Brain death is one of the most serious diagnoses a neurologist can make. Unlike severe forms of coma, a diagnosis of brain death means there is no coming back. Medically, brain death is death.
If the diagnosis is made properly, it can be done just by ensuring the patient is in a coma of known and irreversible cause, and that certain physical exam findings are absent, including brainstem reflexes and any effort to breathe during an apnea test. The apnea test involves giving the patient oxygen but turning off the ventilator to allow carbon dioxide to build up in the system, which normally triggers an attempt to breathe. There are no well-documented cases of a diagnosis of brain death being carefully made in which the patient then had a meaningful recovery.
However, there are times when meeting all the technical qualifications for brain death is impossible. For example, in severe facial trauma, it may be impossible to perform a reliable examination of the cranial nerves. In some patients, it may be impossible to do an apnea test, either because the patient is too unstable or because they have built up a tolerance for carbon dioxide, as is seen in some patients with chronic obstructive pulmonary disease or severe sleep apnea. In these cases, additional testing is called for.
Furthermore, because the diagnosis of brain death is so serious, many families prefer to have additional testing done before making decisions about stopping mechanical ventilation or considering organ donation.
An EEG is used to measure electrical activity in the brain. It is most commonly used when a doctor is concerned that someone has seizures or epilepsy. In brain death, rather than looking for abnormal activity, the EEG is looking for any kind of activity at all. Some small degree of electrical activity may appear to be present, but this actually represents artifact due to signal from nearby devices or the heartbeat, and must not exceed a certain threshold in order to meet criteria for a diagnosis of brain death.
Somatosensory Evoked Potentials (SSEP)
Like an EEG, SSEPs evaluate how electricity flows through the body, including the brain. Rather than just looking at spontaneous brain activity, SSEPs involve the nervous system being stimulated by mild electrical shocks, usually to the median nerve. Normally, these shocks register as a signal received in the brain, which can be measured by an electrode placed on the patient’s head. The absence of these signals indicates that the brain is no longer able to receive these messages.
In a cerebral angiogram, a contrast dye is injected into the vessels of the body, and the brain is observed on a monitor while the patient undergoes a series of X-rays. This allows for close examination of how blood is moving through the body. In brain death, the vessels of the brain do not fill as they normally would.
A transcranial doppler exam uses ultrasound waves to evaluate blood flow in the brain. During brain death, the brain can swell in ways that increase resistance in the blood vessels, minimizing the flow of blood. These changes in blood flow can be seen in the transcranial doppler.
Nuclear Medicine Tests
Nuclear medicine involves the injection of a radioisotope into the brain. This isotope is a chemical that moves along with blood flow. The isotope decays, resulting in a release of energy that is detected by sensors and converted into a digital image. If the brain is healthy and active, it will look like it is lighting up on the monitor as blood flows into the brain tissue. In a brain death exam, the most common isotope is called technetium-99m hexamethylpropyleneamine oxime. If the patient is brain dead, then there will be no signal from the brain in the scan. This is sometimes known as the "hollow skull phenomenon."
Bringing Everything Together
These techniques are widely accepted as additional, although usually unnecessary, tests for a brain death exam. Some technical standards may vary from state to state and even hospital to hospital, however. Like any kind of test, each of the above tests needs to be interpreted carefully and in the context of the patient’s known medical history. No test is perfect, and so it is crucial that close attention is paid to details of how the test is run, so that the chance of misinterpretation of results is minimized.
Brain death of a loved one is a traumatic experience for families, but additional testing can help ensure that surrogate decision makers move forward with confidence that they are respecting what the patient would want.
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.