Quantification of fixation stability of upward gaze in myasthenia gravis by using an eye-tracking system

Abstract

Purpose : In the current examinations to quantify ocular motility and ocular deviation in myasthenia gravis (MG) patients with diplopia, the patients’ characteristic easy fatigability causes rapid fluctuations in the eye positions, thereby making accurate measurement difficult. Eye-tracking systems can record real-time eye position non-invasively. We quantified fixation stability of upward gaze in normal subjects and MG patients by using an eye-tracking system.

Methods : We directed 12 normal subjects (Group 1) and 7 MG patients (Group 2) to keep fixation on a target in 20 degrees upward direction with both eyes for one minute. During the examination, the horizontal (X) and vertical (Y) positions of both eyes were recorded using the eye tracker (ViewPoint EyeTracker, Arrington Research) at a sampling rate of 220Hz. We calculated the standard deviations (SDs) of X and Y eye positions, and the bivariate contour ellipse areas (BCEAs) encompassing 68% and 95% of fixation points. To quantify the fixation stability, these SDs and BCEAs were compared between the two groups. In addition, the whole fixation period (one minute) was divided into three consecutive 20-s fractions and the temporal stability of the fixation in each group was compared between the three fractions.

Results : For the fixation stability of one minute, the SDs (both X- and Y-positions) and BCEAs were significantly larger in Group 2 than in Group 1 (Group1: SD of X=0.51 ± 0.2°, SD of Y=1.32 ± 0.6°, 68% BCEA = 4.21 ± 3.09 deg², 95% BCEA = 11.08 ± 8.12 deg²; Group2: SD of X = 1.28 ± 0.74°, SD of Y = 4.16 ± 2.48°, 68% BCEA = 24.9 ± 16.9 deg², 95% BCEA = 65.53 ± 44.47 deg²; t-test: P < 0.01). In the patient whom we were able to compare before and after treatment, the SD of Y-position and the BCEAs decreased after the treatment, thereby indicating an improvement in the fixation stability. For the temporal fixation stability, Group 1 showed no change during the one minute, while the SD of Y-position for the first 20-s fraction was significantly larger in Group 2 (Tukey-Kramer HSD test: P < 0.01).

Conclusions : Non-invasive quantification of fixation stability with an eye-tracking device is useful in precisely identifying the fatigue characteristics of MG and evaluating the effects of treatments in MG patients with diplopia.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.