Prevalence and associated factors for pterygium in Han and Mongolian adults: a cross-sectional study in inner Mongolian, China

In this cross-sectional study, stratified sampling was conducted according to level of urbanization, and four survey sites were enrolled (Table 1 and Fig. 1): Hohhot, a large city located at 40.83°N that has short sunshine duration and is little affected by sandstorms; Tsining District, a midsize city and commercial trade zone located at 41.03°N (high altitude), has longer sunshine duration and is known for its dry, windy weather and sandstorms; Wuyuan, a county seat located at 40.10°N with the longest sunshine duration among the survey sites; and Xilingol League, a pastoral area located at 44.58°N, which is at the lowest altitude and which is predominantly grassland with sufficient sunshine and precipitation. In these cities and counties, we randomly selected different districts in cities and rural townships. The samples were stratified according to the sex and age distribution of the Inner Mongolian population in the China Population Census 2010. We set the proportion of Han and Mongolian participants according to the proportions of the local population. This study included only residents who had lived in the area for more than one year and excluded patients with mental illness by self-reported, pregnant women, and active military personnel. Only residents whose ethnicity was the same as their parents’ were enrolled in the study; in other words, Han participants had Han parents, and Mongolian participants had Mongolian parents. Out of 3468 eligible residents, 3185 underwent ophthalmological examinations, with an overall response rate of 91.84%. We finally included 2651 people who were 30 years of age or older. This study is based on the principles of the Declaration of Helsinki. Ethical approval was granted by the bioethical committee of the Institute of Basic Medical Sciences, the Chinese Academy of Medical Sciences.

The data collection was conducted in the field from July 2014 to August 2014. A team of three ophthalmologists, medical workers from general hospitals in Beijing and administrative personnel from the region carried out the data collection. Trained counsellors asked each participant questions and presented questionnaires to collect data, including information about age, sex, ethnic group, birthplace, residence (urban or rural), occupation (worker, farmer, management, service and sales, technical work, student, housework, etc.; agricultural work was classified as an outdoor occupation, while the other occupations were considered to be indoor occupations), education level (elementary or lower, middle school to high school, and university or higher), level of exercise [14] (light, moderate or heavy), history of hypertension and diabetes, and lifestyle (e.g., smoking and alcohol consumption). Smoking was divided into two categories: never-smokers and ever-smokers (including current smokers and former smokers). Similarly, alcohol consumption was classified into never-drinkers and ever-drinkers (including current drinkers and former drinkers). The systolic and diastolic blood pressure of the participants was measured three times using an electronic automatic blood pressure monitor (HEM-907, Omron, Japan). The average was used as the mean blood pressure. Height was measured by using a fixed stadiometer. Body weight and body fat percentage (BF%) were measured by bioelectrical impedance analysis (BC-420, Tanita, Japan). BMI (Body mass index) was calculated as weight in kilograms divided by height in metres squared (kg/m²).

Ophthalmologic examinations included visual acuity at 4 m (EDTRS, Wehen Co., Ltd., Guangzhou, China). Data related to refraction, such as corneal curvature radius, were measured with an auto ref-keratometer (ARK- 510A, Nidek Co., Ltd., Tokyo, Japan). Astigmatism was defined as cylinder value<− 0.50 D. We used a portable hand-held slit-lamp to examine the anterior segment of the eye (KJ5S2, Suzhou Kangjie Medical Co., Ltd., Jiangsu, China).

Pterygium (in either eye) was defined as a raised fibrovascular tissue encroaching through the limbus into the cornea or a history of pterygium excision. The examination of pterygium was performed by ophthalmologists with portable slit lamp. The grading was based on the location of pterygium head under standard lighting conditions [4, 10]. Grade 1: at the limbus. Grade 2: between the limbus and the undilated pupil margin. Grade 3: within the pupil margin. Grade 4: beyond the pupil margin. If bilateral pterygium was diagnosed, the higher grade eye was counted.

All statistical analyses were performed using the SPSS software program (Statistical Package for Social Sciences Inc., Chicago, IL, USA, version 21.0.0.0). Figures were created with the GraphPad Prism 7.0 software program. We performed a chi-square test using the pterygium growth site and unilateral and bilateral as variables to calculate the prevalence of pterygium and calculated the age-adjusted prevalence after referring to the China Population Census 2010. Independent sample t-test and chi-square test were used to analyse the demographic characteristics of the Han and Mongolian participants and the grades of pterygium. The univariate analysis between presence of pterygium and factors was performed at first. Subsequently, we performed interaction terms in logistic regression to evaluate the factors which may cause variance inflation. The multivariate regression analysis model was used to assess possible risk factors or protective factors for pterygium afterward. Ultimately, we used sex and ethnicity as subgroups to further analyse relevant factors in a multivariate regression analysis.

The final study population included 2651 participants of 30 years of age or older. Among these, there were 1043 (39.3%) men and 1608 women (60.7%) and 1910 Han Chinese (72%) and 741 Mongolian (28%) adults (Table 2). The average age of the participants was 48.93 ± 11.06 years (range 30–79 years).

