Do the Skull and Dura Exert Influence on Brain Volume Regulation Following Hypo- and Hyperosmolar Fluid Treatment?

Summary

The present studies were performed to determine the response of the brain water and electrolytes to acute hypoosmolality and hyperosmolality in animals with intact skull and dura, in comparison with those subjected to extensive bilateral or unilateral craniectomy and dural opening. Four to 5 weeks following extensive unilateral or bilateral craniectomy and dural opening in rats, 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. However, the tissue water accumulation following “water intoxication” was less (40% of the predicted value) than that predicted for ideal osmotic behaviour. The brain tissue was also found to shrink less than predicted on the basis of ideal osmotic behaviour (40% of the predicted value after mannitol treatment, and 60% after NaCl administration). This non-ideal osmotic response of the brain tissue is consistent with the finding in other studies¹ and indicated a significant degree of volume regulation.

Water and electrolyte changes were not different in operated and non-operated animals, demonstrating no effects of extensive skull and dura defects on tissue volume regulation under hypo- and hyperosmolar conditions of a degree that may be 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 non-pathological tissue volume of craniectomy patients is not influenced by a skull or dura defect.