Pupil size of myopes under different mesopic conditions and its influencing factors

To explore the pupil size distribution of the Chinese myopic population under different mesopic conditions, and to analyze the possible influencing factors.

A cross-sectional study was conducted.Two hundred and fourteen myopic patients (428 eyes) who underwent refractive surgery in Tianjin Eye Hospital from December 2018 to April 2019 were randomly selected.The patients were 17 to 45 years old, with an average age of (22.62±4.88) years old.The patients were divided into astigmatism <-1.5 D group (372 eyes) and astigmatism ≥-1.5 D group (56 eyes) according to their astigmatism measurements.The low mesopic pupil size (LMPS) (0.2 lx) was measured with the infrared Colvard pupillometer, and the high mesopic pupil size (HMPS) (6-12 lx) was obtained through the anterior Pentacam segment analyzer.The pupil size was compared between both eyes, different sexes and different astigmatism measurements.The relationship between pupil size and possible influencing factors, such as age, sex, spherical equivalent, spherical diopter, cylinder diopter, axis, mean keratometry(Km), and central cornea thickness was analyzed.This study adhered to the Declaration of Helsinki.The study protocol was approved by the Ethics Committee of Tianjin Eye Hospital (No.201912). Written informed consent was obtained from each subject or their guardians.

The pupil sizes measured by the Colvard pupillometer and Pentacam were (6.806±0.776)mm and (3.312±0.540)mm, respectively.The pupil size of male subjects was (6.692±0.754)mm, which was larger than (6.668±0.792)mm of females, showing a statistically significant difference (t=2.935, P=0.004). Under the high mesopic condition, the pupil size of astigmatism ≥-1.5 D group was lower than that of astigmatism <-1.5 D group, with a statistically significant difference (t=2.611, P=0.009). Under the low mesopic condition, pupil size was negatively correlated with age and Km (r=-0.213, -0.210; both at P<0.001). Under the high mesopic condition, pupil size was weakly positively correlated with cylinder power (r=0.124, P=0.010) and was weakly negatively correlated with Km (r=-0.142, P=0.003). The multiple linear regression analysis revealed that the LMPS=0.659×HMPS-0.019×age-0.084×Km+ 8.662.About 28% of pupil size under low mesopic conditions could be predicted by Pentacam.LMPS of ≤7 mm could be better predicted when the results were below 3.6 mm.

Age and corneal curvature are influencing factors of mesopic pupil size.Older people with steep curvature have a smaller pupil.At high mesopic conditions, astigmatism affects pupil size.Pentacam measurements can predict LMPS to some degree but are not a substitute for dark-adapted pupil diameter.