Approach Considerations
Electroencephalography (EEG) is an essential part of the workup for Lennox-Gastaut syndrome (LGS). To date, there are no known laboratory investigations to aid in the diagnosis of LGS. Neuroimaging is an important part of the search for an underlying etiology in a patient with LGS.
Electroencephalography
Always perform an EEG in patients with suspected LGS, since the diagnosis depends on the presence of specific EEG findings. A routine 20-minute EEG may not capture the patient both awake and asleep and thus may miss specific important EEG findings. Instead, obtain prolonged video/EEG telemetry, if possible. Record both waking and sleep EEG, to assist in confirming a suspected diagnosis and to capture and classify each of the patient's multiple seizure types.
Video recordings can also be used to educate the parents on which of the patient's "events" are seizures and which are nonepileptic behavioral events. Parental ability to correctly recognize and identify atypical absences is poor. In one study using video/EEG monitoring in a cohort of children with LGS, parental recognition was 27% for atypical absences, while the sensitivity was as high as 80% for myoclonic seizures and 100% for tonic, atonic, tonic-clonic, clonic, and complex partial seizures. [8]
Go to EEG in Common Epilepsy Syndromes, Epileptiform Normal Variants on EEG, and Generalized Epilepsies on EEG for more information on these topics.
Interictal electroencephalography
Interictal EEG is characterized by a slow background that can be constant or transient. Permanent slowing of the background is associated with poor cognitive prognosis.
The hallmark of the awake interictal EEG in patients with LGS is the diffuse slow spike wave (see the image below). This pattern consists of bursts of irregular and generalized spikes or sharp waves followed by a sinusoidal 35-400-millisecond slow wave with an amplitude of 200-800 microvolts, which can be symmetric or asymmetric.
The amplitude often is higher in the anterior region or in the frontal or frontocentral areas, but in some patients the activity may dominate in the posterior head regions. The frequency of the slow spike wave activity commonly is found at 1.5-2.5 Hz.
Slow spike wave pattern in a 24-year-old awake male with Lennox-Gastaut syndrome. The slow posterior background rhythm has frequent periods of 2- to 2.5-Hz discharges, maximal in the bifrontocentral areas, occurring in trains as long as 8 seconds without any clinical accompaniment.
Slow spike waves usually are not activated by photic stimulation. Hyperventilation rarely induces slow spike waves, although mental retardation prevents adequate cooperation in many patients. During non–rapid eye movement (REM) sleep, discharges are more generalized, more frequent, and consist of polyspikes and slow waves. In REM sleep, spike waves decrease. During periods of frequent seizures, the total duration of REM sleep is reduced.
Ictal electroencephalography
During a tonic seizure, the EEG is characterized by a diffuse, rapid (10-13 Hz), low-amplitude activity pattern, mainly in the anterior and vertex areas ("recruiting rhythm") that progressively decreases in frequency and increases in amplitude.
A brief generalized discharge of slow spike waves or flattening of the recording may precede this pattern. Diffuse slow waves and slow spike waves may follow it.
These fast discharges are common during non-REM sleep. Unlike tonic-clonic seizures, no postictal flattening occurs with these seizures. Clinical manifestations appear 0.5-1 second after the onset of EEG manifestations and last several seconds longer than the discharge.
During an atypical absence seizure, the EEG is characterized by diffuse, slow (2-2.5 Hz), and irregular spike waves, which may be difficult to differentiate from interictal bursts. Occasionally, discharges of rapid rhythms may be observed preceded by flattening of the record for 1-2 seconds, followed by progressive development of irregular fast rhythm in the anterior and central regions, and ending with brief spike waves.
During atonic, massive myoclonic, and myoclonic-atonic seizures, the EEG is characterized by slow spike waves, polyspike waves, or rapid diffuse rhythms. Simultaneous video/EEG recording can help differentiate these seizure types. In most patients, these 3 types of seizures coexist.
The EEG during absence status epilepticus reveals continuous spike wave discharges, usually at a lower frequency than at baseline, and rapid rhythms during tonic status epilepticus.
Neuroimaging Studies
In general, a magnetic resonance imaging (MRI) scan is the preferred neuroimaging study for a patient with LGS, rather than a CT scan. CT scans may be preferred in selected situations (eg, evaluation of suspected intracranial injury and/or hematoma in a patient with head trauma resulting from a seizure).
Abnormalities revealed by neuroimaging associated with LGS can include findings of tuberous sclerosis, brain malformations (eg, cortical dysplasias), hypoxia-ischemia injury, or frontal lobe lesions.
No current indication exists for routine positron emission tomography (PET) or single-photon emission computed tomography (SPECT) scanning in patients with LGS. However, PET or SPECT scans may be useful in patients undergoing evaluation as candidates for epilepsy surgery.
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Patient with Lennox-Gastaut syndrome wearing a helmet with face guard to protect against facial injury from atonic seizures
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Slow spike wave pattern in a 24-year-old awake male with Lennox-Gastaut syndrome. The slow posterior background rhythm has frequent periods of 2- to 2.5-Hz discharges, maximal in the bifrontocentral areas, occurring in trains as long as 8 seconds without any clinical accompaniment.
