Examining factors related to accelerated long-term forgetting in epilepsy using ambulatory EEG monitoring

Some patients with epilepsy demonstrate normal memory when this is tested at relatively short intervals (e.g., 30 min), but substantial loss over longer delay periods (e.g., days or weeks) when compared to healthy control subjects. This pattern of "accelerated long-term forgetting" (ALF) affects the everyday lives of patients, yet goes undetected by standard neuropsychological memory tests, and its pathophysiologic basis is poorly understood. By testing memory over a period of concurrent ambulatory electroencephalography (EEG), the current study aimed to investigate possible factors contributing to ALF.

Thirty-nine patients diagnosed with epilepsy or probable epilepsy underwent 5 days of continuous ambulatory EEG: 18 had normal EEG studies, 10 had focal epileptic discharges, 5 had generalized epileptic discharges, and 6 had one or more seizures. Fifteen matched healthy control subjects also participated, but did not undergo EEG. Subjects were taught 13-item word and design lists to criterion, and recall was tested at 30 min, 24 h, and 4 days. Subjects also completed questionnaires pertaining to everyday memory and mood.

Group analyses (excluding patients who experienced seizures during monitoring) indicated that patients who experienced generalized discharges during the 24-h to 4-day delay intervals showed higher rates of forgetting for nonverbal information. Those with focal discharges showed ALF between 30 min and 4 days for verbal information, whereas those with normal EEGs over the 4 days recording had no evidence of ALF. Surprisingly, mood and epilepsy variables (such as duration of disease or number of anticonvulsant medications) showed no significant correlation with ALF. Although no aspect of nighttime sleep architecture was found to be related to recall after the first 24 h, daytime naps were associated with better retention. Self-report of everyday memory functioning was related to recall at longer delays, but not at 30 min.

The present findings indicated that ALF in epilepsy is associated with subclinical discharges rather than antiepileptic drugs (AEDs), mood or sleep disturbance. Measures of longer-term recall can reveal correlations with subjective everyday memory complaints that are not evident when recall is only tested at a standard (30 min) delay interval. These findings have the potential to improve treatment strategies for patients who complain of memory difficulties.