Vignette

A 26-year-old woman with a past episode of right optic neuritis presents with subacute vertigo and diplopia over 3 days. On lateral gaze to the left, the right eye fails to fully adduct and the left abducting eye shows dissociated nystagmus. Vertical gaze-evoked nystagmus is present in upgaze.

Diagnostic question

What is the most likely localisation and diagnosis?

Teaching point

Internuclear ophthalmoplegia, vertical nystagmus, and skew in a young adult with a prior demyelinating episode point to brainstem MS. HINTS findings are central, but the diagnostic framework is the McDonald criteria, not HINTS.

References: ¹²