The incidence of unilateral pterygium was higher than that of bilateral pterygium (P < 0.001). We found a significant difference in prevalence between the right and left eyes (P < 0.001); specifically, 54 cases (43.0%) involved the right eye and 73 cases (57.0%) involved the left eye. Regarding the location of the disease, in the majority of cases, the pterygium was located on the nasal side (n = 38; 1.2%). In 4 cases (0.1%) the pterygium was located on the temporal side. There were no cases with pterygium on both sides.

Regarding the grading of pterygium, 5 (0.2%) cases were classified as were grade 1, 162 (6.2%) were classified as grade 2, and no cases were classified as grade 3 or 4. In general, grade 2 was the most common grade among the two ethnic groups. As Figs. 2 and 3 shown, the Grade 2 were mainly concentrated in Mongolian adults of 50–59 years of age.

Frist of all, we performed the interaction terms to evaluate the relationships between time in rural, occupation and education level (Tables 3, 4 and 5), the relationships between BMI, Low-density lipoprotein cholesterol (LDLC) and Body fat percentage (Tables 6, 7 and 8). Since there was no participant who had university or higher education level worked outdoor, we combined the “middle school to high school” and “university or higher” together as a new group, which called “middle school or higher”. We found that BMI<24–27.9 * LDLC≥4.14 had statistical difference (β = − 1.17, OR = 0.31, 95% CI: 0.10–0.96, P = 0.041). The education level and occupation influence each other with statistical difference (β = 1.01, OR = 2.76, 95% CI: 1.82–4.18, P < 0.001). At the same time, the occupation and time in rural could influence each other with statistical difference (time in rural> 30 years*outdoor: β = − 1.67, OR = 0.19, 95% CI: 0.07–0.19, P = 0.001). The multivariate logistic regression model was based on a univariate analysis and study settings; with the following factors included in the final calculation: ethnicity, gender, age group, smoking status, education level, occupation, BMI, body fat percentage (BF%), low density lipoprotein cholesterol, survey site and time spent in rural areas. The results showed that having an outdoor occupation (OR 1.78, 95% CI: 1.09–2.90, P = 0.020), living in rural areas for more than 30 years (OR 1.98, 95% CI: 1.29–3.03, P = 0.002), and being 50–59 years old (OR 2.86, 95% CI: 1.59–5.16, P < 0.001) or ≥ 60 years old (OR 2.79, 95% CI: 1.48–5.26, P = 0.001) were risk factors for pterygium. Living near an urban survey site such as Hohhot (OR 0.44, 95% CI: 0.26–0.76, P = 0.003) or Tsining District (OR 0.30, 95% CI: 0.17–0.52, P < 0.001) were protective factors. We found that pterygium had no correlation with sex, ethnicity, BF%, low density lipoprotein cholesterol or smoking status. The results of the logistic regression analysis are presented in Table 9.

We also performed a subgroup logistic regression analysis using ethnicity and sex as the classification criteria (Figs. 4 and 5).

From the results of the ethnic subgroups, being older and having lived longer in rural residence areas risk factors among male and female participants (50-59 yrs. Male OR = 3.50, 95% CI: 1.37–8.91, P = 0.009; > 60 yrs. Male OR = 3.94, 95% CI: 1.5–10.39,P = 0.005; 50-59 yrs. Female OR = 2.71, 95%CI: 1.26–5.82, P = 0.011;. Residing in Hohhot and Tsining District were protective factors (Male in Hohhot OR = 0.19, 95% CI: 0.06–0.56, P = 0.003; Male in Tsining District OR = 0.38, 95%CI: 0.17–0.82, P = 0.014; Female in Tsining District OR = 0.30, 95%CI: 0.14–0.66, P = 0.003). In the analysis of male subjects alone, having an outdoor occupation was a risk factor for pterygium (OR = 2.36, 95% CI: 1.14–4.88, P = 0.021). When referring to female subjects, living in rural for more than 30 years was a risk factor (OR = 2.15, 95% CI: 1.22–3.81, P = 0.008).

In the ethnic subgroup analysis, being older and having lived in a rural area for 11–20 years or > 30 years were risk factors for pterygium in Han adults (50-59 yrs. OR = 4.26, 95% CI: 1.89–9.58, P < 0.001; > 60 yrs. OR = 5.69, 95% CI: 2.48–13.08, P < 0.001; time in rural 11-20 yrs. OR = 1.87, 95% CI: 1.00–3.49, P = 0.049; time in rural > 30 yrs. OR = 2.21, 95% CI: 1.29–3.78, P = 0.004). A high education level, BMI ≥ 28, and residing in Hohhot and Tsining District were protective factors in Han participants (Hohhot OR = 0.31, 95% CI: 0.16–0.59, P < 0.001; Tsining District OR = 0.32, 95%CI: 0.18–0.57, P < 0.001). BMI ≥ 24 was a major risk factor for pterygium in the Mongolian adults (BMI = 24–27.9 OR = 2.45, 95% CI: 1.12–5.33, P = 0.024; BMI ≥ 28 OR = 2.41, 95% CI: 1.04–5.61, P = 0.041). BMI ≥ 28 was a risk factor in Han adults as well (OR = 0.42, 95% CI: 0.21–0.81, P = 0.010). None of the Mongolian participants in Tsining District suffered from pterygium.