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) is elevated in normal pressure hydrocephalus (NPH). These studies assume that the cerebrospinal formation rate (CSF) does not vary during the infusion. If the CSF were to increase during the infusion then the R would be overestimated. Previous estimates of the CSF in NPH have suggested a low figure. More recent estimates of the CSF suggest that it is increased, indicating it probably varies with measurement technique. This would bring the estimation of R into doubt. A previous paper using a lumped parameter model suggested the CSF 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.

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 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 by increasing the interstitial fluid production.

CONCLUSIONS

The model suggests that infusion studies and draining CSF increases the CSF in NPH compared to earlier techniques. Allowing for an increase in the CSF suggests that infusion overestimates the R 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.