A patient presents with internuclear ophthalmoplegia (INO). On attempted left lateral gaze, the left eye abducts fully but the right eye fails to adduct. Convergence is intact. Where is the lesion?
- A Right medial longitudinal fasciculus (MLF) between the abducens and oculomotor nuclei ✓
- B Right oculomotor (CN III) nucleus
- C Left medial longitudinal fasciculus
- D Paramedian pontine reticular formation (PPRF) on the left
Explanation
In INO, the lesion lies in the MLF ipsilateral to the adducting eye that fails to follow the gaze command. On left gaze, the left abducens nucleus fires and the left eye abducts; the signal is transmitted via the right MLF to the right medial rectus subnucleus to adduct the right eye. When the right MLF is damaged, this adduction signal fails, causing right eye adduction failure on left gaze. Convergence is intact because the midbrain accommodative vergence pathway bypasses the MLF. The oculomotor nucleus lesion would impair adduction in all contexts including convergence. The left MLF lesion would cause left eye adduction failure on right gaze (left INO). PPRF lesion causes ipsilateral gaze palsy affecting both eyes.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.