[Asymptomatic Moyamoya syndrome. Diagnosis by EEG and magnetic resonance angiography].

Alternating hemiplegias in children suggest the possibility of a Moyamoya syndrome. In the absence of clinical symptoms it is the EEG which will offer the decisive clue in childhood Moyamoya syndrome. The pertinent finding, which consists in progressive frequency decrease and amplitude activation after hyperventilation, has only been described in pediatric patients. It has been termed re-build-up phenomenon. Magnet resonance imaging and magnet resonance angiography (MR angiography) are able to confirm the tentative diagnosis of a Moyamoya syndrome based on the EEG.