The young woman with double vision
A 28-year-old woman reports horizontal double vision that is worse when she looks to the right or to the left. She also has mild unsteadiness when walking. Symptoms began 5 days ago. Two years ago she had a brief episode of left-eye visual blurring with pain on eye movement that resolved after 3 weeks; she never had it evaluated.
A 28-year-old woman reports horizontal double vision that is worse when she looks to the right or to the left. She also has mild unsteadiness when walking. Symptoms began 5 days ago. Two years ago she had a brief episode of left-eye visual blurring with pain on eye movement that resolved after 3 weeks; she never had it evaluated.
On rightward gaze: the LEFT eye adducts slowly and incompletely; the RIGHT eye abducts fully but shows a coarse horizontal nystagmus. On leftward gaze: the RIGHT eye adducts slowly and incompletely; the LEFT eye abducts and shows nystagmus. Convergence is preserved. Visual acuity 20/40 left, 20/20 right; left RAPD present.
"On leftward gaze: adduction of the RIGHT eye is slowed/incomplete, with abducting (dissociated) nystagmus of the LEFT eye."
What is the localizing diagnosis?
Best next investigation?
Bilateral internuclear ophthalmoplegia — clinically isolated syndrome / probable MS
- 1.Rule of thumb: bilateral INO in a young patient (<40) = MS until proven otherwise. Unilateral INO in an older patient = brainstem stroke until proven otherwise.
- 2.INO is named for the side of the LESION = the side of the SLOW ADducting eye.
- 3.The dissociated abducting nystagmus is thought to be a central adaptive overshoot phenomenon (Hering's law of equal innervation drives both eyes, but only the unaffected abducting eye can respond).
- 4.Past episode of painful monocular vision loss with eye-movement pain = classic optic neuritis. Combined with a new CNS event ≥30 days later and disseminated in space (MLF), this meets McDonald 2017 criteria for MS once MRI confirms typical lesions.