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Howard P. Apple, Joseph M. Miller, Erin M. Harvey, Tina K. Green, Dawn H. Messer, J.Daniel Twelker; Accommodative Lag Changes in Uncorrected Astigmatic and Non-Astigmatic Students While Attempting to Read Small and Large Letters. Invest. Ophthalmol. Vis. Sci. 2011;52(14):838.
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To compare accommodative lag in uncorrected non-astigmatic and astigmatic students while binocularly viewing large (20/100) and small (20/25) letters at 40 cm.
The Grand Seiko WAM-5500 was modified to use a bridge-of-the-nose rest, allowing subjects to speak while reading letters displayed on an iPod Touch (Apple Inc) microdisplay. Subjects were 15 6th - 12th grade students from schools where the majority of the children are members of a Native American Tribe (Tohono O’odham) with a high prevalence of astigmatism. Cycloplegic autorefraction (Retinomax K-Plus, Nikon, now Right Mfg) was performed. We excluded myopic subjects whose SEQ was >-2.50D The WAM, a binocular full-field viewing monocular autorefractor, was used in continuous recording mode at 5 samples per second. Subjects read aloud from the display of 5 lines of 5 letters having Snellen equivalent sizes of 20/25, 20/50, and 20/100. All 15 could read the 20/100 display, while 7 could read the 20/25 display.
Eight students had <1 D of refractive astigmatism, and 7 had > 1D. Students able to read the 20/25 line (R25+) had mean (sd) of 0.43 (0.49) D astigmatism (min 0D, max 1.25 D), while those unable to read the 20/25 line (R25-) had 2.5 (1.70) D astigmatism (min 0.75D max 5D) (p=0.0086). SEQ refraction did not differ between the groups (R25+ -0.71 (1.41) D, R25- -0.199 (1.71) D (p=0.51)). Accommodative Lag (AL), defined as the difference between the demand placed by the target distance (2.5D) and SEQ measured by the autorefractor, was measured and pooled across subjects with n=440 for AL determinations while reading 20/25 stimuli, and n=431 while reading 20/100. Those who could read 20/25 (R25+) had 0.36 (1.09) D lag, while R25- had -0.024 (0.94) D while viewing the 20/25 letters (p=0.0001). AL observed while reading 20/100 letters shifted for both groups, with R25+ having 0.170 (0.81) D, and R25- 0.27 (0.88) D of AL (p=0.195).
Binocularly viewing of letters too small to read causes focus to shift closer to the spherical equivalent refraction in predominantly astigmatic subjects. Reading larger letters results in positive lag in both groups of subjects.
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