Aqueductal Stenosis is seen in infants and young adults. It can present later on in the teenage years as well. Essentially, the aqueduct is atretic or sometimes the channel will fork into a blind pouch. Since cerebral spinal fluid (CSF) cannot flow down into the IV Ventricle, obstructive hydrocephalus develops. Characteristically, everything proximal to the aqueduct dilates. The IV Ventricle remains normal in size.

Aqueductal Stenosis : CAT Scan

Notice how the ventricular system has developed obstructive hydrocephalus proximal to the aqueduct. The frontal horns, the III ventricle and the temporal horns are all enlarged. A lower CT slice demonstrates that the IV ventrical which is distal to the aqueduct is normal in size. Compare this CT to the CT below showing communicating hydrocephalus where all the parts of the ventricular system are dilated.

Communicating Hydrocephalus: CAT Scan

Notice how all the ventricles are enlarged.