The fetal subarachnoid cisterns: an ultrasound study with report of a case of congenital communicating hydrocephalus.

The intracranial anatomy of 351 normal fetuses, ranging from 15 to 42 weeks' gestational age, was investigated sonographically in an attempt to define the normal sonographic anatomy of the subarachnoid spaces during intrauterine development. The triangular velum, vein of Galen, ambient, interpeduncular, chiasmatic, Sylvian, and supracerebellar cisterns and the cisterna magna were easily recognized in the vast majority of cases as fluid-filled spaces following the contour of the brain. A large fluid layer, overlying the cerebral convexities, was seen in early gestation and subsequently disappeared. Evaluation of the subarachnoid cisterns in a fetus with ventriculomegaly allowed the prenatal diagnosis of communicating hydrocephalus. Knowledge of the normal sonographic anatomy of the subarachnoid spaces is useful both in avoiding misinterpretation of the normal sonogram and in the differential diagnosis of congenital hydrocephalus.