Brain death is the complete loss of brain function (including involuntary activity necessary to sustain life). It differs from persistent vegetative state, in which the person is alive and some autonomic functions remain. It is also distinct from an ordinary coma, whether induced medically or caused by injury and/or illness, even if it is very deep, as long as some brain and bodily activity and function remain; and it is also not the same as the condition locked-in syndrome. A differential diagnosis can medically distinguish these differing conditions.

Brain death is used as an indicator of legal death in many jurisdictions, but it is defined inconsistently and often confused by the public. Various parts of the brain may keep functioning when others do not anymore, and the term "brain death" has been used to refer to various combinations. For example, although one major medical dictionary considers "brain death" to be synonymous with "cerebral death" (death of the cerebrum), the US National Library of Medicine Medical Subject Headings (MeSH) system defines brain death as including the brainstem. The distinctions are medically significant because, for example, in someone with a dead cerebrum but a living brainstem, spontaneous breathing may continue unaided, whereas in whole-brain death (which includes brainstem death), only life support equipment would maintain ventilation. Patients classified as brain-dead can have their organs surgically removed for organ donation.

Medicolegal history

Differences in operational definitions of death have obvious medicolegal implications (in medical jurisprudence and medical law). Traditionally, both the legal and medical communities determined death through the permanent end of certain bodily functions in clinical death, especially respiration and heartbeat. With the increasing ability of the medical community to resuscitate people with no respiration, heartbeat, or other external signs of life, the need for another definition of death occurred, raising questions of legal death. This gained greater urgency with the widespread use of life support equipment, as well as rising capabilities and demand for organ transplantation.

Since the 1960s, laws on determining death have, therefore, been implemented in all countries with active organ transplantation programs. The first European country to adopt brain death as a legal definition (or indicator) of death was Finland in 1971. In the United States, Kansas had 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 toward what is now known as brain death. In the wake of the 1976 Karen Ann Quinlan case, state legislatures in the United States moved to accept brain death as an acceptable indication of death. In 1981 a Presidential commission issued a landmark 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 has been enacted in 39 states of the United States.The Uniform Determination of Death Act in the United States attempts to standardize criteria. 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 1995, abandoning the 1979 claim that the tests published in 1976 sufficed for the diagnosis of brain death and suggesting a new definition of death based on the irreversible loss of brain stem function alone.This new definition, the irreversible loss of the capacity for consciousness and for spontaneous breathing, and the essentially unchanged 1976 tests held to establish that state, have been adopted as a basis of death certification for organ transplant purposes in subsequent Codes of Practice.The Australia and New Zealand Intensive Care Society (ANZICS) states that the "determination of brain death requires that there is unresponsive coma, the absence of brain-stem reflexes and the absence of respiratory centre function, in the clinical setting in which these findings are irreversible. In particular, there must be definite clinical or neuro-imaging evidence of acute brain pathology (e.g. traumatic brain injury, intracranial haemorrhage, hypoxic encephalopathy) consistent with the irreversible loss of neurological function."

In 2020, an international panel of experts, the World Brain Death Project, published a guideline that "provides recommendations for the minimum clinical standards for determination of brain death/death by neurologic criteria (BD/DNC) in adults and children with clear guidance for various clinical circumstances. The recommendations have widespread international society endorsement and can serve to guide professional societies and countries in the revision or development of protocols and procedures for determination of brain death/death by neurologic criteria, leading to greater consistency within and between countries.

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