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Nancy J Coletta, Lenna E. Walker, Fuensanta Vera-Diaz; Fixation Instability in Myopia. Invest. Ophthalmol. Vis. Sci. 2016;57(12):199.
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© ARVO (1962-2015); The Authors (2016-present)
Myopes exhibit larger visual acuity (VA) loss than emmetropes at low luminance (Coletta and Moskowitz, 2003; Coletta et al., ARVO, 2007). During VA measures in dim light, myopes often complain that letters are difficult to fixate. In this study we evaluated whether myopes have less stable fixations than emmetropes when reading VA charts.
Measurements were made on 19 subjects (age 25.7 ± 2.29 years) with spherical equivalent refraction range +0.63 to -10.88 D (Mean -4.14 ± 4.12 D). Axial lengths and corneal power were used to convert angular units of VA to retinal VA in cyc/mm. Mesopic (0.276 cd/m2) and photopic (325 cd/m2) VA was measured with high contrast letters, 5 letters per line, displayed on an LCD panel viewed at 4 m. Subjects wore contact lenses if correction was needed. Eye movements were recorded using a video-based eye tracker (EyeLink 1000; SR Research) with 0.6 min arc spatial resolution and 2 msec sampling rate. A custom Matlab program controlled the letter presentation and recorded eye position relative to the display. Eye movements were analyzed for letter sizes corresponding to the mesopic and photopic VA levels. Stability of fixation was expressed as the standard deviation (SD) of vertical eye position during VA reading.
The SD of vertical eye position increased with increasing myopia when subjects read letters at their mesopic (p=0.011) and photopic VA limit (p=0.012), as well as when the line of letters corresponding to the mesopic VA was viewed in the photopic condition (p=0.0002). LogMAR VA did not vary significantly with refractive error, although retinal VA in log cycles/mm became significantly lower with increasing myopia (p=0.039 mesopic; p=0.022 photopic). There was a trend towards greater loss of VA in dim lighting with increasing myopia.
Myopes showed less stable fixation than emmetropes during a VA task. This instability may interfere with reading or visual tasks in challenging conditions such as at low light levels. It has been reported that uncorrected myopia increases the amplitude of fixational microsaccades (Ghasia and Shaikh, IOVS, 2015). Although our measurements were made with best refractive correction, it is possible that reduced optical quality in myopic eyes may contribute to their fixation instability.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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