Ventriculomegaly and pericerebral CSF collection in the fetus: early stageof benign external hydrocephalus?

Background and purpose. Mild ventricular dilatation or an asymmetric aspect of the atrium with prominent subrachnoid spaces is a challenging clinical condition in utero that requires prenatal MRI to rule out a destructive les ion or brain malformation. We report five cases that demonstrated benign ex ternal hydrocephalus postnatally, together with the prenatal MRI to define prenatal criteria of so-called benign external hydrocephalus. Methods and r esults: The prenatal MR images of five cases showing typical features of ex ternal hydrocephalus postnatally were reviewed. All cases showed in utero m ild ventricular dilatation at the level of the atrium with enlargement of t he subarachnoid spaces homo-, contra- or bilaterally, and predominantly in the parietooccipital areas. The head circumference was normal in all cases with no family history of macrocephaly. MRI did not demonstrate either abno rmal signal within brain parenchyma or loss of the normal layering of the d eveloping brain. Conclusion. Posterior mild ventricular dilatation and prom inent subarachnoid spaces in a posterior distribution can be considered an early stage of benign external hydrocephalus that is nicely illustrated by MRI.