Consciousness and the Reticular Activating System

Expansion

Consciousness has two components:

• Arousal (wakefulness): the ascending reticular activating system in the brainstem tegmentum, projecting through the thalamus to the cortex
• Awareness (content): the cerebral cortex

Causes of coma therefore fall into three groups:

• Diffuse or bilateral hemispheric dysfunction: metabolic, toxic, hypoxic, seizures, meningitis. By far the commonest, and usually without focal signs
• Brainstem lesion directly affecting the reticular formation
• Mass lesion causing herniation and secondary brainstem compression

Two important states distinguish the components:

• Locked-in syndrome: a ventral pontine lesion destroys motor pathways but spares the reticular formation and cortex. The patient is fully aware but can move only the eyes vertically. Easily mistaken for coma
• Persistent vegetative state: the reverse, with preserved brainstem arousal and sleep-wake cycles but no awareness

Pupillary responses are relatively resistant to metabolic causes, so reactive pupils in a deeply comatose patient point towards a metabolic or toxic cause.