A 28-year-old woman with multiple sclerosis reports diplopia on looking to the left. Examination shows impaired adduction of the left eye on rightward gaze and abduction nystagmus of the right eye. The lesion localizes to which structure?
- A Left medial longitudinal fasciculus ✓
- B Right medial longitudinal fasciculus
- C Left oculomotor nerve
- D Left abducens nerve
Correct answer: A. Left medial longitudinal fasciculus
Explanation
This is internuclear ophthalmoplegia (INO): impaired adduction of the ipsilateral eye and abducting nystagmus of the contralateral eye on lateral gaze. INO results from a lesion in the medial longitudinal fasciculus (MLF) on the side of impaired adduction. The left MLF lesion causes failure of left eye adduction. INO is highly characteristic of multiple sclerosis.
Reference: Clinical Neuroanatomy (Snell), 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.