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Level of consciousness (LOC) is a measurement of a person's arousability and responsiveness to stimuli from the environment.[1] A mildly depressed level of consciousness may be classed as lethargy; someone in this state can be aroused with little difficulty.[1] People who are obtunded have a more depressed level of consciousness and cannot be fully aroused.[1][2] Those who are not able to be aroused from a sleep-like state are said to be stuporous.[1][2] Coma is the inability to make any purposeful response.[1][2] Scales such as the Glasgow coma scale have been designed to measure level of consciousness.

An altered level of consciousness can result from a variety of factors, including alterations in the chemical environment of the brain (e.g. exposure to poisons), insufficient oxygen or blood flow in the brain, and excessive pressure within the skull. Prolonged unconsciousness is understood to be a sign of a medical emergency.[3] A deficit in the level of consciousness suggests that both of the cerebral hemispheres or the reticular activating system have been injured.[4] A decreased level of consciousness correlates to increased morbidity (disability) and mortality (death).[5] Thus it is a valuable measure of a patient's medical and neurological status. In fact, some sources consider level of consciousness to be one of the vital signs.[3][6]


Scales and terms to classify the levels of consciousness differ, but in general, reduction in response to stimuli indicates decreasing level of consciousness:

Levels of consciousness
Level Summary (Kruse)[2] Description
Conscious Normal Assessment of LOC involves checking orientation: people who are able promptly and spontaneously to state their name, location, and the date or time are said to be oriented to self, place, and time, or "oriented X3".[7] A normal sleep stage from which a person is easily awakened is also considered a normal level of consciousness.[8] "Clouding of consciousness" is a term for a mild alteration of consciousness with alterations in attention and wakefulness.[8]
Confused Disoriented; impaired thinking and responses People who do not respond quickly with information about their name, location, and the time are considered "obtuse" or "confused".[7] A confused person may be bewildered, disoriented, and have difficulty following instructions.[8]. The person may have slow thinking and possible memory time loss. This could be caused by sleep deprivation, malnutrition, allergies, environmental pollution, and infection.
Delirious Disoriented; restlessness, hallucinations, sometimes delusions Some scales have "delirious" below this level, in which a person may be restless or agitated and exhibit a marked deficit in attention.[2]
Somnolent Sleepy A somnolent person shows excessive drowsiness and responds to stimuli only with incoherent mumbles or disorganized movements.[7]
Obtunded Decreased alertness; slowed psychomotor responses In obtundation, a person has a decreased interest in their surroundings, slowed responses, and sleepiness.[8]
Stuporous Sleep-like state (not unconscious); little/no spontaneous activity People with an even lower level of consciousness, stupor, only respond by grimacing or drawing away from painful stimuli.[7]
Comatose Cannot be aroused; no response to stimuli Comatose people do not even make this response to stimuli, have no corneal or gag reflex, and they may have no pupillary response to light.[7]

Causes of alteration[]

File:Intracerebral heamorrage.jpg

Intracranial hemorrhage, one cause of altered level of consciousness

A lowered level of consciousness can indicate a deficit in brain function.[4] Level of consciousness can be lowered when the brain receives insufficient oxygen (as occurs in hypoxia); insufficient blood (as occurs in shock); or has an alteration in the brain's chemistry.[3] Metabolic disorders such as diabetes mellitus and uremia can alter consciousness.[9] Hypo- or hypernatremia (decreased and elevated levels of sodium, respectively) as well as dehydration can also produce an altered LOC.[10] A pH outside of the range the brain can tolerate will also alter LOC.[8] Exposure to drugs (eg. alcohol) or toxins may also lower LOC,[3] as may a core temperature that is too high or too low (hyperthermia or hypothermia). Increases in intracranial pressure (the pressure within the skull) can also cause altered LOC. It can result from traumatic brain injury such as concussion.[9] Stroke and intracranial hemorrhage are other causes.[9] Infections of the central nervous system may also be associated with decreased LOC; for example, an altered LOC is the most common symptom of encephalitis.[11] Neoplasms within the intracranial cavity can also affect consciousness,[9] as can epilepsy and post-seizure states.[8] A decreased LOC can also result from a combination of factors.[9]


Illu tentorium

The tentorium (red), above which lesions do not normally alter consciousness significantly

Although the neural science behind alertness, wakefulness, and arousal are not fully known, the reticular formation is known to play a role in these.[8] The ascending reticular activating system is a postulated group of neural connections that receives sensory input and projects to the cerebral cortex through the midbrain and thalamus from the retucular formation.[8] Since this system is thought to modulate wakefulness and sleep, interference with it, such as injury, illness, or metabolic disturbances, could alter the level of consciousness.[8]

