Internuclear Ophthalmoplegia (INO)

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  • Lesion of the medial longitudinal fasciculus (MLF), connecting the nucleus of cranial nerve VI (pons) and the nucleus of cranial nerve III (in the midbrain)

Clinical Findings

  • Unilateral
    • Limitation of adduction ipsilateral with nystagmus in the abducting contralateral eye; no strabismus in primary position; intact convergence if lesion is only in the midbrain
  • Bilateral
    • Bilateral limited adduction, nystagmus in the abducting eye; bilateral exotropia with convergence deficit (WEBINO = “wall-eyed” bilateral internuclear ophthalmoplegia).
  • One and a Half (1 ½) Syndrome
    • Lesion of the MLF + paramedian pontine reticular formation (PPRF)/nucleus of cranial nerve VI
    • Ipsilateral horizontal gaze palsy + limitation of adduction contralateral; convergence still possible if lesion is limited to the Pons


  • Unilateral: Demyelination (in younger patients), ischaemia (older patients), tumors, etc.
  • Bilateral: Demyelination most common, ischaemia, tumors, etc.


  • MRI (demyelinating lesions, tumors, stroke)


  • Treatment according to the cause; often good prognosis
  • For bilateral exotropia in primary position (WEBINO), consider patching, prism glasses, strabismus surgery


  • EyeWiki Internuclear Ophthalmoplegia
  • The Wills Eye Manual: Office and Emergency Room Diagnosis and Treatment of Eye Disease; Kalla Gervasio MD, Travis Peck MD et al; Lippincott Williams&Wilkins; 8th Edition (2021)
  • Kanski’s Clinical Ophthalmology: A Systematic Approach; John E Salmon MD; Elsevier; 9th Edition (2019)
  • Review of Ophthalmology: Neil J. Friedman; Peter K. Kaiser; William B. Trattler; Elsevier, 3rd Edition (2018)