Subdural effusions re-appearing after shunts in patients with non-tumoural stenosis of the aqueduct.

Three patients shunted for non-tumoural stenosis of the aqueduct suffered from progressive clinical symptoms about four months after the shunting. Computed tomography (CT) showed bilateral subdural effusions. The effusions were evacuated, and the shunts revised. One month later all patients suffered from symptoms of increased intracranial pressure, and CT showed enlargement of the supratentorial cerebral ventricles. The effusions had disappeared. After shunt revision the symptoms decreased again. The fluctuation in the ventricular size, the thickness of the subdural effusions, and the clinical deterioration were related to the change in the opening pressure of the shunt valve in all patients. Patients with large supratentorial cerebral ventricles (Evans index over 0.40) should be monitored by intraventricular pressure recording in order to select the exact opening pressure of the shunt valve before inserting a relieving shunt; a clinical check-up and a CT examination should be carried out about three months after the operation in order to investigate any changes in the function of the shunt.