Lesion of the right medial longitudinal fasciculus (MLF) between the abducens nucleus and oculomotor nucleus produces which characteristic eye movement abnormality?

• A Right eye fails to abduct on right gaze; convergence is intact
• B Right eye fails to adduct on left gaze; nystagmus of the left abducting eye ✓
• C Bilateral impaired vertical gaze with convergence retraction nystagmus
• D Right eye exotropia with preserved adduction on convergence

Explanation

The MLF connects the contralateral abducens nucleus to the ipsilateral oculomotor nucleus (medial rectus subnucleus). A right MLF lesion interrupts the signal from the left CN VI nucleus to the right CN III medial rectus, producing internuclear ophthalmoplegia (INO): the right eye fails to adduct when looking left, while the left abducting eye shows nystagmus. Convergence is typically preserved because it uses a separate pathway not relying on MLF. Bilateral impaired vertical gaze with convergence-retraction nystagmus is Parinaud syndrome (dorsal midbrain). A right MLF lesion does not impair right eye abduction.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.