Absence of early epileptiform abnormalities predicts lack of seizures on continuous EEG

Abstract

Objective: To determine whether the absence of early epileptiform abnormalities predicts absence of later seizures on continuous EEG monitoring of hospitalized patients.

Methods: We retrospectively reviewed 242 consecutive patients without a prior generalized convulsive seizure or active epilepsy who underwent continuous EEG monitoring lasting at least 18 hours for detection of nonconvulsive seizures or evaluation of unexplained altered mental status. The findings on the initial 30-minute screening EEG, subsequent continuous EEG recordings, and baseline clinical data were analyzed. We identified early EEG findings associated with absence of seizures on subsequent continuous EEG.

Results: Seizures were detected in 70 (29%) patients. A total of 52 patients had their first seizure in the initial 30 minutes of continuous EEG monitoring. Of the remaining 190 patients, 63 had epileptiform discharges on their initial EEG, 24 had triphasic waves, while 103 had no epileptiform abnormalities. Seizures were later detected in 22% (n = 14) of studies with epileptiform discharges on their initial EEG, vs 3% (n = 3) of the studies without epileptiform abnormalities on initial EEG (p < 0.001). In the 3 patients without epileptiform abnormalities on initial EEG but with subsequent seizures, the first epileptiform discharge or electrographic seizure occurred within the first 4 hours of recording.

Conclusions: In patients without epileptiform abnormalities during the first 4 hours of recording, no seizures were subsequently detected. Therefore, EEG features early in the recording may indicate a low risk for seizures, and help determine whether extended monitoring is necessary.

Figure 1. Seizure rate as a function of time without epileptiform activity

The maximum, minimum, and observed seizure rate on subsequent continuous EEG monitoring as a function of time already passed without epileptiform activity (in the cohort of patients without epileptiform activity to that time). The observed seizure rate is indicated with the solid line. The upper dashed line indicates the maximum possible true seizure rate in the underlying population that could have resulted in the observed seizure rate (with a probability p > 0.05); the lower dashed line indicates the minimum possible true seizure rate that could have resulted in the observed seizure rate (with a probability p > 0.05).