Internuclear ophthalmoplegia is a highly localizing neuro-ophthalmologic sign caused by a lesion of the medial longitudinal fasciculus (MLF) in the brainstem, resulting in impaired adduction of the ipsilateral eye with dissociated abducting nystagmus of the contralateral eye. [1-2] The two most common etiologies are multiple sclerosis (especially in younger patients, particularly when bilateral) and brainstem ischemic stroke (in older patients, typically unilateral). [3-4]
1. History
- Onset and tempo: Acute vs. subacute onset of horizontal diplopia; sudden onset favors stroke, subacute over days favors demyelination [3][5]
- Laterality: Binocular horizontal diplopia worse on lateral gaze to the side opposite the affected MLF
- Associated symptoms: Vertigo, oscillopsia (visual instability with head movement), gait unsteadiness, limb weakness or numbness [6-7]
- Prior neurologic episodes: Previous optic neuritis, transverse myelitis, sensory disturbances, or brainstem symptoms suggesting MS [8]
- Preceding illness: Recent viral/GI illness may suggest anti-GQ1b antibody syndrome (Miller Fisher variant) [9]
- Trauma history: Head injury can cause INO via direct brainstem contusion or herniation [4][10]
- Important negatives: Absence of pain (pain suggests cavernous sinus pathology or Tolosa-Hunt), absence of ptosis/pupil involvement (helps distinguish from CN III palsy)
2. Alarm Features
- Acute onset with other brainstem signs (dysarthria, dysphagia, hemiparesis, ataxia) → posterior circulation stroke until proven otherwise [3]
- Altered consciousness → brainstem compression, herniation, or large stroke [11]
- Bilateral INO with exotropia (WEBINO) → extensive bilateral MLF damage; consider herniation, large pontine lesion [12-13]
- Pupillary abnormalities (anisocoria, fixed dilated pupil) → transtentorial herniation or CN III compression [11]
- Progressive ophthalmoplegia over hours to days → expanding mass, hemorrhage, or anti-GQ1b syndrome [9]
- Fever + INO → brainstem abscess, meningitis, encephalitis [4]
- New INO in a cancer patient → brainstem metastasis [14]
3. Medications
- Drug overdose from various medications (barbiturates, phenytoin, tricyclic antidepressants, opioids, benzodiazepines, lithium) can produce INO-like findings in the absence of structural MLF lesions [4]
- Anticholinergics and antihistamines may worsen blurry vision and should be reviewed [15]
- Treatment of underlying cause:
- MS-related INO: IV methylprednisolone 1000 mg/day for 3–5 days accelerates recovery [16-17]
- Stroke-related INO: Standard acute stroke management (thrombolytics if within window, antiplatelet therapy, anticoagulation as indicated) [12]
- Anti-GQ1b syndrome: IVIG or plasmapheresis [9]
- Contraindicated: Oral prednisone alone (without preceding IV methylprednisolone) was associated with increased relapse risk in the Optic Neuritis Treatment Trial and is generally avoided as monotherapy for acute MS relapses [16]
4. Diet
- No specific dietary triggers for INO
- In Wernicke encephalopathy (a mimic), thiamine deficiency from malnutrition or alcohol use is the cause → urgent thiamine replacement before glucose
- Adequate hydration and nutrition are important in the acute setting, particularly in stroke patients with dysphagia risk
5. Review of Systems
- Neurologic: Limb weakness, numbness/tingling, gait instability, vertigo, hearing changes, facial numbness, dysarthria, dysphagia
- Ophthalmologic: Visual acuity changes, eye pain, color desaturation (optic neuritis in MS)
- Urologic: Bladder dysfunction (MS)
- Constitutional: Fever, weight loss (infection, malignancy)
- Rheumatologic: Joint pain, rash, oral ulcers (SLE, vasculitis as rare causes) [4]
- Recent infections: Preceding GI or respiratory illness (anti-GQ1b syndrome) [9]
6. Collateral History and Family History
