Coma

Medical Author: Benjamin C. Wedro, MD, FAAEM
Medical Editor: William C. Shiel Jr., MD, FACP, FACR

• What is a "coma"?
  • Glasgow Coma Scale
• What are the causes of a coma?
  • Trauma
  • Bleeding (Hemorrhage)
  • Epidural, subdural, and subarachnoid hemorrhages
  • Swelling
  • Lack of oxygen
  • Hypoglycemic coma
  • Poisons
• How is coma assessed?
• What tests are there for coma?
• What is the outcome and prognosis for a patient in a coma?

What is a "coma"?

Coma is a state of unconsciousness whereby a patient cannot react with the surrounding environment. The patient cannot be wakened with outside physical or auditory stimulation. The inability to waken differentiates coma from sleep. Patients can have different levels of unconsciousness and unresponsiveness depending upon how much or how little of the brain is functioning.

The Glasgow Coma Scale

The Glasgow Coma Scale was developed to provide health-caregivers a simple way of measuring the depth of coma based upon observations of eye opening, speech, and movement. Patients in the deepest level of coma:

• do not respond with any body movement to pain,
  
  
• do not have any speech, and
  
  
• do not open their eyes.

Those in lighter comas may offer some response, to the point they may even seem wake, yet meet the criteria of coma because they do not respond to their environment.

The scale is used as part of the initial evaluation of a patient, but does not assist in making the diagnosis as to the cause of coma. Since it "scores" the level of coma, the GCS can be used as a standard method for any health-caregiver to assess change in patient status.

There are many causes of coma, but to understand unconsciousness, we need to know 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 housed. The cerebellum sits under the cerebral hemispheres and is where balance and coordination are located. The brain stem is where automatic responses to the body including heart rate, blood pressure, and breathing are controlled. The reticular activating system (RAS) is located within the brain stem and is the important "on/off" switch of the brain.

• 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 oxygen and glucose supplied by blood flow, then function stops. This is either ischemic (where blood supply is lost) or hemorrhagic (where bleeding occurs and the structures fail).
   
   
2. A pre-death event: increased swelling in the brain pushes down on the brain stem and causes it to fail. To have both cerebral hemispheres fail requires the blood supply to the brain be compromised, or some sort of toxic insult has occurred to all brain tissue.

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