Transhemispheric diaschisis. A review and comment.

We review here the literature in both animal models and humans concerning electrical activity, blood flow, and metabolism in the hemisphere contralateral to unilateral cerebral ischemia. We analyze the data by periods based on the time from initial injury to emphasize the time course of transhemispheric diaschisis. Contralateral electrical activity, such as evoked potential amplitude, is increased in the late stages after unilateral infarction, with the data from the more acute periods being inconclusive. Contralateral blood flow changes probably depend on the magnitude of the ischemic injury, with a larger insult resulting in a decrease not seen with smaller insults. Some studies have shown a decrease in contralateral blood flow over the first week followed by a gradual return toward baseline. Most measures of contralateral metabolism show a time course similar to blood flow, that is, a decrease followed by gradual recovery. The effects of corpus callosum section on transhemispheric diaschisis are not yet established. We provide examples to show that under certain conditions, diaschisis may represent a loss of remote inhibition rather than a loss of remote facilitation, as von Monakow originally suggested. By following the contralateral changes over time, particularly during the first minutes and hours of ischemia, insight will be gained into the brain's responses remote from the focus of ischemic injury. These responses should bear a relation to the brain's defense mechanisms ipsilaterally to the region of ischemia.