In this group, all patients have myoclonic seizures associated with recurrent brief attacks of varying types. Examples of these attacks include atonic seizures, atypical absence, partial seizures, and brief tonic seizures. This is sometimes termed the myoclonic variant of Lennox-Gastaut syndrome. The outcome is worse than in the cryptogenic myoclonic epilepsy group, with more patients having cognitive dysfunction, a significant proportion of whom have severe mental retardation. However, the outcome is better compared to patients with Lennox-Gastaut syndrome.
Myoclonic seizures appear by the second or third year and are often photosensitive.
Atypical absence seizures may be seen without an electrographic correlate. Most patients have fluctuating ataxia and erratic myoclonus. The myoclonic seizures may resolve after a few years, but other seizures tend to be persistent. Nonconvulsive status is common. The EEG is normal initially, despite frequent seizures.