Brain death is defined as a complete and irreversible cessation of brain activity. Absence of apparent brain function is not enough. Evidence of irreversibility is also required.

Traditionally, death has been defined as the cessation of all body functions, including respiration and heartbeat. Since it became possible to revive some people after a period without respiration, heartbeat, or other visible signs of life, as well as to maintain respiration and blood flow artificially using life support treatments, an alternative definition for death was needed. In recent decades, the concept of "brain death" has emerged. By brain-death criteria, a person can be pronounced clinically dead even if the heart continues to beat due to life support measures.

A brain-dead individual has no electrical activity and no clinical evidence of brain function on neurologic examination (no response to pain, no cranial nerve reflexes (pupillary response (fixed pupils), oculocephalic reflex, corneal reflexes), and no spontaneous respirations). It is important to distinguish between brain death and states that mimic brain death (eg. barbiturate intoxication, alcohol intoxication, sedative overdose, hypothermia, hypoglycemia, coma or chronic vegetative states). Some comatose patients can recover, and some patients with severe irreversible neurologic dysfunction will nonetheless retain some lower brain functions such as spontaneous respiration.

In a brain dead individual, the brain tissue itself is necrotic (dead). Conditions leading to brain death involve swelling of brain tissue and a rise in intracranial pressure, eventually shutting off all blood flow within the skull. Although some have proposed that loss of neo-cortical function should be termed "brain death", the term as used clinically includes loss of both cortex and brainstem function. Thus anencephaly, in which there is no higher brain present, is generally not considered brain death, although it is certainly an irreversible condition in which it may be appropriate to withdraw life support.

Note that brain electrical activity can stop completely, or apparently completely (a "flat EEG") for some time in deep anaesthesia or during cardiac arrest before being restored. Brain death refers only to the permanent cessation of electrical activity. Numerous people who have experienced such "flat line" experiences have reported near-death experiences, the nature of which is controversial.

It is presumed that a permanent cessation of electrical activity indicates the end of consciousness. Those who view the neo-cortex of the brain as solely responsible for consciousness, however, argue that only electrical activity there should be considered when defining death. In many cases, especially when elevated intracranial pressure prevents blood flow into the skull, the entire brain is nonfunctional; however, some injuries may affect only the neo-cortex.

The diagnosis of brain death needs to be made quite rigorously to be certain the condition is truly irreversible. Legal criteria vary from place to place, but generally require neurologic exams by two independent physicians showing complete absence of brain function, and may include two isoelectric (flat-line) EEGs 24 hours apart. The proposed Uniform Determination Of Death Act in the United States is an attempt to standardize criteria. The patient should have a normal temperature and be free of drugs that can suppress brain activity if the diagnosis is to be made on EEG criteria. Alternatively, a radionuclide cerebral blood flow scan that shows complete absence of intracranial blood flow can be used to confirm the diagnosis without performing EEGs.

Most organ donation for organ transplantation is done in the setting of brain death. Consent from family members is required in most nations (except, for instance, in Belgium and Portugal, where every citizen is automatically an organ donor, unless you get a special attest stating that you are not an organ donor), and the non-living donor is kept on ventilator support until the organs have been surgically removed. If a brain dead individual is not an organ donor, ventilator and drug support is discontinued and cardiac death is allowed to occur.

References

• Karasawa H, et al. Intracranial electroencephalographic changes in deep anesthesia. Clin Neurophysiol. 2001 Jan;112(1):25-30. (Medline abstract (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11137657))