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What Is Ophthalmoplegia Internuclear (INO)?

Ophthalmoplegia internuclear, or internuclear ophthalmoplegia (INO), is a gaze disorder caused by damage to the medial longitudinal fasciculus, a brainstem tract that links cranial nerve nuclei controlling horizontal eye movements. The classic sign is impaired adduction of the affected eye during horizontal gaze with nystagmus of the abducting fellow eye. Convergence can be preserved or mildly affected. INO can be unilateral or bilateral and often points to demyelinating, vascular, or structural brainstem disease.

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What Is Ophthalmoplegia Internuclear (INO)?

Ophthalmoplegia internuclear, or internuclear ophthalmoplegia (INO), is a gaze disorder caused by damage to the medial longitudinal fasciculus, a brainstem tract that links cranial nerve nuclei controlling horizontal eye movements. The classic sign is impaired adduction of the affected eye during horizontal gaze with nystagmus of the abducting fellow eye. Convergence can be preserved or mildly affected. INO can be unilateral or bilateral and often points to demyelinating, vascular, or structural brainstem disease.

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Clinical Features and Examination

Patients complain of horizontal diplopia, especially when looking to the side opposite the affected medial rectus. Some describe motion blur or imbalance rather than clear double images. On examination, the adducting eye lags or fails to cross midline in lateral gaze, while the abducting eye shows coarse nystagmus. Vertical movements are usually intact, and convergence testing helps distinguish INO from isolated medial rectus palsy. Bilateral INO can produce wall eyed bilateral internuclear ophthalmoplegia with exotropia in primary position.

Common Causes and Age Patterns

In younger adults, multiple sclerosis is a leading cause of INO due to demyelination in the medial longitudinal fasciculus. In older patients, small brainstem infarcts are more frequent culprits. Less common causes include brainstem tumors, trauma, infections, and structural malformations. INO can coexist with other ocular motor deficits such as gaze palsies, skew deviation, or one and a half syndrome when adjacent structures are involved. Detailed neurologic examination often reveals additional signs.

Diagnosis and Neuroimaging

Suspicion of INO arises from characteristic eye movement findings at the slit lamp or bedside. Magnetic resonance imaging of the brainstem with attention to the pons and midbrain is the main tool for confirming a lesion in the medial longitudinal fasciculus and identifying its cause. Additional tests such as evoked potentials, cerebrospinal fluid analysis, or vascular imaging are chosen based on clinical context. The neurologist and neuro–ophthalmologist work together to interpret results and plan management.

Management and Outlook

Treatment focuses on the underlying disorder, such as disease modifying therapy for multiple sclerosis or secondary prevention after stroke. Fresnel prisms or temporary occlusion can reduce diplopia while the condition evolves. Some cases, especially demyelinating INO, improve over months, while others leave persistent deficits if axonal damage is significant. Physical and visual rehabilitation can help patients adapt to residual misalignment. Long term follow up monitors for new neurologic events.

FAQs About Internuclear Ophthalmoplegia

Is INO the same as a sixth nerve palsy?

No, INO affects adduction of one eye with nystagmus of the other, while a sixth nerve palsy limits abduction of the involved eye.

Can INO get better over time?

Yes, especially when due to demyelination, partial or near complete recovery is common, though not guaranteed.

Does INO always mean multiple sclerosis?

No, stroke and other brainstem lesions are also frequent causes, particularly in older adults.

Will I have double vision forever with INO?

Some patients adapt or improve, and optical aids can reduce symptoms, but others have lasting diplopia that needs long term strategies.

References

EyeWiki. ?Internuclear Ophthalmoplegia.? https://eyewiki.org/Internuclear_Ophthalmoplegia

StatPearls (NCBI Bookshelf). ?Internuclear Ophthalmoplegia.? https://www.ncbi.nlm.nih.gov/books/NBK441970/

American Academy of Ophthalmology. ?Internuclear Ophthalmoplegia.? https://www.aao.org/eye-health/diseases/internuclear-ophthalmoplegia

Cleveland Clinic. ?Internuclear Ophthalmoplegia.? https://my.clevelandclinic.org/health/diseases/24818-internuclear-ophthalmoplegia

NCBI Bookshelf. ?Internuclear ophthalmoplegia (EyeWiki/Bookshelf-related search).? https://www.ncbi.nlm.nih.gov/books/?term=internuclear+ophthalmoplegia