Progressive myoclonic epilepsies: it takes a village to make a diagnosis.

The progressive myoclonus epilepsies (PMEs) are a devastating group of rare disorders(1) that manifest with increasing action myoclonus, which is also present at rest but activates with stimuli such as noise, light, or touch. Ultimately, patients become wheelchair-bound and experience early death. Neurologic signs that frequently but not reliably coexist include other seizure types (particularly generalized tonic-clonic), progressive ataxia, and dementia. Typically, presentation is in late childhood or adolescence; however, all ages may be affected. Although distinction from more common forms of genetic generalized epilepsy, particularly juvenile myoclonic epilepsy, may be challenging early on, the presence or evolution of 1) progressive neurologic disability, 2) failure to respond to antiepileptic drug therapy, and 3) background slowing on EEG should suggest PME. Importantly, inappropriate therapy in the genetic generalized epilepsies may result in ataxia, impaired cognition, and uncontrolled seizures, which may mimic PME. PMEs should be distinguished from progressive encephalopathies with seizures (due to degenerative conditions such as GM2 gangliosidosis, nonketotic hyperglycinemia, Niemann-Pick type C, juvenile Huntington and Alzheimer disease) and progressive myoclonic ataxias, which affect predominantly adults with progressive ataxia, myoclonus, few if any tonic-clonic seizures, and without evidence of dementia.(2,3.)