Coma is a state of unconsciousness in which a person
cannot be awakened; fails to respond normally to painful stimuli, light, or
sound; lacks a normal wake-sleep cycle; and does not initiate voluntary
actions. A person in a state of coma is described as being comatose. A
distinction is made in the medical community between a real coma and a
medically induced coma, the former is a result of circumstances beyond the
control of the medical community, while the latter is a means by which medical
professionals may allow a patient's injuries to heal in a controlled
environment.
A comatose person exhibits a complete absence of wakefulness and is unable to consciously feel, speak, hear, or move. For a patient to maintain consciousness, two important neurological components must function. The first is the cerebral cortex—the gray matter that forms the outer layer of the brain. The other is a structure located in the brainstem, called reticular activating system (RAS).
Injury to either or both of these components is sufficient to cause a patient to experience a coma. The cerebral cortex is a group of tight, dense, "gray matter" composed of the nuclei of the neurons whose axons then form the "white matter," and is responsible for perception, relay of the sensory input via the thalamic pathway, and many other neurological functions, including complex thinking.
RAS, on the other hand, is a more primitive structure in the brainstem which includes the reticular formation (RF). The RAS area of the brain has two tracts, the ascending and descending tract. Made up of a system of acetylcholine-producing neurons, the ascending track, or ascending reticular activating system (ARAS), works to arouse and wake up the brain, from the RF, through the thalamus, and then finally to the cerebral cortex. A failure inARAS functioning
may then lead to a coma. The word is from the Greek κῶμα koma, meaning
"deep sleep.”
Coma may result from a variety of conditions, including intoxication (such as drug abuse, overdose or misuse of over the counter medications, prescribed medication, or controlled substances), metabolic abnormalities, central nervous system diseases, acute neurologic injuries such as strokes or herniations, hypoxia, hypothermia, hypoglycemia, eclampsia or traumatic injuries such as head trauma caused by falls, drowning accidents, or vehicle collisions. It may also be deliberately induced by pharmaceutical agents during major neurosurgery, to preserve higher brain functions following brain trauma, or to save the patient from extreme pain during healing of injuries or diseases.
Forty percent of comatose states result from drug poisoning. Drugs damage or weaken the synaptic functioning in theARAS and keep the system from properly
functioning to arouse the brain. Secondary effects of drugs, which include
abnormal heart rate and blood pressure, as well as abnormal breathing and
sweating, may also indirectly harm the functioning of the ARAS and lead to a
coma. Seizures and hallucinations have shown to also play a major role in ARAS malfunction. Given that drug poisoning is the cause
for a large portion of patients in a coma, hospitals first test all comatose
patients by observing pupil size and eye movement, through the
vestibular-ocular reflex.
The second most common cause of coma, which makes up about 25% of comatose patients, occurs from lack of oxygen, generally resulting from cardiac arrest. The Central Nervous System (CNS) requires a great deal of oxygen for its neurons. Oxygen deprivation in the brain, also known as hypoxia, causes neuronal extracellular sodium and calcium to decrease and intracellular calcium to increase, which harms neuron communication. Lack of oxygen in the brain also causes ATP exhaustion and cellular breakdown from cytoskeleton damage and nitric oxide production.
Twenty percent of comatose states result from the side effects of a stroke. During a stroke, blood flow to part of the brain is restricted or blocked. An ischemic stroke, brain hemorrhage, or tumor may cause such cessation of blood flow. Lack of blood to cells in the brain prevents oxygen from getting to the neurons, and consequently causes cells to become disrupted and eventually die. As brain cells die, brain tissue continues to deteriorate, which may affect functioning of theARAS .
The remaining 15% of comatose cases result from trauma, excessive blood loss, malnutrition, hypothermia, hyperthermia, abnormal glucose levels, and many other biological disorders.
Comas can last from several days to several weeks. In more severe cases a coma may last for over five weeks, while some have lasted as long as several years. After this time, some patients gradually come out of the coma, some progress to a vegetative state, and others die. Some patients who have entered a vegetative state go on to regain a degree of awareness. Others remain in a vegetative state for years or even decades (the longest recorded period being 42 years).
