Which sign indicates lack of midbrain function?

• A. Absent pupillary light reflex
• B. Absent corneal reflex
• C. Absent inducible eye movements with ice water
• D. Absent apnea test

Answer: (A)

Midbrain control of the pupil makes the pupillary light reflex a direct indicator of midbrain integrity. When light enters the eye, the signal travels via the optic nerve to the midbrain’s pretectal area, then bilaterally to the Edinger-Westphal nuclei. From there, parasympathetic fibers travel with the oculomotor nerve to the sphincter pupillae, causing pupil constriction. If the midbrain is damaged, this pathway is disrupted, so the pupil fails to constrict in response to light on either side, producing an absent pupillary light reflex.

The other signs depend on different parts of the brainstem. The corneal reflex involves the trigeminal nerve for sensation and the facial nerve for the blink, a circuit largely in the pons. The ice-water test (oculovestibular response) assesses brainstem connections between the vestibular system and eye movements, which involve pathways across the brainstem, not exclusively the midbrain. The apnea test evaluates respiratory drive controlled by medullary centers. While absence of these signs suggests brainstem dysfunction, they do not localize to the midbrain as specifically as the pupillary light reflex does.