How do you diagnose internuclear ophthalmoplegia?

Diagnosing Internuclear Ophthalmoplegia (INO)

The definitive test for internuclear ophthalmoplegia is demonstrating impaired adduction of one eye during horizontal gaze with nystagmus in the abducting contralateral eye, followed by MRI imaging of the brain stem to visualize the lesion in the medial longitudinal fasciculus.

Clinical Examination

Key Diagnostic Features

• Horizontal Gaze Testing: The hallmark finding is:
  • Impaired or slow adduction of one eye during lateral gaze
  • Monocular nystagmus in the abducting eye
  • May be unilateral or bilateral 1

Additional Examination Elements

1. Pupil examination:

   • Check for reactivity and relative afferent pupillary defect 2
   • INO itself does not typically cause pupillary abnormalities

2. Saccadic eye movement testing:

   • Quantitative saccade testing shows markedly reduced velocity of adducting saccades
   • Abducting saccades may have normal velocity but wider distribution 3

3. Vestibulo-ocular reflex testing:

   • Video head impulse test (vHIT) may show decreased horizontal and posterior canal gains
   • Positive posterior canal head impulse test can be a localizing finding in bilateral INO 4

4. Nystagmus evaluation:

   • Document presence of abducting nystagmus
   • Check for vertical nystagmus (both upbeat and downbeat can occur) 3
   • Assess for skew deviation (eye usually higher on the side of the lesion) 5

Imaging Studies

MRI Brain with Focus on Brainstem

• MRI is the gold standard for confirming INO diagnosis 2, 6
• Protocol should include:
  • T2-weighted images to identify high signal intensity in the medial longitudinal fasciculus
  • Small field-of-view high-resolution T2-weighted images of the brainstem 2
  • Contrast-enhanced sequences to identify active demyelination 6

Additional Imaging Considerations

• In younger patients, focus on demyelinating plaques (multiple sclerosis)
• In older patients, focus on ischemic lesions (stroke) 2
• Consider MRA or CTA if vascular etiology is suspected 2

Differential Diagnosis Considerations

Common Causes of INO

1. Multiple sclerosis (most common in young adults)
2. Brainstem infarction (most common in older adults)
3. Other causes: head trauma, brainstem tumors, infections, hydrocephalus, and systemic lupus erythematosus 1

Diagnostic Pitfalls

• Subtle INO may be difficult to diagnose without quantitative testing
• Absence of abducting nystagmus does not rule out INO, especially in subtle cases 4
• CT imaging is inadequate for diagnosing INO 6

Diagnostic Algorithm

1. Perform detailed eye movement examination focusing on horizontal gaze
2. Document adduction weakness and abducting nystagmus
3. Perform quantitative saccade testing if available
4. Order MRI brain with special attention to brainstem and medial longitudinal fasciculus
5. Consider additional testing based on suspected etiology (e.g., lumbar puncture for MS)

By following this systematic approach to testing for internuclear ophthalmoplegia, clinicians can accurately diagnose this condition and identify the underlying pathology affecting the medial longitudinal fasciculus.