Ataxia

Ataxia (from Greek ataxiā(αταξία), meaning "lack of order") is a neurological sign and symptom consisting of gross incoordination of muscle movements. Ataxia is an aspecific clinical manifestation implying dysfunction of parts of the nervous system that coordinate movement, such as the cerebellum. Several possible causes exist for these patterns of neurological dysfunction. The "International Ataxia Awareness Day" is celebrated on September 25th each year.

Types of ataxia

Cerebellar ataxia

The term cerebellar ataxia is employed to indicate ataxia due to dysfunction of the cerebellum. This causes a variety of elementary neurological deficits, such as antagonist hypotonia, asynergy, dysmetria, dyschronometria, and dysdiadochokinesia. How and where these abnormalities manifest depend on which cerebellar structures are lesioned, and whether the lesion is bilateral or unilateral. Vestibulo-cerebellar dysfunction presents with postural instability, in which the person tends to separate the feet on standing to gain a wider base, and avoid oscillations (especially posterior-anterior ones); instability is therefore worsened when standing with the feet together (irrespective of whether the eyes are open or closed: this is a negative Romberg's test). Spino-cerebellar dysfunction presents with a wide-based "drunken sailor" gait, characterised by uncertain start and stop, lateral deviations, and unequal steps. Cerebro-cerebellar dysfunction presents with disturbances in carrying out voluntary movements, including intention tremor (coarse trembling, accentuated over the execution of voluntary movements, possibly involving the head and eyes as well as the limbs and torso), peculiar writing abnormalities (large, unequal letters, irregular underlining), and a peculiar pattern of dysarthria (slurred speech, sometimes characterised by explosive variations in voice intensity despite a regular rhythm).

Sensory ataxia

The term sensory ataxia is employed to indicate ataxia due to loss of proprioception (sensitivity to joint and body part position), which generally depends on dysfunction of the dorsal columns of the spinal cord, since they carry proprioceptive information up to the brain; in some cases, the cause may instead be dysfunction of the various brain parts that receive that information, including the cerebellum, thalamus, and parietal lobes. Sensory ataxia presents with an unsteady "stomping" gait with heavy heel strikes, as well as postural instability that is characteristically worsened when the lack of proprioceptive input cannot be compensated by visual input, such as in poorly lit environments. Doctors can evidence this during physical examination by having the patient stand with his / her feet together and eyes shut, which will cause the patient's instability to markedly worsen, producing wide oscillations and possibly a fall (this is called a positive Romberg's test). Worsening of the finger-pointing test with the eyes closed is another feature of sensory ataxia. Also, when the patient is standing with arms and hands extended toward the examiner, if the eyes are closed, the patient's finger will tend to "fall down" and be restored to the horizontal extended position by sudden extensor contractions ("ataxic hand").

Vestibular ataxia

The term vestibular ataxia is employed to indicate ataxia due to dysfunction of the vestibular system, which in acute and unilateral cases is associated with prominent vertigo, nausea and vomiting. In slow-onset, chronic bilateral cases of vestibular dysfunction, these characteristic manifestations may be absent, and dysequilibrium may be the sole presentation.

Causes of ataxia

The three types of ataxia have overlapping causes, and can therefore either coexist or occurr in isolation.

Focal lesions

Any type of focal lesion of the central nervous system (such as stroke, brain tumour, multiple sclerosis will cause the type of ataxia corresponding to the site of the lesion: cerebellar if in the cerebellum, sensory if in the dorsal spinal cord (and rarely in the thalamus or parietal lobe), vestibular if in the vestibular system (including the vestibular areas of the cerebral cortex).

Exogenous substances

Exogenous substances that cause ataxia mainly do so because they have a depressant effect on central nervous system function. The most common example is ethanol, which is capable of causing overlapping cerebellar and vestibular ataxia. Other examples include both prescription drugs (e.g. most antiepileptic drugs have cerebellar ataxia as a possible unwanted effect) and recreational drugs (e.g. ketamine, PCP).

Vitamin B₁₂ deficiency

Vitamin B₁₂ deficiency may cause, among several neurological abnormalities, overlapping cerebellar and sensory ataxia.

Causes of isolated sensory ataxia

peripheral neuropathies may cause generalised or localised sensory ataxia (e.g. a limb only) depending on the extent of the neuropathic involvement. Spinal disorders of various types may cause sensory ataxia from the lesioned level below, when they involve the dorsal columns.

Non-hereditary cerebellar degeneration

Non-hereditary causes of cerebellar degeneration include chronic ethanol abuse, paraneoplastic cerebellar degeneration, high altitude cerebral oedema, coeliac disease, and normal pressure hydrocephalus.

Hereditary ataxias

Ataxia may depend on hereditary disorders consisting of degeneration of the cerebellum and/or of the spine; most cases feature both to some extent, and therefore present with overlapping cerebellar and sensory ataxia, even though one is often more evident than the other. Hereditary disorders causing ataxia include autosomal dominant ones such as spinocerebellar ataxia, episodic ataxia, and dentatorubropallidoluysian atrophy, as well as autosomal recessive disorders such as Friedreich's ataxia (sensory and cerebellar, with the former predominating), ataxia-telangiectasia (sensory and cerebellar, with the latter predominating), and abetalipoproteinaemia. An example of X-linked ataxic condition is the rare fragile X-associated tremor/ataxia syndrome.

References

• Boon et al, Davidson's Principles & Practice of Medicine, 20th edition, Churchill Livingstone 2006
• Cambier at al, Neurologie, 11th edition, Masson 2004
• Kandel et al, Principles of Neural Science, 4th edition, McGraw-Hill 2000
• Blumenfeld, Neuroanatomy through Clinical Cases, 1st edition, Sinauer Associates Inc, 2002
• Rolak, Neurology Secrets, 4th edition, Elsevier Mosby, 2005