirms that neuropathic pain does exist, and it progresses, in cavaliers with only CM.

In a January 2017 article, UK researchers determined, from its group of 28 cavaliers with pain due to CM but not SM, that they "had a short basicranium (line ab in Figure 1 below) with a resultant compensatory increased cranial height (small angle 7, below) and increased brachycephaly with olfactory bulb more ventrally rotated (p = 0.003) and rostral forebrain flattening compared to Control CKCS. However, in comparison with SM dogs, the CM cohort has a longer line bc and a wider angle 9 increases the volume of the caudal fossa, which may lessen obstruction to CSF flow and the risk of developing SM."

In a November 2017 article, a team of researchers at NC State University and Cornell University prepared and studied the results of a questionnaire answered by owners of 50 cavalier King Charles spaniels, 20 with only Chiari-like malformation (CM) and 30 with both CM and syringomyelia (SM). Of the 50 dogs, 33 were symptomatic and 17 were not. The most common presenting sign was phantom scratching, occurring in 32 dogs, and the most common sign of pain was crying out when being lifted, occurring in 11 dogs. The researchers found that owner-reported findings were not significantly associated with presence or severity of SM or neurologic examination findings. Ten of the 20 CM-only dogs reportedly displayed "classic" signs of neuropathic pain.

The series of questionnaires are linked here: Preliminary Questionnaire -- Dog Diagram (ChiMPS-M) -- Final Questionnaire (ChiMPS-T). The researchers concluded:

"T" The conclusion that SM causes pain in CKCS is complicated by the finding that dogs with CM but no SM can show classic signs of neuropathic pain, as illustrated by 10 dogs in our study. ... To conclude, the full range of signs reported by owners of CKCS includes a variety of manifestations of pain, with phantom scratching as the most commonly reported sign followed by crying out when being lifted. Owner reporting of pain and scratch frequency and severity captured by the ChiMPS-T correlates with the owner-reported surface area affected by these signs in their dogs. Neither the scores nor the surface area reported correlated with the presence or severity of SM, highlighting uncertainty on the source of pain in these dogs."

In Figure 8 below, this comparison series of MRI scans of three cavaliers (from this November 2018 article by Drs. Rusbridge and Knowler) demonstrates differences in the positioning (and crowding) of the brain among dogs with normal, CM-pain-affected, and SM-affected conditions. See, also, our discussion of Syringomyelia -- Expressions of Pain, below.

In a July 2019 article, UK neurology researchers studied the records of 130 cavalier King Charles spaniels diagnosed with CM and some also with SM to determine which symptoms (clinical and behavioral signs) related to CM and to SM and to syrinx diameter, in order to use the data in future studies of diagnosis, treatment, and genetics of CM/SM. Dogs were grouped based upon whether they had no SM (Group 1) up to having a syrinx greater than 4 mm (Group 4).

Signs found NOT RELATED to syrinx presence or size:



• Vocalization (65.4%) (except being picked up under the sternum (breastbone), which was more common among dogs with no or mild SM).

• Spinal pain -- 54.6%

• Reduced activity -- 37.7%

• Reluctance to jump or to climb stairs --35.4%

• Aversion to touch or grooming -- 30.0% (ears, head, and neck region -- 25.4%)

• Change in emotional state (more timid, anxious, withdrawn, or aggressive) -- 28.5%

• Disrupted sleep -- 22.3%

The implication from this study is that the major signs of pain expressed by CM/SM cavaliers definitely are not due to having a syrinx and probably are due to having Chiari-like malformation, but the authors cannot definitely attribute the painful expressions solely to CM.

As the disorder progresses, there usually follows increasingly severe pain around the dog's head, neck, and shoulders, causing it yelp or scream. This is described as "vocalization" -- spontaneous, when picked up under the sternum, or when changing position especially at night. It is believed to be a neuropathic pain, probably due to disordered neural processing in the damaged dorsal horn.

