The routine EEG is useful for diagnosing spells, but the interpretation of the EEG must be cautious. Many normal variants, such as asymmetrical vertex waves, wicket spikes, small sharp spikes, 14- and 6-per second positive spikes, and rhythmical temporal theta bursts of drowsiness (formerly called psychomotor variant), can be mistaken for interictal spikes and sharp waves (109; and see Chapter 1). Additionally, a few percent of the normal American population exhibit interictal epileptiform discharges in a baseline EEG (110,111). The combination of an ambiguous history for a seizure and normal variants in the EEG can be an invitation to inappropriate treatment. Conversely, many individuals who have epilepsy lack abnormalities on an interictal EEG (112). Repeat EEGs to a total of about four EEGs may increase the yield (113). Other procedures useful for eliciting abnormalities in the EEG are activating procedures such as sleep deprivation or use of extra scalp (114) or sphenoidal electrodes (115). Prolonged digital EEG recordings may be performed in the ambulatory setting (116). Ambulatory EEGs are useful in capturing spells, but care must be taken in interpretation, because they are very subject to movement artifact.
As discussed in Chapter 16, sleep studies are of value when the history suggests hypersomnolence as a possible etiology of an episodic disorder of consciousness (18-19). A sleep disorder mimicking epilepsy should not be mistaken for a seizure disorder linked to the sleep cycle (117,118).
Inpatient video-EEG monitoring is one of the most powerful methods for diagnosis of spells with altered neurobehavioral function (119-121). Such monitoring extends the eyes and ears of the clinician. Depending on the question being asked, long-term epilepsy monitoring can be helpful in establishing a diagnosis, in seizure classification, in the localization of seizure foci in presurgical evaluations, and in quantification of seizure activity(121). However, in monitoring units, EEG changes rarely provide a diagnosis for the many imitators of epilepsy.
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