Brain death can be misleading: A person who is brain-dead, meaning they have a complete absence of brain activity, may appear simply comatose, breathing with the help of a ventilator. But is brain death reversible?
Sadly, no: Brain death is permanent. And based on a U.S. law on the books since 1981, it is considered a true death, just as final as death when the heart stops.
Until the 1950s, the distinction of brain death didn't really exist, because when the heart stopped, the oxygen-starved brain died within a few minutes. Conversely, any direct injury to the human brain severe enough to damage the regions controlling breathing inevitably led the heart and respiration to stop. But the advent of cardiopulmonary resuscitation (CPR), artificial ventilation, and extracorporeal membrane oxygenation (ECMO), which uses a heart-lung machine to oxygenate the blood outside the body, broke that inevitable link. Now, even if the brain is nonfunctional, it's possible to keep the body alive and the other organ systems functioning, at least for a while.
The rules around diagnosing brain death can differ from place to place, and there are sometimes controversies when a patient's family disagrees with a doctor's assessment that their relative's condition is irreversible or when medical error yields a misdiagnosis. There are also debates about whether brain death should include more tests or criteria. Finally, some faith traditions also object to brain death on religious grounds. But at its core, brain death is defined by doctors as the irreversible loss of all brain function, including in the brain stem.
How does the brain die?
Brain death, also known as death by neurological criteria, is a relatively rare way to die, said Dr. Panayiotis Varelas, chair of neurology at Albany Medical College. But it can happen in cases of severe brain injury, either due to brain trauma or an extended lack of oxygen.
In the face of trauma or a lack of oxygen, the brain cells begin to die, Varelas told Live Science. This mass die-off causes inflammation and swelling, locking the brain into a vicious feedback loop: Damage leads to swelling, which leads to more damage as delicate nerve cells are pressed against the hard case of the skull. The brain stem gets squeezed through the foramen magnum, the hole at the base of the skull where the spinal cord connects to the brain.This damages the regions that control breathing and the heartbeat. Meanwhile, Varelas said, the increasing pressure inside the skull makes it increasingly harder for blood to reach the brain. More and more brain cells die. Once these cells are lost, so are their complex connections to one another, which can lead to irreversible damage.
Once a patient is far along enough on this spiral, brain death is inevitable. In most cases, death of the rest of the body will follow, even if a patient's other organs are still getting oxygen through a ventilator. The other organs stop responding to medical treatment, Varelas said.
"The whole body shuts down, a sign of how powerful the control of the brain is to the rest of the body," he said. "When the central integrator, the brain, dies, the entire body disintegrates."
How is brain death diagnosed?
The designation of brain death, however, means that the person can be declared dead before the heart stops beating. This can allow for the removal of artificial life support and futile treatment, or enable organ donation in some cases. An experienced neurologist or neurosurgeon must conduct an examination. To get to that point, a few key criteria must be met: There must be a known cause for the patient's condition, consistent with a catastrophic injury to the whole brain, and the patient's unresponsive state must not be due to side effects of medication, sedation or some other potentially reversible cause, according to a 2022 article in the Journal of Intensive Care.
If these prerequisites are met, the physician can conduct a clinical exam testing for reflexes that depend on a functioning brain stem. There are multiple reflexes tested, such as whether the pupils respond to light and whether the eyelid closes when something touches the surface of the patient's eye. Another test involves flushing the inner ear with cold water. In a patient with a functioning brain stem, the eye will move toward that ear as part of a reflex involving the body's sense of balance. There are also tests involving the gag reflex and other automatic muscle movements.
If the patient shows no signs of brain stem reflexes, doctors conduct a final test called the apnea test: They remove the patient from mechanical ventilation to see if the person starts breathing spontaneously. A brain-dead patient cannot breath on their own.
"It's a very, very difficult moment for the families and for us physicians," Varelas said. Brain deaths often occur after sudden medical events or accidents, so the loss of the loved one is often unexpected and extremely difficult to process, he said.
Doctors can try to reduce swelling in an injured brain and prevent brain death, Varelas said, and they treat patients as aggressively as they can until brain death is declared. But once full brain function is lost, there is no hope for repair.
"That's why they call it the coma dépassé, or 'beyond the coma,'" Varelas said, referring to the name given to the condition by two French neurologists in 1959. "Here we are talking about the loss of function of the entire brain."
Originally published on Live Science
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Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.