In a May 2014 report, USA researchers assessed 36 cavalier King Charles spaniels for neurologic pain and dysfunction. They found that 20 of the dogs demonstrated neuropathic pain; that dural bands (compressive lesions caused by abnormally thickened dura matter at the craniocervical junction) were present in 31 of the dogs; that 34 of the dogs had Chiari-like malformation; that 23 of the dogs had syringomyelia (and 21 of those 23 dogs had dural bands). They also found that dural bands were associated with both the presence and severity of clinical signs and the presence of SM, and that higher compression indices were associated with more severe SM. They concluded that:

"Dural bands appear to play a significant clinical role. Compression indices provide a better assessment of dural band severity compared to grading."

Similar forms of neuropathic pain in humans suffering from Chiari type I malformation (the human counterpart to CM and SM include: (a) a burning type pain, pins-and-needles and other odd sensations (called dysaesthesia); (b) pain from a stimulus which is not normally painful, such as light touch or motion (called allodynia); (c) increased pain from stimuli which are normally painful (called hyperpathia,); and (d) a constant, burning type pain (called causalgia). In humans, neuropathic pain also is associated with anxiety, depression, and reduced quality of life.

As the disease destroys portions of the cavalier's spinal cord, the dog may experience so much pain that it may contort its neck and may even sleep and eat only with its head held high. Ultimately, the dog may develop scoliosis, as a result. There may also be progressive weakness in the legs, so that walking becomes increasingly difficult. Some dogs deteriorate to the point of paralysis.

In a June 2007 study of 55 cavaliers, the researchers reported that the wider the syrinx, the stronger the predictor of pain, scratching behavior and scoliosis in dogs with syringomyelia. They stated: "Both pain and syrinx size were positively correlated with syrinxes located in the dorsal half of the spinal cord." They also concluded that such pain is likely to be neuropathic pain, resulting from disordered neural processing in the damaged dorsal horn. Similarly, in an August 2012 study, the researchers found evidence that:

"... the disruption of the dorsal horn structure is a significant event in the production of clinical signs in CKCS. The spinal cord dorsal horn in symptomatic CKCS is significantly more asymmetric than that of control animals, whereas the asymptomatic CKCS have changes that are midway between control and symptomatic CKCS. This suggests the possibility that progression from mild to severe asymmetry in CKCS is associated with development of clinical signs; however such a conclusion cannot be definitively supported by this study because of the cross sectional nature of the data collected."

Syringomyelia can be very deceptive because some symptoms (which may include paw licking, head shaking, head rubbing, circular walking, fly biting, and reluctance to defecate) are common behaviors for many unaffected dogs. One distinction is that dogs suffering from SM engage in these patterns excessively and seemingly compulsively. So, other causes of the dog's symptoms need to be considered and should be ruled out before concluding that SM is the cause. For example, if a syrinx develops in a lower area of the spine, such as the lumbar region, the dog may scoot excessively, even to the extent of rubbing the anal area raw. However, scooting is a common symptom of other disorders, or even of no particular disorder at all.

In a 2009 study of 64 cavaliers affected with CM/SM, Drs. Sofia Cerda-Gonzalez, Natasha J. Olby and others classified clinical signs of pain from grade 0 to grade 5, by which the dogs displayed symptoms of neck scratching, head scratching, neck pain upon neurolgoical examination, as well as ataxia and paresis detected upon examination. See table below:

In a September 2017 study of CM and SM in Chihuahuas, the researchers used a questionnaire for the dogs' owners to complete. The questions for dogs included a grading system for the description of the presence frequency, and severity of CM/SM-related clinical signs, such as:

1.persistent scratching episodes of the ears or shoulders with or without skin contact

2.persistent scratching episodes of the cranial thoracic spine with or without skin contact

3.facial rubbing

4.spinal hyperesthesia

5.vocalization

6.gait incoordination

7.weakness

These were graded from 1 (occurring <2 times a week) to 5 (occurring several times a day). A percentage from the maximum points (7x5 = 35 points) was calculated for each patient.

