What Is a Coma?

A coma is a prolonged state of unconsciousness. During a coma, a person is unresponsive to his or her environment. The person is alive and looks like he or she is sleeping. However, unlike in a deep sleep, the person cannot be awakened by any stimulation, including pain.

What Causes a Coma?

Comas are caused by an injury to the brain. Brain injury can be due to increased pressure, bleeding, loss of oxygen, or buildup of toxins. The injury can be temporary and reversible. It also can be permanent.

More than 50% of comas are related to head trauma or disturbances in the brain's circulatory system. Problems that can lead to coma include:

Trauma: 

Head injuries can cause the brain to swell and/or bleed. When the brain swells as a result of trauma, the fluid pushes up against the skull. The swelling may eventually cause the brain to push down on the brain stem, which can damage the RAS (Reticular Activating System) -- a part of the brain that's responsible for arousal and awareness.

Swelling:

 Swelling of brain tissue can occur even without distress. Sometimes a lack of oxygen, electrolyte imbalance, or hormones can cause swelling.

Bleeding:

Bleeding in the layers of the brain may cause coma due to swelling and compression on the injured side of the brain. This compression causes the brain to shift, causing damage to the brainstem and the RAS (mentioned above). High blood pressure, cerebral aneurysms, and tumors are non-traumatic causes of bleeding in the brain.

Stroke:

When there is no blood flow to a major part of the brain stem or loss of blood accompanied with swelling, coma can occur.

Blood sugar : 

In people with diabetes, coma can occur when blood sugar levels stay very high. That's a condition known as hyperglycemia. Hypoglycemia, or blood sugar that's too low, can also lead to a coma. This type of coma is usually reversible once the blood sugar is corrected.

Oxygen deprivation: 

Oxygen is essential for brain function. Cardiac arrest causes a sudden cutoff of blood flow and oxygen to the brain, called hypoxia or anoxia. After cardiopulmonary resuscitation (CPR), survivors of cardiac arrest are often in comas. Oxygen deprivation can also occur with drowning or choking.

Infection: 

Infections of the central nervous system, such as meningitis or encephalitis, can also cause coma.

Toxins:

Substances that are normally found in the body can accumulate to toxic levels if the body fails to dispose of them correctly. As an example, ammonia due to liver disease, carbon dioxide from a severe asthma attack, or urea from kidney failure can accumulate to toxic levels in the body. Drugs and alcohol in large quantities can also disrupt neuron functioning in the brain.

Seizures: 

A single seizure rarely produces coma. But continuous seizures -- called status epilepticus -- can. Repeated seizures can prevent the brain from recovering in between seizures. This will cause prolonged unconsciousness and coma.

culled: http://www.webmd.com/brain/coma-types-causes-treatments-prognosis