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Purpose: We propose that eyelid squint can reduce the impact of several conditions known to cause eyestrain such as uncorrected refractive error, accommodative dysfunction, presbyopia, and glare. Clinicians commonly accept that squint improves visual acuity (VA) in the presence of refractive error, and even though the benefit of eyelid squint in bright light seems self-evident, data are not available to support either benefit. The purpose of this study was to measure the effect of eyelid squint on VA in the presence of refractive blur and on visual field sensitivity.

Methods: Nineteen subjects with optimal refractive correction were tested, with and without squinting, as follows: (1) distance VA with induced refractive errors of +0.50, +1.00, +2.00, +3.00, +1.00 -1.00 x 90, and +1.00 -1.00 x 180; (2) near VA with induced refractive errors of -1.00 x 90 and -1.00 x 180; and (3) central and peripheral threshold visual fields with a Humphrey Field Analyzer. Differences in visual acuity between squinting and nonsquinting were tested for significance with repeated-measures analysis of variance, and differences in visual fields were tested using mixed model analysis of variance with repeated measures.

Results: Eyelid squint significantly improved (p < 00.016) distance VA measurements for 1.00 to 3.00 D of induced myopia. The squint-induced VA improvement increased from 0.06 logarithm of the minimum angle of resolution for 1.00 D to 0.24 logarithm of the minimum angle of resolution for 3.00 D of myopia. Eyelid squint also significantly reduced visual field sensitivity below the horizontal meridian by an average of 1.4 dB, by 1.6 dB on the horizontal meridian, and with increasing reductions in the vertical field up to 11.6 dB for measurements 40 degrees above fixation. This pattern of decreased superior field sensitivity would decrease visual discomfort from overhead glare.

Conclusions: The results provide empirical support that eyelid squint improves visual acuity for subjects with refractive error and reduces glare in the superior visual field.