Ataxia (clinical sign)
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Ataxia denotes the impaired coordination of voluntary muscle function. It is not a specific disease, but a clinical sign that can have diverse etiologies. It is typically caused by either cerebellar damage or impaired vestibular or proprioceptive afferent sensory input to the cerebellum.
The physical findings depend on the location of the insult. Unilateral cerebellar damage results in ipsilateral, whilst diffuse injuries lead to symmetric ataxia. Hemispherical lesions cause ataxia in the extremities, whilst that of the midline (paleocerebellum) produce truncal ataxia and gait abnormalities 1,2.
Hereditary ataxias typically manifest as chronic, progressive diseases whilst some acquired forms (e.g. those caused by intoxications, infections) have an acute onset 2.
Common clinical symptoms of ataxia 1:
- gait ataxia: impaired coordination of the lower limbs resulting in a characteristic wide-based stance
- sensory ataxia: gait impairment combined with a positive Romberg sign
- truncal ataxia: involuntary oscillating movements of the truncal muscles
- limb ataxia: impaired coordination of the upper extremities
- dysdiadochokinesia: difficulty performing rapid alternating movements (e.g. rapid alternating pronation and supination of the hand)
- intention (kinetic) tremor: impaired coordination of the proximal limb resulting in increasing involuntary oscillations nearing the end of a voluntary movement (e.g. during the finger-to-nose approach test)
- dysmetria: missing a target object during voluntary movement of a limb (e.g. during a shin-tap test)
- dysarthria: impaired articulation, slurred, slow, difficult speech
- nystagmus: commonly encountered in cerebellar injuries, e.g. evoked by lateral gaze
- saccadic intrusions (e.g. ocular flutter, opsoclonus): various forms of involuntary, rapid eye movements secondary to cerebellar pathology
Ataxia can arise due to diverse innate genetic or secondary etiologies.
Acquired (non-genetic) ataxia
Acquired forms include ischemic and hemorrhagic stroke, CNS infections (cerebellitis, abscess), intoxication (e.g. alcohol, solvents, sedatives), systemic infections (HIV, CJD, neurosyphilis), vitamin deficiencies (e.g. B1 and B12), hypothyroidism, paraneoplastic syndromes, cerebellar masses, neurodegenerative disorders (e.g. MSA).
A list of hereditary forms have been described, which are divided into the autosomal dominant (e.g. spinocerebellar ataxias), autosomal recessive (e.g. Friedreich ataxia, ataxia telangiectasia) and mitochondrial categories 1,2.
In hereditary forms of ataxia, atrophy of the cerebellum is the commonest imaging finding. The atrophy may be global, whilst in some forms of autosomal recessive ataxias the vermis is more severely affected 2.