Brain death describes the irreversible loss of all brain stem functions which, in turn, deprives the forebrain of all expressions of conscious awareness, behavior and, shortly thereafter, of life itself. Mechanisms Coma always arises as a result of an acute disorder that produces sudden functional depression or damage to specific and nonspecific cerebral, diencephalic or upper brain stem arousal mechanisms. Its most common and reversible example is the state of general anesthesia. Examples of such conditions include ischemic infarcts, the inflammatory lesions of encephalitis and the development of either downward or upper herniation of midline brain structures through the tentorial incisura. Anatomically, these latter arousal-generating structures lie predominantly in the subcortical brain's paramedian gray matter extending from the level of the nucleus parabrachialis of the pontine tegmentum forward as far as the ventral posterior hypothalamus and adjacent pretectal areas.
Some authorities believe that the intralaminar nuclei also play a critical role in arousal.
Most pathological coma reflects acute or subacute but always profound involvement of one or more of the above mechanisms. Acute abnormalities that are similarly distributed but less severe cause delirium , a condition marked by fluctuating levels of attention and arousal associated with impaired memory, confusion, distractibility and, sometimes, delusions or hallucinations. Cyclic sleep wake patterns characterize the day to day behavior of all mammals. Accordingly, nearly all humans who survive illnesses causing coma reawaken within a matter of days or weeks but may retain varying degrees of cognitive impairment. For example, sustained coma following a traumatic head injury has a better prognosis than post-anoxic coma at almost any age.