Intracranial volume adjustments and cerebrospinal fluid pressure in the osmotically swollen rat brain.

Acute dilutional hyponatremia (DH) (plasma [Na+] approximately equal to 110 meq/l over 3 h) was produced by intraperitoneal dialysis with distilled water in pentobarbital-anesthetized rats with either closed (SC) or open (SO) skulls. SO rats showed marked brain protrusion during DH, but the loss of brain Na+ and Cl- and gain in brain water was the same as that seen in SC rats. During DH, cerebrospinal fluid pressure (PCSF) rose five to six times within 90 min postdialysis and then fell toward control values in both SC and SO rats. This pattern did not correlate with changes in brain water, suggesting that volume adjustments in another intracranial compartment, possibly bulk CSF, are partially responsible for changes in PCSF. The pressure-volume characteristics of the intracranial space, as measured by the pressure-volume index (PVI), were unaffected by osmotic brain swelling. Comparison of PCSF values observed during DH with values predicted from PVI data showed that actual PCSF is, at most, half the predicted value. This again suggests reduction of a nontissue intracranial compartment, which, from relative size considerations, is probably bulk CSF.