- Collateral: Witnesses to onset timing (critical for stroke window), baseline neurologic function, medication/substance use, recent trauma
- Family history: MS (modest familial risk), autoimmune diseases, stroke risk factors, neurodegenerative conditions (progressive supranuclear palsy) [18]
- Social context: Alcohol use (Wernicke encephalopathy), IV drug use (endocarditis with septic emboli), smoking and vascular risk factors
7. Risk Factors
- For MS-related INO: Young age (20–40), female sex, Northern European descent, higher latitude, vitamin D deficiency, family history of MS [8]
- For stroke-related INO: Older age, hypertension, diabetes, atrial fibrillation, hyperlipidemia, smoking, prior stroke/TIA [1][3]
- For traumatic INO: Head injury, particularly with brainstem contusion or tentorial herniation [4][10]
- For infectious INO: Immunosuppression, HIV, TB exposure [4]
- For vasculitic INO: SLE, Sjögren syndrome, temporal arteritis [4]
8. Differential Diagnosis
- Myasthenia gravis (pseudo-INO) — the most common mimic; fatigable weakness of medial rectus can simulate INO; distinguished by positive edrophonium test, fatigability, and acetylcholine receptor antibodies [4][10]
- CN III palsy — involves ptosis, pupil dilation (if compressive), and limitation of adduction, elevation, and depression; INO spares these other movements and convergence is typically preserved
- Miller Fisher syndrome / anti-GQ1b antibody syndrome — ophthalmoplegia + ataxia + areflexia; can include INO pattern [9]
- Wernicke encephalopathy — ophthalmoplegia + confusion + ataxia; thiamine deficiency
- Brainstem tumor or metastasis — progressive course, additional cranial nerve deficits [14]
- Progressive supranuclear palsy — chronic, progressive vertical gaze palsy with eventual horizontal involvement [18]
- Thyroid eye disease — restrictive myopathy mimicking adduction deficit; distinguished by proptosis, lid retraction, and forced duction testing
- Convergence spasm — voluntary or functional; pupils constrict during attempted lateral gaze
9. Past Medical History
- Prior episodes of optic neuritis, transverse myelitis, or other demyelinating events (MS)
- Prior stroke or TIA
- Autoimmune conditions (SLE, Sjögren, sarcoidosis)
- Malignancy (brainstem metastasis) [14]
- Head trauma history [10]
- Myasthenia gravis
- Vascular risk factors (HTN, DM, hyperlipidemia, atrial fibrillation)
10. Physical Exam
- Cardinal finding: Impaired/slowed adduction of the ipsilateral eye on horizontal saccade testing with abducting nystagmus of the contralateral eye [2][5][19]
- Convergence: Typically preserved in INO (distinguishes from CN III palsy); if impaired, suggests a more rostral lesion or WEBINO [2]
- Naming convention: INO is named for the side of the adduction deficit (e.g., left INO = impaired left eye adduction)
- Subtle INO: Only velocity of adduction is reduced, range may be full — best detected by having the patient make rapid horizontal saccades and comparing the speed of the adducting vs. abducting eye [2]
- WEBINO variant: Bilateral adduction deficits + primary gaze exotropia ("wall-eyed") + impaired convergence [12-13]
- One-and-a-half syndrome: INO + ipsilateral horizontal gaze palsy (lesion involving MLF + ipsilateral PPRF or abducens nucleus) [20]
- Additional brainstem signs: Assess for facial weakness, hearing loss, nystagmus patterns (gaze-evoked, upbeat, downbeat), limb ataxia, long tract signs [6][19]
- Pupillary exam: Should be normal in isolated INO; abnormalities suggest alternative or additional pathology [11]
11. Lab Studies
- Stroke workup (older patients): CBC, BMP, glucose, lipid panel, HbA1c, coagulation studies, ESR/CRP
- MS workup: CSF analysis for oligoclonal bands and IgG index; serum AQP4-IgG and MOG-IgG antibodies to differentiate from NMOSD [8][21]
- Infectious etiologies: RPR/VDRL, HIV, Lyme serologies, TB testing as clinically indicated [4]
