Coma
Overview
A coma is a deep state of unconsciousness, during which an individual is not able to react to his or her environment. Someone in a coma cannot consciously respond to stimulation. Coma can be caused by an underlying illness, or it can result from head trauma. A comatose person is still very much alive, but he or she is not simply asleep. The brain wave activity in a comatose person is very different from that of a sleeping person; you can wake up a sleeping person, you can't wake a person in a coma.
A coma usually does not last for more than a few weeks. Many people recover their full physical and mental functioning when they emerge from a coma. Others require various forms of therapy to recover as much functioning as possible. Some patients never recover anything but very basic body functions.
Sometimes, following a coma, a person may enter what is known as a persistent vegetative state; patients in persistent vegetative state have lost all cognitive neurological function but are still able to breathe and may exhibit various spontaneous movements. They may even be awake and appear to be normal but, because the cognitive part of their brain no longer functions, they are not able to respond to their environment. A vegetative state can last for years.
There are other terms, in addition to coma and vegetative state, that are used to describe varying levels of unconsciousness and a person's ability to respond to stimuli. These include stupor, in which a person is unconscious but will eventually respond to repeated, vigorous stimulation; and obtundation and lethargy, which are used to describe a person who is not entirely unconscious but does not respond to stimuli.
Usually, coma and other altered states of unconsciousness are considered neurological emergencies, and actions need to be taken quickly to avoid permanent damage.
The Human Brain
The human brain is made up of three basic parts: the cerebrum, the cerebellum, and the brainstem. The cerebrum is the largest part of the brain and makes up about 85% of the brain's total weight. It's divided into two hemispheres, the cerebral hemispheres, one on each side of the head. The cerebrum is where all of the body's most complicated mental and sensory functions are controlled-intelligence, reasoning, memory, emotions, vision, the ability to feel, etc.
The cerebellum is a smaller part of the brain that lies behind the cerebrum. It plays an essential role in coordination, posture and balance. The brainstem is the stemlike part of the brain that connects the cerebral hemispheres to the spinal cord and is responsible for controlling many basic bodily functions, such as breathing, blood pressure, and being awake and alert.
Unconsciousness
As with most mental processes that occur in the brain, the biology of consciousness-also called arousal-is very complicated and not well understood. There are many tissues located deep within the brain that play a role in how conscious and alert a person is.
Researchers believe that one of the important physiological processes that keeps a person conscious is the transfer, or neurotransmission, of chemical signals from the brainstem to the cerebral hemispheres of the brain. This continuous neurotransmission needs to be happening in order for a person to be aware of their environment. Abnormalities that interrupt it can lead to coma or other states of unconsciousness.
Abnormalities that can cause coma include injury or damage to the brain that leads to swelling (edema) in the brain, which results in an increased intracranial pressure (pressure within the skull). Increased pressure, whether it is localized in one particular spot or spread over the whole brain, decreases the flow of blood and can lead to unconsciousness.
Injury or damage to the brain can also cause some areas of the brain to shift within the skull and exert pressure on surrounding tissues and structures, including blood vessels. When a part of the brain shifts position like this, the event is called a brain herniation, and it can lead to coma and death if not treated immediately.
Signs and Symptoms
All of the various states of unconsciousness are symptoms of severe neurological dysfunction. Coma can begin suddenly, such as when a person suffers a brain hemorrhage or severe head trauma, or can develop slowly over time. In a person who is intoxicated with alcohol or other drugs, for example, the early signs of an impending state of unconsciousness develop slowly. Usually the progression of the state of unconsciousness and how quickly it develops, provides clues about what is causing it.
In a slowly developing coma, patients may be only mildly confused and drowsy in the beginning, and/or their personality may change. If the underlying cause is an infection in the central nervous system, patients may feel a headache or other bodily discomfort, a fever, rash, muscular pains, or dizziness, before the actual coma develops. As time passes, they may begin to show signs of lethargy and obtundation, during which they are not quite unconscious but are not responding to external stimuli.
Sometimes a person who is falling into unconsciousness may not recognize what is happening and may not be able to complain. Or, it may happen so quickly that they don't have time to notice and complain.
After a person has entered into a state of unconsciousness, he or she may not respond to external stimuli, depending on how deep the coma is, and/or may show abnormal body movements. An unconscious person may lie still and not respond to anything, or may move spontaneously. These movements can include shaking, tremors, and jerking movements. The eyes may move abnormally. If the breathing muscles are affected, the patient's breathing may be irregular, and a respirator may be required.
Causes
There are many things that can cause coma or other states of unconsciousness. Some of the causes are treatable and reversible, and others are not. Some of them are focal processes-localized abnormalities that exist only in one part of the brain; others are diffuse processes that affect large parts of the brain.
Focal Processes
Focal processes that are localized to a specific spot in the brain and can lead to coma include: brain hemorrhage, an abnormal flow of blood that occurs in a specific place in the brain; ischemic stroke, a stroke due to the obstruction of a blood vessel in the brain; a brain tumor; and brain abscesses, infections in the brain.
Diffuse Processes
Diffuse processes that are widespread and affect large parts of the brain include the following:
- Head trauma associated with an increased intracranial pressure
- Various toxins, including poisons, alcohol and other drugs (e.g., barbiturates, opiate narcotics, sedatives, amphetamines, cocaine, aspirin)
- Metabolic abnormalities that lead to either elevated or reduced glucose levels in the blood
- Liver or kidney failure
- Hypoxia (poor oxygenation)
- Imbalance of electrolytes (substances like salts that are found in the blood and tissues and play essential roles in normal body function)
- Central nervous system infections (e.g., meningitis, encephalitis)
- Hemorrhage in one of the membranous layers covering the brain
- Seizure disorders
- Extreme elevation in blood pressure
Psychiatric Cause
Sometimes, it is difficult to know if a person's unresponsiveness is due to psychiatric problems rather than a medical illnesses.
Diagnosis
Medical History
As with most neurological problems, the first step in diagnosing what is causing a coma or other state of unconsciousness is to obtain a complete medical history.Obviously, there is no way to get this
information from the comatose patient, so physicians must rely on friends and family for these details. It is important to know how the coma developed, whether it was sudden or slow. It is also helpful to know about any medical problems that the comatose person has, including diabetes mellitus, high blood pressure, kidney disease, liver disorders, or a history of seizures.
Physical Exam & Neurological Evaluation
The physical exam will evaluate a comatose person's breathing pattern and cardiovascular function, as well as look at the skin and limbs for any noticeable abnormalities. Sometimes, particular breathing patterns are related to specific abnormalities in the brain, and they can be used to identify the underlying problem.
Eye Examination
The eyes will be examined carefully. Like a person's breathing pattern, the way the eye moves is sometimes related to specific abnormalities in the brain and can be used to identify the underlying problem. A fundoscope is used to examine the optic nerve in the back of the eye for any signs of swelling, which indicates that there may be increased intracranial pressure.
Laboratory Tests
Usually, a full set of laboratory tests are done on the comatose patient. These include blood tests that check the liver, kidney and thyroid function, glucose levels, and the presence of any toxins. The results of the various lab tests may provide clues as to what has caused the coma.
Imaging Studies
A CT (computerized axial tomography) and MRI (magnetic resonance imaging) can both be used to visualize most abnormalities in the brain. In some cases, a lumbar puncture, known as a spinal tap, may be done to remove some of the cerebrospinal fluid for evaluation.
EEG
If a seizure is suspected, an electroencephalography (EEG) can be done to confirm it or rule it out. An EEG tests the electrical activity in the brain.