Apart from "brain death", which is easily dealt with (it being equivalent to decapitation), ICU staff are constantly confronted by difficult situations wherein they are compelled to care for patients who for all intents and purposes are inert tissue with little evidence of consciousness. This chapter deals with differentiating the various states which cause the patient to remain uncommunicative and motionless after the resolution of their acute illness. The question of which particular vegetative state is present, and how to tell between them, is explored in Question 16 from the second paper of 2007.

The canonical resource for all things pertaining to unconsciousness is Chapter 49  (pp. 549) from Oh's Manual, Disorders  of  consciousness  by Balasubramanian  Venkatesh.

Non-canonical sources are also available. An ideal reference for the time-poor candidate would have to be this 2010 article by Monti et al, from the BMJ. An alternative for somebody with little else to do would be this 2003 position statement from the Royal College of Physicians; Table 2 from the latter was a major influence on the discussion presented below. Lastly, a person with truly infinite time resources could spend some weeks submerged in the 72-page NHMRC "information paper" from 2004, which is a truly definitive resource.

In summary:

Clinical Features to Distinguish States of Persistent Unconsciousness
Category Chronic Coma Persistent vegetative state (PVD) Minimally conscious state
Awareness
Sleep-wake cycle
Response to pain
GCS
Motor function
Respiration
EEG features
PET: cerebral metabolism
Category Locked-in syndrome Alinetic mutism Brain death
Awareness
Sleep-wake cycle
Response to pain
GCS
Motor function
Respiration
EEG features
PET: cerebral metabolism

Chronic coma

This is not seen very often, as it is typically a short-lived condition. It is characterised by the prolonged absence of consciousness. There are no sleep-wake cycles. The patient may not have any autonomic reflexes, and may require 24/7 ventilation. Motor performance is limited to reflex responses only. There is no evidence of visual fixation or pursuit. The definition was coined in 1966 by Plum and Posner; and since then it has remained essentially unchanged.

Any sort of global cerebral injury or damage to large volumes of brain matter can result in such a state, so it is meaningless to discuss aetiology. Examples include hypoglycaemia, severe hypoxic brain injury and severe traumatic brain injury (particularly diffuse axonal injury).

Persistent vegetative state

This is the persistence of autonomic functions (such as repiration and thermoregulation) in the absence of higher cortical functions. There may be periods of spontaneous eye opening. These patients may appear to be awake, but give no sign that they are able to interact with their environment. The definition for this condition has never been formally agreed upon to my knowledge. The British Royal College of Physicians defines it as "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 opening which may simulate sleep and waking". Most authors' definitions involve the absence of awareness in an otherwise "awake" patient, with "persistence" defined as a duration greater than 1 month after the initial injury.

The characteristic feature of this state is the presence of an apparent sleep-wake cycle, and the preservation of gross autonomic regulation. These patients do not require a home ventilator, but they may have some degree of dysautonomia. There may be non-purposeful movements, but the response to noxious stimuli is usually a startle response, withdrawal, or characteristic motor posturing.

Minimally conscious state

Unlike the persistent vegetative state, these patients do occasionally demonstrate episodes of seemingly purposeful behaviour. These episodes are fleeting and incosistent. Some view this as a transitional state, on the path to recovery.

The key diagnostic feature is the presence of "clearly discernable behavioural evidence of consciousness". These people inconsistently obey commands, localise to noxious stimuli, perform automatic movements (eg. scrating themselves) and may be able to track you with their eyes.

Locked-in syndrome

This state preserves the cognitive function. However, most motor control is lost. The terrified patient is left to spontaneously open their eyes and to move them up or down.

This lesion is usually caused by basial artery thrombosis, and involves the ventral pons. Complete paralysis of all four limbs and orobuccal musculature is the result.

Characteristic findings are a totally normal EEG, with preservation of normal consciousness.

Akinetic mutism

The seminal 1941 article describes this disorder in beautiful terms.

"The patient sleeps more than normally, but he is easily roused. In the fully developed state he makes no sound and lies inert, except that his eyes regard the observer steadily, and follow the movement of objects, and they may be diverted by sound"

These people may not be completely mute or completely akinetic, but they certainly dont answer readily, or perform any sort of complex voluntary actions. The lesion tends to destroy the frontal lobes bilaterally, or damage the midbrain (as in the case of that third ventricle tumour). Occasionally, bilateral damage to the thalmus or to the cingulate gyrus can result in this sort of picture but this is even more rare.

Thus, the characteristic features are motor immobility in spite of complete consciousness. These people are not paralysed, but rather lack any volition to move, as the centres normally responsible for the initiation of volutary motor activity are destroyed.

 

References

Oh's Intensive Care manual: Chapter 49   (pp. 549) Disorders  of  consciousness  by Balasubramanian  Venkatesh

Plum, Fred, and Jerome B. Posner. The diagnosis of stupor and coma. Vol. 19. Oxford University Press, 1982. - warning! This link takes you to a download of the whole 9.0Mb file, which represents the entire volume of the 1980 second edition.

Maramattom, Boby Varkey, and Eelco FM Wijdicks. "Uncal herniation."Archives of neurology 62.12 (2005): 1932-1935.

Skrzat, J., et al. "The clinical significance of the petroclinoid ligament." Folia morphologica 66.1 (2007): 39-43.

Conomy, John P., and Michael Swash. "Reversible decerebrate and decorticate postures in hepatic coma." New England Journal of Medicine 278.16 (1968): 876-879.

Bateman, David E. "Neurological assessment of coma." Journal of Neurology, Neurosurgery & Psychiatry 71.suppl 1 (2001): i13-i17.

Feldman, Martin H., and Shirley Sahrmann. "The Decerebrate State in the Primate: II. Studies in Man." Archives of neurology 25.6 (1971): 517-525.

Teasdale, Graham, and Bryan Jennett. "Assessment of coma and impaired consciousness: a practical scale." The Lancet 304.7872 (1974): 81-84.

Dowell, Anthony R., et al. "Cheyne-Stokes respiration: a review of clinical manifestations and critique of physiological mechanisms." Archives of internal medicine 127.4 (1971): 712.

Andreas, Stefan, et al. "Cheyne-Stokes respiration and prognosis in congestive heart failure." The American journal of cardiology 78.11 (1996): 1260-1264.

Douglas, C. Gordon, and J. S. Haldane. "The causes of periodic or Cheyne-Stokes breathing." The Journal of physiology 38.5 (1909): 401-419.

Tomlinson, B. E. "Brain-stem lesions after head injury." Journal of Clinical Pathology. Supplement (Royal College of Pathologists). 4 (1970): 154.

Perman, Sarah M., et al. "Timing of neuroprognostication in postcardiac arrest therapeutic hypothermia." Critical care medicine 40.3 (2012): 719.

Towne, A. R., et al. "Prevalence of nonconvulsive status epilepticus in comatose patients." Neurology 54.2 (2000): 340-340.

TEASDALE, GRAHAM, and BRYAN JENNETT. "Assessment of coma and severity of brain damage." Anesthesiology 49.3 (1978): 225.

Green, Steven M. "Cheerio, laddie! Bidding farewell to the Glasgow Coma Scale." Annals of emergency medicine 58.5 (2011): 427-430.

Gill, Michelle R., David G. Reiley, and Steven M. Green. "Interrater reliability of Glasgow Coma Scale scores in the emergency department." Annals of emergency medicine 43.2 (2004): 215-223.

Riechers, Ronald G., et al. "Physician knowledge of the glasgow coma scale."Journal of neurotrauma 22.11 (2005): 1327-1334.