Fourth ventricular entrapment caused by rostrocaudal herniation following shunt malfunction.

The subacute development of isolated fourth ventricle (IFV) is a recognized complication following shunting of the lateral ventricles for congenital and acquired hydrocephalus. We present an unusual case of acute IFV in a clinical setting which has not previously been described. Subsequent to rostrocaudal herniation caused by an obstructed frontally placed ventricular catheter, IFV developed in our patient 24 h following shunt revision, necessitating placement of an additional fourth ventricle shunt system. No signs of intraventricular hemorrhage or cerebrospinal fluid (CSF) infection were detected at the time of shunt revision and there was no documentation of similar events in the perinatal history. Dependent upon the actual underlying etiology of this child's hydrocephalus, we hypothesize that two mechanisms may have accounted for this unusual and precipitous development of IFV. Following rostrocaudal herniation and caudal shift of the brainstem, progressive edema in the pons developed. If communicating hydrocephalus was the primary etiology, then midbrain edema occluded the aqueduct of Sylvius, preventing retrograde flow of CSF to the shunt. A distinctly different mechanism for acute IFV must be invoked if aqueductal stenosis was the preexisting cause for congenital hydrocephalus. Following herniation, brainstem displacement and edema resulted in obliteration of the lateral pontine and ambient cisterns, preventing the normal rostral migration of CSF around and over the mesencephalon. Cerebellar tonsillar herniation with impaction of the tonsils into the foramen magnum may have also contributed to obstruction of fourth ventricular outflow in both settings. This unusual case of acute onset IFV is presented in detail. The underlying etiologies and clinical settings in which IFV may develop is reviewed as well.(ABSTRACT TRUNCATED AT 250 WORDS)