Optic neuritis is one of the most common forms of presentation in multiple sclerosis: in 25% of new MS cases optic neuritis is the initial clinical event. Moreover, studies show that 50-70% of all patients with MS will experience optic neuritis at some stage of the condition. Optic neuritis presents with acute unilateral vision loss^{1 - 3}. Visual field defects are present in almost all patients with optic neuritis (97.5%)¹. Relative afferent pupillary defects (RAPD) occur in 96% of acute unilateral optic neuritis cases⁴. 

The most frequently observed eye movement disorders in MS are saccadic dysmetria (91%), INO (68%), vestibulo-ocular reflex abnormalities and gaze-evoked nystagmus (36%), fixation instability, and impaired smooth pursuit ^1,5 . Up to 71% of INOs are not detected in routine clinical assessment⁶.

1 Frohman EM, Frohman TC, Zee DS, McColl R, Galetta S. The neuro-ophthalmology of multiple sclerosis. Lancet Neurol. 2005;4(2):111-121. 
2 Dhanapalaratnam R, Markoulli M, Krishnan AV. Disorders of vision in multiple sclerosis. Clin Exp Optom. 2022;105(1):3-12.
3 Sheehy CK, Beaudry-Richard A, Bensinger E, Theis J, Green AJ. Methods to Assess Ocular Motor Dysfunction in Multiple Sclerosis. J Neuroophthalmol. 2018;38(4):488-493.
4 Cox TA, Thompson HS, Corbett JJ. Relative afferent pupillary defects in optic neuritis. Am J Ophthalmol. 1981;92(5):685-690. 
5 Niestroy A, Rucker JC, Leigh RJ. Neuro-ophthalmologic aspects of multiple sclerosis: Using eye movements as a clinical and experimental tool. Clin Ophthalmol. 2007;1(3):267-272.
6 Frohman TC, Frohman EM, O'Suilleabhain P, et al. Accuracy of clinical detection of INO in MS: corroboration with quantitative infrared oculography. Neurology. 2003;61(6):848-850.