Changes in intracranial pulse pressure amplitudes after shunt implantation and adjustment of shunt valve opening pressure in normal pressure hydrocephalus.

BACKGROUND: We have previously reported that the intracranial pulse pressure amplitudes were elevated in idiopathic normal pressure hydrocephalus (NPH) patients responding to shunt surgery. Whether or not shunt implantation or adjustment of the shunt valve opening pressure modifies the intracranial pulse pressure amplitudes in NPH patients remains to be established. This report summarises our observations.

PATIENTS AND METHODS: Thirteen patients with NPH (idiopathic in nine and secondary in four) are presented in whom continuous intracranial pressure (ICP) monitoring was done before and after shunt implantation. In two, ICP monitoring was also done during adjustment of shunt valve opening pressure. The mean ICP and mean ICP wave amplitude (i.e. pulse pressure amplitudes) were determined in 6-s time windows.

RESULTS: After shunt implantation there was a fall in both mean ICP and mean ICP wave amplitude; the reduction in the two ICP parameters correlated significantly. However, mean ICP in the supine position was normal (i.e. <15 mmHg) in 12 of 13 patients before shunt placement, and remained normal after shunting. According to our criteria, the mean ICP wave amplitudes were elevated before shunting in 12 of 13 patients and became "normalised" the day after shunting in nine patients. The reduction in mean ICP wave amplitude after shunt was highly significant at the group level. Moreover, adjustment of shunt valve opening pressure modified the levels of mean ICP wave amplitudes.

CONCLUSIONS: The present observations in 13 NPH patients indicate that shunt implantation reduces mean ICP wave amplitudes. Moreover, the level of reduction can be tailored by adjustment of the shunt valve opening pressure.