Makindo Medical Notes

🧠 Internuclear Ophthalmoplegia (INO) is a brainstem disorder characterised by impaired horizontal eye movement due to a lesion in the medial longitudinal fasciculus (MLF). 👁️ The key finding: failure of adduction of one eye with nystagmus of the abducting eye when looking laterally.

📖 About

• The medial longitudinal fasciculus (MLF) is a paired white matter tract that coordinates horizontal gaze by linking the abducens nucleus (cranial nerve VI) in the pons to the oculomotor nucleus (cranial nerve III) in the midbrain.
• This interconnection ensures synchronous movement of both eyes — essential for binocular vision and depth perception.
• Disruption of the MLF causes failure of one eye to adduct while the other abducts normally with nystagmus.

🧩 Aetiology & Pathophysiology

• Normal coordination: When looking left, the left abducens nucleus activates the left lateral rectus and simultaneously sends a signal via the right MLF to the right oculomotor nucleus (to contract the right medial rectus).
• Lesion: Damage to the right MLF prevents transmission of this signal → right eye fails to adduct on left gaze.
• Because the abducting eye receives unopposed input, it overshoots and develops nystagmus.
• In bilateral lesions (often in MS), both MLF tracts are affected, producing bilateral INO — patients often complain of oscillopsia (visual “shaking”).

🧠 Diagram

🩺 Causes

• Multiple Sclerosis (MS): The most common cause in younger adults. Demyelination interrupts conduction within the MLF.
• Brainstem Stroke: Common cause in older adults; usually unilateral and due to infarction of pontine perforators.
• Brainstem Tumours: E.g. glioma, metastasis — compress or infiltrate the MLF.
• Trauma: Brainstem contusion or diffuse axonal injury.
• Inflammatory/Infective: Encephalitis, sarcoidosis, or lupus cerebritis.

🧭 Clinical Features

• Primary gaze: Eyes usually aligned.
• On attempted lateral gaze:
  • Abducting eye moves fully but shows nystagmus.
  • Adducting eye fails or lags (impaired medial rectus function).
• Lesion localisation: Defective adduction in the left eye = left MLF lesion.
• Bilateral INO: Typically due to MS — both eyes show impaired adduction and bilateral nystagmus.
• “One-and-a-half syndrome”: INO + conjugate gaze palsy to one side due to larger pontine lesion involving both the MLF and ipsilateral PPRF (paramedian pontine reticular formation).
• Convergence: Usually preserved (the medial rectus pathway for convergence bypasses the MLF).

⚖️ Differentials

• Myasthenia Gravis: Can mimic INO (“pseudo-INO”) due to fatigable medial rectus weakness — distinguish by normal convergence and variable findings.
• Third Nerve Palsy: Produces adduction failure but also ptosis and pupil involvement.
• Pontine Lesion: May produce associated contralateral hemiparesis or facial weakness.

🔍 Investigations

• MRI Brainstem: Preferred imaging modality to detect demyelination, infarct, or mass lesion affecting the MLF.
• Consider CSF analysis if demyelination or infection suspected (e.g. oligoclonal bands in MS).
• Autoimmune and vasculitic screen if cause unclear.

🩹 Management

• Treat the underlying cause:
  • MS: Corticosteroids for relapse; disease-modifying therapy long term.
  • Stroke: Antiplatelet or anticoagulant therapy as indicated.
  • Infection/inflammation: Appropriate antimicrobial or immunosuppressive therapy.
• Diplopia can be managed with prism glasses or occlusion therapy in selected patients.
• Physiotherapy and neuro-ophthalmology review for persistent visual dysfunction.

📖 Educational Summary

Internuclear ophthalmoplegia is a classic localising sign of brainstem pathology. The MLF acts as the “communication cable” between cranial nerves VI and III — when cut, the eyes lose synchrony. A simple rule for students: the eye that won’t adduct is on the same side as the lesion. INO also exemplifies how demyelination impairs conduction rather than destroying neurons — explaining why recovery may be good in MS but limited after stroke. Always differentiate from myasthenia and third nerve palsy, and link the lesion to its neuroanatomical substrate.

📚 References

• BNF: Neurology & MS Management
• BMJ Best Practice: Internuclear Ophthalmoplegia (2023)
• Clinical Neuroanatomy, Snell (9th ed.) – Brainstem Pathways
• Adams & Victor’s Principles of Neurology, 12th ed.