The relation between CSF pressure and ventricular dilatation in hydrocephalic HTx rats

The relation between increased cerebrospinal fluid (CSF) pressure and ventricular enlargement in infantile hydrocephalus is uncertain, variable, and probably dependent on the etiology of the condition. The HTx rat has early-onset hydrocephalus due to aqueduct stenosis in late gestation with ventricular dilatation present in fetuses. Increased CSF pressure, however, is first detected at postnatal day 10 or later. How the transition from low pressure to raised pressure hydrocephalus takes place, is not clear. In order to study this, we investigated how CSF pressure is related to the magnitude of ventricular dilatation using hydrocephalic and control HTx rats at postnatal days 15 and 21. At 15 days, the CSF pressure for hydrocephalics was 31.1 +/- 3.6 mm H2O (mean +/- SEM) which was not significantly higher than the controls at 19.9 +/- 2.8 mm H2O. At 21 days, the pressure was significantly higher in hydrocephalics at 50.9 +/- 7.5 mm H2O compared to 24.3 +/- 3.6 mm H2O for controls, p < 0.05. The mean volume of the lateral ventricles was 409.3 mm3 at day 15 and 478.8 mm3 at day 21, whereas age-matched control rats in a previous study had ventricles of 14 and 25 mm3. At each age there was a significant linear relationship between CSF pressure and ventricle volume, p < 0.05 at day 15 and p < 0.01 at day 21. Closer examination of the data, however, showed that 5/10 hydrocephalics at day 15 and 4/10 at day 21 had CSF pressures that were within the 95% confidence limits of the mean pressures for the control groups. The ventricle volumes of these low pressure hydrocephalics were between 100 and 280 mm3 which was several times larger than the ventricles in control rats. CSF pressure in the remaining 15 day rats ranged from 30 to 53 mm H2O and in the 21 day rats from 46 to 90 mm H2O and the ventricles were between 280 and 1050 mm3. The results suggest that up to a critical volume the infant rat brain can accommodate additional CSF without an increase in pressure. At this volume there is a "break point" beyond which additional fluid results in further expansion along with an increase in CSF pressure.