Purpose: Current literature suggests that longer duration of EEG recording increases the yield of detecting interictal epileptiform discharges. However, optimal duration for a repeat study in patients with initially normal 30-minute EEG is not clear. Thus, the purpose of this study is to determine whether a 2-hour EEG has a diagnostic advantage over a routine 30-minute EEG in detecting epileptiform abnormalities in patients who had a first normal 30-minute EEG.
Methods: This is a single-center, retrospective study done at UT Southwestern Medical Center at Dallas and Parkland Memorial Hospital. The data from 1997 to 2015 were extracted from the existing EEG report database for patients who had a first normal 30-minute EEG recording. EEG was interpreted by board-certified clinical neurophysiologists, who classified each EEG as normal or abnormal, with relevant subsequent subclassification.
Results: Over 18 years, a total of 12,425 individual 30-minute EEGs were performed. Of these, 1,023 patients had at least one repeated EEG after the first normal EEG. Among these patients, 763 had a 30-minute EEG as the second study and 260 had a 2-hour EEG as the second study. The yield of epileptiform discharges was 3.3% in the 30-minute EEG group and 4.2% in the 2-hour EEG group (P = 0.5) in the repeating studies.
Conclusions: Two-hour EEG has a similar yield as 30-minute EEG to detect epileptiform discharges in patients with a normal 30-minute EEG.