Relationship between valve opening pressure, body position, and intracranial pressure in normal pressure hydrocephalus: paradigm for selection of programmable valve pressure setting.

Abstract

There is little scientific basis for guidance in selecting the optimal valve for the treatment of normal pressure hydrocephalus. The aim of this study was to determine the programmable valve opening pressure setting that would result in a slight reduction in intracranial pressure (ICP) after a ventriculoperitoneal shunt is implanted. We also assessed whether shunt-induced ICP could be predicted on the basis of a simple hydrodynamic equation. In this prospective study of 11 patients with normal pressure hydrocephalus, ICP was measured before and after implantation of a shunt incorporating a programmable valve without an antisiphon device. Pressure measurements, including intraperitoneal pressure, were recorded at body angles ranging from 0 to 55 degrees and at valve settings ranging from 30 to 200 mm H(2)O. Measured ICP values were compared with values computed using a simple hydrodynamic equation. Even at a valve setting greater than the mean baseline ICP (200 mm H(2)O), the supine ICP was significantly lower than the baseline value (baseline ICP, 164 +/- 64 mm H(2)O; postoperative ICP, 125 +/- 69 mm H(2)O, P = 0.04). Valve pressure did not equate 1:1 with the measured postoperative ICP. Comprehensive ICP measurements at upright body positions demonstrated a stepwise reduction in ICP rather than a precipitous decline as a result of so-called siphoning. This study indicates that very high valve opening pressure settings may be optimal for the initial treatment of normal pressure hydrocephalus. The relationship between ICP and opening pressure valves is linear but not predicted by simple hydrodynamics.