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Double vision on lateral gaze

A 27-year-old woman with diplopia, mild vertigo, and a curious eye-movement finding.

The case

A 27-year-old PhD student presents with horizontal binocular diplopia that has been present for 5 days and is worse when looking to either side. She has also noticed mild dizziness and a feeling of unsteadiness when walking. There is no headache. She had an episode of right-sided eye pain with reduced colour vision 18 months ago that resolved spontaneously over several weeks; she was told at the time it was 'something with the optic nerve' but did not pursue investigation.

On examination, on left gaze, the right eye fails to adduct beyond the midline and the left eye shows abducting nystagmus. On right gaze, the left eye fails to adduct beyond the midline and the right eye shows abducting nystagmus. When she fixates on a target moved close to her nose, both eyes adduct symmetrically and normally.

In primary gaze, her eyes appear slightly divergent. There is no ptosis, no pupillary abnormality, and no other cranial-nerve deficit. Cerebellar testing reveals mild dysmetria on the right. Reflexes are brisk throughout, with upgoing plantars bilaterally.

Question

What is the most likely underlying diagnosis?

Teaching point

Bilateral internuclear ophthalmoplegia in a young patient — particularly with exotropia in primary gaze (WEBINO) — is almost pathognomonic for multiple sclerosis. The diagnostic fingerprint of INO is the preservation of convergence: the medial rectus muscle and its third-nerve innervation are intact, but the MLF that yokes them to the contralateral abducens nucleus during conjugate horizontal gaze is demyelinated. A history of unexplained transient visual loss 18 months ago is a probable previous attack of optic neuritis — together with the current attack, this provides dissemination in time and space. The 2024 McDonald criteria revisions formally recognise the optic nerve as a fifth topographic site for dissemination in space and allow time-criterion waivers when sufficient imaging biomarkers are present.

References

  1. [1]Frohman EM, Frohman TC, Zee DS, McColl R, Galetta S. The neuro-ophthalmology of multiple sclerosis. Lancet Neurology 2005;4(2):111–121. doi:10.1016/S1474-4422(05)00992-0
  2. [2]Montalban X, Lebrun-Frénay C, Oh J, Arrambide G, Ayrignac X, Banwell B, Barkhof F, Bourdette D, Bermel R, Carroll WM, et al.. Diagnosis of multiple sclerosis: 2024 revisions of the McDonald criteria. Lancet Neurology 2025;24(10):851–870. doi:10.1016/S1474-4422(25)00270-4
  3. [3]Serra A, Chisari CG, Matta M. Eye movement abnormalities in multiple sclerosis: pathogenesis, modeling, and treatment. Frontiers in Neurology 2018;9:31. doi:10.3389/fneur.2018.00031
Related moduleMultiple Sclerosis →