Unilateral megalencephaly: correlation of MR imaging and pathologic characteristics.

Unilateral megalencephaly is a rare and poorly understood malformation resulting in the enlargement of all or part of a cerebral hemisphere. The clinical and radiologic features of 12 patients with unilateral megalencephaly are presented; pathologic correlation was available in four. All patients had seizures and developmental delay. Two were in congestive heart failure as a result of arteriovenous shunting through the abnormal hemisphere. The affected hemispheres showed a wide spectrum of involvement. Anomalies of neuronal migration were present, and there was a roughly inverse correlation between the severity of hemispheric involvement and the magnitude of enlargement. This correlation is explained via a proposed mechanism of a mild hemispheric insult in the middle-to-late second trimester. One patient had an extremely anomalous hemisphere that did not have characteristics of a neuronal migration anomaly and may have been a hamartomatous malformation. Our correlation of the clinical, radiologic, and pathologic features of unilateral megalencephaly, together with a theory of pathogenesis, should help elucidate this rare malformation.