The cranial nerves are 12 pairs of nerves that arise directly from the brain and brainstem rather than from the spinal cord. Numbered I to XII, each pair has distinct functions: some are purely sensory, some purely motor, and some carry both types of fibres. They supply the head, face, neck and several internal organs, controlling functions including smell, vision, eye movement, facial sensation and movement, hearing, balance, swallowing, speech, taste, and regulation of the heart and digestive system.
Several cranial nerves are clinically important in the context of cardiac arrest and critical illness. The vagus nerve (X) is the longest cranial nerve and plays a central role in autonomic regulation of the heart rate and rhythm; vagal stimulation slows the heart. The glossopharyngeal (IX) and vagus (X) nerves together control swallowing, and damage to these can cause dysphagia. The hypoglossal nerve (XII) controls tongue movement, and its impairment contributes to speech and swallowing difficulties. The oculomotor (III), trochlear (IV) and abducens (VI) nerves control eye movements, and their function is assessed during neurological examination in the intensive care unit as part of brainstem evaluation.
In patients with severe hypoxic-ischaemic brain injury following cardiac arrest, the function of brainstem cranial nerve pathways is tested as part of prognostic assessment. Absent pupillary light reflexes (testing cranial nerve II input and III output) and absent corneal reflexes (testing V and VII) at 72 or more hours after rewarming from targeted temperature management are among the multimodal prognostic indicators used to help assess likelihood of neurological recovery.
Cranial nerve function may also be impaired by prolonged critical illness, position-related nerve compression during intensive care, or as a late effect of hypoxic injury. Difficulties with swallowing, eye movement, facial expression or speech after cardiac arrest should be assessed by the relevant specialist, as many cranial nerve deficits improve with targeted rehabilitation.
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