Objective: Hydrodynamic theories of idiopathic normal pressure hydrocephalus (iNPH) favor a decrease in compliance as an important underlying principle of the disease's immanent pathophysiology. ICP overnight monitoring (ONM) and lumbar infusion study (LIS) are both established methods that aid as supplemental tests in the selection of shunt-responsive patients. Pulse wave amplitude (AMP) and RAP index are measures derived from ONM and are related to intracranial compliance. Elastance (E) and pressure volume index (PVI), parameters derived from LIS, also describe the compliance of the system. We investigated whether the parameters of the two methods correlat with each other.
Methods: Thirty-three patients with probable/possible NPH underwent ICP overnight monitoring and a lumbar infusion study in supine position with ICP recorded from the intracranial compartment.
Results: E and PVI from the lumbar infusion study did not correlate at all with RAP or AMP from ICP monitoring. A qualitative change of the RAP and E pointing towards the same direction of either increased or decreased compliance, however, was found in 60.6% of our patients.
Conclusion: Although data from the infusion test and from overnight monitoring of ICP describe the underlying pathophysiology in at least 60% of patients qualitatively in the same way; there is no direct correlation of parameters. This indicates that the underlying mechanisms of RAP are different from those in principle or in reaction time that are responsible for E.