The effect of skull and dura on brain volume regulation after hypo- and hyperosmolar fluid treatment

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Abstract
This study was performed to determine the response of brain water and electrolytes to acute hypo-osmolality and hyperosmolality in animals with intact skulls and dura, in comparison with those subjected to extensive bilateral or unilateral craniectomy and opening of the dura. In rats, 4 to 5 weeks after extensive unilateral or bilateral craniectomy and opening of the dura, a 50-mOsm/kg decrease in plasma osmolality was produced by systemic administration of distilled water ("water intoxication"), or a 28-mOsm/kg increase in plasma osmolality was produced by systemic administration of either 1 M NaCl or 1 M mannitol in 0.34 M NaCl. Tissue water, Na, and K contents were determined after 120 minutes. Tissue water accumulation or water loss was proportional to the decrease or increase in plasma osmolality. The tissue water accumulation after "water intoxication," however, was less (40% of the predicted value) than that predicted for ideal osmotic behavior. The brain tissue was also found to shrink less than predicted on the basis of ideal osmotic behavior (40% of the predicted value after mannitol treatment, and 60% after NaCl administration). This nonideal osmotic response of the brain tissue is consistent with the finding in other studies and indicated a significant degree of volume regulation. Water and electrolyte changes did not differ between animals operated on and those not operated on, a fact which demonstrates that there are no effects of extensive skull and dura defects on tissue volume regulation under hypo- and hyperosmolar conditions encountered under clinical circumstances. The results support the view that the volume of brain tissue is controlled by an internal "osmometer" consisting of the capillary endothelium, which is not affected by alterations in the brain''s container. These observations indicate that the effect of osmotic therapy on the nonpathological tissue volume of craniectomy patients is not influenced by the skull and dura defect.