Effects of pupil dilation on objective refraction.

Abstract

Refractive correction, including glasses, contact lenses or refractive surgery, is common procedure in modern clinical ophthalmology practice. However, frequent sequela of refractive correction is refractive error, which greatly affects the patient's quality of vision. Improving the accuracy of the pre- and postexamination is important in minimizing refractive error. In the refraction test, we inspect one eye while covering the other eye. However, this test differs from the everyday visual environment, in which both eyes are kept open. Although it has been reported that the pupil dilates when one eye is covered (Kawamorita & Uozato 2014), it has not yet been reported how this affects visual function. In this study, we investigated the effects of pupil dilation on objective refraction. This study included 30 healthy volunteers with a mean age of 20.5 ± 1.5 years. We certify that all applicable institutional regulations concerning the ethical use of human volunteers were followed. We used a binocular open auto ref/keratometer (WAM-5500; Grand Seiko, Ltd., Fukuyama City, Hiroshima Prefecture, Japan) to measure the chronological change in pupil diameter and objective refraction associated with covering one eye. We had patients fixated on an angular vision visual acuity chart (0.1) at a distance of 5 m. We measured, in order, binocular (15 s), monocular (30 s, using an occluder) and binocular (15 s) vision (Fig. 1A). Statistical analysis was performed using the Scheffé test, with values of P < 0.01 considered to be significant. The chronological changes in pupil diameter and objective refraction associated with covering one eye are shown in Fig. 1B. When one eye was covered, the pupil slowly dilated. The average pupil diameter before, during and after covering was 4.32 ± 0.013 mm, 5.25 ± 0.26 mm and 4.14 ± 0.26 mm, respectively. The pupil diameter was significantly larger with one eye covered than it was before (P = 0.0002) and after covering (P = 0.0008). There was no significant difference in the pupil diameter measurements taken before and after covering (P = 0.94). The average objective refraction before, during and after covering was −2.18 ± 0.07 diopters (D), −2.32 ± 0.09 D and −2.22 ± 0.09 D, respectively. The objective refraction with one eye covered was significantly myopic compared with before (P = 0.0003) and after (P = 0.0018) covering, with differences of 0.14 D (maximum, 0.57 D) and 0.10 D (maximum, 0.51 D), respectively. There was no significant difference in the objective refraction measurements taken before and after covering (P = 0.90). When one eye was covered, the pupil dilated. As a result, the objective refraction became myopic. The depth of focus decreased because of pupil dilation, and the retinal image became blurred (Campbell 1957). It has been shown that the retinal image blurs in response to accommodation (Kaufman & Alm 2003); it is assumed that accommodation causes the retinal image to blur. Our results suggest that covering one eye causes refractive error after refractive correction. As our study was to investigate chronological changes of objective refraction and pupil diameter, we could not conduct it in the condition of cycloplegic refraction. However, the condition of cycloplegia is required for studying refraction in patients until the age of 20 years (Sanfilippo et al. 2014). Moreover, the condition of cycloplegia is also required for epidemiological studies of refraction in patients aged over 50 years (Morgan et al. 2015). Therefore, we need to examine how pupil dilation due to covering one eye affects the refraction in the condition of cycloplegic refraction. By examining the pupil diameter under everyday visual conditions, it may be possible to improve refractive error after refractive correction.