Clinical characteristics: Spinocerebellar ataxia type 1 (SCA1) is characterized by progressive cerebellar ataxia, dysarthria, and eventual deterioration of bulbar functions. Early in the disease, affected individuals may have gait disturbance, slurred speech, difficulty with balance, brisk deep tendon reflexes, hypermetric saccades, nystagmus, and mild dysphagia. Later signs include slowing of saccadic velocity, development of upgaze palsy, dysmetria, dysdiadochokinesia, and hypotonia. In advanced stages, muscle atrophy, decreased deep tendon reflexes, loss of proprioception, cognitive impairment (e.g., frontal executive dysfunction, impaired verbal memory), chorea, dystonia, and bulbar dysfunction are seen. Onset is typically in the third or fourth decade, although childhood onset and late-adult onset have been reported. Those with onset after age 60 years may manifest a pure cerebellar phenotype. Interval from onset to death varies from ten to 30 years; individuals with juvenile onset show more rapid progression and more severe disease. Anticipation is observed. An axonal sensory neuropathy detected by electrophysiologic testing is common; brain imaging typically shows cerebellar and brain stem atrophy.

Diagnosis/testing: The diagnosis of SCA1 is established in a proband with characteristic clinical findings and an abnormal CAG repeat expansion in ATXN1 identified by molecular genetic testing. Affected individuals usually have 39 or more CAG repeats.

Management: Treatment of manifestations: Supportive care including adaptive devices, physical therapy, occupational therapy, avoidance of obesity; intensive rehabilitation (coordinative physiotherapy) may be beneficial; speech therapy and communication devices for dysarthria; video esophagram to help identify the consistency of food least likely to trigger aspiration and feeding devices may be indicated with recurrent aspiration; caloric support for those with weight loss; vitamin supplementation as needed; psychotherapy, neuropsychologic rehabilitation, and/or standard psychiatric treatments for cognitive and psychiatric manifestations; pharmacotherapy and/or referral to pain management as needed for pain.

Surveillance: Every three to six months: neurologic assessment for progression of ataxia, and physiatry, occupational therapy, and physical therapy assessment for mobility and self-help skills; at each visit: assessment of access to communication, speech needs, aspiration risk, feeding needs, mood, psychiatric manifestations, cognition, and family needs.

Agents/circumstances to avoid: Alcohol, medications known to cause nerve damage (e.g., isoniazid, large-dose vitamin B₆), and circumstances that could lead to physical harm, such as operating machinery or climbing to great heights.

Genetic counseling: SCA1 is inherited in an autosomal dominant manner. Offspring of an affected individual have a 50% chance of inheriting the expanded allele. Anticipation has been observed in SCA1; expansions are more likely to occur when the pathogenic ATXN1 allele is paternally transmitted, and contractions are more typical of maternal transmissions. Once an abnormal CAG trinucleotide expansion in ATXN1 has been identified in an affected family member, prenatal and preimplantation genetic testing are possible.