Electrophysiological studies

Patients in whom a diagnosis of ALS is suspected on clinical grounds should have electrophysiological studies primarily to document lower motor dysfunction in clinically involved and uninvolved regions, and secondarily to exclude other disease processes. The first published criteria for electrodiagnosis of ALS were by Lambert in 1957 and 1969 [186, 187]. The revised El-Escorial criteria [9] have proposed electrophysiological criteria for the diagnosis of ALS, which have been future refined in December 2006 at an consensus conference on Awaji Island, Japan [10]. It is important to bear in mind that clinical neurophysiological examination is used in the diagnosis of ALS when the diagnosis is clinically suspected, and suggestive neurophysiological abnormalities alone cannot clinch the diagnosis without clinical support.

1. Nerve conduction studies (motor and sensory)

Nerve conduction studies are required for the diagnosis principally to define and exclude other disorders of peripheral nerve, neuromuscular junction and muscle that may mimic or confound the diagnosis of ALS, and these studies should generally be normal or near normal, unless the compound muscle potential is small [9]. In ALS, the distal motor latency (DML) and motor conduction velocity (MCV) remain almost normal, never falling below 70% of the upper or lower limit of normal [188–190]. Motor studies are also important in excluding multifocal motor neuropathy, by the detection of partial conduction block. A marked reduction of proximal amplitude or negative-peak area as compared with the distal ones (over 50%), in short segments, (excluding entrapment sites) implies partial conduction block [191]. F-wave studies are particularly useful in assessing proximal conduction and abnormalities have been reported in ALS. These include increased F-wave latency with normal frequency and increased amplitude, and slowing of F-wave velocity with decreased F-wave frequency. Prominent UMN features may be associated with an increased F-wave frequency [188].

The sensory nerve conduction studies can be abnormal in the presence of entrapment syndromes and coexisting peripheral nerve disease [9]. There is also recent evidence sub-clinical involvement of the sensory system in 10–20% of patients with ALS, suggesting an additional polyneuropathy or sensory ganglionopathy [192, 193].

2. Conventional electromyography

Concentric needle electromyography (EMG) provides evidence of LMN dysfunction which is required to support a diagnosis of ALS, and should be found in at least two of the four CNS regions: brainstem (bulbar/cranial motor neurons), cervical, thoracic, or lumbosacral spinal cord (anterior horn motor neurons). For the brainstem region it is sufficient to demonstrate EMG changes in one muscle (e.g. tongue, facial muscles, jaw muscles). For the thoracic spinal cord region it is sufficient to demonstrate EMG changes either in the paraspinal muscles at or below the T6 level or in the abdominal muscles. For the cervical and lumbosacral spinal cord regions at least two muscles innervated by different roots and peripheral nerves must show EMG changes [9].

The revised El-Escorial criteria require that both evidence of active or ongoing denervation and chronic partial denervation is required for the diagnosis of ALS, although relative proportions vary from muscle to muscle [9]. Signs of active denervation consist of:

1. fibrillation potentials

2. positive sharp waves

Signs of chronic denervation consist of:

1. large motor unit potentials of increased duration with an increased proportion of polyphasic potentials, often of increased amplitude

2. reduced interference pattern with firing rates higher than 10 Hz (unless there is a significant UMN component, in which case the firing rate may be lower than 10 Hz)

3. unstable motor unit potentials.

Fasciculation potentials are an important characteristic finding in ALS, although they can be seen in normal muscles (benign fasciculations) and are not present in all muscles in ALS patients. In benign fasciculations the morphology of the fasciculation potentials are normal, whereas in fasciculation potentials associated with neurogenic change there are abnormal and complex morphology [10, 194]. The Awaji group suggest that the presence of abnormal complex fasciculation potentials in a muscle showing neurogenic change, can be considered equivalent in importance to fibrillation potentials or positive sharp waves [10].

3. Transcranial magnetic stimulation and Central motor conduction studies

Transcranial magnetic stimulation (TMS) allows a non-invasive evaluation of corticospinal motor pathways, and allows detection of UMN lesions in patients who lack UMN signs. Motor amplitude, cortical threshold, central motor conduction time and silent periods can be easily evaluated using this method [195]. Central motor conduction time (CMCT) is often marginally prolonged to muscles of at least one extremity in ALS patients. Electrophysiological features compatible with UMN involvement include [9]:

1. Up to a 30% increase in central motor conduction time determined by cortical magnetic stimulation and

2. Low firing rates of motor unit potentials on maximal effort.

Marked prolongation in the CMCT is seen in FALS patients with D90A SOD1 mutations and patients with the flail arm and flail leg variants [196–198].

4. Quantitative electromyography

Motor unit number estimation (MUNE) is a special electrophysiological technique that can provide a quantitative estimate of the number of axons innervating a muscle or group of muscles. MUNE consists of a number of different methods (incremental, multiple point stimulation, spike-triggered averaging, F-wave, and statistical methods), with each having specific advantages and limitations. Despite the lack of a perfect single method for performing MUNE, it may have value in the assessment of progressive motor axon loss in ALS, and may have use as an end-point measure in clinical trials [199].

Neuroimaging studies

The most important use of neuroimaging is in the diagnosis of ALS to exclude treatable structural lesion that mimics ALS by producing varying degrees of UMN and LMN signs, especially in those with clinically probable or possible ALS. The WFN revised criteria state that imaging studies are not required in cases that have clinically definite disease with bulbar or pseudobulbar onset as it is unlikely that structural lesions can mimic clinically definite disease [9]. Magnetic resonance imaging (MRI) can be used in revealing lesions in the corticospinal tracts in ALS. The most characteristic finding in ALS is hyperintensity of the corticospinal tracts on T2-weighted, proton density weighted and FLAIR-weighted MRI, and is best visualised in the brain and brainstem and to a lesser extent in the spinal cord [200–203]. T2 weighted MRI may also show hypointensity of the primary motor cortex, usually along the posterior bank of the precentral gyrus, although this is an inconsistent and non-specific finding [204].

More advanced neuroimaging modalities such as magnetic resonance spectroscopy, diffusion weighted imaging (DWI)/diffusion tensor imaging (DTI), magnetic resonance voxel-based morphometry and functional imaging techniques (fMRI, PET and SPECT) have a limited role in routine clinical practice but have shown promise in understanding pathophysiology of the disease in vivo, identification of potential biomarkers of disease progression and identifying disease changes earlier in the course of the disease facilitating earlier diagnosis [205–210].

Muscle biopsy and neuropathalogical studies

Biopsy of skeletal muscle or other tissues is not required for diagnosis, unless to rule out a mimic syndrome (e.g. Inclusion body myositis). In addition, muscle biopsy may be used to demonstrate LMN dysfunction in a body region when clinical or electrophysiological findings do not support this. Histological findings that are compatible with the diagnosis of ALS include [9]:

• Scattered hypertrophied muscle fibres.

• No more than a moderate number of target or targetoid fibres.

• Fibre type grouping of no more than mild-to-moderate extent.

• The presence of a small number of necrotic muscle fibres.

Other laboratory studies

There are few other investigations that may be considered mandatory in the work-up of an ALS patient. Clinical laboratory tests that may be abnormal in otherwise typical case of ALS include [9]:

• Muscle enzymes (serum creatine kinase [unusual above ten times upper limit of normal], ALT, AST, LDH)

• Serum creatinine (related to loss of skeletal muscle mass)

• Hypochloremia, increased bicarbonate (related to advanced respiratory compromise)

• Elevated CSF protein (uncommonly more than 100 mg/dl)