February 2013
Volume 54, Issue 2
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Clinical and Epidemiologic Research  |   February 2013
10-Year Incidence and Associations of Pterygium in Adult Chinese: The Beijing Eye Study
Author Affiliations & Notes
  • Liang Zhao
    From the Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China; and the
  • Qi Sheng You
    From the Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China; and the
  • Liang Xu
    From the Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China; and the
  • Ke Ma
    From the Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China; and the
  • Ya Xing Wang
    From the Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China; and the
  • Hua Yang
    From the Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China; and the
  • Jost B. Jonas
    From the Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China; and the
    Department of Ophthalmology, Medical Faculty Mannheim of the Ruprecht-Karls-University, Heidelberg, Germany.
  • Corresponding author: Liang Xu, Beijing Institute of Ophthalmology, 17 Hougou Lane, Chong Wen Men, 100005 Beijing, China; xlbio1@163.com
Investigative Ophthalmology & Visual Science February 2013, Vol.54, 1509-1514. doi:10.1167/iovs.12-11183
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      Liang Zhao, Qi Sheng You, Liang Xu, Ke Ma, Ya Xing Wang, Hua Yang, Jost B. Jonas; 10-Year Incidence and Associations of Pterygium in Adult Chinese: The Beijing Eye Study. Invest. Ophthalmol. Vis. Sci. 2013;54(2):1509-1514. doi: 10.1167/iovs.12-11183.

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Abstract

Purpose.: To assess the 10-year incidence of pterygium and associated factors in adult Chinese.

Methods.: The population-based, longitudinal Beijing Eye Study, which included 4439 subjects (age: 40+ years) in 2001, was repeated in 2011 with 2695 subjects participating (66.4% of the survivors). Eyes with a pterygium in 2001 (n = 90) were additionally excluded. The study participants underwent a detailed ophthalmic examination, including slit-lamp examination for detection of pterygia and corneal photography for measurement of the pterygia.

Results.: Incident pterygia were detected in 157/5300 eyes (2.9 ± 0.3%) of 129/2628 persons (4.9 ± 0.4%). In 101 (78.3%) subjects, the incident pterygia had occurred unilaterally. Of 44 participants with unilateral pterygium in 2001, six (13.6%) subjects developed an incident pterygium in the fellow eye during the 10-year follow up. In the age groups of 40 to 49, 50 to 59, 60 to 69, and 70+ years, the 10-year incidence of pterygia was 5.4 ± 0.7%, 4.7 ± 0.7%, 4.5 ± 0.8%, and 5.0 ± 1.5%, respectively. The length of pterygium was 30.4 ± 14.7% of the corneal diameter. In multivariate analysis, the 10-year incidence of pterygia was associated with the rural region of habitation (P < 0.001; odds ratio [OR]: 0.24; 95% confidence interval [CI]: 0.14, 0.42) and lower fasting blood concentration of glucose (P = 0.009; OR: 0.82; 95% CI: 0.71, 0.95), whereas the level of education (P = 0.90), cognitive score (P = 0.64), occupation as farmer versus nonfarmer (P = 0.47), diastolic blood pressure (P = 0.11), and systolic blood pressure (P = 0.10) were not significantly associated.

Conclusions.: The 10-year incidence of pterygium in adult Chinese was 4.9%. It was significantly associated with the rural region of habitation and lower fasting blood glucose concentration.

