Coma
Overview
Coma is a state of profound unconsciousness in which a patient is unresponsive to people or events. It is a medical emergency because secondary brain injury from hypoxaemia, hypotension or raised intracranial pressure (ICP) is common and potentially reversible.
Consciousness requires an intact ascending reticular activating system (RAS) and a functional cerebral cortex (at least one hemisphere); disruption of either produces coma.
Pathophysiology Two principal mechanisms produce coma:
- Failure of the ascending RAS (brainstem): examples include brainstem stroke, haemorrhage, direct trauma, or compression from raised ICP or herniation.
- Bilateral cortical dysfunction: examples include global hypoxic/ischaemic injury, severe metabolic or toxic encephalopathy, diffuse infection (encephalitis), or multifocal structural disease.
Most comas reflect a mix of primary structural pathology and secondary physiological insults (hypoxaemia, hypotension, hypoglycaemia, electrolyte disturbance). Early identification and reversal of reversible factors is the first priority.
Common causes - practical classification Structural
- Traumatic brain injury (contusion, intracranial haemorrhage, diffuse axonal injury)
- Intracerebral haemorrhage and subarachnoid haemorrhage
- Brainstem infarction or haemorrhage (pontine, midbrain)
- Space‑occupying lesions with mass effect (tumour, abscess)
- Large bilateral ischaemic strokes