Evaluation of saccades show ocular-motor fatigue in internuclear ophthalmoparesis due to multiple sclerosis

Abstract

Purpose : To investigate the fatigue-based changes of saccadic properties produced by subjects with multiple sclerosis who displayed uni- or bi-lateral intranuclear ophthalmoplegia (INO), as compared to those made by unaffected subjects.

Methods : Subjects were selected from patients to the MS clinic. They were included in this study if they had a clinically apparent INO in either eye, and had no other neurological conditions. We recorded fixation, saccades and smooth pursuit in 30 MS patients (median age xx, median EDSS 3) with INO. We tested fatigue using a saccadic stimulus requiring tracking of a +/-10 degree, high-contrast laser target (diameter 0.1 deg) at 0.5Hz. Data were recorded using the Eyelink 2 video system, sampled at 500 Hz and 16-bit resolution. Subjects were asked to fixate a straight-ahead target for sixty seconds. They then performed the 10-minute saccadic fatigue test before being asked to perform an additional sixty-second fixation of the central target. We measured pulse-size ratio (PSR) and interocular timing of saccade onset (PTD) using a 10 deg/sec velocity threshold for saccades made during the first ("time 1") and last ("time 2") 90 seconds. Mean and proportion differences were calculated with paired t-test and chi-square test respectively.

Results : Overall, the INO group showed increased PSR [average from 1.596 (range 1.001-3.70) at time 1 to 1.67 (range 1.11-5.56) deg/sec at time 2), p=0.01] and increased PTD [average from 0.015 (range 0.001-0.218) at time 1 to 0. 021 (range 0.001-0.318) msec at time 2), p=0.002]. After fatigue 17 INOs showed increased PSR with 20 INOs showing PTD increase ≥ 2 msec. 10 INOs showed increased PTD without increased PSR.

Conclusions : Ocular-motor fatigue in MS-related INO may be secondary to decreased size (increased PSR) and/or delayed delivery (increased PTD) of the saccadic pulse for the adducting eye. Adding a measure of interocular timing of saccade onset (PTD) may expand the ability to capture ocular-motor fatigue in INO. Ocular-motor fatigue may be due to compromised axonal transmission in the MLF under high-demand ocular-motor tasks. INO may represent an accessible model to understand motor fatigue in MS and test therapies intended to ameliorate axonal transmission and improve INO-related visual disability.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.