Objective: To investigate whether internuclear ophthalmoparesis (INO) due to demyelination of the medial longitudinal fasciculus (MLF) provides a model for studying the poorly understood symptom of fatigue in multiple sclerosis (MS). We asked whether repetitive horizontal saccades increased eye movement disconjugacy in patients with MS with INO, but not in healthy subjects.
Methods: We compared conjugacy of horizontal saccades in 9 patients with INO (4 bilateral, total 13) and 8 controls during minute 1 and minute 10 of a fatigue test; we measured the ratio of abducting/adducting peak velocity (versional disconjugacy index [VDI]).
Results: VDI values were greater in patients than controls. During the fatigue test, controls showed no changes of VDI, but patients did (p < 0.005) for 10/13 INOs, with increased ratios in 5 cases and a decrease in the other 5.
Conclusion: Fatigue-induced worsening of conjugacy was observed in milder internuclear ophthalmoparesis (INO), and may reflect deteriorated fidelity of saccadic pulse transmission along demyelinated medial longitudinal fasciculus. Improved conjugacy was observed in the more severe INOs, and may be due to adaptive mechanisms, such as recruitment of vergence to aid gaze shifts. INO may provide an accessible, reductionist model to study how decreased neural transmission influences fatigue in multiple sclerosis, how the brain adapts to it, and whether drugs may prove therapeutic.