Abstracts

Rationale: Findings on the interictal electroencephalogram (EEG) can provide evidence for a diagnosis of epilepsy and support for a specific epilepsy syndrome. Current guidelines recommend that a EEG be recorded for at least 20 minutes. A limitation of the EEG is that many patients with epilepsy will not exhibit epileptiform abnormalities during a single 20 minute EEG. We hypothesized that extended recording outpatient EEGs would have a higher yield for epileptiform abnormalities then a routine 20 minute EEG, and that a significant number of the extended studies would have epileptiform abnormalities seen only after the first 20 minutes.Methods: We retrospectively reviewed all adult, outpatient, routine EEGs of greater then 60 minutes duration acquired at Loma Linda University from January 2003 to March 2006.All EEG were acquired to evaluate for evidence of a seizure disorder. In studies where epileptiform abnormalities were identified the time to the first epileptiform abnormality was recorded. The mean and median times to first abnormality were calculated.Results: A total of 172 EEGs performed on 156 patients were identified. The length of the EEG ranged from 65-384 minutes with an average of 186.6 minutes. 45 (26%) of the EEGs captured epileptiform abnormalities. Of these studies, 53% showed epileptiform abnormalities in the first 20 minutes and 47% showed abnormalities only after 20 minutes of recording. The average time to first abnormality was 32.8 minutes with a range of 1-216 minutes and a standard deviation (SD) of 48.2 min. Results were skewed to the right (skew 2.22) with a median time to first abnormality of 10 minutes. 93% of the abnormal studies demonstrated the first epileptiform abnormality within the first 90 minutes of the EEG. Conclusions: An extended recording EEG can capture abnormalities that would otherwise be missed by a routine 20 minute EEG.