Introduction
Pterygium is a dysplasia of the bulbar conjunctiva. It is located most commonly in the nasal horizontal region and grows from the conjunctiva onto the corneal surface. In its advanced stage, a pterygium can induce significant astigmatism and restriction of ocular motility, and cause visual impairment. Recent population-based studies have suggested that countries closer to the equator have a higher prevalence of pterygia. 111 In Australia, an epidemiologic survey in the Victoria region in 1984 showed a prevalence of pterygia in aborigines of 3.4%, of 1.1% for non-aborigines, and of 1.2% for rural areas. 5 A study in Northern Europe showed a prevalence of 9.6% in Greenland. 1 In equatorial tropical Asia, in the region of Singapore, the prevalence of pterygium in Chinese people over the age of 40 years was 6.9%. 2 In the Japanese Kumejima Islands, a population-based survey found a prevalence of pterygia of 30.8%. 11 A similar figure was reported from South China from the Guangdong province with a prevalence of 37.4%. 8 In 2001, the Beijing Eye Study from North China reported on a prevalence rate of 2.9% in 40+ -year-old subjects. 10 Studies suggested that the higher prevalence of pterygia closer to the equator may be associated with higher levels of ultraviolet radiation exposure. 111 The exact relationship between ultraviolet exposure and pterygium as well as the role of other causal mechanisms and risk factors, including genetic predisposition to the disease, have remained elusive so far. 12,13 Although the prevalence of pterygia has been examined in several previous cross-sectional population-based studies, information on the incidence of the disorder is scarce. The Barbados Eye Study has so far been the only major population-based investigation that addressed the new development of pterygia in a follow-up examination. 14 It was, therefore, the purpose of our study to assess the incidence of pterygia and their associated factors in Northern China, which differs remarkably in climate and population from the Caribbean region. 
Methods
The Beijing Eye Study is a population-based prospective cohort study in a rural area and an urban region of Greater Beijing. The Medical Ethics Committee of Beijing Tongren Hospital approved the study protocol. Of all 5324 individuals with an age of 40+ years who were eligible, 4439 individuals participated in the baseline study in 2001 (response rate: 83.4%), with 1918 subjects (43.8%) from the rural region. The mean age was 56.4 ± 10.4 years (range, 40–101 years). In 2006 and 2011, the study was repeated by inviting all participants from the survey of 2001. The details of participants and nonparticipants at baseline and the 5-year follow-up examination as well as the examinations performed have been described elsewhere. 15,16  
All study participants underwent an interview by trained health staff with standardized questions on their family status, level of education, income, quality of life, psychic depression, physical activity, known major systemic diseases such as arterial hypertension and diabetes mellitus, and intake of systemic medication. Fasting blood samples were taken for measurement of blood lipids, glucose, and glycosylated hemoglobin HbA1c. Blood pressure was measured. Body height and weight and the circumference of the waist and hip were recorded. The ophthalmic examination included measurement of visual acuity, pneumotonometry, slit-lamp examination of the anterior ocular segment, optical low-coherence reflectometry (Lensstar 900 Optical Biometer; Haag-Streit, Koeniz, Switzerland), biometry, digital photography of the cornea, lens (Neitz CT-R camera; Neitz Instruments Co., Tokyo, Japan), optic disc, and macula (fundus camera CR6-45NM; Canon Inc., Lake Success, NY). Cognitive function was assessed using the Mini–Mental State Examination scale. 17  
A pterygium was diagnosed during the slit-lamp examination of the cornea carried out by an experienced ophthalmologist (QSY). The corneal photographs were taken for quantitative analysis of the pterygia. The length of a pterygium was measured as the ratio of the pterygium diameter from the limbus to the pterygium head divided by the total cornea diameter. Additionally, the limbal circumference covered by the pterygium was measured in degrees of arc. 
Statistical analysis was performed using a commercially available statistical software package (SPSS for Windows, version 20.0; IBM-SPSS, Chicago, IL). In a first step, we determined the mean values (presented as mean ± SD) and median values of the main outcome parameters. In a second step, we performed univariate analyses of the associations between the incidence of pterygia and other systemic parameters and ocular parameters. In a third step, we carried out multivariate binary regression analyses with the incidence of pterygia as the dependent parameter and with all those variables as independent parameters that were significantly associated with the incidence of pterygia in the univariate analyses. Odds ratios (ORs) and 95% confidence intervals (CIs) were presented. A value of P < 0.05 was considered to indicate statistical significance. 
Results
In 2011, of all 4439 participants who participated in the baseline examination, 2695 subjects (1139 [42.3%] males) were re-examined, whereas 379 participants were dead, and 1365 subjects did not agree to be reexamined or had moved away. The response rate was 60.7% of the original cohort, or 66.4% of the survivors. The mean age of the study participants in 2001 was 54.6 ± 9.8 years (range: 40–83 years), and the mean refractive error (in 2001) was −0.33 ± 2.21 diopters (D) (median: 0 D; range: −20.1 to + 7.5 D). Compared with nonparticipants, the study participants were significantly younger (P < 0.001), lived more often in the rural region (P < 0.001), were more often peasants (P < 0.001), and had higher systolic blood pressure (P = 0.005) (Table 1). They did not differ in refractive error (P = 0.53), sex (P = 0.051), level of education (P = 0.12), blood glucose concentration (P = 0.53), and diastolic blood pressure (P = 0.93) (Table 1). 
Table 1. 
 
