How Comas Work

Illnesses that affect the brain and brain injuries can both cause comas. If a person suffers severe head trauma, the impact can cause the brain to move back and forth inside the skull. The movement of the brain inside the skull can tear blood vessels and nerve fibers, which causes swelling in the brain. This swelling presses down on blood vessels, blocking the flow of blood (and with it, oxygen) to the brain. The oxygen- and blood-starved parts of the brain begin to die. Some infections of the brain and spinal cord (such as encephalitis or meningitis) can also cause swelling in the brain. Conditions that cause an excess of blood inside the brain or skull, such a skull fracture or a burst aneurysm, can also lead to swelling and further brain injury.

A type of stroke, called an ischemic stroke, can also lead to a coma. This stroke occurs when an artery that supplies the brain with blood is blocked. The blockage starves the brain of blood and oxygen. If it is very large, the person can fall into a stupor or coma.

In people with diabetes, the body does not produce enough of the hormone insulin. Because insulin helps cells use glucose for energy, a lack of the hormone causes blood glucose levels to rise (hyperglycemia). Conversely, when insulin isn't in the right proportion, blood sugar can drop too low (hypoglycemia). If the blood sugar is either extremely high or low, it can cause a person to fall into a diabetic coma.

Comas can also be caused by brain tumors, alcohol or drug overdoses, seizure disorders, lack of oxygen to the brain (such as from drowning) or extremely high blood pressure.

A person can become comatose immediately or gradually. If an infection or other illness causes the coma, for example, the person might run a high fever, feel dizzy or seem lethargic before falling into a coma. If the cause is a stroke or severe head trauma, they can become comatose almost immediately.

We'll see how doctors determine if someone is comatose next.