This was the first cross-sectional study of Han and Mongolian adults in the Inner Mongolia Autonomous Region of China. A stratified sampling method was used to include 3,185 participants who underwent ophthalmologic examination at the four survey sites. The mean age was 48.93±11.06 years, and the overall prevalence after age adjustment was 6.38%. Pterygium most frequently occurred on the nasal side (38/2,651, 1.4%), and the most common grade was grade 2 (162/2,651, 6.1%). Having an outdoor occupation, living in a rural area for >30 years, and being >50 years of age were risk factors for pterygium. Having a university or higher education level and living near an urban survey site (Hohhot and Tsining District) were protective factors.
Numerous epidemiological surveys have confirmed that aging is an important risk factor for pterygium [4,10,15]. The prevalence of pterygium in people over 80 years of age has been found to be as high as 19.5% [1]. Similar to the results of the above studies, our results showed that participants over 50 years of age had a significantly increased risk of pterygium.
In a 2004 study of the relationship between the onset of pterygium and UV exposure time in Hainan Province, China [16], a research group confirmed that length of pterygium was positively correlated with UV exposure time. The study participants were divided into three groups:youth, middle-aged, and old age. The length of pterygium was positively correlated with UV exposure time, with those in the old group having had pterygium the longest. Accordingly, we hypothesize that the increase in the prevalence of pterygium in people over 50 years of age may be related to an increase in the cumulative UV exposure time.
There is currently no consistent conclusion on the relationship between sex and pterygium. Many studies, such as those in China, India, Japan, Singapore, Iran and Ethiopia [9,11,17-21], have found that being a man was a risk factor for pterygium. Nevertheless, the study of Dali and the Tibet Autonomous Region of China confirmed that being a woman was risk factor; this may be related to the social division of men and women in the cultures of different regions. In most parts of Asia and Africa, men represent the main labour force in the family, women spend most of their time doing indoor housework. In recent years, with the process of non-agriculturalization in western China, the social division of labour among Inner Mongolian farmers and herdsmen has changed [22,23]. More men choose to go to work in large cities, while women stay in rural areas for agricultural activities or livestock farming. During the busy farming season, men return to the countryside to participate in agricultural work. Thus, in general, men and women spend roughly the same amount of time doing outdoor work. Our data analysis also confirmed that there was no significant differences in the prevalence of pterygium between men and women, which is similar to the results of a study conducted in Spain in 2011 (prevalence: 4.8% in men, 6.5% in women, P=0.346) [24].
Previous multiethnic studies on pterygium have found differences in the prevalence among ethnicities; for example, the prevalence of pterygium was significantly increased among people of Han ethnicity in Xinjiang and among people of Yi ethnicity in Yunnan [4,10]. Our study found no significant difference in the prevalence of pterygium between Han and Mongolian people (5.8% vs 7.8%, P=0.06). There was no significant association between ethnicity and pterygium after adjustment for age and sex. According to the China Population Census 2000, the intermarriage rates between Han and Uygur, Han and Yi were 0.62%, 16.29% respectively, which were much lower than Han and Mongolian (37.49%). Mongolian ethnicity is a large ethnic group in Inner Mongolia and used to have mixed multi-ethnic regions with Han ethnicity since Yuan dynasty [25,26]. According to the China Population Census 2000, the mixed ethnic household rate in Inner Mongolia was 11.70%. Meanwhile, the Uygur ethnicity is Caucasian, Mongolian and Han ethnicity both belong to Mongolian. We hypothesised that the Han and Mongolian ethnicities had similar lifestyle.
