Disproportionately Large Fourth Ventricle as a Complication of Skull Base Surgery

Objective: To raise awareness of a disproportionately large fourth ventricle (DLFV) as a variant of hydrocephalus complicating skull base surgery in which management significantly varies from other forms of hydrocephalus.

Design: Retrospective case series.

Patients/Materials and Methods: The study included three patients with progressive cerebellar and/or cranial nerve dysfunction after skull base procedures incorporating the foramen magnum expansion. One patient had foramen magnum meningioma, one had clipping of PICA aneurysm, and one had Chiari malformation. Hydrocephalus was diagnosed on imaging. Dilatation of all ventricles was observed, but a disproportionate enlargement of the fourth ventricle was also noted.

Results: All but one patient had multiple CSF shunt procedures. It was falsely assumed that failure of clinical and radiological improvement was secondary to technical failure. In the last case, DLFV was recognized at presentation and multiple procedures were avoided.

In the author's experience, DLFV is associated with altered compliance of the brain parenchyma. Resolution requires CSF drainage at sub-physiological pressures. In the first case, resolution was achieved with an unvalved ventriculoperitoneal shunt; the others required pleural drainage to achieve sufficient negative pressure.

When DLVF was recognized at presentation, an external drain was placed as the first option then converted to a pleural shunt with successful resolution of clinical and radiological features.

Conclusions: DLVF has only once been reported in the English literature, but skull base surgeons should be aware of the phenomenon to avoid mistaking the entity for an encysted fourth ventricle, and to prevent inappropriate management with shunt systems incorporating unnecessary valves or anti-siphon devices.