Updated October 12, 2014.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

A person’s level of consciousness is a measure of how alert and aware they are. Consciousness is a spectrum with many shades. To some degree, you experience these shades as you slowly wake up in the morning. Diseases can expand that spectrum so that you are far more difficult to rouse than you would be from a normal sleep (as in a coma) or are overly awake and alert (hypervigilance), as can be seen in mania or amphetamine intoxication.

Levels of Consciousness

Because altered levels of consciousness can be associated with worse outcomes, some experts have even suggested that a patient’s level of consciousness be considered another vital sign, like heart rate and body temperature. Physicians have many ways of describing levels of consciousness and how it has changed in a patient.

An older way of describing consciousness was to use terms like "clouded consciousness, obtundation, stupor" and "coma," each signifying a progressively more severe state. However, these terms have largely been discarded as not specific or descriptive enough and even negative.

The most widely used method of describing consciousness now is the Glasgow coma scale (GCS), which ranks a person's level of consciousness on a scale from one to 15, with larger numbers representing greater alertness. The GCS is not perfect. Other scales have been proposed, but doctors' familiarity with the GCS keeps this one the most widely used.

Coma

The best known alteration of consciousness is the infamous coma - meaning that someone cannot be roused and their eyes are closed.

There are many causes of coma, with different degrees of severity. For example, coma can be caused intentionally by medications given prior to a surgery, or it can be caused by a severe stroke. In severe cases, coma may be replaced by a persistent vegetative state or even brain death. Other times, someone may wake up from a coma.

In addition to coma, there are many other ways that a person’s consciousness can be impaired:

Acute Confusional States

One of the most common disorders of consciousness in hospitals is the acute confusional state, also known as delirium. Some estimates are that about 50 percent of people in hospital settings experience this condition to some degree. In an acute confusional state, consciousness fluctuates so that a person may seem fine one moment and a few minutes later may seem an entirely different person. They may not know where they are, may not know the time or date and may not recognize familiar faces at the bedside. Hallucinations are not uncommon. People in an acute confusional state may develop paranoid delusions, fearing that hospital staff or family are going to harm them. Sometimes confused patients will pull out lines providing medication and may even try to get out of bed and escape the hospital.

Acute confusional states are usually caused by problems that affect the entire body, such as toxins, medications, infections, pain and more. The good news is that while it can take a while, these states tend to resolve on their own after the underlying medical problem is corrected.

Hypersomnia

Some people have excessive sleepiness. This may be due to any number of problems, including neurological diseases like narcolepsy and idiopathic hypersomnia. The result is that someone will be sleepy when awake and may sleep through a great deal of the day. Although people in a coma may appear to be sleeping, real sleep is actually quite different. For example, while sleeping, you are able to turn over or move your arm. Comatose patients cannot do this.

Akinetic Mutism

Lesions such as a stroke in certain parts of the brain, including the anterior cingulate gyrus, result in someone who appears alert but does not understand what is going on around them and does not move spontaneously. Advanced stages of dementia result in akinetic mutism.

Abulia

Abulia is a kind of extreme lack of incentive due to damage to the pathways that are responsible for motivation. This damage may be sudden, as in the case of stroke, or slow and progressive, as in advanced Alzheimer’s disease. The result is a person who does not, and cannot, want to do much of anything. The degree of abulia can vary, but in extreme cases the person will not move, speak or even eat or drink, thereby resembling akinetic mutism. In less severe cases, an abulic person can be coaxed to follow simple commands, though they will not do this without encouragement.

Catatonia

Catatonia is a psychiatric disorder in which a person appears unresponsive but has an otherwise normal neurological exam. People with catatonia may demonstrate unusual behavior, such as catalepsy, which is keeping seemingly uncomfortable limb positions for a prolonged amount of time; or waxy flexibility, meaning that someone can position the patient’s limb. People with catatonia may also have repetitive motions that appear similar to a seizure, even though their electroencephalograph (EEG) is normal. Catatonia can result from psychiatric disorders such as bipolar disorder or schizophrenia.

Locked-In Syndrome

Technically, locked-in syndrome is not an impairment of consciousness, although it can mimic one. In fact, that’s what makes this disorder particularly horrifying. A person who is locked in is unable to move or communicate with the outside world but remains perfectly awake and alert. For instance, a stroke in the brainstem can cause nearly entire body paralysis and may cause the patient to appear comatose. Depending on the cause, the person may be able to communicate with eye movements. While it may be difficult, every effort must be made to distinguish a comatose or vegetative patient from one who is locked in.

How well a patient does with any of these conditions depends on many factors, not the least of which is a correct diagnosis. Neurologists must take care to correctly diagnose these conditions, as each is caused by different underlying diseases and may respond to different treatments.

<sub>Sources:

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</sub>