Coma (Medical)

  • Medical Author:
    Benjamin Wedro, MD, FACEP, FAAEM

    Dr. Ben Wedro practices emergency medicine at Gundersen Clinic, a regional trauma center in La Crosse, Wisconsin. His background includes undergraduate and medical studies at the University of Alberta, a Family Practice internship at Queen's University in Kingston, Ontario and residency training in Emergency Medicine at the University of Oklahoma Health Sciences Center.

  • Medical Editor: Melissa Conrad Stöppler, MD
    Melissa Conrad Stöppler, MD

    Melissa Conrad Stöppler, MD

    Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

Dementia Quiz


While trauma can make the brain swell, other types of injury or insult can cause brain swelling (cerebral edema). Whether the insult is lack of oxygen or abnormal electrolytes, it may ultimately result in edema of the brain tissue. As with bleeding, the skull limits the space available for brain swelling to occur. As a result, the brain tissue is damaged and its function decreases the more it is compressed against the bones of the skull.


Infection and inflammation, such as encephalitis and meningitis of the brain and surrounding tissues may be associated with coma. Encephalitis is an infection of the brain tissue itself, while meningitis is an infection of the linings surrounding the brain. Infections may also be associated with abscesses or collections of pus within the brain. Infections may alter brain function and cause coma even without any swelling that might be detected on CT scan.

Lack of Oxygen

The brain requires oxygen to function; and without it the brain shuts down. There is a very short time to return oxygen supply back to brain tissue before there is permanent damage. Most research suggests that the time window is only four to six minutes.

The body provides oxygen to the brain through the lungs. The lungs extract oxygen from the air, hemoglobin in red blood cells pick up the oxygen, and the heart pumps blood through normal blood vessels to cells in the brain and the rest of the body. If any part of the system fails, the oxygen supply to the brain can be interrupted.

The most common failure occurs with heart rhythm disturbances. The coordinated electrical beat of the heart is lost and the heart muscle doesn't squeeze blood adequately; no blood is pumped to the brain and it stops functioning almost immediately.

Lungs can also fail causing hypoxemia; examples include pneumonia, emphysema, or asthma. In each case, inflammation in the lung tubes (bronchi or bronchioles) or lung tissue makes it difficult for oxygen to get into the lungs and transferred into the blood stream. Hemoglobin also takes carbon dioxide, the waste product of metabolism, and returns it to the lungs to be exhaled. Elevated levels of carbon dioxide in the blood aslo can affect brain function to cause coma.

Anemia, or low red blood cell count, can cause the brain to fail directly, or more likely it causes other organs like the heart to fail. The heart, like any other muscle requires oxygen to function. Anemia can occur chronically or it can be due to an acute blood loss (examples include trauma or bleeding from the stomach). If the blood loss is slow, the body is better able to adapt and tolerate low hemoglobin levels; if the bleeding occurs quickly, the body may be unable to compensate, the result being inadequate oxygen supply to tissues including the brain.

Medically Reviewed by a Doctor on 3/11/2016

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