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Brain death, also known as death by neurologic criteria, involves the permanent and complete absence of human brain function.
Brain death involves loss of function of both the cerebrum, the “thinking” part of the brain, and the brainstem, the deep brain structure responsible for reflexes and breathing.
Causes of Brain Death
Brain death results from a devastating brain injury, commonly due to head trauma, bleeding in the brain, stroke, or loss of blood flow to the brain after the heart stops (cardiac arrest). In the US in 2016 there were an estimated 15 405 cases, about 2% of all hospital deaths.
Factors to Be Assessed Before Brain Death Consideration
Patients must have a normal core body temperature because hypothermia can make it difficult to perform a brain death assessment. Sedating or paralyzing drugs, either taken by a patient prior to hospitalization or prescribed by doctors during hospitalization, also interfere with assessment of coma. Sufficient time for the body to clear the effects of these medications is required before brain death evaluation. Certain medical conditions such as very low blood pressure or severe abnormalities in blood glucose or other blood electrolytes need to be corrected before brain death assessment.
Diagnosis of Brain Death
Brain death is typically diagnosed in an intensive care unit by a doctor trained in brain death evaluation. Brain death diagnosis requires presence of 3 conditions: persistent coma, absence of brainstem reflexes, and lack of ability to breathe independently. Coma is confirmed when a painful stimulus causes no eye opening, no verbal response, and no limb movement in a patient. Brainstem function is assessed by testing multiple reflexes, including pupil responsiveness to light and coughing or gagging with throat suctioning. If coma and absence of brainstem reflexes are confirmed, the final step is an apnea test—temporarily removing a patient from mechanical ventilation and observing for spontaneous breaths. If after 10 minutes no breathing is witnessed and the blood carbon dioxide level increases by 20 millimeters of mercury or more, the patient meets criteria for brain death. Patients who cannot have an apnea test because of very low blood pressure or low oxygen levels must have additional radiographic testing, such as radionuclide studies, transcranial Doppler ultrasound, or cerebral angiography, to confirm brain death.
Consequences of and Considerations in Brain Death
Once brain death has been diagnosed, a patient is declared dead. A single brain death examination, including the apnea test, is the minimum standard for diagnosing brain death in adults. However, in children, recent guidelines recommend 2 separate brain death examinations as the minimum standard. Organ support, including mechanical ventilation and medications to maintain adequate blood pressure, may be continued after a declaration of brain death if the patient is a candidate for organ donation or is pregnant and a decision is made to continue support for the fetus.
United Network for Organ Sharingwww.unos.org/wp-content/uploads/unos/PartneringWithTransplantTeam.pdf
Conflict of Interest Disclosures: None reported.
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Greer DM, Shemie SD, Lewis A, et al. Determination of brain death/death by neurologic criteria. JAMA. Published online August 3, 2020. doi:10.1001/jama.2020.11586
Walter K. Brain Death. JAMA. 2020;324(11):1116. doi:10.1001/jama.2020.15898
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