The multivariate analysis in the present study demonstrated that living in towns such as Hohhot and Tsining District were protective factors for pterygium. This may be related to their urbanization, residents’ living habits and the geographical environment, in which the sunshineduration is short and there is little sandy wind. In a study in Jordan in 2004 [27], living in a dry, dusty environment with long-term exposure to large amounts of particles was identified as a risk factor for pterygium. Although Tsining District has an extremely dry climate and sandy wind, during the epidemiological investigation, we observed that the local residents paid attention to self-protection and went outside wearing sunglasses and hats. Interestingly, after pooling the influence of environmental and social factors on pterygium, the results of multivariate analysis showed that living in Tsining District was a protective factor. This reminds us that in a dry and dusty environment, using more protective measures and avoiding long-term exposure to ultraviolet light will greatly reduce the occurrence of pterygium. Protective factors and risk factors are influenced by culture, geography and health awareness.
Our results confirmed a positive correlation between the incidence of pterygium and education level. The prevalence of pterygium in university or higher education level was lower than that of middle or high school education level (3.4% VS 6.3%). This is similar to the results of a multiethnic study in Malaysiaand a Latin American study in 2009[28, 29]. In the present study, there was no people who had university or higher education level had outdoor occupation. We hypothesize that because people with higher education levels have a higher likelihood of working indoors, they have less exposure to sunlight and would therefore be exposed to ultraviolet light for a shorter period of time.
Outdoor activities, including surfing and fishing have been reported to be risk factors for pterygium in previous studies[8,30]. At the same time, high-intensity exposure to ultraviolet light during youth increases the risk of pterygium. A case-control study in Brisbane in 1992 and a study on Norfolk Island in 2013 suggested that the cumulative duration of UV exposure had a greater impact on pterygium [31, 32]. Consistent with these results, we found that living in rural areas longer than 30 years and an outdoor occupation were risk factors for pterygium.
We usually use BMI to quantitatively analyse obesity, but the effect of high BMI on pterygium is still inconclusive [33, 34]. In our subgroup analysis, BMI≥28 was a protective factor for Han (OR 0.42, 95% CI 0.21-0.81, P=0.010), while it was a risk factor for Mongolian (OR 2.41, 95% CI 1.04-5.61, P=0.041). However, BMI is not the only index used to evaluate obesity. In recent years, the body fat percentage (BF%), which assesses the fat mass more effectively than the BMI, has also been used to evaluate the degree of obesity [35, 36]. Previous studies have found that the Chinese have lower BMI and higher BF% [37]. There were great differences in the diet and living habits of the Han and Mongolian ethnicities [38, 39]. In comparison to the Han ethnicity, Mongolians had a higher meat and salt intake and a lower intake of fruits and vegetables, which meant that Mongolians ingested more fat and protein[40]. It is well recognized that high-fat diets can increase the oxidative stress level in the body, and the conclusion that oxidative stress is caused by high-protein diets remains controversial [41, 42]. The oxidative stress caused by obesity may have an effect on pterygium [43]. Hence, we used participants' BF% to more accurately assess obesity. The BF% of the Han ethnicity was 29.58±7.70%, and the BF% of the Mongolian ethnicity was 30.99±7.78%, which represented a significant difference (P<0.001). Therefore, we believe that although the BMI value of both ethnic groups were elevated, the different dietary structures lead to different fat content and therefore different degrees of oxidative stress. However, the effects of systemic oxidative stress on the ocular surface need to be confirmed by further studies.
The association between smoking and pterygium has been controversial. Certain studies have suggested that smoking is a risk factor for pterygium [44], but a meta-analysis in 2014 and a survey in Israel in 2016 showed that smoking had a protective effect [45, 46]. The biological impact of smoking on pterygium remains unclear. Some researchers have speculated that smoking could inhibit the expression of pro-inflammatory cytokines, reduce ocular surface inflammation, and inhibit vasoconstriction through various receptors [47, 48]. In our study, we found that smoking was not a factor related to pterygium in people of Han or Mongolian ethnicity.
The present study was associated with some limitations. First, information concerning aspects such as medical history and living habits was collected by a questionnaire, which allowed for a recall bias. Second, we did not quantitatively measure the intensity or duration of UV exposure. Finally, as this was a cross-sectional study, we were unable to determine the causal relationship between these factors and pterygium. The next step should be undertake a cohort study.