In a traumatic brain injury, unequal pupils result from pressure on which nerve?

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Multiple Choice

In a traumatic brain injury, unequal pupils result from pressure on which nerve?

Pupillary size is controlled by parasympathetic fibers in the oculomotor nerve (CN III) that constrict the pupil via the sphincter pupillae. In traumatic brain injury, pressure from swelling or herniation can press on CN III, disrupting its parasympathetic input. When the constriction pathway is blocked but the sympathetic system remains, the pupil on the affected side loses the ability to constrict and becomes dilated, often with accompanying ptosis from eyelid muscle weakness. This pattern—unequal pupils due to a compressed oculomotor nerve—is well described in brain injury scenarios. The optic nerve affects vision, not pupil size; the trochlear nerve mainly causes vertical diplopia when damaged; the vagus nerve does not typically alter pupil diameter in this context.

Pupillary size is controlled by parasympathetic fibers in the oculomotor nerve (CN III) that constrict the pupil via the sphincter pupillae. In traumatic brain injury, pressure from swelling or herniation can press on CN III, disrupting its parasympathetic input. When the constriction pathway is blocked but the sympathetic system remains, the pupil on the affected side loses the ability to constrict and becomes dilated, often with accompanying ptosis from eyelid muscle weakness. This pattern—unequal pupils due to a compressed oculomotor nerve—is well described in brain injury scenarios. The optic nerve affects vision, not pupil size; the trochlear nerve mainly causes vertical diplopia when damaged; the vagus nerve does not typically alter pupil diameter in this context.

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