Severe brain edema associated with cumulative effects of hyponatremic encephalopathy and ischemic hypoxia.

Hyponatremia in cats produced brain edema, detectable by both magnetic resonance imaging (MRI) and increased brain water, with a compensatory decrease of brain sodium. Sodium transport was measured in synaptosomes from hyponatremic cat cerebral cortex. The sodium efflux via Na(+)-K(+)-ATPase was significantly higher (144%) than control, while sodium influx via the Na+/H+ antiporter was significantly decreased (74%). Both responses tend to decrease brain intracellular sodium and thus, brain cell osmolality. Ischemia following unilateral middle cerebral artery occlusion also resulted in brain edema. However, the efflux of sodium via both Na(+)-K(+)-ATPase and sodium channels actually decreased, both maladaptive responses. Furthermore, when ischemia was superimposed upon hyponatremia, all of the cerebral adaptive changes which had been induced by hyponatremia alone were rendered ineffective. This resulted in further elevations of brain water and sodium. Hyponatremia superimposed upon ischemia thus worsens the brain edema associated with ischemia alone. Thus, ischemia impairs the ability of the brain to adapt to hyponatremia, probably by eliminating the compensatory mechanisms of brain sodium transport initiated by hyponatremia.