The word coma has terrifying connotations to most people. As a physician, I sometimes struggle when people ask, "Doctor, is my loved one in a coma?" This is a difficult question to answer -- not necessarily because it is complicated, but because people do not understand what it means when the answer is "yes."
Many people have learned almost all they know about coma by watching television, where coma is a condition from which recoveries are both predictable to viewers and miraculous to the characters. In reality, depending on the cause and severity of the coma, a recovery can be almost guaranteed or extremely unlikely.
The definition of coma is any condition where the patient is unconscious with their eyes closed, and unable to be aroused by even vigorous or painful stimulation. This is not the same thing as sleeping, since the brain does not go through the normal activity associated with sleep during a coma. Whereas someone who is sleeping may move if they are uncomfortable, a comatose patient will not, except for spinal reflexes.
Note that by this definition, doctors frequently put people into coma intentionally every time they use general anesthesia for a surgical procedure. Similarly, many people in hospitals take a long time to rid their body of foreign substances, whether those substances are medications or infections. In these cases, we would expect the person to wake up when the body finally rids itself of the infection, medication or toxin.
On the other hand, there are forms of coma from which it may be impossible to wake. Contrary to what we used to think, nerve cells can regenerate, but they do so only in specific parts of the brain, and even then, very slowly. If enough nerve cells die in a region that is essential for maintaining wakefulness, such as the thalamus, brainstem, or large areas of the cerebral cortex, then the person will probably never regain normal consciousness.
Other States of Unconsciousness
While everyone seems to focus on coma, there are even more severe states of unconsciousness. For example, some types of coma are eventually replaced by what is called a vegetative state. Whereas comatose patients appear to be sleeping, people in a vegetative state regain some degree of crude arousal, resulting in the eyes opening. The eyes may even reflexively move, appearing to gaze at things in the room. However, people in a vegetative state do not show any true awareness of themselves or their environment. If the brainstem remains intact, the heart, lungs, and gastrointestinal tracts continue to function. If this condition lasts for months, the patient is considered to be in a persistent vegetative state.
Brain death is an even more severe situation in which the functions of the brainstem are compromised in a comatose patient, and someone can no longer even breathe on their own. The unconscious patient's ability to increase or decrease their heart rate appropriately may also be affected. There have been no well-documented cases of people accurately diagnosed with brain death who have had any kind of meaningful recovery. While a qualified physician can make a diagnosis of brain death based off the physical exam alone, given the seriousness of the diagnosis, some families prefer to have additional tests done as well. However, if the bedside exam can be done completely and accurately, additional tests are unlikely to show any new or more hopeful information. If an autopsy is done on a brain-dead patient, many cells of the brain will have wasted away.
Minimally Conscious States
Because of the very bad prognosis of these conditions, neurologists hope to find a sign that their patient may actually not be in a true coma or vegetative state, but rather in a minimally conscious state. Minimally conscious states still signify a severe deficit in awareness, but there is at least some glimmer of preserved awareness of the self or the surrounding environment. This may be a clearly reproducible ability to follow simple commands, appropriately give yes/no responses, demonstrate purposeful behavior such as appropriate smiling or crying, or adjusting their hands to the size and shape of held objects. In general, people in minimally conscious states have much better outcomes than patients in sustained comas.
Determining whether a person is in a minimally conscious state or a coma is more difficult than one would initially think. A comatose person may move in ways that seem like they are awake, misleading friends and family. For example, comatose patients may grimace if a painful stimulus is applied to a finger or toe. They may even appear to pull the limb back away from such pain. In what is called Lazarus syndrome, an especially strong reflex can lead a comatose patient to sit upright. However, these responses are just reflexes, similar to what happens to your leg when a neurologist taps your knee with a hammer. Such movements do not necessarily mean that someone is awake.
Recovering from a Coma
When most people ask if their loved one is in a coma, what they really want to know is how soon the patient will wake up, if ever. As you’ve seen, this can vary depending on the cause and severity of the unconscious state. For example, coma due to traumatic brain injury tends to have a better prognosis than coma due to cardiac arrest. Younger patients tend to do better than older ones. Someone in a drug-induced coma may wake naturally as the drug is cleared from their system, whereas someone with a permanent brain lesion may progress to a persistent vegetative state, or even brain death. In general, the longer someone remains unconscious, the less likely they are to recover their alertness.
However, even the guidelines above can be something of an oversimplification. Neurologists can make predictions about the future, but this is not the same as a metaphorical crystal ball. Unfortunately, the only way of knowing for sure whether someone will recover from a coma is to wait for a reasonable amount of time and see. How much time to wait can be a hard decision, depends on the unique circumstances of the patient and their family, and should be discussed carefully with the entire medical team.
Jerome B. Posner and Fred Plum. Plum and Posner's Diagnosis of Stupor and Coma. New York: Oxford University Press, 2007.
Hal Blumenfeld, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002