Huntington's disease (HD) is a genetically inherited neurodegenerative disorder that affects about 1 in 10,000 people in the United States. It is typically characterized by adult-onset chorea. However, clinical presentations can be highly variable, posing significant diagnostic challenges. While chorea is the most common motor manifestation, a parkinsonian phenotype is a well-recognized alternative presentation, particularly in late-onset cases.

A 74-year-old male presented with tremor, progressive reduction in the amplitude of repeated movements, and lack of balance. History was significant for depression and psychosis for three years. Family history was notable for hereditary chorea in two sisters. Neurological examination demonstrated asymmetric tremor, rigidity, hypertonicity in extremities, and bradykinesia. Mini-Mental-State-Examination score was 27/30, with impaired recent memory. Computed-tomography (CT) of the brain was normal. The patient was initially diagnosed with Parkinson's disease, however, the patient's age, clinical course and family history prompted further investigation. Genetic testing confirmed HD, revealing a pathogenic HTT allele with a cytosine-adenine-guanine (CAG) repeat expansion >39, consistent with a high-penetrance allele for HD.

This case underscores that HD can present atypically with predominant parkinsonian features, deviating from the classic choreiform presentation. Such cases highlight the importance of comprehensive clinical evaluation and consideration of genetic testing in atypical or late-onset movement disorders. The variability in clinical phenotype and disease trajectory, especially in elderly patients, emphasizes the need for early recognition and multidisciplinary management. HD should be considered in the differential diagnosis of late-onset movement disorders, even when chorea is not the primary manifestation. Optimal care in such cases requires a comprehensive and multidisciplinary management approach.