Terminology

There are many different terms used to describe different types of altered consciousness, for example coma, stupor, confusion or obtundation. Unfortunately these mean different things to different people and so are of limited use. This in part reflects the complicated nature of consciousness, making it unsurprising that this cannot be summed up in a single word. It therefore is more useful to try and quantify consciousness individually using specific descriptions or scales (see below). Altered consciousness and coma are a spectrum. The differential diagnosis and management vary according to the level of consciousness. Within that spectrum, however, coma can be recognized as being very much at one end. Coma can be practically defined as unrousable unconsciousness and is discussed in this section.

Aetiology and pathogenesis

Consciousness depends on the intact functioning of the reticular activating system in the brain stem and the cerebral cortex. Coma and altered consciousness arise from disturbance of these elements (Table 1). This can arise from:

infratentorial lesions that disrupt the reticular activating system directly

supratentorial lesions – large enough to cause herniation (p. 48) or large enough to disrupt the cortex

diffuse cerebral insult that affects the function of the cortex and reticular activation system.

Table 1 Some causes of coma

Coma

The clinical assessment of coma is outlined in Box 1. Coma is usually readily recognizable. Potential differential diagnoses include:

‘Locked in syndrome’: This results from a high midbrain lesion or a severe generalized neuropathy, which prevent the patient from all movement despite having a normal consciousness.

Catatonia: This is a psychiatric state of unresponsiveness.

Vegetative state: In patients with diffuse bihemispheric disease but normal brain stem function, this may lead to a vegetative state. This is not usually relevant to the acute diagnosis of coma.

The next issue is to determine the cause so as to direct future management of the coma (Box 1). This can be broadly divided into the following categories:

Coma with neck stiffness. This can arise from meningitis, subarachnoid haemorrhage and, rarely, as a result of foramen magnum herniation.

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