Myoclonic jerks can occur alone, but they more commonly are associated with generalized clonic seizures. Rhythmic jerks can occur during sleep, with associated dystonic posturing during wakefulness. The prognosis for meaningful cognitive function is poor, although the myoclonic jerks may be medically controlled. This group includes all idiopathic cases that primarily display recurrent myoclonic attacks. Some patients may have infrequent generalized tonic-clonic seizures.
Most myoclonic seizures are axial, which sometimes results in falls.
The outcome in these patients is usually favorable, although about half the patients may have behavioral or cognitive dysfunction. 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.