Participants and setting
The Kailuan Eye Study is a cross-sectional study included participants who had undergone ophthalmologic and general examinations from the longitudinal Kailuan Study. The study population included employees and retirees of a coal mining company (Kailuan Group Company). The community of Kailuan located in the city of Tangshan with approximately 7.2 million inhabitants. The Tangshan city is situated approximately 150 km southeast of Beijing and is a center of the coal mining industry. At baseline, the study population consisted of 14440 individuals with an age ranging between 21 and 97 years. The examination in the present study were performed repeatedly at 2-year intervals.[17, 18] The study followed the tenets of the Declaration of Helsinki. The present study was approved by the Medical Ethics Committee of the Beijing Tongren Hospital, and informed consent was obtained from the individuals after explanation of the nature and possible consequences of the study.
Ophthalmological and body examinations
The ophthalmologic examinations[19] which described in detail previously included measurement of visual acuity, tonometry, slit-lamp assisted biomicroscope of the anterior segment of the eye, ocular biometry applying optical low-coherence reflectometry (Lenstar 900 Optical Biometer; Haag-Streit, Koeniz, Switzerland) for the determination of the central corneal thickness, anterior chamber depth, lens thickness and axial length (AL). Using a nonmydriatic fundus camera (CR6-45NM; Canon, Inc., Osta, Tokyo, Japan), Simultaneous stereoscopic 45° color fundus photographs centered on the optic disc and on the macula were taken for each eye. If the pupil diameter did not allow taking fundus photographs with sufficient photographic quality, we dilated the pupil medically by applying eye drops containing 0.5% tropicamide and 0.5% phenylephrine hydrochloride.
Body height and weight and the circumference of the waist and hip were measured. The body mass index (BMI) and waist–hip ratio was calculated. The smoking index was calculated by multiplying the number of cigarettes per day by the number of years of smoking.
Blood pressure was assessed with the participants sitting for at least 5 minutes. Blood samples were collected under fasting conditions to determine the blood glucose, high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglyceride (TC), and total cholesterol (TG) concentrations.[20]
For all participants, an interview was performed with standardized questions about known major systemic diseases and lifestyle parameters. Systemic diseases included hypertension, diabetes mellitus (DM), hyperlipidemia. Lifestyle parameters included smoking status, nature of job, and smart phone usage.
The diagnosis of DM was based on any of the following three criteria: measurement of the fasting blood glucose concentration of 7.0 mmol/L, a self-reported history of DM, or a history of medication with hypoglycemic agents.
The diagnostic criteria for hypertension were blood pressure of ≥ 140/90 mmHg, positive history of hypertension, or the use of antihypertensive drugs.
The diagnostic criteria for hyperlipidemia included hypercholesterolemia (TG ≥ 6.2 mmol /L), hypertriglyceridemia (TC ≥ 2.3 mmol/L), mixed hyperlipidemia (TG ≥ 6.2 mmol /L and TC ≥ 2.3 mmol/L), and low HDL-C levels (< 1.0 mmol/L). The definitions above have been described in detail in previous literature.[20]
Definitions and MM grading
High myopia was defined as AL equal to or more than 26.5mm[12, 21–23] in this study. MM was graded among myopic participants using the International Photographic Classification and Grading System for MM.[24] In brief, MM was classified into five categories based on its severity: no myopic retinal degenerative lesion—category 0 (C0); tessellated fundus only—category 1 (C1); diffuse chorioretinal atrophy—category 2 (C2); patchy chorioretinal atrophy—category 3 (C3); macular atrophy—category 4 (C4). Additional lesions including lacquer cracks (LCs), Fuch’s spot and myopic choroidal neovascularisation, that is, ‘plus’ lesions, were also recorded. The presence of MM was defined as C2 or greater, and/or any additional lesions. To evaluate the interobserver agreement of two experienced ophthalmologists (Z.J.Q., W.H.W.), a test set with 100 images from 100 participants with high myopia (60 had C2 or greater MM) was used to test the ophthalmologists. The unweighted kappa was 0.81 for C0/C1, 0.77 for C2, 0.83 for C3, 0.85 for C4, 0.69 for LC, and 0.67 for Fuch’s spot. The two ophthalmologists interpreted each retinal photograph independently using the above-described criteria. In the case of any discrepancy, the images were evaluated by the third specialist (W.W.B.) for final decision.
Statistical analysis
The prevalence of MM with 95% confidence intervals (CIs) were evaluated. Pearson χ2 tests and Mann-Whitney U test were adopted to compare the characteristics between those participants with and without MM. Univariate and multiple logistic regression were adopted to evaluated risk factors for the presence of MM. Odds ratios (ORs) with 95% CIs were calculated. Visual impairment was classified into moderate (decimal: 0.1–0.3), severe (decimal: 0.05–0.1), blindness (decimal: 0.02–0.05) and severe blindness (decimal: less than 0.02). The influence of MM on visual acuity was also evaluated between participants with MM and without these disorders using Pearson χ2 tests. The statistical analysis was performed using SPSS software (version 21.0; IBM/SPSS, Chicago, IL, USA). A p value < 0.05 was regarded as statistically significant.