EEG Features in Idiopathic Generalized Epilepsy: Clues to Diagnosis

Abstract

To the Editor: We read with interest the article by Betting et al., 2006 entitled “EEG Features in Idiopathic Generalized Epilepsy: Clues to Diagnosis (1).” The authors investigated the contribution of EEG recordings for diagnosis and management of 180 patients with a clinical diagnosis of idiopathic generalized epilepsy (IGE). They found that the first EEGs yielded 45% with normal activity, 55% with abnormal activity, and only 33% indicated typical abnormalities. Patients with absence epilepsy were most likely to exhibit specific EEG abnormalities. At the time of EEG evaluation 92% of the patients were taking an AED and 80% of the patients had remained seizure free for ≥ 1 year. From this information the authors concluded that clinical history contributes more to the diagnosis of IGE and that treatment may be inappropriately delayed while “waiting” for a typical abnormal pattern on EEG. Significant studies on the predictive value of EEG after the first unprovoked seizure have revealed contradictory results. Some studies have shown EEG abnormalities in up to 70% of patients after an initial unprovoked seizure (2). Others have shown that obtaining an EEG provides little additional information in further treatment of the patients (3). In fact, our group has previously shown that extended EEG monitoring is important in establishing the diagnosis of the type of epilepsy and making an appropriate choice of antiepileptic drug (AED) therapy in the future care of patients (4, 5). Indeed, the accurate diagnosis of idiopathic generalized epilepsy was confirmed after prolonged EEG monitoring in only 29% of patients (5). Further controversy exists on whether risk for seizure recurrence is increased by the presence of an abnormal EEG on first examination. Several studies have demonstrated that the risk of recurrence doubles with an abnormal EEG (6, 7). We recommend caution when interpreting the results of this study. Our main reservation is that EEG interpretations were performed independent of whether the patient was receiving AED treatment or not. Studies have demonstrated normalization of EEGs while on AED therapy (8). Moreover, the EEG data was not assessed in patients with new onset seizures only. Initial EEGs were performed in some patients up to 44 years after seizure onset. We understand that in situations where resources are limited, there may be delays before performing an EEG. In this situation, we agree with the authors that AED treatment should not be withheld from a patient with seizures while “waiting” for an EEG. However, EEG is and will continue to be an important investigation in the diagnosis, treatment, and prognosis of patients with epilepsy.