The word Coma derives from the ancient Greek word for sleep, but as applied in modern medical terminology, coma refers to another state altogether. The comatose person lies unmoving, except for shallow breathing, indefinitely. People can be in comas for a few hours up to years. They appear dead to the outside world, except upon close inspection.
Comas are usually caused by brain injuries or other severe trauma. They are largely a mystery and people can emerge from comas at times unforeseen by doctors. Healing can occur during a coma, but it appears to be a slow process. People in a coma do not demonstrate external signs of sleep. They do not move (as people in NREM sleep do), and their EEG readings are inconsistent with sleep. The person cannot be woken up, even with powerful stimuli. Doctors use the Glasgow Coma Scale in their assessment of coma patients. (An alternative is the Rancho Los Amigos Scale..)
In extreme medical situations, doctors use chemicals to induce coma in patients as part of a treatment strategy. Sometimes coma patients can actually hear and remember things people say to them when they are in the coma. Medical intervention is required to maintain life if the coma persists for days; patients are given nutrition intravenously.
The term vegetative state refers to something else. The person in a vegetative state is not in a coma. Also called a "coma vigil" or Apallic Syndrome, this state poses ethical dilemmas because patients do not recover. Their brainstem continues to function, and with artificial hydration and nutrition they can live indefinitely. They may even open their eyes and they show a sleep cycle (albeit not a normal one). But the higher order brain functions are gone. They do not respond to stimuli and cannot be aroused. There is an effort to
rename this state Unresponsive Wakefulness Syndrome
Anesthesia reduces the sensation of pain produces temporary amnesia, and stops movement of the skeletal muscles. The only stage of sleep where skeletal muscles are paralyzed is REM, but brain activity differs substantially between REM and anesthesia.
General anesthesia is when the person is made unconscious. It is so common that 60,000 surgical patients go under every weekday just in the United States. The person under anesthesia does not respond - even the deepest sleep is not as deep (measured by response to stimuli) as anesthesia.
Colloquially this is called "going to sleep", but it is strictly speaking not a form of sleep. The doctor may even tell the patient and his or her family that the surgery will occur while the patient is asleep, but this is a simplification and the doctor knows it. In a way, general anesthesia is a reversible coma. EEG readings of brain activity are not similar to those of any stage of sleep. Indeed, readings are closer to those of a comatose patient.
Physiologists have created a measure called the bispectral index to measure the depth of sedation and anesthesia. It indicates coherence among different frequencies measured in nervous system rhythms. The scale runs from 0 to 100, and a high number reflects good cortical integration that occurs during waking. The deeper the anesthesia state, the lower the bispectral index number. In medical anesthesia uses, the index is typically 40 to 45.
Of course anesthesia is not normal sleep and when under anesthesia the person does not experience REM. This is true for most anesthesia drugs used in medical procedures. Subjects accumulate a “REM debt” when under anesthesia and experience rebound REM in the day following. This suggests that there is a REM homeostat the same way there is an overall sleep homeostat (process C in the two-process theory). Some drugs (e.g. isoflurane and sevoflurane ) allow the brain to get its needed NREM sleep, but the anesthesia drug halothane does not. The drug propofol is an exception to the anesthesia rule, as people do not seem to accumulate an REM debt while under its effects, and indeed, sleep debt present upon going under propofol disappear while the anesthesia is in effect.
They make a big deal about risk in hospitals when it comes to anesthesia. The large majority of patients weather it, but the reduced heart rate and blood pressure and general metabolism increase the chances of death. About one in a thousand patients wakes up during anesthesia, leading to sometimes psychologically horrifying consequences. More scary is when patients do not wake up from anesthesia on schedule. Some with neurological conditions spend hours unconscious before waking while the doctors worry. Narcoleptics can take 8 hours to wake up, while a healthy person takes a few minutes. Animal experiments suggest that the orexin deficiency associated with narcolepsy affects waking from anesthesia, not going under.
Even in healthy people, there is an inertia – analogous to sleep inertia perhaps. Levels of the anesthesia drug must fall to a lower level for the patient to awaken that it takes to put him or her to sleep.
|Will last less than 12 hours||Yes||No||Yes|
|Can be induced by drugs||Yes||Yes (in rare medical procedure)||Yes|
|Person can feel pain||Yes||No||No|
|Inertia upon awakening||Under 1 hour||Days or weeks||Up to a day|
|Person awakens in response to sounds or shaking||Yes||No||No|
|Experienced as refreshing||Yes||No||Not usually|
|Indicates neurological damage||No (except some hypersomnia)||Yes||No|
Surprisingly, doctors have been able to awaken some coma patients by giving them zolpidem. The mechanism for this phenomenon is not known. Another mystery of the brain and consciousness.