Internuclear Ophthalmoplegia for Boards

Master internuclear ophthalmoplegia for NBEO and OKAP: localize the MLF lesion, recognize classic signs and avoid common exam traps.

Reviewed for accuracyJuly 27, 20263 min read

Four-panel diagram of a left internuclear ophthalmoplegia showing eye positions from the examiner's view: normal primary gaze, full conjugate movement on gaze to the patient's left, failure of the left eye to adduct with abducting nystagmus of the right eye on gaze to the patient's right, and preserved convergence in which the same left eye adducts normally for a near target

Warm-up

Internuclear ophthalmoplegia localizes to a lesion of the:

Lesson

Internuclear Ophthalmoplegia in 30 Seconds

Internuclear ophthalmoplegia is an MLF syndrome that disrupts conjugate horizontal gaze. The affected eye adducts slowly or incompletely and the fellow eye develops abducting nystagmus. For NBEO and OKAP, recognize the pattern, name the side correctly and connect age plus laterality to the likely cause.

The Lesion and the Pathway

A lesion of the medial longitudinal fasciculus interrupts fibers from internuclear neurons in the abducens nucleus to the contralateral medial rectus subnucleus of the oculomotor nucleus. This causes slowed or limited adduction of the ipsilateral eye during horizontal gaze and abducting nystagmus in the fellow eye. Name the INO by the side of the adduction deficit, which is also the side of the MLF lesion. Horizontal binocular diplopia, worse on lateral gaze, is the typical presenting symptom.

The Classic Signs

Ipsilateral adduction deficit or slowing: Rapid horizontal saccades can expose a subtle adduction lag.
Abducting nystagmus of the fellow eye: This dissociated nystagmus is a key localization clue.
Convergence is often preserved: This supports a lesion of the MLF rather than the medial rectus muscle or oculomotor nerve, since convergence pathways may bypass the MLF. It may be impaired in some lesions, so it is not always present.
Preserved vertical eye movements: The lesion selectively affects horizontal conjugate gaze, so vertical gaze is generally spared.
Bilateral INO: Both eyes show adduction slowing or limitation with abducting nystagmus on lateral gaze.

4 Facts Exams Always Ask

Multiple sclerosis is a classic cause in younger patients and bilateral INO is common in this group. Stroke or ischemia is a classic cause in older patients and is often unilateral.
Convergence is often preserved in INO because convergence pathways may bypass the MLF, though it can be impaired depending on lesion location.
WEBINO means wall-eyed bilateral internuclear ophthalmoplegia. It combines bilateral INO with a large-angle primary-position exotropia.
Unlike a third nerve palsy, isolated INO does not cause ptosis or pupillary involvement, and it produces the characteristic fellow-eye abducting nystagmus while often preserving convergence.

The Classic Trap

Do not label every adduction deficit a third nerve palsy. In INO, look for slowed adduction during horizontal saccades and abducting nystagmus in the opposite eye. Unlike a third nerve palsy, isolated INO does not produce ptosis or pupillary abnormalities, and convergence is often preserved, which further favors an MLF lesion. In a younger patient, bilateral findings should make multiple sclerosis a leading concern.

Clinical Pearl

Test fast horizontal saccades between two peripheral targets. A mild INO may show adduction slowing before an obvious duction deficit appears, since adduction slowing is often more apparent during rapid saccades than during smooth pursuit. This is a high-yield bedside maneuver when the history suggests intermittent horizontal diplopia.

Check yourself

Checkpoint 1

Which additional finding often helps distinguish internuclear ophthalmoplegia from a partial third nerve palsy?

Checkpoint 2

Wall-eyed bilateral internuclear ophthalmoplegia is characterized by bilateral adduction deficits and:

Clinical case

A 27-year-old patient reports binocular horizontal diplopia when looking to the right. On right gaze, the left eye adducts slowly and incompletely while the right eye shows horizontal abducting nystagmus. Convergence is intact. Vertical eye movements are normal.

A patient presents with adduction deficit of the left eye and abducting nystagmus of the right eye on right gaze, with intact convergence. Where is the lesion?

Explanation

This is a classic internuclear ophthalmoplegia (INO), caused by a lesion in the medial longitudinal fasciculus ipsilateral to the adduction deficit. Preserved convergence localizes the lesion to the MLF rather than the oculomotor nucleus/nerve, since the convergence pathway is separate.

Case discussion

This is a left internuclear ophthalmoplegia. The left eye has the adduction deficit and the right (abducting) eye shows the dissociated nystagmus, so the lesion localizes to the left medial longitudinal fasciculus, the side of the adduction deficit. Intact convergence supports an MLF lesion rather than a medial rectus or third nerve disorder. In a younger patient, an INO should raise concern for multiple sclerosis.

Sources & Reviewer Info

Reviewed by Vasilis Inglezis: Optometrist and Ocular Oncology Imaging Specialist, Ocular Oncology Center, Athens, Greece. Last updated July 27, 2026.

Diagram created with AI image generation, clinically reviewed for accuracy.

References

American Academy of Ophthalmology. Basic and Clinical Science Course, Section 5: Neuro-Ophthalmology.
Walsh and Hoyt's Clinical Neuro-Ophthalmology.
American Academy of Ophthalmology EyeWiki. "Internuclear Ophthalmoplegia."

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