In a 2010 Canadian study, researchers found a significant linear correlation between the severity of neurologic dysfunction and size of the syrinx, with a larger syrinx being associated with more severe neurologic signs.

In an April 2012 study, Geoffrey Skerritt and Dr. Luca Motta observed that the level of neurological pain a dog experiences can only be based upon subjective evaluations of the dog's behaviors, which includes the dog's owner's subjective observations.

In order to evaluate changes in the level of a dog's discomfort as objectively as possible, to determine whether their surgical procedure on the dog was successful (see "syringosubarachnoid shunt" below) these neurosurgeons devised a Pain Score Scheme (see table below) for the dog's owners to measure pain which the dog experiences from CM and SM, particularly following surgery. Their pain score scheme was created on the basis of different neurological grade classifications previously suggested by other researchers, including the Cerda-Gonzalez / Olby 2009 study above. Mr. Skerritt and Dr. Motta evaluated their patients’ histories and found some specific information that could be used to create an objective pain score (i.e., frequency of scratching episodes and site of scratching, screaming episodes). However, because of the inherent subjectivity of relying upon reports from the dog's owners, they concluded that the design of a more robust scoring system together with prospective studies was warranted.

In a 2012 study, UK researchers of cavaliers with neuropathic pain report on the results of far more extensive questionnaires (using a 5-point scale), completed by the owners of 122 CM/SM-affected CKCSs. They found that owners who noticed evidence of neuropathic pain in their dogs also found the dogs to have increased fear-related behaviors (such as acting more fearfully when approached by strangers, or when in unfamiliar situations, or when sudden loud noises occurred, such as thunderstorms). These dogs also were more clingy to their owners and appeared to be more fearful when left alone. They also showed decreased willingness to exercise, and problems in settling, including sleep disturbances. Not surprisingly, the study also showed that owners found that their affected dogs had reduced quality of life.

In an effort to pinpoint the locus of the pain caused by SM, in an August 2012 study, researchers compared the expression of two pain-related neuropeptides* in the spinal cord dorsal horn of normal dogs with the peptides' expression in cavaliers with and without clinical signs of syringomyelia. They discovered that there was a decrease in expression of both peptides in CKCSs with symptomatic syringomyelia.

In an April 2013 study of 26 cavaliers (11 dogs without clinical signs of pain; 6 dogs with pain and symmetrical syrinxes; 9 dogs with pain and asymmetrical syrinxes), German researchers found "an association" between pain and SM asymmetry, and they found "a strong association" between pain and dorsal horn involvement of SM. CKCSs with clinical signs of pain showed either asymmetrical syrinxes or involvement of the dorsal horn gray matter. They also found that cavaliers with clinical signs of pain showed a presence of interleukin-6 (a key component of the nervous system’s response to injury) and substance P (a neurokinin that regulates the immune functions of spinal glial cells) in their cerebrospinal fluid (CSF). The researchers conclude that the release of interleukin-6 and substance P is a factor in the development of persistent pain in cavaliers with SM. They suggest that this information could offer new diagnostic and treatment options for CKCSs with SM. (In the study's photo above, the top three syrinxes are asymmetrical; the bottom three are symmetrical.)

In a May 2016 article, UK researchers tested the electronic von Frey aesthesiometer (eVF) (at right) on twelve cavaliers to determine if they could quantify the dogs' cervical skin sensitivity. They decided that the number of dogs was insufficient to reach any conclusions, and they announced plans to study a larger group of CKCSs and divide them by CM/SM status. Their aim is to establish a protocol to quantify neck pain in cavaliers with neuropathic pain.

