Cyclopentolate

After corneal anesthesia induced with proparacaine (0.5% Alcaine, Alcon Laboratories, Ft. Worth, TX), 3 drops of 1% cyclopentolate (Alcon Laboratories, Ft. Worth, TX) were administered at 5 minutes a part to induce cycloplegia. Five consecutive, reliable autorefraction measurements were obtained 30 minutes after the third drop was administered. The child was asked to sit in front of the autorefractor (Canon RK-F1) and to look at the fixation target, which was designed to obtain the smallest accommodative response.^{[36 (link)]} Both before and after cycloplegia, the right eye was measured (in 0.25 D steps) first and then the left eye.

All the twins underwent autorefraction under cycloplegia at every visit. Cycloplegia was induced with 1% Cyclopentolate (1% Cyclogyl, Alcon Labs, Fort Wroth, Texas). Refraction was measured afterwards using an autorefractor (KR8800, Topcon Corp, Tokyo, Japan). An interviewer-administered questionnaire was used to collect information about demographic characteristics, near work activities and outdoor time for twins. Near work activity includes reading and homework on weekdays and weekends. Time of outdoor activities referred to the number of hours spent on morning exercise, travelling to school, returning back home and other activities such as walking or gardening on weekdays and weekends. The biological parents of the twins received autorefraction measurements in both eyes without cycloplegia. The sampling and methodology has been described in detail elsewhere39 (link)41 (link). Written informed consent was obtained from all the participants and their parents. The study obtained ethical committee approval from the Zhongshan University Ethical Review Board and Ethics Committee of Zhongshan Ophthalmic Center, and all examinations were conducted in accordance with the Tenets of the World Medical Association’s declaration of Helsinki.

All children received a NCR and CR (Canon RK-F1, Canon Inc., Tokyo, Japan), with an average of three consecutive readings. For cycloplegia, 1% cyclopentolate (Alcon, Fort Worth, TX, USA) and 0.5% tropicamide/0.5% phenylephrine mixed eye drops (Mydrin-P, Santen, Osaka, Japan) were administered to each eye three times with a 5-min interval. After 30 min, eyes were checked for pupillary light reflection to determine whether cycloplegia is completed. If the pupil was dilated to 6 mm or more, the eye was considered as cycloplegic.

After one drop of topical anaesthetic agent (Alcaine, Alcon), cycloplegia process was performed by two drops of 1% cyclopentolate (Alcon) and one drop of Mydrin P (Santen, Japan), once every 5–8 minutes¹⁶ . To press the lacrimal sac for 3 min after each time, and wait for at least 30 min after three times until the pupillary reaction to light disappeared or only the weak light reflex remained. If the pupillary light reflex was still present or the pupil size was less than 6.0 mm, a third drop of cyclopentolate was instilled. Objective refraction (NIDEK Automated Refractor/Keratometer (ARK-1) , NIDEK, Japan) were completed to analyze the refractive status and corneal curvature of the participants under cycloplegic conditions^{11 (link)}. Multiplying the curvature by 0.3375 to calculate the corneal power. Measured two times, took the mean value of the results with confidence ≥ 8. RA is the cylindrical part of refractive error at the corneal plane. ACA is the difference in power between the steep and flat meridians. The NIDEK ARK-1 measured the radius of corneal curvature within 3 mm optical zone of the cornea by Placido ring principle.

The protocols for measuring refractive error and ocular biometry were the same between the baseline and follow-up visit. Each participant’s refractive status was measured before and after cycloplegia using an autorefractor (RM-8000; Topcon Corp., Tokyo, Japan) by optometrists or trained technicians. For cycloplegia, each participant was first administered two drops of 1% cyclopentolate (Alcon) after a 5-min interval. Thirty minutes later, a third drop was administered if pupillary light reflex was still present or the pupil size was less than 6.0 mm. The first five valid readings of autorefraction were used and averaged using vector methods to generate a single estimate of refractive error. All five readings should be at most 0.50 D apart in both the spherical and cylinder components. Myopia was defined as spherical equivalent (SE) less than − 0.50D. An IOL Master (Carl Zeiss Meditec AG, Jena, Germany) was used to measure ocular biometric parameters including AL, CP and ACD. LT was measured by using Lenstar LS900 (Haag-Streit Koeniz, Switzerland). Three repeated reading were obtained and averaged before cycloplegia.