Why valve opening pressure plays a relatively minor role in the postural ICP response to ventricular shunts in normal pressure hydrocephalus: modeling and implications.

OBJECTIVES: In normal pressure hydrocephalus, there is no clear evidence that the opening pressure of differential pressure valves influences outcome. In this study, the objective was to determine whether the relationship between valve opening pressure and post-operative postural ICP changes was predictable.

METHODS: Eight adult-onset hydrocephalus patients were evaluated using an intraparenchymal ICP microsensor. Postural ICP measurements were obtained before and after placement of a ventricular shunt (VP or VA) with a Codman Hakim programmable valve.

RESULTS: Valve opening pressure had a poor correlation with ICP at any head-of-bed elevation across subjects (best r = 0.25). The degree of negative ICP encountered for any given patient did not appear to be determined by the valve pressure, and varied significantly between patients at the same valve pressure. Each individual's ICP correlated well to opening pressure (r > 0.76), but the response is more sigmoidal than linear, and the opening pressure at which ICP changed most was markedly different between patients.

CONCLUSIONS: The hydrodynamic pressures generated by shunts with different pressure valves are unpredictable. Our data supports the use of programmable valves in patients with adult onset hydrocephalus and suggests that ICP be measured post-operatively to achieve an optimal clinical outcome.