In a November 2016 abstract, UK researchers (H. Williams, S. Sanchis, H. A. Volk, L. Pelligand, J. Murrell, N. Granger) tested 70 cavalier King Charles spaniels for skin sensitivity using the eVF. The dogs were categorized in three classes: (i) 37 dogs had syringomyelia and clinical signs (syringomyelia-symptomatic – SM-S); (ii) 15 dogs had syringomyelia without clinical signs (syringomyelia-asymptomatic – SM-A); and (iii) 18 dogs had no syringomyelia (syringomyelia-free - SM-F). The researchers found that eVF assessment of skin sensitivity does not differ significantly by syringomyelia status.

In a July 2019 article, a team of UK neurology researchers studied the records of 130 cavalier King Charles spaniels diagnosed with Chiari-like malformation (CM) and some also with syringomyelia (SM) to determine which symptoms (clinical and behavioral signs) related to CM and to SM and to syrinx diameter, in order to use the data in future studies of diagnosis, treatment, and genetics of CM/SM. Dogs were grouped based upon whether they had no SM (Group 1) up to having a syrinx greater than 4 mm (Group 4).

Signs found TO BE RELATED to syrinx presence or size:



• Phantom scratching -- 67% of Group 4 dogs and none in other groups

• Scratching or rubbing of the head or ears -- 28% but less common in Group 4 dogs

• Scoliosis -- 27% of Group 4 dogs and none in other groups

• Postural defects -- 15% of Group 4 dogs and none in other groups

• Weakness -- 39% of Group 4 dogs and none in other groups

The researchers stated that their findings suggest that "phantom scratching is highly unlikely with small syrinxes." They also found that among the dogs in their study, PSOM (primary secretory otitis media) is common in symptomatic CM-affected dogs and in SM-affected dogs, thereby raising the possibility of confusing the cause of some signs and behaviors, since PSOM-affected dogs tend to engage in ear-rubbing. They concluded:

"The study further suggests that SM-specific signs are phantom scratching, scoliosis, and sensory and motor signs that can be related to spinal cord damage by the syrinx and are associated with large syringes (transverse width ≥4 mm). Non-SM-specific signs include vocalization (described as without obvious trigger, when shifting position when recumbent and when being lifted under the sternum to a height), spinal pain, head and ear rubbing or scratching, aversion to touch, refusal or difficulty jumping or doing stairs, exercise intolerance/reduced activity, sleep disruption, or behavioral change described as becoming more anxious, timid, aggressive, or withdrawn. These non-SM-specific signs could reflect CM-P [CM associated pain]."

The implication from this study is, therefore, that the major signs of pain expressed by CM/SM cavaliers definitely are not due to having a syrinx and probably are due to having Chiari-like malformation, but the authors cannot definitely attribute the painful expressions solely to CM.

Several of these signs are so common that they may be for other reasons and unrelated to CM or SM at all. For example, prior studies have shown that approximately 25% of dogs that display clinical signs of SM are not found to have a syrinx on MRI scans. See, e.g., this December 2011 article.

Another disorder common to cavaliers and with symptoms similar to SM is Primary Secretory Otitis Media (PSOM), which is a highly viscous mucus plug which fills the middle ear and causes the tympanic membrane to bulge. Because the pain and other sensations in the head and neck areas, resulting from PSOM, are so similar to symptoms due to SM, the possibility that the cavalier has PSOM and not SM should be determined before diagnosing SM.

In a brief July 2009 article, UK researchers Dr. Richard J Piercy and Gemma Walmsley disclosed that they had identified a genetic form of muscular dystrophy in the cavalier, with symptoms (weakness and exercise intolerance) similar to some of those of SM. However, these other symptoms of this muscular dystrophy may clearly distinguish it from SM: muscle atrophy, difficulty swallowing, and an enlarged tongue. Also, the researchers have found that only males are affected by this form of muscular dystrophy, and the females are only carriers of the mutation.

Dr. Curtis Dewey has reported that in the course of his examination of MRIs of cavaliers with Chiari-like malformation, he also has discovered cerebellar infarcts (strokes). He has written that CKCSs appear to be pre-disposed to infarcts due to the presence of CM and that the existence of both CM and infarcts "is common in the CKCS." See Cerebellar Infarcts for details.

