Interictal (Between Seizures)

The background activity in wake is often slow for the patient’s age. This can be constant or transient and permanent slowing has been associated with a poorer cognitive or developmental prognosis. Normal sleep architecture and patterns may not be recognizable due to the frequent epileptiform (meaning seizure-like) discharges.

The classic EEG feature of LGS is a slow spike-wave complex, seen on both sides or over the entire head, repeating at 1-2 per second. The same pattern may be seen during an atypical absence seizure. The frequent presence of the slow spike-wave complexes and the waxing and waning presence can make it difficult to distinguish between the interictal (between seizure) and ictal (during seizure) pattern. The slow spike-wave complexes may not be present when the child is first diagnosed with seizures, and only becomes apparent on a subsequent EEG, making the initial diagnosis difficult in some children. The occurrence of slow spike-wave complexes may decrease in adolescents and adults. In patients whose seizure frequency decreases, the pattern can change to single spike-and-wave complexes during sleep and then their subsequent disappearance. Focal spike (epileptiform discharges) or focal slowing may be seen. Bursts of generalized polyspikes at 10 per second or more are seen frequently enough to be considered an additional criterion for LGS. These are seen best in sleep, and may be associated with a nocturnal tonic seizure (as the ictal pattern). As children reach early adult years, only 30% to 50% still have the characteristic EEG and clinical characteristics.

Ictal (During Seizures)

The EEG during a tonic seizure reveals generalized, fast (10-15 per second), low-amplitude activity, seen best over the anterior head regions. This slows in frequency and increases in amplitude as the seizure progresses. This may be preceded by a single generalized spike-and-wave complex. The clinical features typically begin within one second of the EEG manifestations. Diffuse, slow (less than 2 – 2.5 per second), spike-and-wave complexes characterize the EEG during an atypical absence seizure. This may be difficult to distinguish from the interictal bursts. Atonic, myoclonic and myoclonic-atonic seizures have an EEG characterized by bilateral slow spike-and-wave complexes, polyspike-and-wave complexes or rapid polyspikes.

Author: James Wheless, M.D.
Topic Editor: Robert Fisher, M.D., Ph.D.
Last Reviewed: 8/7/09