811 Relationship between Valve Opening Pressure, Body Position, and Intracranial Pressure in Normal-pressure Hydrocephalus: Paradigm for Selection of Programmable Valve Pressure Setting

Abstract

INTRODUCTION: There is little scientific basis for guidance in selecting the optimal valve for the treatment of normal-pressure hydrocephalus (NPH). The aim of this study was to determine the programmable valve opening pressure (OPV) setting that would result in a slight reduction in intracranial pressure (ICP) after a ventriculoperitoneal (VP) shunt. We also assessed whether shunt-induced ICP changes could be predicted on the basis of a simple hydrodynamic equation. METHODS: In this prospective study of 11 patients with NPH, ICP was measured before and after implantation of a shunt incorporating a programmable valve without an antisiphon device. Pressure measurements, including intraperitoneal pressure (IPP), and shunt hydrostatic column lengths (HC) were recorded at body angles ranging from 0 to 55 degrees and at valve settings ranging from 30 to 200 mm H2O. Measured ICP values were compared with values computed using a simple hydrodynamic equation (ICPPredicted = OPV − HC + IPP). RESULTS: Even at the highest OPV setting of 200 mm H2O, which exceeded the mean baseline ICP (164 ± 64 mm H2O), there was a significant reduction in the postoperative ICP in the supine position (125 ± 69 mm H2O, P = 0.04). The OPV did not equal the measured postoperative ICP. Linear regression revealed a nonunity slope (0.39) and a nonzero y intercept (47 mm H2O versus measured IPPSupine of 69 ± 23 mm H2O). A precipitous decline of ICP at upright body positions, predicted by so-called siphoning, was not seen. Instead, postural ICP profile nearly paralleled that of the baseline measurements. CONCLUSION: This study indicates that very high OPV settings may be optimal for the initial treatment of NPH. The relationship between ICP and OPV is linear but not predicted by simple hydrodynamics. In NPH, VP shunts seem to lower ICP in a “physiological” manner without significant siphoning.