OcuLearning › Neuro

Internuclear Ophthalmoplegia (INO)

URGENT — Neuro

Symptoms

horizontal binocular diplopia (worse on lateral gaze)

blurry vision or vague visual complaints

oscillopsia (world moving) due to abducting nystagmus

may be asymptomatic and detected only on examination

Clinical Findings

Hallmark exam

ipsilateral adduction deficit: the eye on the side of the mlf lesion cannot move inward
contralateral abducting nystagmus: the other eye shakes when looking away from the lesion
convergence is preserved: adduction works at near (uses a different pathway)
adduction lag: the weak eye moves slower than the abducting eye during saccades

Neuro-localization

lesion of the medial longitudinal fasciculus (mlf)
ipsilateral to the eye with the adduction deficit
one-and-a-half syndrome: ino plus ipsilateral horizontal gaze palsy

Associated findings

skew deviation: vertical misalignment of the eyes
upbeat nystagmus on upgaze (especially in bilateral cases)
webino: wall-eyed bilateral ino (exotropia in primary position)

Etiology

Multiple sclerosis (MS)

most common cause in young patients; frequently bilateral
bilateral ino in a young adult is ms until proven otherwise

Brainstem stroke

most common cause in elderly patients; usually unilateral
lacunar infarct of the paramedian pontine or midbrain perforators

Other causes

brainstem tumors, aums, or trauma
infectious/inflammatory: sarcoidosis, behçet, cryptococcosis, or syphilis

Red Flags

bilateral ino in a young patient (multiple sclerosis)

unilateral ino in an older patient with vascular risks (stroke)

one-and-a-half syndrome (indicates a larger pontine lesion)

ino with other brainstem signs (ataxia, facial weakness, altered consciousness)

Investigations

Clinical diagnostics

horizontal saccade testing: look for slow adduction (most sensitive)
near-point convergence: confirms adduction works for non-lateral gaze
optokinetic drum: unmasks subtle adduction lag
exclude myasthenia gravis ("pseudo-ino"): check lid twitch and fatigability

Imaging (mandatory)

mri brain with gadolinium: look for t2/flair signal in the paramedian brainstem
dwi sequence: to identify acute ischemic infarcts

Systemic workup

young patients: mri spine, lp for oligoclonal bands, and visual evoked potentials
older patients: mra/cta head and neck, echocardiogram, and lipid/a1c labs

Treatment

Primary

treat the underlying etiology (ms therapies, stroke prevention, or tumor resection)

Symptomatic

prism glasses or monocular occlusion for persistent diplopia
strabismus surgery: reserved for stable, chronic, non-resolving cases

Prognosis

many lesions resolve over weeks to months as inflammation or ischemia settles

Clinical Pearl

INO in a young adult is MS; in an older adult, it's a lacunar stroke — the adducting eye fails, the abducting eye has nystagmus, and the MLF is ipsilateral to the weak adduction.

Differential Diagnosis

myasthenia gravis (pseudo-ino)
third nerve palsy (medial rectus weakness + ptosis/pupil signs)
thyroid eye disease (ted) or orbital myositis
partial medial rectus palsy