Thirty percent of cases are of unknown etiology, that is, no previous history of seizures and normal neurological function prior to the onset. The initial presentation is usually in early infancy, and is manifested by head drop or atony of the neck. This syndrome is characterized by presenting multiple types of seizures. The most common are tonic seizures, atypical absence seizure and atony. It could also present with generalized tonic-clonic, partial or myoclonic seizures.
Conscience is altered briefly and is fully recuperated as soon as the attack is over.
Myoclonic seizures can occur before, during, or after the atonic seizures. They are not as abrupt as in petit mal seizures. The EEG is characteristic with spikes and slow frequency waves of 1.5 to 2.5 Hz. The background rhythm is slow with multifocal spikes. The maximal voltage area is usually anterior.