A Lumped Parameter Model Suggests That Infusion Studies Overestimate the Cerebrospinal Fluid Outflow Resistance in Normal Pressure Hydrocephalus.

Background/objectives: Cerebrospinal infusion studies indicate that cerebrospinal fluid outflow resistance (R _out ) is elevated in normal pressure hydrocephalus (NPH). These studies assume that the cerebrospinal formation rate (CSF _fr ) does not vary during the infusion. If the CSF _fr were to increase during the infusion then the R _out would be overestimated. Previous estimates of the CSF _fr in NPH have suggested a low figure. More recent estimates of the CSF _fr suggest that it is increased, indicating it probably varies with measurement technique. This would bring the estimation of R _out into doubt. A previous paper using a lumped parameter model suggested the CSF _fr could vary with the capillary transmural pressure (TMP) in this disease, suggesting a possible solution to this problem. The current study investigates the possibility that the intracranial pressure manipulation occurring during an infusion study may vary the capillary TMP and CSF _fr .
Methods: A lumped parameter model previously developed to describe the hydrodynamics of NPH was modified to investigate the effect of CSF pressure manipulation during infusion studies and to describe how the CSF _fr could vary depending on the technique used.
Results: The model indicates the capillary transmural pressure is normal in NPH and increases during an infusion study. CSF drainage at the end of an infusion study similarly increases the capillary TMP and, presumably, the CSF _fr by increasing the interstitial fluid production.
Conclusions: The model suggests that infusion studies and draining CSF increases the CSF _fr in NPH compared to earlier techniques. Allowing for an increase in the CSF _fr suggests that infusion overestimates the R _out by between 23 and 33%. This study indicates that further research may be required into the utility and accuracy of infusion studies and their ability to diagnose NPH.