Diplopia due to a lesion in the brainstem is most consistent with involvement of which cranial nerves, potentially without ptosis or pupil abnormalities?

• A. Optic nerve lesion
• B. Oculomotor nerve (CN III)
• C. Cranial nerves III or VI (oculomotor or abducens)
• D. Cranial nerves II or IV

Answer: (C)

Diplopia from a brainstem lesion most often reflects disruption of the motor nerves that control eye movements. The oculomotor (III) and abducens (VI) nerves carry the commands to most of the eye muscles, and a brainstem lesion affecting these pathways commonly produces misalignment of the eyes and double vision. The optic nerve is a visual pathway, so its damage causes vision loss rather than misalignment. The trochlear nerve (IV) can cause diplopia, but in the brainstem context the pattern most classically described involves III or VI. Additionally, involvement of oculomotor fibers can occur without pupil or eyelid signs if the parasympathetic fibers are spared, so diplopia may appear without ptosis or pupil abnormalities.