I Movimenti oculari saccadici come modello sperimentale per lo studio della fatica nei pazienti affetti da sclerosi multipla

Background:INO in MS causes slowed adduction due to demyelination of the medial longitudinal fasciculus(MLF). Objective: To determine whether changes in horizontal eye-movement conjugacy during repetitive saccades provides a model for the phenomenon of fatigue in MS. Methods:We measured horizontal eye movements in 9 MS patients with INO(4 bilateral,total 13 INOs)and 8 normals as they made saccades between targets separated 20 degrees horizontally for 10 minutes(fatigue test).Conjugacy of horizontal saccades was measured during the first and last minute of the trial from:(1)amplitude ratio(AR)of abducting to adducting movement;(2)ratio of abducting/adducting peak velocity (versional disconjugacy index, VDI);(3)normalized, interocular velocity difference(IVD)at 20% of eye displacement on phase-plane plots. Results:AR changes in patients were variable and inconsistent.Velocity VDI and IVD values showed consistent changes after fatigue in 10/13 INOs,with increase in 5 cases and decrease in other 5.Worsening of conjugacy(increased VDI and IVD values)was generally observed in those INOs with initial VDI<1.9,while improvement of conjugacy (decreased VDI and IVD values)was observed in those with initial VDI>1.9. Conclusions: A saccade fatigue test induced changes of conjugacy in MS patients as measured by velocity criteria.Worsening of conjugacy may reflect deterioration in transmission of the saccadic pulse signal along demyelinated MLF,and occurred in patients with milder INO. Improvement of conjugacy occurred in patients with more severe INO, and may be due to adaptive mechanisms, such as recruitment of vergence to aid gaze shifts.The reductionist model for INO provides a promising circuit to study the phenomenon of fatigue due to reduced neural transmission,as well as the brain’s adaptive responses.