Coma (cont.)

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What are the causes of a coma?

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Coma occurs when the brain does not have enough nutrients. For example, if inadequate amounts of oxygen or blood sugar (glucose) are delivered to it, the brain can "turn off." Trauma, bleeding, or swelling of the brain can affect blood delivery, various poisons can also directly injure the brain, and brain inflammation and infection can also alter mental status and lead to 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 "sides" to the brain, the right and left cerebral hemispheres. They each contain frontal, parietal, temporal, and occipital lobes where movement, sensation, speech, and thought are processed. The cerebellum sits beneath the cerebral hemispheres and controls balance and coordination. 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 for 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 and the 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 may be due to lack of blood flow, for example when a rhythm disturbance of the heart such as ventricular fibrillation causes the to heart stop beating. 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 of both cerebral hemispheres. Trauma is another potential cause of coma.

Trauma

Minor head injuries can cause brief loss of consciousness, but the brain is able to turn itself back on. Similarly, patients with seizures become unconscious - but gradually waken relatively quickly as the brain recovers from the seizure's "electrical storm." Those people with significant trauma or prolonged seizures may not wake quickly, and may remain in coma.

The skull is a rigid box that protects the brain. Unfortunately, if the brain is injured and begins to swell (edema), there is no space to accommodate additional fluid. As a result, increased pressure within the brain compresses the brain tissue against the skull bones and may cause a decreased level of consciousness. If the intracranial pressure continues to increase without being treated, the swelling will push the brain downward toward the opening at the base of the skull where the brain stem is locate. This crisis causes the brain to herniate, and turns off the reticular activating system. This can affect the ability of the brain to stimulate breathing and control blood pressure, leading to death.

The effect of trauma on the brain is not predictable. Relatively minor force to the skull and brain can be catastrophic. There does not have to be obvious injury visualized with computerized tomography (CT scan) imaging of the brain to cause significant damage. In fact, when shear injury occurs, the scan may be normal, even if the patient is in coma. Shear injury is microscopic injury to individual nerve connections within the brain without obvious swelling or bleeding.

Head trauma can cause different types of brain injury. The injury can occur to the brain tissue itself or it may cause bleeding to occur in the space between the brain and the skull, or in the ventricles deep within the brain tissue. Computerized tomography (CT) of the head may be able to identify most bleeding from trauma.

Medically Reviewed by a Doctor on 11/15/2013

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