Effect of osmolarity on CSF volume during ventriculo-aqueductal and ventriculo-cisternal perfusions in cats.

The effect of cerebrospinal fluid (CSF) osmolarity on the CSF volume has been studied on different CSF/brain tissue contact areas. It has been shown, on anesthetized cats under normal CSF pressure, that the perfusion of CSF system (12.96 μl/min) by hyperosmolar CSF (400 mOsml/l) leads to significantly higher outflow volume (μl/min) during ventriculo-cisternal perfusion (29.36 ± 1.17 and 33.50 ± 2.78) than the ventriculo-aqueductal perfusion (19.58 ± 1.57 and 22.10 ± 2.31) in experimental period of 30 or 60 min. Both of these hyperosmolar perfusions resulted in significantly higher outflow volume than the perfusions by isoosmolar artificial CSF (12.86 ± 0.96 and 13.58 ± 1.64). These results suggest that the volume of the CSF depends on both the CSF osmolarity and the size of the contact area between CSF system and surrounding tissue exposed to hyperosmolar CSF. However, all of these facts imply that the control of the CSF volume is not in accordance with the classical hypothesis of cerebrospinal fluid hydrodynamic. According to this hypothesis, the CSF volume should be regulated by active formation of CSF (secretion) inside the brain ventricles and passive CSF absorption outside of the brain. Obtained results correspond to the new hypothesis which claims that the volume of CSF depends on the gradients of hydrostatic and osmotic forces between the blood on one side and extracellular fluid and CSF on the other. The CSF exchange between the entire CSF system and the surrounding tissue should, therefore, be determined by (patho)physiological conditions that predominate within those compartments.