Seizure and acute osmotic change: clinical and neurophysiological aspects

There are a number of clinical situations where overhydration may occur. If the reduction in plasma osmolality is acute, passive water influx swells brain cells, shrinking the extracellular space around them. It is during this time that susceptibility to generalized tonic-clonic seizure dramatically increases. Common clinical examples include hastened rehydration therapy, the dialysis disequilibrium syndrome, compulsive polydipsia, the syndrome of inappropriate ADH secretion (SIADH) and post-TURP syndrome. Treatments that tend to restore normal cellular volume (dehydration, mannitol infusion) help protect against this form of seizure. Support for a correlation between plasma osmolality and seizure susceptibility is scattered amongst the literature of several medical disciplines and spans almost 70 years. However a cellular basis to explain how overhydration might promote epileptiform activity has been examined only recently. The neocortical and hippocampal brain slice preparations permit an examination of how acute osmotic change alters cortical excitability independent of vascular damage, brain compression or other factors secondary to brain swelling. Electrophysiological evidence indicates that hyposmolality promotes epileptiform activity by strengthening both excitatory synaptic communication in neocortex and field effects among the entire cortical population. Moreover there is little evidence that associated hyponatremia in itself leads to increased CNS excitability. Such findings help in understanding how rapid lowering of plasma osmolality in clinical situations can promote the hyperexcitability associated with generalized tonic-clonic seizure.