Progressive myoclonus epilepsy: a differential diagnosis from juvenile chorea Huntington disease

Huntington disease (HD) is a neurodegenerative, dominantly inherited genetic disorder caused by the expansion of a CAG triplet repeat (> 36 CAG) of the Huntington gene on chromosome 4p16.3. The juvenile variant of the disorder is rare (5 to 7%) and shows a phenotype with myoclonic jerks, seizures, behavioral disturbances, ataxia, dystonia, and parkinsonism. Choreatic movements are rare.

We describe a case of a female patient from Ecuador who was diagnosed with HD at the age of 10 years. The father's positive HD was not mentioned on presentation owing to the daughter's negative genetic results. Progressive myoclonic epilepsy had been suspected in Ecuador because of epilepsy and progressive myoclonic jerks in the presence of only mild and unspecific MRT changes. Valproic acid and levetiracetam led to amelioration of the epilepsy. However, the emergence and slow progression of other symptoms such as dysarthria, saccadic eye movements, tremors, and ataxia leading to a loss of free ambulation were also observed.

Owing to the suggestive clinical phenotype, it was decided to repeat the molecular genetic assessment to verify or exclude HD. This showed a triple nucleotide repeat expansion of 112 CAG repeats and confirmed the diagnosis, which was also double-checked at a second laboratory. At the same time, all relevant differential diagnoses with the leading symptom myoclonic epilepsy were considered and relevant diagnostic steps taken.

This case report demonstrates that a positive diagnosis in a father is an important hint regarding suspicion of juvenile HD in his child. Should a PCR-based test produce a negative result, southern-blot analysis is a valuable alternative method.