Atypical EEG abnormalities in genetic generalized epilepsies

Objective: Bilateral, symmetric and synchronous generalized epileptiform activity is considered to be the typical electroencephalographic (EEG) abnormality in genetic generalized epilepsy (GGE). We sought to study atypical EEG abnormalities in a systematic way based on 24-h ambulatory EEG recordings. 

Methods: The diagnosis of GGE was validated and classified into syndromes according to the International League against Epilepsy criteria. All participants underwent 24-h ambulatory EEG recording. Epileptiform discharges were counted and detailed information was entered into an electronic database. Amplitude asymmetry, focal onset/offset of paroxysms, focal discharges, atypical morphology and generalized paroxysmal fast rhythm were defined as atypical abnormalities. Results: Of the total of 120 patients, 107 had abnormal EEGs, of which 66.4% had at least one atypical epileptiform abnormality on EEG. Atypical morphology was the most frequent abnormality in 93.4% of patients, followed by amplitude asymmetry (28.0%), focal discharges (21.5%), focal onset of paroxysms (13.1%), focal offset of paroxysms (8.2%) and generalized paroxysmal fast rhythm (1.9%). The analysis of individual discharges revealed that 76% of paroxysms were of atypical morphology. Significant associations were found between (a) amplitude asymmetry and state of arousal (p <0.001) as well as seizure-free duration (p 0.013); (b) atypical morphology and state of arousal (p <0.001). 

Conclusion: In GGE, there are both common and rare atypical epileptiform EEG abnormalities that may vary according to the state of arousal and seizure-free duration. 

Significance: Awareness of these variations is important to avoid misdiagnosis.