Normally, stupor and coma are produced by interference with the brain stem, such as can be caused by a lesion or indirect effects, such as brain herniation.[8] Mass lesions in the brain stem normally cause coma due to their effects on the reticular formation.[12] Mass lesions that occur above the tentorium cerebelli (pictured) normally do not significantly alter the level of consciousness unless they are very large or affect both cerebral hemispheres.[8]


Assessing LOC involves determining an individual's response to external stimuli.[9] Speed and accuracy of responses to questions and reactions to stimuli such as touch and pain are noted.[9] Reflexes, such as the cough and gag reflexes, are also means of judging LOC.[9] Once the level of consciousness is determined, clinicians seek clues for the cause of any alteration.[8]


One tool for measuring LOC objectively, Glasgow Coma Scale (GCS), has come into almost universal use for assessing people with brain injury.[2] Verbal, motor, and eye-opening responses to stimuli are measured, scored, and added into a final score on a scale of 3–15, with a lower score being a more decreased level of consciousness.

The Grady Coma Scale classes patients on a scale of I to V along a scale of confusion, stupor, deep stupor, abnormal posturing, and coma.[8]

The AVPU scale is another means of measuring LOC: patients are assessed to determine whether they are alert, responsive to verbal stimuli, responsive to painful stimuli, or unresponsive.[3][6] To determine responsiveness to voice, a caregiver speaks to, or, failing that, yells at the paitent.[3] Responsiveness to pain is determined with a mild painful stimulus such as a pinch; moaning or withdrawal from the stimulus is considered a response to pain.[3] The ACDU scale, like AVPU, is easier to use than the GCS and produces similarly accurate results.[13] Using ACDU, a patient is assessed for alertness, confusion, drowsiness, and unresponsiveness.[13]


  1. 1.0 1.1 1.2 1.3 1.4 Kandel ER, Jessell, Thomas M.; Schwartz, James H. (2000). Principles of neural science, 901, New York: McGraw-Hill. URL accessed 2008-07-03.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Porth C (2007). Essentials of Pahtophysiology: Concepts of Altered Health States, 835, Hagerstown, MD: Lippincott Williams & Wilkins. URL accessed 2008-07-03.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Pollak AN, Gupton CL (2002). Emergency Care and Transportation of the Sick and Injured, 140, Boston: Jones and Bartlett. URL accessed 2008-07-04.
  4. 4.0 4.1 Porth, p. 838
  5. Scheld et al.. p. 530
  6. 6.0 6.1 Forgey WW (1999). Wilderness Medicine, Beyond First Aid, 5th Edition, 13, Guilford, Conn: Globe Pequot. URL accessed 2008-07-04.
  7. 7.0 7.1 7.2 7.3 7.4 Kruse MJ (1986). Nursing the Neurological and Neurotrauma Patient, 57–58, Totowa, N.J: Rowman & Allanheld.
  8. 8.00 8.01 8.02 8.03 8.04 8.05 8.06 8.07 8.08 8.09 8.10 8.11 8.12 Tindall SC (1990). "Level of consciousness" Walker HK, Hall WD, Hurst JW Clinical Methods: The History, Physical, and Laboratory Examinations, Butterworth Publishers. URL accessed 2008-07-04.
  9. 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 von Koch CS, Hoff JT (2005). "Diagnosis and management of depressed states of consciousness" Doherty GM Current Surgical Diagnosis and Treatment, 863, McGraw-Hill Medical. URL accessed 2008-07-04.
  10. Johnson AF, Jacobson BH (1998). Medical Speech-language Pathology: A Practitioner's Guide, 142, Stuttgart: Thieme. URL accessed 2008-07-04.
  11. Scheld WM, Whitley RJ, Marra CM (2004). Infections of the Central Nervous System, 219, Hagerstown, MD: Lippincott Williams & Wilkins. URL accessed 2008-07-04.
  12. Tindall SC (1990). "Level of consciousness" Walker HK, Hall WD, Hurst JW Clinical Methods: The History, Physical, and Laboratory Examinations, Butterworth Publishers. URL accessed 2008-07-04. "Mass lesions within the brainstem produce coma by virtue of direct effects on the reticular formation"
  13. 13.0 13.1 Posner JB, Saper CB, Schiff ND, Plum F (2007). Plum and Posner's Diagnosis of Stupor and Coma, 41, Oxford University Press, USA.

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