The outcome for coma and vegetative state depends on the cause, location, severity and extent of neurological damage. A deeper coma alone does not necessarily mean a slimmer chance of recovery, because some people in deep coma recover well while others in a so-called milder coma sometimes fail to improve.
People may emerge from a coma with a combination of physical, intellectual, and psychological difficulties that need special attention. Recovery usually occurs gradually—patients acquire more and more ability to respond. Some patients never progress beyond very basic responses, but many recover full awareness. Regaining consciousness is not instant: in the first days, patients are only awake for a few minutes, and duration of time awake gradually increases. This is unlike the situation in many movies where people who awake from comas are instantly able to continue their normal lives. In reality, the coma patient awakes sometimes in a profound state of confusion, not knowing how they got there and sometimes suffering from dysarthria, the inability to articulate any speech, and with many other disabilities.
Predicted chances of recovery are variable owing to different techniques used to measure the extent of neurological damage. All the predictions are based on statistical rates with some level of chance for recovery present: a person with a low chance of recovery may still awaken. Time is the best general predictor of a chance of recovery: after four months of coma caused by brain damage, the chance of partial recovery is less than 15%, and the chance of full recovery is very low.
The most common cause of death for a person in a vegetative state is secondary infection such as pneumonia, which can occur in patients who lie still for extended periods.
There are reports of patients coming out of coma after long periods of time. After 19 years in a minimally conscious state, Terry Wallis spontaneously began speaking and regained awareness of his surroundings.
A brain-damaged man, trapped in a coma-like state for six years, was brought back to consciousness in 2003 by doctors who planted electrodes deep inside his brain. The method, called deep brain stimulation (DBS) successfully roused communication, complex movement and eating ability in the 38-year-old American man who suffered a traumatic brain injury. His injuries left him in a minimally conscious state (MCS), a condition akin to a coma but characterized by occasional, but brief, evidence of environmental and self-awareness that coma patients lack.
Comas lasting seconds to minutes result in post-traumatic amnesia (PTA) that lasts hours to days; recovery plateau occurs over days to weeks. Comas that last hours to days result in PTA lasting days to weeks; recovery plateau occurs over months. Comas lasting weeks result in PTA that lasts months; recovery plateau occurs over months to years.
A comatose person exhibits a complete absence of wakefulness and is unable to consciously feel, speak, hear, or move. For a patient to maintain consciousness, two important neurological components must function. The first is the cerebral cortex—the gray matter that forms the outer layer of the brain. The other is a structure located in the brainstem, called reticular activating system (RAS).
Injury to either or both of these components is sufficient to cause a patient to experience a coma. The cerebral cortex is a group of tight, dense, "gray matter" composed of the nuclei of the neurons whose axons then form the "white matter," and is responsible for perception, relay of the sensory input via the thalamic pathway, and many other neurological functions, including complex thinking.
RAS, on the other hand, is a more primitive structure in the brainstem which includes the reticular formation (RF). The RAS area of the brain has two tracts, the ascending and descending tract. Made up of a system of acetylcholine-producing neurons, the ascending track, or ascending reticular activating system (ARAS), works to arouse and wake up the brain, from the RF, through the thalamus, and then finally to the cerebral cortex. A failure in
Signs and Symptoms of a Coma
Generally, a
person who is unable to voluntarily open the eyes, does not have a sleep-wake
cycle, is unresponsive in spite of strong tactile (painful) or verbal stimuli,
and who generally scores between 3 and 8 on the Glasgow Coma Scale is
considered in a coma. Coma may have developed in humans as a response to injury
to allow the body to pause bodily actions and heal the most immediate injuries
before waking. It therefore could be a compensatory state in which the body's
expenditure of energy is not superfluous. The severity and mode of onset of
coma depends on the underlying cause. For instance, severe hypoglycemia (low
blood sugar) or hypercapnia (increased carbon dioxide levels in the blood) initially
cause mild agitation and confusion, but progress to obtundation, stupor, and
finally, complete unconsciousness. In contrast, coma resulting from a severe traumatic
brain injury or subarachnoid hemorrhage can be instantaneous. The mode of onset
may therefore be indicative of the underlying cause.
