The large temporal horn: MR analysis in developmental brain anomalies versus hydrocephalus.

PURPOSE To utilize MR to delineate the morphologic abnormalities of the temporal horn and adjacent structures in patients with congenital brain anomalies and to differentiate these findings from the temporal horn alterations in obstructive hydrocephalus. PATIENTS AND METHODS Thirty-six patients were included in this retrospective study, including eight with agenesis of the corpus callosum (ACC), four with lissencephaly (lis), four with lobar holoprosencephaly (holo), as well as 20 with isolated obstructive hydrocephalus due to tumor (17 patients) or aqueductal stenosis (three patients). Twenty patients with normal scans were included as controls. RESULTS Coronal plane image analysis showed that 75% of patients with brain anomalies had enlargement of the temporal horns, most prominently involving the inferolateral aspects of the ventricle (8/8 ACC, 4/4 lis, 0/4 holo). Hippocampal formations were small in 62% (6/8 ACC, 3/4 lis, 1/4 holo). An abnormal, vertical orientation (incomplete inversion) of the hippocampal formations was observed in 82% (8/8 ACC, 4/4 lis, 1/4 holo). Focal thinning of the white matter lateral to the temporal horn was seen in 50% (8/8 ACC, 0/4 lis, 0/4 holo). All patients with isolated obstructive hydrocephalus showed enlargement of the temporal horns, most pronounced in the superior-lateral region. Hippocampal formations showed normal, horizontal orientation (complete inversion) and were of normal size in 17 of 20 patients; the only exceptions included patients with severe hydrocephalus where the hippocampi were flattened along the inferior margin of the temporal horn. Temporal lobe white matter was normal in the 17 patients with mild or moderate hydrocephalus: diffuse white matter thinning inferolaterally was observed in the three patients with severe hydrocephalus. Distinct differences were present in the morphology of the temporal horn and surrounding brain in congenital brain disorders compared with those in patients with hydrocephalic. The anomalous brains showed enlargement as a result of incomplete development, and the hydrocephalic brains showed enlargement as a result of increased intraventricular pressure. CONCLUSION Temporal horn enlargement in lissencephaly and agenesis of the corpus callosum should not be misinterpreted as hydrocephalus. Analysis of temporal lobe morphology will allow differentiation if doubt exists.