Internuclear ophthalmoplegia vs Sixth nerve palsy
How to tell Internuclear ophthalmoplegia and Sixth nerve palsy apart when a board question makes them look alike.
INO is adduction failure with preserved convergence; sixth nerve palsy is abduction failure.
Side by side
Internuclear ophthalmoplegia causes impaired adduction of the affected eye during horizontal gaze with abducting nystagmus of the other eye and preserved convergence from a medial longitudinal fasciculus lesion.
Sixth nerve palsy causes impaired abduction of the affected eye, horizontal diplopia worse looking toward the lesion, and an eye that may rest medially.
MLF lesion disconnects the abducens nucleus from the contralateral oculomotor medial rectus subnucleus during conjugate gaze.
Abducens nerve or nucleus dysfunction weakens the lateral rectus muscle needed for abduction.
Adduction failure with preserved convergence and contralateral abducting nystagmus supports INO, often from MS in young adults or stroke in older adults.
Isolated abduction deficit of one eye supports sixth nerve palsy, often from microvascular ischemia or raised intracranial pressure.
The trap
Both cause horizontal diplopia, but they fail in opposite directions. INO is failure of ADDUCTION on the side of the MLF lesion with contralateral abducting nystagmus, and convergence is preserved because convergence does not need the MLF; CN VI palsy is failure of ABDUCTION of the affected eye, and convergence is normal because both eyes can still adduct. INO in a young patient is MS until proven otherwise (bilateral INO is essentially pathognomonic for MS); INO in an older patient is more often pontine stroke. CN VI has the longest intracranial course, so it is the first to fail with raised intracranial pressure (false localizing sign).
The repair move
For horizontal diplopia, identify which eye cannot move and whether convergence is preserved.
Try a free question
CoreA patient looking left has failure of the right eye to adduct and abducting nystagmus of the left eye. Convergence is preserved. Which diagnosis is most likely?
Show answer and explanation
Answer: A. Internuclear ophthalmoplegia. Adduction failure with contralateral abducting nystagmus and preserved convergence supports INO from an MLF lesion.
Why the other choices are tempting
- B. Sixth nerve palsy causes abduction failure of the affected eye.
- C. Third nerve palsy causes ptosis and impaired adduction/elevation/depression, often with pupil findings.
- D. Horner syndrome causes ptosis, miosis, and anhidrosis.
- E. Optic neuritis causes painful vision loss, not this gaze pattern.
Flashcards
Tap a card to flip it.
What lesion causes INO?
A medial longitudinal fasciculus lesion causes internuclear ophthalmoplegia.
What eye movement pattern points to INO?
Adduction failure with contralateral abducting nystagmus points to INO.
Quick answers
What is the fastest way to tell Internuclear ophthalmoplegia from Sixth nerve palsy?
INO is adduction failure with preserved convergence; sixth nerve palsy is abduction failure.
What is the key clue for Internuclear ophthalmoplegia?
Internuclear ophthalmoplegia causes impaired adduction of the affected eye during horizontal gaze with abducting nystagmus of the other eye and preserved convergence from a medial longitudinal fasciculus lesion.
What is the key clue for Sixth nerve palsy?
Sixth nerve palsy causes impaired abduction of the affected eye, horizontal diplopia worse looking toward the lesion, and an eye that may rest medially.
What is the common trap with Internuclear ophthalmoplegia vs Sixth nerve palsy?
Both cause horizontal diplopia, but they fail in opposite directions. INO is failure of ADDUCTION on the side of the MLF lesion with contralateral abducting nystagmus, and convergence is preserved because convergence does not need the MLF; CN VI palsy is failure of ABDUCTION of the affected eye, and convergence is normal because both eyes can still adduct. INO in a young patient is MS until proven otherwise (bilateral INO is essentially pathognomonic for MS); INO in an older patient is more often pontine stroke. CN VI has the longest intracranial course, so it is the first to fail with raised intracranial pressure (false localizing sign).