Purpose: This study aimed to evaluate the sensitivity of the EEG in Angelman syndrome (AS), to verify the age at onset of suggestive EEGs and to study EEG patterns, analyzing variations and comparing our findings with nomenclature previously used.

Methods: Seventy EEG and 15 V‐EEGs of 26 patients were analyzed. Suggestive EEG patterns of AS were classified in delta pattern (DP), theta pattern (TP), and posterior discharges (PDs). Generic terms were used to simplify the analysis.

Results: Suggestive EEGs were observed in 25 (96.2%) patients. DP occurred in 22 patients with four variants—hypsarrhythmic‐like: irregular, high‐amplitude, generalized delta activity (DA) with multifocal epileptiform discharges (EDs); slow variant: regular, high‐amplitude, generalized DA with rare EDs; ill‐defined slow spike‐and‐wave: regular, high‐amplitude, generalized DA with superimposed EDs characterizing a slow wave, with notched appearance; triphasic‐like: rhythmic, moderate‐amplitude DA over anterior regions with superimposed EDs. TP was observed in eight patients, as generalized or over the posterior regions. PDs were seen in 19 patients as runs of sharp waves or runs of high‐amplitude slow waves with superimposed EDs. TP was the only age‐related pattern (younger than 8 years) and observed only in patients with deletion. In 15 patients who had an EEG before the clinical diagnosis, 60% had a suggestive tracing.

Conclusions: Although some EEG descriptions are not very detailed, and every author describes findings in a slightly different manner, obviously a common denominator must exist. In this context, EEG seems to be a very sensitive method for the diagnosis of AS, offering an opportunity to corroborate this etiologic diagnosis. Conversely, we do not believe that these patterns may be accounted as specific, except for the delta pattern, which seems to be extremely unusual in other syndromes. Other EEG patterns observed in AS, such as theta activity and PDs, occur in a wide variety of disorders. Nonetheless, their importance for the EEG diagnosis of AS is supported by the fact that they are associated with other features and may be helpful in a proper clinical setting.