Lennox-Gastaut syndrome (LGS) is characterized by a mixed seizure disorder. Tonic seizures and a slow spike-and-wave EEG pattern are major components. The syndrome always begins in childhood. Mental retardation is considered a component by some authors.51

The child with LGS typically has a mixture of seizure types. The most frequently occurring52,53-59 are:

• tonic
• tonic-clonic
• myoclonic
• atypical absences
• "drop attacks"-a form of atonic, tonic, or myoclonic seizures

Tonic seizures are a key component of LGS.55,58 They are usually brief, lasting from a few seconds to 1 minute, with an average duration of about 10 seconds. The seizures may cause falls and injury. Eyelid retraction, staring, mydriasis, and apnea are commonly associated and may be the most prominent features.55 During tonic seizures the patient is unconscious, although arousal from light sleep may occur. Because they are often very brief, the seizures frequently go undetected. Tonic seizures in LGS typically are activated by sleep and may occur repetitively throughout the night. They are much more frequent during non-REM sleep than during the awake state and usually do not occur during REM sleep.

Atonic seizures are common in LGS but they occur less often than tonic and myoclonic seizures. Most atonic seizures last only 1 to 4 seconds. In the shortest attacks, patients may show only head nodding or sagging at the knees. The seizures are so brief that it is difficult to determine whether consciousness is lost. If a fall occurs, the patient usually gets up immediately and resumes what he or she was doing. Many children with drop attacks have myoclonic or tonic seizures. In a study of 48 drop attacks in 15 children with LGS,60 only 4% of the seizures were of the atonic type.

Myoclonic seizures, occurring either in isolation or as a component of absence seizures, may occur in LGS but usually are not the most common seizure type. However, occasionally the myoclonus is so prominent that some investigators have described a myoclonic variant of LGS.55

Generalized tonic-clonic (GTC) seizures and atypical absence seizures are seen in over half of the patients with LGS.56,57 GTC seizures usually cause the most concern to parents and may precipitate hospitalization. Atypical absences are generally longer than typical absences and are more likely to include changes in postural tone and myoclonic jerks.51

Patients with LGS typically have very frequent seizures. Markand56 found that 60% of his patients had seizures daily, whereas Papini and colleagues,61 in a longitudinal study of 16 patients with LGS, found the mean daily frequency of seizures to range from 9 to 70. Some children with this syndrome have hundreds of seizures daily.

Mental retardation is present before seizure onset in 20% to 60% of patients.55 Some patients whose seizures have idiopathic or cryptogenic etiologies have normal IQ scores or developmental histories before the onset of their seizures, but deterioration often occurs in LGS.55 Only few patients escape mental retardation.55 Marked fluctuations in cognitive abilities occur in LGS patients. To some degree, these are correlated with the intensity of EEG abnormalities.

In addition to cognitive difficulties, behavioral problems are very common in LGS, from hyperactivity with aggressive behavior to frank psychotic and autistic behavior.

Other neurologic abnormalities have been reported in 30% to 88% of patients with LGS.56,57,62

Electroencephalography

The identifying EEG finding in LGS is a slow spike-and-wave discharge superimposed on an abnormal, slow background. The slow spike-and-wave or sharp-and-slow-wave complexes consist of generalized discharges occurring at a frequency of 1.5 to 2.5 Hz. The morphology, amplitude, and repetition rate may vary both between bursts and during paroxysmal bursts of spike-and-wave activity. Transient and shifting asymmetries of the discharge frequently occur.

The area of maximum voltage, although variable, is usually frontal or temporal. Hyperventilation and photic stimulation rarely activate them.

Sleep increases the frequency of the discharges. During non-REM sleep, slow spike-and-wave discharges may be replaced by multiple spike-and-wave discharges. In REM sleep, the paroxysmal activity decreases markedly.

The typical EEG manifestation of tonic seizures is the occurrence of fast-rhythm discharges of 10 to 20 Hz and of progressively increasing amplitude, at times followed by a few slow waves or spike-waves. This pattern was previously called the grand mal discharges of Gibbs. Patients may also have bursts of multiple spike-and-wave discharges during tonic seizures.

EEG patterns characteristic of other seizure types in LGS include63:

• atonic seizures-usually a fast-rhythm discharge, but sometimes bursts of slow spike-wave complexes or high-amplitude 10-Hz discharges
• myoclonic seizures-bursts of arrhythmic, multiple spike-wave or irregular spike-wave activity
• atypical absence seizures-slow (<2.5 Hz) and often asymmetric and irregular spike-and-wave activity

Adapted from: Holmes GL. Classification of seizures and the epilepsies. In: Schachter SC, Schomer DL, eds. The comprehensive evaluation and treatment of epilepsy. San Diego, CA: Academic Press; 1997. p. 1-36.
With permission from Elsevier (www.elsevier.com)
Reviewed and revised January 2004 by Gregory L. Holmes, MD, Dartmouth Medical School

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