Updated October 12, 2014.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

There is more to a loss of consciousness than simply not being awake. States such as coma, persistent vegetative state and normal sleep all involve a loss of consciousness, and all have different degrees of severity. Even in cases as severe as persistent vegetative state, there are rare cases of people waking up. Recent research suggests that some may have a greater degree of retained consciousness than previously recognized.

Brain death is different. As the term suggests, in brain death there is no hope of recovery. Medically, brain death is death.

Unlike the other forms of lost consciousness, brain death involves a complete loss of brainstem functions as well. This means that the reticular activating system responsible for consciousness is irreparably damaged. It also means that the centers for breathing and the control of heart rate have also been destroyed.

Most families come to an understanding of the concept of brain death, but it is not easy. Instinctively, we associate death with a heart that has stopped beating. But the heart's electrical system is somewhat independent from the rest of the nervous system. In fact, the heart can continue to beat even when removed from the rest of the body. Of course, this does not mean that the rest of the body is alive. Similarly, it is possible for machines to pump blood through the body without the help of the heart, as is sometimes done in cardiac surgery. Simply put, there is more to life than a beating heart.

In an intensive care unit, the rest of the body can be kept alive without the brain for some period of time. Mechanical ventilators can provide oxygen and remove carbon dioxide. IV fluids and tube feedings can replace normal means of nutrition. Ultimately, though, the rest of the body will follow the brain in death.

It is extremely important that brain death not be misdiagnosed. There are no well-documented cases of a correct diagnosis of brain death in which the patient has had any meaningful recovery. While the media will sometimes report such a case, further investigation reveals a misstep in the diagnosis, meaning the patient should not have been declared brain dead at all. So how do doctors diagnose brain death?

Diagnosing Brain Death

There are three main steps to diagnosing brain death. There are variations in the requirements for this diagnosis between states and even between different hospitals, but these three basic conditions must always be met.

1) The brain must be in an intractable loss of consciousness of known and irreversible cause.

The patient must not demonstrate any true intentional movement. It is important to be aware that many reflexes may mimic intentional movement, even to the point of the patient sitting up in bed (the Lazarus sign) or having multiple jerking movements of one or more limbs (myoclonus).

There are many mimics of brain death. Certain drugs and toxins can cause severe coma, even to the point of robbing the brainstem of any function. However, this will be corrected as the body rids itself of the toxins. The same could be said for many infections or blood pressure that is too low or too high. Hypothermia can mimic brain death, leading to the medical saying, "Nobody is dead until they're warm and dead." Because of these mimics, it is critical that nobody diagnosed with brain death can have any of these factors present.

It is also important that the cause of brain death be known and irreversible. For example, stroke is not reversible, but if it is relatively small, it will not lead to a complete loss of brain function. Some swelling associated with the stroke can be controlled and will eventually diminish. Depending on where the stroke is located, the brainstem will remain intact. On the other hand, if the neurons of the brainstem have been destroyed by the initial stroke, there is no way to replace those neurons. In this case, brain death may be inevitable.

2) The patient must have no brainstem reflexes.

Brainstem reflexes are automatic responses similar to the way your knee jerks when it is tapped with a neurologist's hammer. The knee-jerk reflex tests the nerves that come and go from the spinal cord; brainstem reflexes test whether or not the brainstem is intact. These reflexes are as follows:

• The pupillary reflex - the neurologist shines a light into the patient's eyes and sees if the pupils constrict. In a brain dead patient, the pupils will not respond.
• The corneal reflex - the neurologist touches the patient's eyes with either a cotton swab or drop of water to see if the patient blinks. A brain dead patient will not blink.
• The oculocephalic or "dolls eyes" reflex - the patient's head is moved from side to side to see if the eyes remain fixated on the examiner's face. A brain dead patient's eyes will not fixate.
• The gag reflex - the back of the patient's throat is touched with a cotton swab, or a suction device may be used instead. Normal patients will gag or cough. A brain dead patient will not.
• Cold caloric testing - In this test, ice water is squirted into the patient's ear. In normal and awake patients, this causes a sensation of nausea, since it tricks the inner ear into believing the patient is spinning in a circle. As a result, the eyes will automatically move in the opposite direction to try to keep vision steady. A brain dead patient's eyes will not move.

3) The apnea test.

We breathe for two reasons: to rid the body of carbon dioxide and to take in oxygen. The primary drive to breathe is actually a build-up of carbon dioxide, which rises quickly without breathing. If carbon dioxide levels reach above a certain level and there is no attempt made to breathe, those centers of the brainstem are not functioning.

To test this, the ventilator is turned off and the patient is watched to see if any attempts to breathe are made. Blood tests are taken to ensure that the level of carbon dioxide has reached levels that would normally stimulate breathing (more than 20 percent of the patient's baseline carbon dioxide level). In order to avoid the body becoming hypoxemic (too little oxygen), the patient is given 100% oxygen for 15 minutes before the apnea test, and oxygen is also administered into the respiratory tract during the test.

Reasons for Additional Testing

Some people have an increased tolerance for increased levels of carbon dioxide. For example, patients with chronic obstructive pulmonary disease (COPD) may have built up a tolerance because their lungs have not functioned well for a long period of time. In these cases, some physicians may prefer to do additional testing.

As we've touched on, while these conditions for brain death are basically the same throughout all healthcare institutions, other requirements may be enforced based on state and individual hospital regulations. For example, because brain death is such a serious diagnosis, most hospitals require two different physical exams by two different qualified physicians to confirm the diagnosis. Some space of time is usually required between these exams, though this varies. In addition, these guidelines do not necessarily apply to children.

If all the requirements above are met, then the patient has no chance of meaningful recovery and is declared brain dead. If all the above steps are done correctly, then additional tests are not helpful. But there are many times where all the steps above can't be taken. For example, perhaps an apnea test cannot be performed because the patient is too unstable. Some families also request additional tests just for the added assurance that their loved one is brain dead before making an important decision, such as organ donation.

Coming to Terms with Brain Death

Like any other kind of death, brain death leads to a difficult time for families and loved ones. It is important that they meet with everyone involved in the patient's medical care, and that everyone's questions and concerns are addressed. A chaplain or counselor is often involved in these discussions. One of the most important thing to focus on is what the patient would have wanted under these conditions. Everyone present in these talks wishes to do what's best for their loved one, and this can help ensure that their death is managed with the dignity and respect they deserve.

<sub>Sources:

Eelco F.M. Wijdicks, MD, PhD, Panayiotis N. Varelas, MD, PhD, Gary S. Gronseth, MD David M. Greer, MD, Evidence-based guideline update: Determining brain death in adults, Report of the Quality Standards Subcommittee of the American Academy of Neurology Neurology 74, June 8, 2010.

Jerome B. Posner and Fred Plum. Plum and Posner's Diagnosis of Stupor and Coma. New York: Oxford University Press, 2007.</sub>