Internuclear Ophthalmoplegia

A lesion of the medial longitudinal fasciculus (MLF) in the pons or midbrain interrupts the signal from the abducens nucleus to the contralateral medial rectus subnucleus during horizontal gaze.

• The result is impaired adduction on the side of the lesion with abducting nystagmus in the opposite eye.
• Bilateral INO in a young adult is multiple sclerosis until proven otherwise. Unilateral INO in an older patient is usually brainstem infarction.
• Other causes: brainstem haemorrhage, tumour, Wernicke encephalopathy, Chiari malformation, drug toxicity (opioids, barbiturates, tricyclics, phenytoin), trauma, infection, and myasthenia gravis (pseudo-INO).

• Slowed or absent adduction of the ipsilateral eye on lateral gaze; convergence is preserved in pontine lesions and lost in midbrain lesions.
• Abducting (dissociated) nystagmus in the contralateral eye.
• Mild exotropia in primary position in bilateral cases (WEBINO: wall eyed bilateral INO).
• Skew deviation and vertical gaze evoked nystagmus; impaired vertical pursuit.
• One-and-a-half syndrome: a pontine lesion involving the MLF and paramedian pontine reticular formation or sixth nucleus produces ipsilateral horizontal gaze palsy and INO, leaving only abduction of the contralateral eye.
• Symptoms: oscillopsia, horizontal diplopia, blurred vision during head turns; some are asymptomatic.

• Bedside: horizontal saccades (slow adducting saccade, a more sensitive sign than limitation), convergence, vertical eye movements, head impulse test, pupils.
• Exclude myasthenia (ice pack test, fatigability, acetylcholine receptor antibodies, single fibre EMG).
• MRI brain with thin cuts through the brainstem, FLAIR, DWI and contrast: demyelinating plaque, infarct, haemorrhage or mass.
• If demyelination suspected: MRI spine, CSF oligoclonal bands, visual evoked potentials.
• Stroke workup: MR or CT angiography, ECG, cardiac monitoring, lipids, glucose.

• Multiple sclerosis relapse: methylprednisolone 1 g IV daily for 3 to 5 days (or oral 1250 mg daily); disease modifying therapy for long term control, selected by the neurology team.
• Acute ischaemic stroke: alteplase 0.9 mg/kg within 4.5 hours when eligible, aspirin 300 mg after imaging, then antiplatelet and statin secondary prevention, blood pressure and glucose control.
• Wernicke: thiamine 500 mg IV 3 times daily for 3 days then 250 mg daily for 5 days.
• Drug induced: withdraw the offending agent.
• Symptomatic: occlusion or Fresnel prism for diplopia; botulinum toxin to the lateral rectus (2.5 to 5 units) or surgery for persistent exotropia in primary position; gabapentin 300 mg increasing to 900 to 1800 mg daily or memantine 10 to 20 mg for troublesome nystagmus and oscillopsia.
• Prognosis: INO from demyelination often recovers within weeks to months; stroke related INO recovers over months.