Also called micropsia, this condition distorts visual perception so that objects that are close appear disproportionately tiny, as though viewed through the wrong end of a telescope. It owes its name to Lewis Carroll's fictional protagonist, Alice, who perceived things as too small or too big after taking magical medicines. Usually temporary in nature, the syndrome is associated with migraines. Carroll suffered migraines, so perhaps he was describing his own experiences. PICA

This is a compulsive appetite for non-edible items, including clay, stones, cigarette ash, paint, glue, laundry starch, ice and even hair. Found among pregnant women and young children, particularly in poorer areas, it's thought to relate to nutritional deficiencies and may be treated with mineral supplements. However, that's just one of many theories about pica, whose precise causes are unknown. It's also found among brain damaged or mentally ill people, among whom it can take particularly dangerous turns including swallowing sharp objects (known as acuphagia). Undigested items can also form bezoars - calcified "stones" - in the stomach. FOREIGN ACCENT SYNDROME Imagine suddenly putting on a convincing French accent. Or Scottish. Or Italian. It sounds like fun, but it's no joke for the victims of Foreign Accent Syndrome, which can set in after strokes or other brain trauma. Without warning, they'll start speaking their native tongue with a different accent, which could sound anything from Swedish to South African. Victims need never have heard the accent in question, according to Oxford University researchers Dr Jennifer Gurd and Dr John Coleman, who believe it arises from damage to areas of the brain responsible for language production, altering pitch, pronunciation and speech patterns. So people with the syndrome aren't putting on a foreign accent, it just sounds that way.

ONDINE'S CURSE A wonderful name for a nasty problem, this is a sleep disorder resulting from a malfunctioning autonomic nervous system. Its victims are unable to breathe spontaneously but must consciously will each breath, so will suffocate if they fall asleep. Respirators may help. Also known as congenital central alveolar hypoventilation syndrome, Ondine's Curse derives its name from the legend of a water nymph, Ondine, who fell in love with a human, thereby forfeiting her immortality. Though he had pledged his undying love, Ondine discovered him snoring in the arms of another woman. She cursed him, declaring that as he had pledged his love with every waking breath, he would die the moment he fell asleep.

ALIEN HAND SYNDROME Another condition arising from brain trauma, this bizarre syndrome involves losing control of one hand, which can do anything from gesticulating to unbuttoning clothes its owner is trying to put on with his or her other hand. The condition is also called Dr Strangelove Syndrome, thanks to Peter Sellers' inspired performance as Dr Strangelove in Stanley Kubrick's 1964 film. Sellers's mechanical hand alternated between throttling himself and throwing Nazi salutes. While victims can still experience sensation in the affected hand, they say it seems to have a mind of its own. The only solution is to keep it busy, for example by holding onto something - just not, hopefully, your own throat. POLYDACTYLISM

Still on the subject of hands, polydactylism is a congenital abnormality involving being born with too many digits, ranging from rudimentary nubbins to fully formed fingers or toes. While rare, polydactylism is prevalent among communities given to intermarriage, such as Philadelphia's Old Order Amish. Notables with the condition have included Henry VIII's ill-fated second wife Anne Boleyn (whose extra pinky sparked rumours of her being a witch), war photographer Robert Capa and cricketer Garry Sobers. According to The Guinness Book of Records, the record for extra digits goes to Indian brothers Tribhuwan and Triloki Yadav, who boast 20 fingers, four thumbs and 24 toes between them. CAPGRAS SYNDROME

