Abstracts

RATIONALE: Hemimegalencephaly (HME) is a rare asymmetrical hamartomatous brain malformation. Epilepsy is usually severe and begins early. We attempt to establish diagnostic criteria integrating clinical, imaging and neuropathological parameters and correlations with epilepsy.
METHODS: We studied 12 children with HME, confirmed by imaging: 5 boys and 7 girls, ages 3 days to 2.5 years when first seen. all patients presented with seizures with onset before 1 year in 11, 4 newborns. The epilepsy was partial in 10 patients, tonic and infantile spasms in 1 patient and 1 had epilepsia partialies continua. All were studied with CT, MRI and EEG. Three patients underwent hemispherectomy for intractable seizures and one died during the procedure. We performed neuropathological investigations in these 3, including immunocytochemistry for neuronal and glial maturation.
RESULTS: Six cases of HME were isolated and 6 were syndromic, associated with epidermal nevus; 2 presented with hemifacial lipoma. Of the 12 patients, 5 exhibited hemiparesis, 4 had generalized hypotonia and 3 had no overt motor deficit. The clinical and imaging features corresponded to moderate or severe HME; none of our cases were mild. EEG studies showed asymmetrical paroxysmal activity in all. MRI showed colpocephaly in 5 patients, reduced ventricles in 1 and asymmetrical cortical dysplasia in 11. The [dsquote]occipital sign[dsquote] (displacement of the occipital lobe to the opposite side) was observed in 6. Neuropathological studies using synaptophysin demonstrated that single heterotopic neurons in white matter are not [dsquote]isolated[dsquote] as they appear histologically, but are connected with grey matter neurons by axonal projections and synapses with afferent axons. These heterotopic connected neurons included some, but not all,, balloon cells. Many cells were of mixed lineage, coexpressing both neuronal and glial proteins.
CONCLUSIONS: Epilepsy affected all of our patients with HME. Clinical, EEG and imaging data are essential for the selection of patients for hemispherectomy. Neuropathological examination with immunocytochemistry provides new information in the pathogenesis of HME and insight into epileptogenesis. Heterotopic individual neurons and balloon cells and others with mixed lineage in white matter may contribute to epilepsy because they are not synaptically isolated. No pathological studies of mild cases are available.