As defined by the International League Against Epilepsy classification, Lennox-Gastaut syndrome (LGS)

. . . manifests itself in children aged 1–8 years but appears mainly in preschool-age children. The most common seizure types are tonic-axial, atonic, and absence seizures, but other types such as myoclonic, generalized tonic clonic seizures (GTCS), or partial seizures frequently are associated with this syndrome. Seizure frequency is high, and status epilepticus is frequent (stuporous states with myoclonias, tonic and atonic seizures). The EEG usually has abnormal background activity, slow spike-waves <3 Hz and, often, multifocal abnormalities. During sleep, bursts of fast rhythms (10 Hz) appear. In general, there is MR. Seizures are difficult to control, and development is unfavorable. In 60% of cases, the syndrome occurs in children suffering from a previous encephalopathy but is primary in other cases.15

A triad of basic elements usually is needed to make a diagnosis of LGS, based on the preceding definition coupled with clinical experience and research. This triad consists of:7,15–20

1. Multiple types of seizures including tonic, atypical absence, and atonic seizures
2. An EEG pattern consisting of interictal diffuse slow spike and wave discharges occurring at a 1.5- to 2-Hz frequency
3. Diffuse cognitive dysfunction or mental retardation

Some investigators do not consider cognitive dysfunction or mental retardation to be indispensable for diagnosis, especially at onset, if the seizures and EEG pattern are typical.17,21–23 Other authors use a stricter EEG criterion, requiring that the diagnostic EEG pattern include a burst of generalized fast spikes (10 Hz) during non–rapid eye movement (non-REM) sleep.24 Debate continues about the minimal necessary and sufficient criteria needed to diagnose LGS.

Adapted from: Glauser TA and Morita DA. Lennox-Gastaut syndrome. In: Devinsky O and Westbrook LE, eds. Epilepsy and Developmental Disabilities. Boston: Butterworth-Heinemann; 2001;65–78.
With permission from Elsevier (www.elsevier.com).
Reviewed and revised May 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board, and Tracy A. Glauser, MD, Children's Hospital Medical Center, Cincinnati, Ohio.