Scratching the belly or chewing the paws are not signs of CM or SM. If a dog engages in phantom scratching but does not have a wide syrinx extending to the superficial dorsal horn (SDH) in the C3-C6 spinal segments, then that dog most likely is not displaying a symptom of SM.

Seizures are not caused by CM or SM. Progressive paralysis of the hind legs is not caused by CM or SM. CM/SM-affected dogs may display some weakness in their hind legs, and they may stumble when they walk, but they keep on walking nonetheless. So, paralysis of the hind legs indicates, more likely, intervertebral disc disease or degenerative myelopathy.

Behaviors and physical signs customarily attributed to epilepsy, fly catching, facial nerve paralysis, vestibular (balance) disease, intervertebral disc disease, and degenerative myelopathy (severe and progressive hind limb weakness) are either not caused by CM or SM or are highly unlikely to be associated with either CM or SM. These disorders do not result in structural changes on MRI scans.

The most accurate way of diagnosing the disease is said to be through the use of magnetic resonance imaging (MRI) scanning. Clinic charges for MRI examinations of canines have been known to vary from $400.00 to over $2,000.00.* Accurate MRI results require that usually the dog be anesthetized. In view of the high cost of MRI scans, the examing veterinary specialist usually will attempt to rule out other causes of the symptoms first. Veterinarians who perform MRIs of should consider following this MRI Screening Protocol devised by Dr. Rusbridge.

The MRI allows the veterinary neurologist or neurosurgeon to study the skull and spine for the presence of any abnormality which might obstruct the flow of the cerebrospinal fluid. When examined by MRI, the syringomyelia appears as a tubular cavity of fluid, called a syrinx, within the spinal cord. In severe cases, the syrinx is so wide that only a thin rim of the spinal cord is visible. An MRI scan of a dog without any syrinxes at all still may show that the dog has Chiari-like malformation.

While MRI is considered "the gold standard" in diagnosing both CM and SM, as well as the degree of each, MRI is not without error. It requires the examiner to make certain viewing decisions, such as the angle for transverse images, and if that angle is not perpendicular to the dog's spinal cord, the images may artificially enlarge the central canal and result in an incorrect diagnosis. Also, the abilities and experience levels of the examiners have been shown in studies to play an important role in the accuracy of their analyses. Radiology diplomates with decades of experience are much better equipped to accureately diagnose the disorders and grade their degrees than are less experienced examiners. See this April 2020 article for a discussion of the comparisons in abilities of MRI examiners regarding cavaliers with or without CM and SM.

The MRI scan of a cavalier below at the right shows the occipital malformation, with the cerebellum being squeezed out of the occipital bone and into the area of the foramen magnum (red-outlined area). It also shows pockets of white cerebrospinal fluid in the spinal cord (yellow-outlined area). See Karen Kennedy's Basic Canine NeuroAnatomy and MRI Imaging Planes, for further information about MRI scans.

In a 2011 study conducted by Drs. Rusbridge and Knowler, in a sample of seventy "unaffected" cavaliers from Europe and North America, which were MRI-scanned only for breeding purposes, 70% of them had syringomyelia, 17% were "at risk", meaning were young dogs with Chiari-like malformation but no syringomyelia yet, and only 13% were "clear" of both the malformation and SM. In February 2010, Dr. Georgina Child, board certified veterinary neurologist in Australia, reported that of 60 asymptomatic cavaliers scanned as potential breeding stock, 50% had SM syrinxes.

In MRI studies of 49 cavaliers, reported in 2011 in the Veterinary Journal, Dr. Rusbridge and others found that "Syrinx formation was present in the C1–C4 region and in other parts of the spinal cord. The maximal dorsoventral syrinx size can occur in any region of the spinal cord." Seventy-six per cent of CKCS with a a cranial cervical syrinx also had a syrinx in more caudal spinal cord regions. Therefore, so-called "mini-MRI-scans" of only the cervical region, such as those scans for breeding protocol purposes, may not necessarily locate all syrinx which an SM-affected cavalier may have.