- Autoimmune/vasculitic: ANA, anti-dsDNA, SSA/SSB, ESR, CRP, ANCA (if vasculitis suspected) [4]
- Anti-GQ1b antibodies: If Miller Fisher syndrome suspected (ophthalmoplegia + ataxia + areflexia) [9]
- Acetylcholine receptor antibodies: If myasthenia gravis (pseudo-INO) is considered [10]
- Thiamine level: If Wernicke encephalopathy is in the differential
12. Imaging
- First-line: MRI brain with contrast — the imaging modality of choice; focus on brainstem with thin-cut axial sequences through the pons and midbrain [3][22-23]
- In MS: T2/FLAIR hyperintense lesions in the MLF, often with additional periventricular, juxtacortical, and spinal cord lesions; gadolinium enhancement indicates active inflammation [8][24]
- In stroke: Restricted diffusion (DWI) in the paramedian pontine or midbrain tegmentum [24]
- Multiple MLF lesions favor MS; isolated mesencephalic lesion favors ischemia [24]
- MRI sensitivity: Approximately 75% for demyelinating INO and 67–90% for ischemic INO; a normal MRI does not exclude INO [22][25]
- CT head: Useful in the acute ED setting to rule out hemorrhage but insufficient for MLF lesion detection
- CT/MR angiography: If posterior circulation stroke is suspected
- When imaging is unnecessary: Chronic, stable, known MS patient with prior documented INO and no new symptoms
13. Special Tests
- Infrared oculography / video-oculography (VOG): Quantifies adduction slowing using the versional dysconjugacy index (VDI); more sensitive than clinical exam for subtle INO; prevalence of INO in MS by oculography is ~34% [25-26]
- Video head impulse test (vHIT): Decreased horizontal and posterior canal VOR gains are characteristic of MLF lesions; can aid diagnosis of subtle bilateral INO [5][7]
- Ice pack test / edrophonium test: If myasthenia gravis (pseudo-INO) is suspected
- Lumbar puncture: For CSF oligoclonal bands (MS), cell count and protein (infection, inflammation), cytology (leptomeningeal disease)
- Nerve conduction studies/EMG: If anti-GQ1b syndrome or GBS overlap is suspected [9]
14. ECG
- Not directly indicated for INO itself
- Obtain ECG in stroke-related INO to evaluate for atrial fibrillation or other cardioembolic sources
- Telemetry monitoring if posterior circulation stroke is confirmed or suspected
- Baseline ECG before initiating certain MS disease-modifying therapies (e.g., fingolimod requires cardiac monitoring due to risk of bradycardia and heart block) [8]
15. Assessment
INO is a clinical diagnosis based on the hallmark finding of impaired adduction with contralateral abducting nystagmus on horizontal saccade testing. It has exquisite localizing value to the MLF. [4][19]
- Etiologic breakdown (large case series of 410 inpatients): infarction 38%, MS 34%, unusual causes 28% (trauma, herniation, infection, tumor, iatrogenic, hemorrhage, vasculitis) [4]
- Unilateral INO is more common in stroke (87% unilateral) while bilateral INO strongly favors MS (73% bilateral) [1][4]
- Severity spectrum: Ranges from subtle velocity reduction (detectable only by oculography) to complete adduction paralysis [2][26]
- Complications: Persistent diplopia, oscillopsia during head movements, reduced vision-related quality of life [7][26]
- Resolution: ~49% resolve within 9 months; 51% may persist beyond 12 months [20]
16. Treatment Plan
Initial stabilization
- ABCs; assess for signs of brainstem compression or herniation
- If acute stroke suspected: activate stroke protocol, assess for thrombolytic eligibility
Etiology-specific treatment
- MS-related INO:
- Acute relapse: IV methylprednisolone 1000 mg/day for 3–5 days (accelerates recovery but may not change long-term outcome) [16-17]
- Refractory cases: IVIG or plasma exchange [27]
- Long-term: Initiate or optimize disease-modifying therapy (e.g., ocrelizumab, natalizumab, fingolimod) [8][28]
- Stroke-related INO:
