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Susana T L Chung, Girish Kumar, Roger Winghong Li, Dennis M Levi; Fixation Stability and Fixational Eye Movements in Amblyopia. Invest. Ophthalmol. Vis. Sci. 2014;55(13):812.
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Fixation stability is known to be poorer in the amblyopia eye (AE) than in the fellow eye (FE) for people with amblyopia. However, little is known about how the characteristics of the components of fixational eye movements — fixational saccades and slow drifts — compare between AE, FE, and normal controls, and which component of the fixational eye movements plays the major role in limiting fixation stability.
20 adults with amblyopia (10 anisometropic, 10 strabismic, age: 19-75) participated in this study. Acuity ranged from 0.0 to -0.22 logMAR in FE, and 0.1 to 1.46 logMAR in AE. Observers were asked to maintain fixation at the center of a 1° cross projected on the retina of each eye using a scanning laser ophthalmoscope, for trials of 30s. The retinal image and the position of the cross were recorded digitally. Three trials were collected for each eye in an alternate sequence. Eye movements were recovered from the recorded videos at a sampling rate of 540 Hz using a cross-correlation technique. A velocity criterion of 8°/s was used to differentiate between fixational saccades and slow drifts. For comparison, fixational eye movements were measured for a randomly selected eye for 16 adults with normal vision (controls) using identical procedures.
In general, fixation stability, quantified by the bivariate contour ellipse area (BCEA), and other characteristics of fixational eye movements, were not different between FE and normal controls. Comparisons between AE and FE revealed that AE exhibited greater fixation instability (BCEA: 0.78±0.58 [95%CI] vs. 0.11±0.04 deg2) that showed a significant correlation with logMAR acuity (r=0.66). Although the rate and velocity of fixational saccades were similar between AE and FE, fixational saccades made by AE had larger amplitude (29.3±6.4 vs. 17.5±2.2 min) and larger vector error of saccade landings (0.45±0.11 vs. 0.21±0.03°). Other measurements such as the length and velocity of slow drifts, and inter-saccadic interval, were not different between AE and FE. Among all measurements, BCEA showed a significant correlation only with the amplitude of fixational saccades (r=0.72).
Fixation instability is a limiting factor on acuity in AE. The greater fixation instability cannot be explained by the slow drifts of fixational eye movements. Instead, it is likely that the larger amplitude of fixational saccades contributes to the increased fixation instability in the AE.
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