Dr. Curtis Dewey has reported that in the course of his examination of MRIs of cavaliers with Chiari-like malformation, he also has discovered cerebellar infarcts (strokes). He has written that CKCSs appear to be pre-disposed to infarcts due to the presence of CM and that the existence of both CM and infarcts "is common in the CKCS." See Cerebellar Infarcts for details.

Also, benign, small syrinxes are a common incidental finding on MRI examinations. Therefore, clinical

correlation is important in order to determine if the syrinx is associated with the Chiari-like malformation and/or symptoms.

The following MRI photographs, and their descriptive text, are courtesy of Dr. Clare Rusbridge and Dr. Penny Knowler of Stone Lion Veterinary Centre:

Left: This image shows mild Chiari-like malformation – the cerebellum is very slightly indented, the kinking of the medulla is normal for a toy breed and there is displacement of the cerebellum into and just out of the foramen magnum. The ventricular system is slightly dilated.

Right: Although the cerebellum is not coming through the foramen magnum, this dog has a greater degree of Chiari-like malformation than the first dog. The cerebellum is indented, and the medulla is kinked. The central canal is dilated above the first disc space – this is the first sign of syringomyelia developing. There is also mild ventricular dilatation.

Left: This dog has descent of the cerebellum towards the foramen magnum and the cerebellum is indented. The medulla is normal for a toy breed; there is mild ventricular dilatation and a small syrinx/central canal dilatation in the upper cervical spinal cord.

For more MRI views of cavaliers with syringomyelia or the Chiari-like malformation, see Karen Kennedy's* Understanding Canine Chiari Malformation and Syrningomyelia and Related Links below.

In a November 2018 article, Drs. Clare Rusbridge and Susan P. Knowler and Felicity Stringer have thoroughly summarized the current research knowledge about symptomatic Chiari-like malformation (CM) and syringomyelia (SM) in cavalier King Charles spaniels and other affected breeds. Their work includes handy charts covering the topics of skull changes, craniocervical junction and cervical changes, neuroparenchymal changes, syrinx features, MRI protocols, and interpretations of MRI scans for diagnosing CM pain and symptomatic SM. They also have comparison MRI scan views of cavaliers with varying stages of CM and SM, including Figure 8, below, comparing the MRI scans of three CKCSs.

Dynamic susceptibility contrast perfusion MRI: Dynamic susceptibility contrast perfusion magnetic resonance imaging (DSC-PMRI) enables the quantification of the volume of blood passing through the brain tissue. In an August 2017 article, researchers studied the ventricle system of 42 cavalier King Charles spaniels -- 32 CKSCs with ventriculomegaly and 10 control CKCSs -- using DSC-PMRI.

Computed tomography (CT) is an imaging method using digital geometry processing to generate a three-dimensional image of the inside of an object from a large series of two-dimensional x-ray images taken around a single axis of rotation. Researchers have been studying the value of CT scans to detect Chiari-like malformations and syrinxes in cavaliers and comparing the results with MRIs and other resources. In a very preliminary 2008 French study, researchers CT scanned sixteen CKCS to measure the size of their caudal fossas and to determine standard computed tomography dimensions of the caudal fossa.

Dr. Dominic J. Marino of Long Island Veterinary Specialists (LIVS) reported in October 2007 that evaluation of the entire skull shape and size utilizing Spiral CT technology with 3D reconstruction is currently underway to identify additional mechanisms of syrinx formation. He wrote that CT scanning may enable surgeons to focus on correcting the flow of CSF as the malformation affects its normal passage around the brain and spinal cord and leads to the syrinx formation known as syringomyelia.

In a 2013 study of nine cavaliers with neurological disorders, a team of Ghent University (Belgium) veterinary 