- Acute ischemic stroke protocol: IV alteplase/tenecteplase if within window; antiplatelet therapy (aspirin ± clopidogrel); anticoagulation if cardioembolic source [12]
- Secondary prevention: Statin, antihypertensives, glycemic control
- Anti-GQ1b / Miller Fisher syndrome:
- Traumatic INO: Supportive care; neurosurgical consultation if associated hemorrhage or herniation [4][10]
- Symptomatic management of diplopia: Temporary prism glasses or monocular occlusion for comfort
17. Disposition
- Admit if:
- Acute stroke suspected or confirmed (stroke unit)
- New-onset INO with unknown etiology requiring urgent MRI and workup
- Associated brainstem signs suggesting large or expanding lesion
- Altered mental status or signs of herniation [11]
- Severe MS relapse requiring IV steroids
- Observation if:
- Discharge if:
- Known MS patient with chronic, stable INO and no new deficits
- Completed workup with benign etiology and appropriate follow-up arranged
- Specialist consultation triggers:
- Neurology (all new-onset INO)
- Neuro-ophthalmology (persistent diplopia, diagnostic uncertainty)
- Neurosurgery (mass lesion, hemorrhage, herniation)
- Stroke team (if within thrombolytic window)
18. Follow Up / Return Precautions
- Follow-up timing: Neurology follow-up within 1–2 weeks for new-onset INO; sooner if MS diagnosis is being established
- Return immediately for: Worsening diplopia, new weakness or numbness, difficulty speaking or swallowing, severe headache, altered consciousness, vision loss, gait instability
- Patient counseling:
- INO from MS often improves over weeks to months with treatment; ~49% resolve within 9 months [20]
- INO from stroke: recovery depends on infarct size; small pontine infarcts often have favorable prognosis
- Driving restrictions until diplopia resolves
- Prism glasses or patching can provide symptomatic relief during recovery
- Expected recovery: Demyelinating INO often responds to IV steroids within days to weeks; ischemic INO recovery is variable [5][16]
References
1. Internuclear and Supranuclear Disorders of Eye Movements: Clinical Features and Causes. — Karatas M. European Journal of Neurology. 2009.
2. The Neuro-Ophthalmology of Multiple Sclerosis. — Frohman EM, Frohman TC, Zee DS, McColl R, Galetta S. The Lancet. Neurology. 2005.
3. ACR Appropriateness Criteria Orbits Vision and Visual Loss. — Kennedy TA, Corey AS, Policeni B, et al. Journal of the American College of Radiology : JACR. 2018.
4. Internuclear Ophthalmoplegia: Unusual Causes in 114 of 410 Patients. — Keane JR. Archives of Neurology. 2005.
5. Mild Bilateral Internuclear Ophthalmoplegia: The Diagnostic Role of the Vertical Posterior Canal Vestibulo-Ocular Reflex in Acute Brainstem Demyelination, a Clinical-Radiologic Correlation. — Manrique LG, Zhang X, Kathryn L, Marie C, Kattah JC. Journal of Neuro-Ophthalmology : The Official Journal of the North American Neuro-Ophthalmology Society. 2022.
6. Internuclear Ophthalmoplegia Plus Ataxia Indicates a Dorsomedial Tegmental Lesion at the Pontomesencephalic Junction. — Lee SU, Kim HJ, Park JJ, Kim JS. Journal of Neurology. 2016.
7. Vestibulo-Ocular Reflex Deficits With Medial Longitudinal Fasciculus Lesions. — Aw ST, Chen L, Todd MJ, Barnett MH, Halmagyi GM. Journal of Neurology. 2017.
8. Diagnosis and Treatment of Multiple Sclerosis: A Review. — McGinley MP, Goldschmidt CH, Rae-Grant AD. The Journal of the American Medical Association. 2021.
9. Expanding Clinical Spectrum of Anti-GQ1b Antibody Syndrome: A Review. — Lee SU, Kim HJ, Choi JY, Choi KD, Kim JS. JAMA Neurology. 2024.
10. Internuclear Ophthalmoplegia Following Head Injury. Case Report. — Baker RS. Journal of Neurosurgery. 1979.
11. Pearls & Oy-Sters: INO Plus From Downward Herniation-a Cautionary Tale Regarding Neuro-Ophthalmologic Signatures of Brainstem Compression. — Hartig J, Nickl V, Vollmuth C, et al. Neurology. 2024.
