Electroencephalographic aspects of hemimegalencephaly.

The authors retrospectively studied the electroencephalograms (EEGs) of 12 patients with hemimegalencephaly-a unilateral brain malformation which is often overlooked, despite having typical CT-scan and MRI aspects. They were characterized by three types of abnormal tracing, which correlated with prognosis: triphasic complexes of large amplitude were observed in patients with the earliest onset of seizures, and were associated with the most severe prognosis; unilateral, rhythmic 'alpha-like' activity was recorded in patients with seizures occurring after three months of age, and was associated with a relatively favourable outcome; asymmetrical suppression-bursts characterised by 'alpha-like' activity on the abnormal side were seen in patients with infantile spasms. The EEG pattern seems to make an important contribution to both diagnosis and prognosis.