The medial longitudinal fasciculus (MLF) is the final common pathway for all conjugate adducting horizontal eye movements, as well as for the vertical–torsional vestibulo-ocular reflex (VOR). MLF lesion causes adduction paresis of ipsilesional (adducting) eye with dissociated nystagmus of contralesional (abducting) eye—the well-known clinical syndrome of internuclear ophthalmoplegia (INO). We measured the VOR stimulation and also any catch-up saccades, from individual semicircular canal (SCC) evoked by the head impulse test (HIT), using head and binocular 3-dimensional scleral search coils in 27 multiple sclerosis (MS) patients, 8 with unilateral, 19 with bilateral INO. In unilateral INO, VOR gain (normal >0.90) from ipsilesional lateral SCC stimulation was 0.48 for the adducting eye and 0.81 for the abducting eye; 0.61 from contralesional anterior SCC stimulation and only 0.29 from contralesional posterior SCC stimulation. In bilateral INO, there were VOR gain deficits from all six SCCs: lateral SCC gains were asymmetrically reduced to 0.45 in the adducting eye and 0.66 in the abducting eye; anterior SCC gain was 0.48 and posterior SCC gain was only 0.19. Horizontal VOR versional dysconjugacy between adducting and abducting eyes at 0.66 was less severe than horizontal catch-up saccade versional dysconjugacy (0.44); normal >0.80. Unexpected partial preservation of horizontal VOR with greater catch-up saccade impairment from the adducting than abducting eye suggests that the ascending tract of Deiters (ATD), an extra-MLF pathway, also mediates the horizontal VOR, but not adducting horizontal saccades. Vertical VOR deficits will produce vertical oscillopsia with any vertical head movement and measurement of the vertical VOR could help with the diagnosis and quantitative evaluation of MLF lesions in suspected MS. Horizontal VOR deficits and catch-up saccade versional dysconjugacy in INO will cause gaze instability and horizontal oscillopsia during active horizontal head movements.
Internuclear ophthalmoplegia Multiple sclerosis Vestibulo-ocular reflex Catch-up saccades Head impulse test
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We thank all the subjects for participating in the study. This work was supported by the National and Medical Health Research Council Australia [GTN500200], the Garnett Passe and Rodney Williams Memorial Foundation and Royal Prince Alfred Hospital Neurology Department Trustees.
Compliance with ethical standards
Conflicts of interest
Dr. Chen was supported by Garnett Passe and Rodney Williams Memorial Foundation, Research Training Fellowship, and reports personal fees from Bayer, Merck Serono and Novartis. Dr. Aw was supported by National Health and Medical Research Council Australia [GTN500200] and previously supported by [GTN511900]. Mr. Todd has nothing to declare. Dr. Barnett has served on scientific advisory boards for Biogen-Idec, Novartis and Genzyme, and his institution has received research support from Biogen-Idec, Merck-Serono and Novartis. Dr. Halmagyi has acted as an unpaid consultant for GN Otometrics for the development of a video head impulse system.
The study was approved by Sydney Local Health District (No: X10-0085) and Sydney University (No: 13076) Ethics Committees Australia with written informed consents obtained from all subjects in accordance with the Declaration of Helsinki of 1964 and its later amendments.
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