How Comas Work

Doctors often use MRI scans to check the brain tissue damage of a comatose patient.
Doctors often use MRI scans to check the brain tissue damage of a comatose patient.
© Carlos Torres

In December 1999, a nurse was straightening the bed sheets of a patient when she suddenly sat up and exclaimed, "Don't do that!" Although this may not sound unusual, it was pretty surprising to her friends and family -- Patricia White Bull had been in a deep coma for 16 years. Doctors told them that she would never emerge from it.

How can a comatose person emerge after so long? What causes people to fall into comas in the first place? What's the difference between being in a coma and being in a vegetative state? There are a lot of misconceptions and confusion about the state of unconsciousness known as a coma. In this article, you'll learn the physiological processes that trigger a coma, how a real-life coma differs from television depictions and how often people awaken after months or even years of being in a coma.


The word coma comes from the Greek word koma, which means "state of sleep." But being in a coma is not the same as being asleep. You can awaken someone who is asleep by talking to them or touching them. The same is not true for a comatose person -- he is alive and breathing, but so unconscious that he can't respond to any stimuli (such as pain or the sound of a voice) or perform any voluntary actions. The brain is still functioning, but at its most basic level. To understand this, we first need to review the parts of the brain and how they work.

Parts of the Brain

The brain is made up of three major parts: the cerebrum, the cerebellum and the brain stem. The cerebrum is the largest part of the brain. It makes up most of the total brain. The cerebrum controls cognitive and sensory functions such as intelligence, memory, reasoning and emotions. The cerebellumin the back of the brain controls balance and movement. The brain stem connects the two hemispheres of the cerebrum to the spinal cord. It controls breathing, blood pressure, sleep cycles, consciousness and other body functions. In addition, there is a large mass of neurons beneath the cerebrum, called the thalamus. This small, but critical area relays sensory impulses to the cerebral cortex. For a more detailed explanation of the brain's function, see How Your Brain Works.

Scientists believe that consciousness depends on the constant transmission of chemical signals from the brainstem and thalamus to the cerebrum. These areas are connected by neural pathways called the Reticular Activating System (RAS). Any interruptions to these messages can put someone into an altered state of consciousness.


A vegetative state is a type of coma that represents an awake but unresponsive state of consciousness. Many of these patients were comatose previously and after a few days or weeks emerge to an unconscious state in which their eyelids are open, giving the impression that they are awake. Patients in this state of consciousness may exhibit behaviors that lead family members to incorrectly believe they are becoming awake and communicative. These behaviors can include grunting, yawning and moving the head and limbs. However, these patients do not actually respond to any internal or external stimuli and evidence of extensive brain damage still persists. The outcome of patients in whom a vegetative state lasts for a month or more is generally poor and doctors use the term persistent vegetative state.

Next, we'll find out how someone can become comatose.

How Does Someone Become Comatose?

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.

How Do You Know if Someone is in a Coma?

A coma can look different depending on the situation. Some people will lie completely still and be unresponsive. Others will twitch or move involuntarily. If the breathing muscles are affected, the person may be unable to breathe on his or her own.

Doctors evaluate potentially comatose patients based on one of two scales: The Glasgow Coma Scale and the Rancho Los Amigos Scale. identifies the degree of mental impairment by assigning a score ranging from three to 15, with three being the deepest coma and 15 being normally awake and alert. The points are based on three main parameters:


The Rancho Los Amigos Scale, developed by doctors at Rancho Los Amigos Hospital in California, helps doctors follow the progression of a head injury survivor recovering from a coma. It is most useful during the first weeks or months after the injury.

Based on the results of these two scales, doctors put patients in one of four awareness states:

  • Comatose and unresponsive - The patient cannot move or respond to stimuli.
  • Comatose but responsive - The patient does respond to stimuli, with reactions such as movement or increased heart rate.
  • Conscious but unresponsive - The patient can see, hear, touch, and taste, but cannot respond.
  • Conscious and responsive - The patient is out of his or her coma and can respond to commands.

In the next section, we'll look at how doctors care for comatose patients.

How Do Doctors "Treat" Coma Patients?

There is no one treatment that can cause someone to come out of a coma. Treatments can prevent further physical and neurological damage, however.

First, doctors ensure that the patient isn't in immediate danger of dying. This may require placing a tube in the patient's windpipe through the mouth, and hooking up the patient to a breathing machine, or ventilator. If there are other serious or life-threatening injuries to the rest of the body they will be dealt with in order of decreasing severity. If excess pressure in the brain caused the coma, doctors can relieve it by surgically placing a tube inside the skull and draining the fluid. A procedure called hyperventilation, which increases the rate of breathing to constrict blood vessels in the brain, can also relieve pressure. The doctor may also give the patient medication to prevent seizures. If a drug overdose or condition such as very low blood sugar is responsible for the coma, doctors attempt to correct this as soon as possible. Patients with acute ischemic strokes may undergo procedures or receive special clot-busting medication in an effort to restore blood flow to the brain.


