Huntington disease (HD) is an autosomal dominant, neurodegenerative disorder that results from an abnormal CAG triplet repeat expansion in the huntingtin (HTT) gene on chromosome 4. Disease prevalence is higher in populations of European ancestry and lower in Japan, China, Korea, and Finland. Clinical manifestations can begin at any age, but onset typically occurs before age 50 and may begin with subtle motor, cognitive, or behavioral changes. As the disease progresses, signs and symptoms include chorea, dystonia, bradykinesia, rigidity, impaired eye movements, dysarthria, dysphagia, ataxia, dementia, irritability, and psychiatric disease. Patients survive for about 15-20 years after symptom onset.

HD inheritance is characterized by anticipation. Successive generations, especially with paternal inheritance, tend to have a greater number of CAG repeats and thus an earlier age at onset.

Juvenile HD, also called the Westphal variant, presents before age 21 years and accounts for less than 10% of all cases of HD. Juvenile HD typically presents with dysarthria, difficulty walking, and cognitive and behavioral changes. Over time, parkinsonism (bradykinesia or akinesia and rigidity) and seizures develop. Chorea and dystonia may occur later in the disease course. The disease progresses more rapidly in children than it does in adults, and patients survive on average 8-10 years following symptom onset.