Wide cavum septum pellucidum: a marker of disturbed brain development. A wide cavum septum pellucidum defined as a separation of greater than 1 cm of the leaves occurs uncommonly. Nine children with wide cavum septum pellucidum were studied; 8 were abnormal. Observed abnormalities included cognitive impairment (8), seizures (4), hypoplasia of the corpus callosum (4), optic nerve hypoplasia (2), and growth failure (4). The incidence of intellectual dysfunction, the association with midline anomalies of the brain, and growth failure all suggest that wide cavum septum pellucidum may represent part of a spectrum of midline brain anomalies. 