There is a continuum from the individual in full consciousness to the patient in deep coma. The terminology which is most usually employed derives from the Brain Injuries Committee of the Medical Research Council (MRC).7

1. Confusion. "Disturbance of consciousness characterised by impaired capacity to think clearly and to perceive, respond to and remember current stimuli; there is also disorientation." Confusion involves a generalised disturbance of cortical cerebral function which is usually associated with considerable electroencephalographic (EEG) abnormality. Some authors describe an intervening state between normal consciousness and confusion, that of "clouding of consciousness".2

2. Delirium. "A state of much disturbed consciousness with motor restlessness, transient hallucinations, disorientation and delusions."

3. Obtundation. "A disturbance of alertness associated with psychomotor retardation."

4. Stupor. "A state in which the patient, though not unconscious, exhibits little or no spontaneous activity." Although the individual appears to be asleep he or she will awaken to vigorous stimulation but show limited motor activities and usually fail to speak.

5. Coma. "A state of unrousable psychologic unresponsiveness in which the subject lies with eyes closed and shows no psychologically understandable response to external stimulus or inner need." This may be shortened to "a state of unrousable unresponsiveness" which implies both the defect in arousal and in awareness of self or environment manifest as an inability to respond. A more useful assessment of coma is that derived from the hierarchical Glasgow Coma Scale.8 Patients who fail to show eye opening in response to voice, perform no better than weak flexion in response to pain, and make, at best, only unrecognisable grunting noises in response to pain, are regarded as being in coma. This allows the patient to have an eye opening response of two or fewer, a motor response of four or fewer, and a verbal response of two or fewer. The sum Glasgow Score of eight should not be regarded as being definitive of coma since the total score can be achieved in several different ways (see below).

6. Vegetative state. "A clinical condition of unawareness of self and environment in which the patient breathes spontaneously, has a stable circulation and shows cycles of eye closure and eye opening which may simulate sleep and waking. This may be a transient stage in the recovery from coma or it may persist until death."9 When the cortex of the cerebral hemispheres of the brain recovers more slowly than the brain stem or when the cortex is irreversibly damaged there will arise a situation in which the patient enters a vegetative state without cognitive function. This may be a transient phase through which patients in coma pass as they recover or deteriorate but, commonly after anoxic injuries to the brain, a state develops in which the brain stem recovers function but the cerebral hemispheres are not capable of recovery. This is the "persistent vegetative state" described by Jennett and Plum.10 Such patients may survive for long periods, on occasion for decades, but never recover outward manifestations of higher mental activity and the condition, which is comparatively newly recognised, relates to the development of modern resuscitative techniques. Other terms have, in the past, been used to identify similar conditions including coma vigile, the apallic syndrome, cerebral death, neocortical death, and total dementia.

7. Akinetic mutism. This term has been used to define a similar condition of unresponsiveness but apparent alertness together with reactive alpha and theta EEG rhythms in response to stimuli. The major difference from the vegetative state, in which there is tone in the muscles and extensor or flexor responses, is that patients with akinetic mutism have flaccid tone and are unresponsive to peripheral pain. It is thought that this state is due to bilateral frontal lobe lesions, diffuse cortical lesions, or lesions of the deep grey matter.11

8. The locked-in syndrome. Feldman12 described a de-efferented state caused by a bilateral ventral pontine lesion involving damage to the cortico-spinal, cortico-pontine, and cortico-bulbar tracts. The patient has total paralysis below the level of the third nerve nuclei and, although able to open, elevate and depress the eyes, has no horizontal eye movements and no other voluntary movements. The diagnosis depends upon the clinician being able to recognise that the patient can open the eyes voluntarily and allow them to close and can signal assent or dissent, responding numerically by allowing the eyelids to fall. Similar states are occasionally seen in patients with severe polyneuropathy, myasthenia gravis, and after the use of neuromuscular blocking agents.

9. Pseudo-coma. Rarely, patients who appear in coma without structural, metabolic, toxic, or psychiatric disorder being apparent can be shown by tests of brain stem function to have intact brain stem activity and corticopontine projections and not to be in coma.

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