James P. Kelly, MD, FAAN, responds:

After a severe injury or illness that affects the brain, patients can fall into a persistently unconscious state, rendering them unaware of themselves or their environment. Depending on their level of consciousness, patients are considered to be comatose, in a minimally conscious state, in a persistent vegetative state, or brain dead. In the first three situations, these states can dictate treatment as well as how to prepare family members for decisions regarding the patient's prognosis and quality of life.

Coma

In this state, either the thalamus, the brainstem, or both hemispheres of the brain are damaged. Patients cannot be awakened, are unresponsive to stimuli such as pain, sound, or touch, and do not experience sleep-wake cycles. If the lower part of the brainstem has also been damaged, the patient often requires a ventilator to breathe.

Coma is caused by a severe brain injury such as a traumatic head injury, brainstem stroke, or a brain hemorrhage that affects the surrounding tissue and brain structure, as well as severe hypothermia, drug overdose, drowning, and cardiac arrest. Regaining consciousness depends on the cause and severity of the injury and how long the coma lasts. Most patients eventually awaken to some degree. Patient care is individualized, and includes physical therapy and rigorous nursing care.

Minimally Conscious State

In this state, patients are intermittently aware of their surroundings and may respond to commands. Although they are mostly drowsy and unresponsive, they may have moments where they mouth words, react to pain, hold an object, respond to commands, or track movement with their eyes. Patients may improve gradually, especially those who have some ability to speak. It is still difficult to predict, however, how much they can improve beyond a major disability.

Persistent Vegetative State

In this state, patients exhibit no signs of perception and communication or awareness of self. Because brainstem functions that affect breathing and involuntary movements remain intact, patients may be able to breathe on their own and open their eyes, experience sleep-wake cycles, grind their teeth, thrash, or make facial movements. If a patient is comatose for several weeks, which rarely happens, he or she is considered to be in a persistent vegetative state, which can become permanent. Patients may remain in this state for decades if all medical complications are treated aggressively.

Brain Death

This condition, which can be caused by severe injury or lack of oxygen to the brain, involves these critical factors: coma; a permanent cessation of activity in the brain and brainstem, the cause of which is known and for which there are no other explanations; and no ability to generate a breath. The hearts of brain-dead patients may continue to beat, but removing supportive equipment will lead to cardiac arrest.

Dr. James Kelly, a Fellow of the American Academy of Neurology, is a professor of neurology and executive director of the Marcus Institute for Brain Health at the University of Colorado School of Medicine in Denver. He is also a senior fellow at the George W. Bush Institute's Military Service Initiative.