EEG Duration: The Long and the Short of It

Current American Clinical Neurophysiology Society guidelines require a minimum of 20 minutes of artifact-free EEG recording; however, the optimum duration for routine EEGs is not established. Our hypothesis was that an EEG recording of 40 minutes' duration would yield more information than a 20-minute EEG in capturing epileptiform abnormalities and in obtaining sleep. We retrospectively studied 150 consecutive EEGs of 40 minutes' duration performed at St Christopher's Hospital for Children. Although the majority (89%) of interictal EEG abnormalities can be identified within the first 20 minutes of a routine EEG, extending the time of a routine EEG increases the yield significantly by identifying an additional 11% of abnormal studies (P = .0001), precluding the need for further long-term monitoring in these patients. Forty-three percent of interictal epileptiform abnormalities were found during sleep. We recommend that routine EEGs be performed for 40 minutes, whenever possible, to improve yield in a cost-effective manner.