External hydrocephalus. Early finding in congenital communicating hydrocephalus.

Dilation of the subarachnoid channels overlying the cerebral hemispheres, ie, external hydrocephalus, can be an early anatomic-radiologic finding in some infants with congenital communicating hydrocephalus. This structural change may progress to ventricular dilation or internal hydrocephalus. Enlargement of the subarachnoid channels appears to occur during the first stage in the development of congenital communicating hydrocephalus, which is to say that the subarachnoid compartment allows accumulation of CSF before there is significant ventricular distention, at least in some infants. When a pneumoencephalogram or a CT scan is performed at this stage, the findings may falsely suggest "cerebral atrophy." In six patients who received this grave neuroradiologic diagnosis, five subsequently developed normally and the sixth has static incoordination or clumsiness and mild intellectual deficiency.