A loved one has been stolen by a doppelganger; sounds like a movie about alien abduction. But for sufferers of Capgras Syndrome, the action occurs only in their brains, not outer space. This syndrome involves the delusion that a significant other, such as a parent, spouse or other relative, is being impersonated by an imposter. Sufferers sometimes attack the supposed double. The delusion can also extend even to oneself, with the person convinced that the reflection in the mirror is that of an imposter. While extremely rare, it is linked with brain damage, psychotic disorders and various neurological problems that somehow interfere with normal face recognition abilities. The syndrome owes its name to the French psychiatrist who first described it. RILEY-DAY SYNDROME Feeling no pain; it doesn't sound like a problem, but it can be lethal for the victims of a syndrome involving, among other symptoms, insensitivity to pain. Caused by a chromosomal abnormality found among Ashkenazi Jews - people of Eastern European Jewish descent - the syndrome makes its victims exceptionally accident-prone because they simply don't register warning signs of tissue damage such as wounds, bruising and burns. They are even oblivious to oxygen deprivation, which means that when they hold their breath, as infants often do, they do so until they black out. Riley-Day patients tend to die young - around half before the age of 30 - from their injuries.

JERUSALEM SYNDROME This one sounds like the title of a Robert Ludlum thriller, but it's actually a religious psychosis triggered by a visit to Jerusalem. Observed since medieval times, its victims may believe that they are prophets and parade around the city proclaiming the Holy Writ or exhorting sinners to repent. Affecting around 100 tourists per year, it generally clears up once they leave town. However, it can have serious repercussions, as with the widespread rioting that ensued when Australian man Michael Rohan set fire to the al-Aqsa Mosque on the Temple Mount in 1969, convinced he was the "Lord's emissary".

HYPERTRICHOSIS People with hypertrichosis, a congenital condition involving hair growing all over the body - including eyelids and even ears, which can sprout long curls - have always attracted enormous interest, especially as sideshow stars. Probably the most famous was JoJo the Dog-Faced Boy (aka Fedor Jeftichew, a Russian recruited by showman P.T. Barnum), who toured widely during the latter half of the 19th century. There are different forms of hypertrichosis, distinguished by varying hair type, quantity and distribution. Some cases also have a little hairy appendage called a faun tail. PENIS PANIC

Koro is one of a number of names for a hysterical condition known medically as Genital Retraction Syndrome, whose victims become convinced that their genitals are disappearing into their bodies. It can be contagious, sparking off "penis panics", such as the one that overtook Singapore in 1967 in which thousands of men became convinced that their penises were being stolen; it was contained by a complete media blackout on the condition. Often blamed on witchcraft, Koro typically strikes in less developed parts of the world, including Africa and Asia, where belief in sorcery remains strong. It's thought to be an extreme overreaction to normal genital shrinking from cold or other causes. Koro can be treated with medical reassurance and anti-anxiety medications. PROTEUS SYNDROME

Named after Proteus, the Greek god famous for changing his shape, this is a progressive disorder causing disfiguring tumours and abnormal bone development. It's extremely rare, with just over 100 cases confirmed since it was first identified in 1979. Its most celebrated victim was Joseph Merrick, aka "The Elephant Man", a grotesquely deformed man befriended by Dr Frederick Treves, a physician at London Hospital. At first, Merrick was believed to have suffered from the nerve disorder neurofibromatosis, but in 2003 DNA testing on his remains showed that he in fact had Proteus Syndrome. His story inspired the 1980 film The Elephant Man, starring John Hurt as Merrick. EMOTIONAL BLINDNESS This one is included simply for light relief, as it isn't a rare illness so much as a universal phenomenon.

Emotional blindness occurs when people exposed to confronting images - violent or erotic - fail to register other stimuli for a short (about one-fifth of a second) but critical period thereafter - a hiatus that doesn't occur after looking at more innocuous images. This phenomenon, dubbed "emotional rubbernecking" by US researchers who reported it in The Psychonomic Bulletin and Review last August, could have evolutionary value.

From a survival point of view, it pays to focus more on things that signal physical danger or reproductive possibilities, while filtering out less emotionally charged input. Emotional rubbernecking undoubtedly worked for cavemen, who never knew when the next sabre-tooth tiger (or sexual partner) might turn up. Nowadays, given the capacity of sexy billboards to distract drivers from road conditions, for example, it's more likely to be a health hazard.