A patient with a lesion of the medial longitudinal fasciculus (MLF) between the abducens nucleus and the contralateral oculomotor nucleus will exhibit which specific pattern on lateral gaze testing?
- A One-and-a-half syndrome: failure of abduction on the side of the lesion and failure of adduction of both eyes
- B Complete gaze palsy: inability to move both eyes to the side of the lesion
- C Internuclear ophthalmoplegia: failure of adduction on the side of the lesion with nystagmus in the abducting eye ✓
- D Skew deviation with hypertropia ipsilateral to the lesion
Correct answer: C. Internuclear ophthalmoplegia: failure of adduction on the side of the lesion with nystagmus in the abducting eye
Explanation
The MLF connects the ipsilateral abducens nucleus to the contralateral oculomotor nucleus's medial rectus subnucleus. A unilateral MLF lesion produces internuclear ophthalmoplegia (INO): when the patient looks contralateral to the lesion, the adducting eye (on the lesion side) fails to adduct, while the abducting eye shows dissociated nystagmus. Bilateral MLF lesions are highly characteristic of multiple sclerosis. One-and-a-half syndrome requires additional ipsilateral PPRF/abducens nucleus lesion.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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