brain death
a state of extreme and irreversible unconsciousness in which neurological activity and vital physiologic functions have ceased, as opposed to cardiac death, in which cessation of natural cardiac activity and breathing indicates a patient is deceased. Although several professional organizations in neurology and related fields have specified their own guidelines for determining brain death, globally standardized criteria do not yet exist. For example, brainstem death is considered sufficient in the United Kingdom whereas whole-brain death is required in the United States. Despite such variability, many clinicians (as well as legal statutes) establish brain death through the concurrent presence of the following: nonresponsiveness to noxious or other stimuli, absence of breathing, absence of reflexes and spontaneous movement, and absence of electroencephalogram activity. Exact methods of evaluating these factors differ significantly across facilities—for
example, different U.S. hospitals require different ancillary tests for cerebral blood flow and other characteristics, including angiography, positron emission tomography, sensory evoked potential recording, single photon emission computed tomography, and sonography (see ultrasound). Additionally, medical conditions and other complicating factors that temporarily depress brain function, such as hypothermia, drug overdose, and hyperglycemia or hypoglycemia, must be excluded before a definitive determination of brain death can be made. It must also be distinguished from the similar conditions of coma and a vegetative state. Also called cerebral death; irreversible coma.