AKA isolated fourth ventricle: 4th ventricle that neither communicates with the 3rd ventricle (through Sylvian aqueduct) nor with the basal cisterns (through foramina of Luschka or Magendie). Usually seen with chronic shunting of the lateral ventricles, especially with post-infectious hydrocephalus (fungal, in particular) or in those with repeated shunt infections. Possibly as a result of adhe- sions forming from prolonged apposition of the ependymal lining of the aqueduct due to the