Brain death is the irreversible end of all brain activity (including involuntary activity necessary to sustain life) due to total necrosis of the cerebral neurons following loss of brain oxygenation. It should not be confused with a persistent vegetative state.
Brain death, either of the whole brain or the brain stem, is used as a legal indicator of death in many jurisdictions.
 Legal history
Traditionally, both the legal and medical communities determined death through the end of certain bodily functions, especially respiration and heartbeat. With the increasing ability of the medical community to resuscitate people with no respiration, heart beat, or other external signs of life, the need for a better definition of death became obvious. This need gained greater urgency with the widespread use of life support equipment, which can maintain body functions indefinitely, as well as rising capabilities and demand for organ transplantation.
Since the 1960s, laws on determining death have been therefore been implemented in all countries with active organ transplantation programs. The first country to adopt brain death as a legal definition (or indicator) of death was Finland in 1971. In the United States, Kansas enacted a similar law earlier.
An ad hoc committee at Harvard Medical School published a pivotal 1968 report to define irreversible coma. The Harvard criteria gradually gained consensus towards what is now known as brain death. In the wake of the 1976 Karen Ann Quinlan controversy, state legislatures moved to accept brain death as an acceptable indication of death. Finally, a presidential commission issued a landmark 1981 report ' Defining Death: Medical, Legal, and Ethical Issues in the Determination of Death ' that rejected the "higher brain" approach to death in favor of a "whole brain" definition. This report was the basis for the Uniform Determination of Death Act, which is now the law in almost all fifty states. Today, both the legal and medical communities in the US use "brain death" as a legal definition of death, allowing a person to be declared legally dead even if life support equipment keeps the body's metabolic processes working.
In the UK the Royal College of Physicians reported in 1976 and 1977, rejecting the whole brain death criterion as scientifically worthless, and adopting the notion of irreversible brain stem dysfunction as an indicator of death.
 Religious views
Religious views on organ donation are generally very favourable, although there is a debate in certain religious groups on the validity of current brain death criteria. Accordingly, the more theologians are accepting of current brain death criteria, the more they are likely to support organ donation. It is clearly not compatible with some Shinto beliefs,
and is controversial in certain Buddhist, Christian, Jewish and Muslim circles. Japan has been a very late adopter of brain centric indicators of death as a result.
 Medical criteria
A brain-dead individual has no clinical evidence of brain function upon physical examination. This includes no response to pain and no cranial nerve reflexes. Reflexes include pupillary response (fixed pupils), oculocephalic reflex, corneal reflex, no response to the caloric reflex test and no spontaneous respirations.
It is important to distinguish between brain death and states that may mimic brain death (e.g., barbiturate overdose, alcohol intoxication, sedative overdose, hypothermia, hypoglycemia, coma or chronic vegetative states). Some comatose patients can recover, and some patients with severe irreversible neurological dysfunction will nonetheless retain some lower brain functions such as spontaneous respiration, despite the losses of both cortex and brain stem functionality. Thus, anencephaly, in which there is no higher brain present, is generally not considered brain death, though 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 drop to such a low level as to be undetectable with most equipment. An EEG will therefore be flat, though this is sometimes also observed during deep anaesthesia or cardiac arrest. Although in the United States a flat EEG test is not required to certify death, it is considered to have confirmatory value. In the UK it is not considered to be of value.
The diagnosis of brain death needs to be rigorous, in order to be certain that the condition is irreversible. Legal criteria vary, but in general in the USA they require neurological examinations by two independent physicians. The exams must show complete absence of brain function (brain stem function in UK), and may include two isoelectric (flat-line) EEGs 24 hours apart (less in other countries where it is accepted that if the cause of the dysfunction is a clear physical trauma there is no need to wait that long to establish irreversibility). The widely-adopted Uniform Determination of Death Act in the United States attempts 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. The case of Zach Dunlap, who was declared brain dead but later recovered , may be seen to undermine this presumption. However, since he was declared dead only a few hours after presentation, he did not yet meet the American Academy of Neurology's brain death criteria, so he should not have been declared dead and would not have met UK test conditions in any case. This is clearly a case of negligent misdiagnosis. 
Medical science argues 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 electrical activity there should be the only consideration when defining death. In many cases, especially when elevated intracranial pressure prevents blood flow into the brain, the entire brain is nonfunctional; however, some injuries may affect only the neo-cortex. During the death process, brain function can be lost gradually. When going through such a change, a small proportion of subjects have reported a variety of "near-death experiences".
 Organ donation
Brain death may result in legal death, but still with the heart beating, and with mechanical ventilation all other vital organs may be kept completely alive and functional, providing optimal opportunities for organ transplantation.
Most organ donation for organ transplantation is done in the setting of brain death. In some nations (for instance, Belgium, Poland, Portugal and France) everyone is automatically an organ donor, although some jurisdictions (such as Singapore, France, or New Zealand) allow opting out of the system. Elsewhere, consent from family members or next-of-kin is required for organ donation. 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.
 See also
- Murray, Stephen. 'Brain Death: Some of the Questions and Answers,' The Philosopher (Journal of the English Philosophical Society), Spring 1990, 1-12. http://www.the-philosopher.co.uk/contents.htm
- Lock M. Twice Dead: Organ Transplants and the Reinvention of Death. 2002, University of California Press, Berkeley, CA.
- Howsepian AA. In defense of whole-brain definitions of death. Linacre Quarterly. 1998 Nov;65(4):39-61. PMID 12199254
- Karasawa H, et al. Intracranial electroencephalographic changes in deep anesthesia. Clin Neurophysiol. 2001 Jan;112(1):25-30. PMID 11137657
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