Morphology of acallosal brains as assessed by MRI in six patients leading a normal daily life.

The pattern of anatomical features of the brain revealed by magnetic resonance imaging (MRI) is described in six patients incidentally identified as having acallosal brains. The complex of morphological features associated with complete agenesis of the corpus callosum included lateral displacement of slitlike anterior horns of the lateral ventricles (bullhorn-like shape), dilatation of the posterior horns of the lateral ventricles, absence of the septum pellucidum, lateral displacement of the cingulate gyri, complete separation of fornices and the presence of the anterior commissure and longitudinal callosal bundles (Probst's bundles). No compensatory enlargement of the anterior commissure was seen in the patients. The planimetrically measured cross-sectional areas of the anterior commissures were between 2.0 and 4.2 mm2 (mean 3.1) (in ten normal subjects they were 4.5, SD 0.4; range 3.8-5.2 mm2) and were reduced in four and normal in two patients. Inconstant morphological features were an absence of the posterior commissure and a radial pattern of the sulci and gyri on the medial aspect of the hemispheres. Conventional clinical testing revealed no abnormalities except a slight impairment of walking heel-to-toe in two patients. None of the patients had subjective restrictions of activities of daily life, which shows the efficacy of unknown compensatory processes.