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

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.

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.

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.

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.