12. Case Report: A Variant of Wall-Eyed Bilateral Internuclear Ophthalmoplegia From Unilateral Pons Infarction. — Wang T, Cao D, Han J. Frontiers in Neuroscience. 2022.
13. Wall-Eyed Bilateral Internuclear Ophthalmoplegia (WEBINO) in a Patient With Idiopathic Intracranial Hypertension. — Keereman V, Platteau E, Crevits L, Algoed L. The Neurologist. 2018.
14. Unilateral Internuclear Ophthalmoplegia as an Isolated Presentation of Metastatic Melanoma. — Kumar S, Kent SS, Sundaram AN, Sharma M. Journal of Neuro-Ophthalmology : The Official Journal of the North American Neuro-Ophthalmology Society. 2015.
15. Multidisciplinary Collaborative Consensus Guidance Statement on the Assessment and Treatment of Neurologic Sequelae in Patients With Post-Acute Sequelae of SARS-CoV-2 Infection (PASC). — Melamed E, Rydberg L, Ambrose AF, et al. PM & R : The Journal of Injury, Function, and Rehabilitation. 2023.
16. Management of Multiple Sclerosis. — Rudick RA, Cohen JA, Weinstock-Guttman B, Kinkel RP, Ransohoff RM. The New England Journal of Medicine. 1997.
17. Clinically Isolated Syndromes Suggestive of Multiple Sclerosis, Part 2: Non-Conventional MRI, Recovery Processes, and Management. — Miller D, Barkhof F, Montalban X, Thompson A, Filippi M. The Lancet. Neurology. 2005.
18. Progressive Supranuclear Palsy With Walleyed Bilateral Internuclear Ophthalmoplegia Syndrome. — Matsumoto H, Ohminami S, Goto J, Tsuji S. Archives of Neurology. 2008.
19. Central Ocular Motor Disorders, Including Gaze Palsy and Nystagmus. — Strupp M, Kremmyda O, Adamczyk C, et al. Journal of Neurology. 2014.
20. Internuclear Ophthalmoplegia: Causes and Long-Term Follow-Up in 65 Patients. — Bolaños I, Lozano D, Cantú C. Acta Neurologica Scandinavica. 2004.
21. Internuclear Ophthalmoplegia Characterizes Multiple Sclerosis Rather Than Neuromyelitis Optica Spectrum Disease. — Hamza MM, Alas BF, Huang C, et al. Journal of Neuro-Ophthalmology : The Official Journal of the North American Neuro-Ophthalmology Society. 2022.
22. Sensitivity of Magnetic Resonance Imaging of the Medial Longitudinal Fasciculus in Internuclear Ophthalmoplegia. — Tien CW, Donaldson L, Parra-Farinas C, Micieli JA, Margolin E. Journal of Neuro-Ophthalmology : The Official Journal of the North American Neuro-Ophthalmology Society. 2024.
23. Management of Acute Cranial Nerve 3, 4 and 6 Palsies: Role of Neuroimaging. — Tamhankar MA, Volpe NJ. Current Opinion in Ophthalmology. 2015.
24. MRI Topography of Lesions Related to Internuclear Ophthalmoplegia in Patients With Multiple Sclerosis or Ischemic Stroke. — Kleinsorge MT, Ebert A, Förster A, et al. Journal of Neuroimaging : Official Journal of the American Society of Neuroimaging. 2021.
25. Lesion Follows Function: Video-Oculography Compared With MRI to Diagnose Internuclear Ophthalmoplegia in Patients With Multiple Sclerosis. — Omary R, Bockisch CJ, De Vere-Tyndall A, et al. Journal of Neurology. 2023.
26. Diagnosing and Quantifying a Common Deficit in Multiple Sclerosis: Internuclear Ophthalmoplegia. — Nij Bijvank JA, van Rijn LJ, Balk LJ, et al. Neurology. 2019.
27. Paediatric Multiple Sclerosis and Antibody-Associated Demyelination: Clinical, Imaging, and Biological Considerations for Diagnosis and Care. — Fadda G, Armangue T, Hacohen Y, Chitnis T, Banwell B. The Lancet. Neurology. 2021.
28. FDA Orange Book. — FDA Orange Book. 2026.