Expanding the Phenotype of Extremely Early Onset Juvenile Huntington's Disease: A Case Report and Review of Previously Published Cases.

Extremely early-onset juvenile Huntington's disease (HD) has been described in three patients with onset at approximately 18 months to 2 years of age. Herein, we report a patient with, to our knowledge, the youngest age of onset with the largest reported explicit expansion size. We also summarize the previously reported cases of extremely early-onset juvenile HD. This information is important to gain insight into this phenotype for earlier diagnosis and in the hopes of future lifesaving treatments.