4TH VENTRICULAR ENTRAPMENT CAUSED BY ROSTROCAUDAL HERNIATION FOLLOWING SHUNT MALFUNCTION

The subacute development of isolated fourth ventricle (IFV) is a recog nized complication following shunting of the lateral ventricles for co ngenital and acquired hydrocephalus. We present an unusual case of acu te IFV in a clinical setting which has not previously been described. Subsequent to rostrocaudal herniation caused by an obstructed frontall y placed ventricular catheter, IFV developed in our patient 24 h follo wing shunt revision, necessitating placement of an additional fourth v entricle shunt system. No signs of intraventricular hemorrhage or cere brospinal fluid (CSF) infection were detected at the time of shunt rev ision and there was no documentation of similar events in the perinata l history. Dependent upon the actual underlying etiology of this child 's hydrocephalus, we hypothesize that two mechanisms may have accounte d for this unusual and precipitous development of IFV. Following rostr ocaudal herniation and caudal shift of the brainstem, progressive edem a in the pons developed. If communicating hydrocephalus was the primar y etiology, then midbrain edema occluded the aqueduct of Sylvius, prev enting retrograde flow of CSF to the shunt. A distinctly different mec hanism for acute IFV must be invoked if aqueductal stenosis was the pr eexisting cause for congenital hydrocephalus. Following herniation, br ainstem 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 hernia tion with impaction of the tonsils into the foramen magnum may have al so contributed to obstruction of fourth ventricular outflow in both se ttings. This unusual case of acute onset IFV is presented in detail. T he underlying etiologies and clinical settings in which IFV may develo p is reviewed as well. Although the precise sequence of events leading to acute IFV cannot be resolved for this case, the importance of reco gnizing the potential for acute IFV following herniation is underscore d.