Doctors may use imaging studies, such as magnetic resonance imaging (MRI), or computed tomography (CT) scans, to look inside the brain and identify a tumor, pressure, and any signs of damage to the brain tissue. Electroencephalography (EEG) is a test used to detect any abnormalities in the brain's electrical activity. This can also show brain tumors, infections, and other conditions that might have caused the coma. If the doctor suspects an infection such as meningitis, he may perform a spinal tap to make the diagnosis. To perform this test, a doctor inserts a needle into the patient's spine and removes a sample of cerebrospinal fluid for testing.

Once the patient is stable, doctors will concentrate on keeping him or her as healthy as possible. Coma patients are susceptible to pneumonia and other infections. Many comatose patients stay in the hospital's intensive care unit (ICU), where doctors and nurses can continually monitor them. People who are in a coma for a long time may receive physical therapy to prevent long-term muscle damage. Nurses will also move them periodically to prevent bedsores -- painful skin wounds caused by lying in one position for too long.

Because patients who are in a coma can't eat or drink on their own, they receive nutrients and liquids through a vein or feeding tube so that they don't starve or dehydrate. Coma patients may also receive electrolytes -- salt and other substances that help regulate body processes.

If a coma patient continues to be dependent on a ventilator to breathe, they may receive a special tube that goes directly into their windpipe through the front of the throat (a tracheotomy). The tracheotomy tube can be left in place for extended periods of time because it requires less maintenance and does not injure the soft tissues of the oral cavity and upper throat. Because patients who are in a coma can't urinate on their own, they will have a rubber tube called a catheter inserted directly into their bladder to remove the urine.

How Do People "Come Out" of Comas?

How fast a person comes out of a coma depends on what caused it and the severity of the damage to the brain. If the cause was a metabolic problem such as diabetes, and doctors treat it with medication, he can come out of the coma relatively quickly. Many patients who overdose on drugs or alcohol also can recover once the substance clears their system. A massive brain injury or brain tumor can be more difficult to treat, and can lead to a much longer or irreversible coma.

Most comas don't last more than two to 4 weeks. Recovery is usually gradual, with patients becoming more and more aware over time. They may be awake and alert for only a few minutes the first day, but gradually stay awake for longer and longer periods. Research shows that a comatose patient's outcome relates very closely to his or her Glasgow Coma Scale score. The majority of people (87 percent) who score a three or a four on the scale within the first 24 hours of going into a coma are likely to either die or remain in a vegetative state. On the other end of the scale, about 87 percent of those who score between 11 and 15 are likely to make a good recovery [ref].


Some people come out of a coma without any mental or physical disability, but most require at least some type of therapy to regain mental and physical skills. They may need to relearn how to speak, walk, and even eat. Others are never able to recover completely. They may regain some functions (such as breathing and digestion) and transition into a vegetative state, but will never respond to stimuli.

Lots More Information

Related Articles

More Great Links


  • Catastrophe of Coma: A Way Back, by Edward Allen Freeman
  • Recalled to Life: The Story of a Coma, by Esther Goshen-Gottstein
  • Coma, A Healing Journey: A Guide for Family Friends and Helpers, by Amy Mindell
  • Write to Death : News Framing of the Right to Die Conflict, from Quinlan's Coma to Kevorkian's Conviction, by Elizabeth Atwood Gailey

  • "Awake But Not Aware: A Persistent Vegetative State.", March 22, 2005.
  • Casarett, D., et al. "Epidemiology and prognosis of coma in daytime television dramas." BMJ (British Medical Journal), November 8, 2005.
  • Centers for Disease Control and Prevention:Glasgow Coma Scale
  • "Coma." Neurology Channel.
  • "Coma." PBS Transcript, October 7, 1997.
  • "Coma: Some Facts.", May 14, 2004.
  • "Diabetic Coma." Mayo Clinic, May 25, 2004.
  • "Doctors Induce Comas to Fight Brain Disorders.", May 5, 2005.
  • "Fireman Who Spoke After Being in Coma Dies." The America's Intelligence Wire, Feb 22, 2006.
  • Ischemic Stroke. Merck, February 1, 2003.
  • "The Miracles Around Us." Canada Free Press, January 10, 2000.
  • NINDS Coma and Persistent Vegetative State Information Page
  • "Severe Brain Injury and Coma." Brain Injury Association of America.
  • Teasdale G., Jennett B.. "Glasgow Coma Score."
  • "What is Coma?" Northern Alberta Brain Injury Society.
  • "What is a Coma?" KidsHealth, 2006.
  • "What's a Medically Induced Coma?", January 10, 2006.