Internuclear ophthalmoplegia (INO) is caused by a lesion in the medial longitudinal fasciculus (MLF). In left INO, which gaze abnormality is expected?
- A Failure of right eye adduction on left gaze, with nystagmus of the abducting left eye
- B Failure of left eye adduction on right gaze, with nystagmus of the abducting right eye ✓
- C Bilateral failure of adduction with bilateral abducting nystagmus (WEBINO)
- D Failure of upward gaze only on both sides
Explanation
In INO, the lesion is in the MLF on the side of the adduction deficit. Left INO means the left MLF is damaged, disrupting the signal from the right gaze centre (right PPRF/abducens nucleus) to the left medial rectus subnucleus. On attempted right gaze, the right eye abducts normally but the left eye fails to adduct — plus ataxic nystagmus of the abducting right eye (due to internuclear dissociation). Near response (convergence) is typically preserved. Bilateral INO (WEBINO — Wall-Eyed Bilateral INO) is seen with rostral pontine/midbrain lesions. In young patients, bilateral INO strongly suggests multiple sclerosis; in older patients, vascular brainstem infarction.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.