Coma, from the Greek ‘deep sleep’ or ‘trance’, is a state of profound unrousable unresponsiveness in which the patient lies with their eyes closed and does not respond appropriately to external or internal stimuli. There is a total absence of awareness of self and environment even when the patient is externally stimulated. The patient may grimace in response to noxious stimuli and the limbs may demonstrate stereotyped withdrawal responses, but the patient does not make localizing responses or discrete defensive movements. As coma deepens, the patient's responsiveness, even to painful stimuli, may diminish or disappear. Motor responses do not easily correlate with the depth of coma because the neural structures that regulate motor responses differ from those that regulate consciousness, and they may be differentially impaired by specific brain disorders. Knowledge of the underlying mechanisms of coma can help accurate diagnosis.

Consciousness is a state of awareness of self and one's relationship to the environment that gives significance to stimuli from the internal and external environment. It relies on two critical inter-related functions: arousal and alertness. Arousal reflects the integrity of the ascending reticular activating system (RAS) and the brainstem, predominantly the dorsal pons and midbrain; neurones from these areas connect in the thalamus and project to several cortical areas. The cortex processes, integrates and gives context to the information it receives to generate awareness. Cognitive function is facilitated by the cerebral hemispheres and coma can be caused by damage to both hemispheres or by a structural or metabolic lesion affecting the brainstem RAS or its projections to the thalamus and cortex. Unilateral dysfunction or disease of the cerebral hemispheres does not, of its own accord, cause coma.

Coma has many causes and is a neurological emergency. The first priority is the stabilize the patient by treating them for life-threatening conditions. Several causes are easily identifiable. A history of head injury, cardiac arrest or hypoglycaemia, or brain imaging with CT will clarify the diagnosis in many cases. However, some cases require extensive testing to determine the aetiology. Causes of coma can be classified into three groups:

structural brain disease

diffuse neuronal dysfunction (secondary to various conditions that produce a general state of depressed neuronal function)

psychogenic unresponsiveness.

Apart from psychiatric causes, all these diagnoses affect the cerebral cortex, the RAS, or their connections. Coma has been adopted as an Emergency Neurological Life Support (ENLS) protocol and work-up of such patients is critical to establish a correct diagnosis (Figure 1).