Coma (cont.)

Medical Author:

Benjamin Wedro, MD, FACEP, FAAEM

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.

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Medical Editor:

William C. Shiel Jr., MD, FACP, FACR

William C. Shiel Jr., MD, FACP, FACR

Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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Generally, coma is commonly a result of trauma, bleeding and/or swelling affecting the brain. Inadequate oxygen or blood sugar (glucose) and various poisons can also directly injure the brain to cause coma. Brain inflammation and infection are also causes of altered mental status and coma.

To understand unconsciousness, it is important to understand why a person is awake. The brain is a large organ with many parts. There are two main portions when separated down the middle (right and left cerebral hemispheres) containing the frontal, parietal, temporal and occipital lobes where movement, sensation, speech and thought are processed. The cerebellum sits under the cerebral hemispheres and is where balance and coordination are processed. The brain stem processes automatic, unconscious controls of the body including heart rate, blood pressure, and breathing. The reticular activating system (RAS) is located within the brain stem, and is the important "on/off" switch consciousness and sleep.

• To be awake, the reticular activating system (RAS) must be functioning, as well as at least one cerebral hemisphere.
• If a person loses consciousness, either the RAS has stopped working, or both cerebral hemispheres have shut down.

The reticular activating system stops working in two situations:

1. Brain stem stroke: cells in that area of the brain stem have lost their blood supply, oxygen, and glucose that it delivers. This shuts off the reticular activating system. This event is either ischemic (blood supply is lost), or hemorrhagic (bleeding occurs and damages the reticular activating system).
2. A pre-death event: increased swelling in the brain pushes down on the brain stem and causes it to fail.

For both cerebral hemispheres to fail at once, there must be loss of function of the whole brain. This is often due to lack of blood flow, for example when there is a rhythm disturbance of the heart such as ventricular fibrillation or if the heart stops beating. Also, severely decreased levels of oxygen in the blood due to lung failure can cause brain damage. Poisoning or inflammation of the brain can cause coma with loss of function from both cerebral hemispheres.

Source: MedicineNet.com
http://www.medicinenet.com/coma/article.htm