Biomechanical and hydrodynamic characterization of the hydrocephalic infant.

The pressure-volume index (PVI) technique of bolus manipulation of cerebrospinal fluid (CSF) was used to measure neural axis volume-buffering capacity and resistance to the absorption of CSF in 16 hydrocephalic infants prior to shunting. The mean steady-state intracranial pressure (ICP) was 11.7 +/- 5.7 mm Hg (+/- standard deviation (SD], representing a modest elevation of ICP in infants. The mean measured PVI was 28.1 +/- 1.5 ml (+/- standard error of the mean (SEM] compared to the predicted normal level for these infants of 12.1 +/- 2.7 ml (+/- SD) (p less than 0.001). This resulted from an enhanced volume storage capacity in the hydrocephalic infants. The PVI was not related to ventricular size in these hydrocephalic infants. Although absorption of the additional bolus of fluid did not occur at steady-state ICP, it was readily absorbed once ICP was raised above a mean threshold pressure of 16.0 +/- 5.0 mm Hg (+/- SD) in 13 of the 16 infants. Above this pressure, the mean CSF absorption resistance was 7.2 +/- 1.3 mm Hg/ml/min (+/- SEM) which is twice the normal values as measured by the bolus injection technique. The biomechanical profile of infantile hydrocephalus described in this study indicates that two factors are required for progression of ventricular volume. While an absorptive defect may initiate the hydrocephalic process, progressive volume storage requires an alteration in the mechanical properties of the intracranial compartment.