Causes of a Coma
Coma may result from a variety of conditions, including intoxication (such as drug abuse, overdose or misuse of over the counter medications, prescribed medication, or controlled substances), metabolic abnormalities, central nervous system diseases, acute neurologic injuries such as strokes or herniations, hypoxia, hypothermia, hypoglycemia, eclampsia or traumatic injuries such as head trauma caused by falls, drowning accidents, or vehicle collisions. It may also be deliberately induced by pharmaceutical agents during major neurosurgery, to preserve higher brain functions following brain trauma, or to save the patient from extreme pain during healing of injuries or diseases.
Forty percent of comatose states result from drug poisoning. Drugs damage or weaken the synaptic functioning in the
The second most common cause of coma, which makes up about 25% of comatose patients, occurs from lack of oxygen, generally resulting from cardiac arrest. The Central Nervous System (CNS) requires a great deal of oxygen for its neurons. Oxygen deprivation in the brain, also known as hypoxia, causes neuronal extracellular sodium and calcium to decrease and intracellular calcium to increase, which harms neuron communication. Lack of oxygen in the brain also causes ATP exhaustion and cellular breakdown from cytoskeleton damage and nitric oxide production.
Twenty percent of comatose states result from the side effects of a stroke. During a stroke, blood flow to part of the brain is restricted or blocked. An ischemic stroke, brain hemorrhage, or tumor may cause such cessation of blood flow. Lack of blood to cells in the brain prevents oxygen from getting to the neurons, and consequently causes cells to become disrupted and eventually die. As brain cells die, brain tissue continues to deteriorate, which may affect functioning of the
The remaining 15% of comatose cases result from trauma, excessive blood loss, malnutrition, hypothermia, hyperthermia, abnormal glucose levels, and many other biological disorders.
Prognosis
Comas can last from several days to several weeks. In more severe cases a coma may last for over five weeks, while some have lasted as long as several years. After this time, some patients gradually come out of the coma, some progress to a vegetative state, and others die. Some patients who have entered a vegetative state go on to regain a degree of awareness. Others remain in a vegetative state for years or even decades (the longest recorded period being 42 years).
The outcome for coma and vegetative state depends on the cause, location, severity and extent of neurological damage. A deeper coma alone does not necessarily mean a slimmer chance of recovery, because some people in deep coma recover well while others in a so-called milder coma sometimes fail to improve.
People may emerge from a coma with a combination of physical, intellectual, and psychological difficulties that need special attention. Recovery usually occurs gradually—patients acquire more and more ability to respond. Some patients never progress beyond very basic responses, but many recover full awareness. Regaining consciousness is not instant: in the first days, patients are only awake for a few minutes, and duration of time awake gradually increases. This is unlike the situation in many movies where people who awake from comas are instantly able to continue their normal lives. In reality, the coma patient awakes sometimes in a profound state of confusion, not knowing how they got there and sometimes suffering from dysarthria, the inability to articulate any speech, and with many other disabilities.
Predicted chances of recovery are variable owing to different techniques used to measure the extent of neurological damage. All the predictions are based on statistical rates with some level of chance for recovery present: a person with a low chance of recovery may still awaken. Time is the best general predictor of a chance of recovery: after four months of coma caused by brain damage, the chance of partial recovery is less than 15%, and the chance of full recovery is very low.
The most common cause of death for a person in a vegetative state is secondary infection such as pneumonia, which can occur in patients who lie still for extended periods.
There are reports of patients coming out of coma after long periods of time. After 19 years in a minimally conscious state, Terry Wallis spontaneously began speaking and regained awareness of his surroundings.
A brain-damaged man, trapped in a coma-like state for six years, was brought back to consciousness in 2003 by doctors who planted electrodes deep inside his brain. The method, called deep brain stimulation (DBS) successfully roused communication, complex movement and eating ability in the 38-year-old American man who suffered a traumatic brain injury. His injuries left him in a minimally conscious state (MCS), a condition akin to a coma but characterized by occasional, but brief, evidence of environmental and self-awareness that coma patients lack.
Comas lasting seconds to minutes result in post-traumatic amnesia (PTA) that lasts hours to days; recovery plateau occurs over days to weeks. Comas that last hours to days result in PTA lasting days to weeks; recovery plateau occurs over months. Comas lasting weeks result in PTA that lasts months; recovery plateau occurs over months to years.
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