BASICS

DESCRIPTION

• Internuclear ophthalmoplegia (INO) is an ocular motility disturbance manifest by intact abduction of ipsilateral eye but impaired adduction of contralateral eye during ipsilateral horizontal gaze attempts. It is always named after the impaired adducting eye (i.e., a right INO is only observed with gaze to left). It is due to interruption of the pathway [medial longitudinal fasciculus (MLF)] from the ipsilateral abducens nucleus to the contralateral oculomotor nucleus. The abducting eye usually develops coarse end-gaze horizontal nystagmus. Convergence is generally preserved.

– Partial INO, manifest by slowed saccadic velocity in the adducting eye, is much more common than full INO.

– If the lesion encompasses both MLFs and the eyes are exotropic in primary position, the condition is termed wall-eyed bilateral INO (WEBINO). Convergence is usually lost with WEBINO.

– If the lesion encompasses both the MLF and the ipsilateral abducens nucleus/PPRF complex, ipsilateral gaze palsy and contralateral INO occurs; the only normal movement is abduction of contralateral eye. This syndrome is called one-and-a-half syndrome (3 of the 4 horizontal half-movements, 2 in each eye, are affected).

EPIDEMIOLOGY

Incidence

No data available on incidence

Prevalence

INO prevalence in multiple sclerosis (MS) patients is common; 30% of fifty patients in a VA clinic were found to have INO. (1) (C). Exact prevalence is unknown.

RISK FACTORS

Strokes have been associated with 38%, MS with 34%, and unusual causes with 28% of INO cases in a retrospective study in Los Angeles (2)

Genetics

No genetic pattern

GENERAL PREVENTION

Prevention consists of medical control of underlying condition (e.g., stroke risk, MS).

PATHOPHYSIOLOGY

Demyelinating or vascular occlusion affecting the MLF in its course between the abducens and contralateral oculomotor nucleus

ETIOLOGY

• The most common etiology of INO is MS, which often results in bilateral INOs. Asymmetric involvement is the rule.

– In the elderly, small pontine or midbrain strokes often lead to unilateral INO.

– Medications such as anticonvulsants, opioids, etc.

COMMONLY ASSOCIATED CONDITIONS

• Multiple sclerosis

• Optic neuritis

• Small-vessel (lacunar) strokes

• Brainstem hemorrhage

• Brainstem tumors

• Trauma

• Overdose of sedative medications

DIAGNOSIS

HISTORY

• Most patients with partial INO are asymptomatic, although they may complain of transient horizontal diplopia during rapid eye movements.

– As skew deviation may accompany INO, patients may also complain of vertical or oblique diplopia.

PHYSICAL EXAM

• Attempt to gaze in contralateral direction of INO results in normal abduction, but impaired adduction. This latter can range from inability to move eye past midline to slower velocity of the adduction saccade. The abducting eye often has coarse horizontal nystagmus.

• In one-and-a-half syndrome, contralateral gaze is impaired; attempts at ipsilateral gaze result in abduction of ipsilateral eye only.

• In WEBINO, INO is seen with attempts to look in either horizontal direction. Exotropia is usually present, and convergence is usually impaired.

• The presence of ptosis, fatiguable weakness, or other extraocular muscle weakness points to a different etiology. The most common confounder of INO is pseudo-INO, which is usually associated with myasthenia gravis.

DIAGNOSTIC TESTS & INTERPRETATION

Lab

No specific testing is available

Imaging

Initial approach

MRI of the brain is the preferred imaging method. CT scan of the brain does not have sufficient resolution to identify the lesion. The usual finding is a small white matter lesion, best seen on FLAIR imaging. For acute demyelinating lesions in patients with MS, the lesion may enhance with gadolinium contrast. Acute lacunar strokes may show up on diffusion-weighted MR imaging. Often the brainstem is normal on imaging.

Follow-up & special considerations

Repeat imaging for stable or improving INO is not necessary. Infrared oculography can be used to confirm the presence of INO, although the diagnosis is usually made by clinical examination alone.

Diagnostic Procedures/Other

INO commonly occurs as a consequence of a small lacunar infarct or an inflammatory demyelinating lesion. Diagnostic procedures are aimed at finding the causes of stroke or to document the presence of MS or other demyelinating CNS disease.

DIFFERENTIAL DIAGNOSIS

Pseudo-INO due to myasthenia can occur.

TREATMENT

ADDITIONAL TREATMENT

General Measures

Not applicable

Issues for Referral

Selected patients may need to be referred to a neurologist specializing in MS or stroke management.

ONGOING CARE

FOLLOW-UP RECOMMENDATIONS

Patients should be re-examined in 3–6 months.

DIET

If the cause of the INO is determined to be due to a lacunar stroke, a diet resulting in lowering of vascular disease risk is indicated.

PATIENT EDUCATION

Appropriate to the etiology, usually MS or stroke

PROGNOSIS

INO prognosis is dependent on the etiology; those due to MS, trauma, tumors, strokes, or hemorrhages tend to persist, whereas those due to infections (e.g., brainstem encephalitis) or drug overdoses improve or resolve with treatment.

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