Myoclonus in Angelman syndrome.

Angelman syndrome (AS) is a neurogenetic imprinting disorder caused by loss of the maternally inherited Ube3a gene and is characterized by generalized epilepsy, limited expressive speech, sleep dysfunction, and movement disorders. Myoclonic seizures are often the first seizure type to appear, and myoclonic status, associated with developmental regression, may occur in the first few years of life. Additionally, there have been rare reports of prolonged episodes of myoclonus without electrographic correlate in adults with AS. The medical records of 200 individuals seen in the Angelman Syndrome Clinic at the Massachusetts General Hospital and the Lurie Center for Autism were retrospectively reviewed to identify and characterize myoclonic seizures and episodes of nonepileptic myoclonus. Myoclonic seizures were reported in 14% of individuals with age of onset occurring before 8years. These are brief events, unless the individual was experiencing myoclonic status, and electroencephalographs show interictal generalized spike and wave activity. Nonepileptic myoclonus occurred in 40% of individuals over 10years of age, and prevalence appears to increase with age. The episodes of nonepileptic myoclonus arise during puberty or later, with age of onset ranging from 10 to 26years. These events were captured on 5 video electroencephalographs and had no electrographic correlate. They can last from seconds to hours, always occurring in the hands and spreading to the face and all extremities in some individuals. Episodes of nonepileptic myoclonus have a discrete beginning and end, lacks a postictal period, and are not associated with significant alteration of consciousness or developmental regression. These episodes can be difficult to treat and are often refractory to medication; however, levetiracetam, clobazam, and clonazepam appear to be effective for some individuals. Myoclonic seizures are common in AS, typically occurring in young children and associated with epileptiform changes on electroencephalographs. Prolonged episodes are associated with developmental regression. In contrast, nonepileptic myoclonus typically begins in adolescence or early adulthood and has no electroencephalogram (EEG) correlate, alteration in consciousness, or regression but can significantly impact quality of life.