This study is a rare cross-sectional study of Han and Mongolian adults in Inner Mongolia Autonomous Region of China. A stratified sampling method was used to include 3,185 participants who completed the 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 occurred mainly on the nasal side (38/2,651, 1.2, and the most common grade was grade 2 (162/2,651, 6.1%). Having an outdoor occupation, living in a rural area for more than 30 years, and being older than 50 years old 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 ageing is an important risk factor for pterygium[4,10,15]. The prevalence of pterygium in people over 80 years old has been found to be as high as 19.5%. Similar to the results of the above studies, our results showed that participants over 50 years old had a significantly increasing risk of pterygium.
In a study of the relationship between the onset of pterygium and UV exposure time in Hainan Province, China, in 2004,a research group confirmed that length of pterygium was positively correlated with UV exposure time. The study participants were divided into three groups according to 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 speculate that the increase in the prevalence of pterygium in people over 50 years old may be related to the increase in the cumulative UV exposure time.
There is currently no consistent conclusion on the relationship between gender and pterygium. Many studies, including investigations in China, India, Japan, Singapore, Iran and Ethiopia[9,11,17-21], have identified being male as a risk factor for pterygium. Nevertheless, the study of Dali and 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, taking responsibility for going out to work. Women spend most of their time at home doing housework. In western China, especially in Yunnan Province, lifestyle is affected by the matriarchal clan society. Women are the main labour force and carry out more outdoor work. 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. Therefore, 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 in Spain in 2011 (4.8% for men, 6.5% for women, P=0.346).
Previous multi ethnic studies of pterygium have found differences in the prevalence among ethnicities; for example, the prevalence of pterygium among people of Han ethnicity in Xinjiang and among people of Yi ethnicity in Yunnan were significantly increased[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 age and gender correction.
The multivariate analysis of this study showed 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 sunshine time is short and there is little sandy wind. In a study in Jordan in 2004, living in a dry, dusty environment with long-term exposure to large amounts of particles was 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 pterygium and education level. In this study, especially among people of Han ethnicity, the prevalence of pterygium was greatly affected by education level. The OR value of middle or high school education level was twice that of university or higher level. This is similar to the results of a multiethnic study in Malaysia in 2012 (university education level OR 0.5, 95% CI 0.3–0.7, P=0.001).We speculate that because people with higher education levels have a higher likelihood of indoor work, they have less exposure to sunshine and are therefore exposed to ultraviolet light for a shorter period of time. A pterygium epidemic survey in 2009 in which 4,774 Latin American individuals older than 40 years were enrolled found that people who were educated for less than 6 years were 2.81 times more likely to suffer from pterygium than those who were educated for 12 years or longer,a result that is in accordance with our findings.
Activities such as outdoor activity, surfing and fishing have been proven to be risk factors for pterygium in prior literature[8,28]. At the same time, high-intensity exposure to ultraviolet light during youth increases the risk of pterygium. According to a case-control study in Brisbane in 1992, those who lived at low latitudes (30°N)for the first five years after birth had a 40-fold higher prevalence of pterygium than patients who lived at higher latitudes (40°N).This suggests that the cumulative duration of UV exposure had a greater impact on pterygium. Another study on Norfolk Island in 2013 reported that the youngest group had the highest conjunctival ultraviolet autofluorescence.Owing to the higher UV exposure level, the prevalence of pterygium in the youngest group was similar to that in adults older than 30. Consistent with these results, we found that living in rural areas longer than 30 years and outdoor occupation were risk factors for pterygium.
Oxidative stress caused by obesity may have an effect on pterygium; a study by Kormanovski discovered higher nitric oxide levels and total antioxidant levels in individuals in the primary pterygium group compared with normal subjects. The level of oxidation state increased, and all antioxidant enzyme levels decreased. We usually use BMI to quantitatively analyse obesity, but the effect of high BMI on pterygium is still inconclusive. For example, in an Australian study in 2015, the prevalence of pterygium was lower for obese people (BMI>5) than for normal-weight subjects (6.3% vs. 33.8%). A large sample in a Korean study on pterygium and obesity yielded the opposite results. Being a women and having a higher BMI were risk factors for pterygium (OR=1.43, P=0.008). In our subgroup analysis, differences between two ethnic groups were found. BMI≥28 was a protective factor for Han individuals (OR 0.42, 95% CI 0.21-0.81, P=0.01) but a risk factor for Mongolian individuals (OR 2.39, 95% CI 1.02-5.58, P=0.044). We speculate that people with higher BMI may engage in less outdoor activity and therefore face less UV radiation.
However, BMI is not the only evaluation index of obesity. In recent years, body fat percentage (BF%) has also been used to evaluate the degree of obesity, and it assesses fat mass more effectively than BMI[34,35]. Previous studies have found that the Chinese have relatively low BMI and relatively high BF%.At the same time, two surveys have found great differences in the diet and living habits of individuals of Han and Mongolian ethnicities[37,38]. Compared with people of the Han ethnicity, Mongolians had higher meat and salt intake and lower intake of fruits and vegetables. Having high meat intake means that Mongolians ingest more fat and protein. 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[39,40]. 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%, representing a significant difference (P<0.001). Therefore, we believe that although the BMI of both ethnic groups is 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, but a meta-analysis in 2014 showed that smoking had a protective effect. 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[43,44]. In our study, we found that smoking was not a factor related to pterygium in people of Han or Mongolian ethnicity.
This study has some limitations. First, information concerning aspects such as medical history and living habits was collected by questionnaire, allowing recall bias. Second, we did not quantitatively measure the intensity and duration of UV exposure. Finally, as a cross-sectional study, we were unable to determine the causal relationship between these factors and pterygium. The next step should be a cohort study.