Baseline Data of the Participants of the Beijing Eye Study 2001, Stratified by the Follow-Up Characteristics of 2011
Table 1. 
 
Baseline Data of the Participants of the Beijing Eye Study 2001, Stratified by the Follow-Up Characteristics of 2011
Seen in 2011
(n = 2695)
Did Not Participate in 2011 (n = 1744) P Value (1) Died before 2011
(n = 379)
Did Not Return for 10-y Examination (n = 1365) P Value (2)
Age in 2001, y 54.6 ± 9.8 58.7 ± 11.3 <0.001 66.6 ± 10.6 56.5 ± 10.5 <0.001
Males/females 1139/1556 789/955 0.051 202/177 587/778 <0.001
Rural/urban region 1319/1376 654/1090 <0.001 247/132 407/958 <0.001
Level of education 3.92 ± 1.08 3.83 ± 1.24 0.12 3.02 ± 1.39 4.06 ± 1.10 <0.001
Refractive error, diopters −0.33 ± 2.21 −0.43 ± 2.48 0.53 −0.09 ± 2.54 −0.51 ± 2.45 0.006
Profession peasant/no peasant 1293 (48.0%) 631 (36.2%) <0.001 232 (61.2%) 399 (29.2%) <0.001
Blood glucose concentration, mmol/mL (measured in 2006) 5.50 ± 1.60 5.60 ± 1.83 0.53 5.86 ± 2.17 5.55 ± 1.76 0.06
Systolic blood pressure,
mm Hg (measured in 2006)
133.5 ± 11.1 134.8 ± 11.8 0.005 139.4 ± 13.5 134.0 ± 11.3 <0.001
Diastolic blood pressure,
mm Hg (measured in 2006)
79.0 ± 5.9 79.0 ± 6.1 0.93 78.7 ± 7.3 79.0 ± 5.9 0.87
Of the 2695 participants, 67 subjects (90 eyes) had a pterygium in 2001 who were excluded for the current incidence study. An incident pterygium was detected in 157/5300 eyes (mean ± SE) (2.9 ± 0.3%) or 129/2628 persons (4.9 ± 0.4%) (Table 2). In 101 (78.3%) of the 129 subjects, the incident pterygia had occurred unilaterally, and in 28 (21.7%) subjects, the incident pterygium had developed bilaterally. In 156 (99.4%) eyes, the incident pterygium was located nasally, whereas in one eye, the incident pterygium was located nasally and temporally. Of 44 participants with unilateral pterygium in 2001, six (13.6%) subjects developed an incident pterygium in the fellow eye during the 10-year follow up. 
Table 2. 
 
The 10-Year Incidence (Mean ± SE) of Pterygia, Stratified by Age and Sex in the Beijing Eye Study 2001/2011
Table 2. 
 
The 10-Year Incidence (Mean ± SE) of Pterygia, Stratified by Age and Sex in the Beijing Eye Study 2001/2011
Age Group, y Unilateral Pterygium Bilateral Pterygium Total
Males Females Total Males Females Total Males Females Total
40 to 49 10/370 (2.7 ± 1.1%) 32/600 (5.3 ± 1.3%) 42/930 (4.3 ± 0.9%) 2/370 (0.5 ± 1.1%) 8/600 (1.3 ± 1.3%) 10/970 (1.0 ± 0.9%) 12/370 (3.2 ± 0.9%) 40/600 (6.7 ± 1%) 52/970 (5.4 ± 0.7%)
50 to 59 12/293 (4.1 ± 1.6%) 16/481 (3.3 ± 1.2%) 28/774 (3.6 ± 1.0%) 3/293 (1.0 ± 1.6%) 5/481 (1.0 ± 1.2%) 8/774 (1.0 ± 1.0%) 15/293 (5.1 ± 0.3%) 21/481 (4.4 ± 0.9%) 36/774 (4.7 ± 0.7%)
60 to 69 19/328 (5.8 ± 1.7%) 5/355 (1.4 ± 1.2%) 24/683 (3.5 ± 1.0%) 4/328 (1.2 ± 1.7%) 3/355 (0.8 ± 1.2%) 7/683 (1.0 ± 1.0%) 23/328 (7.0 ± 1.4%) 8/355 (2.3 ± 0.8%) 31/683 (4.5 ± 0.8%)
>70 4/112 (3.6 ± 3.0%) 3/89 (3.4 ± 2.9%) 7/201 (3.5 ± 2.1%) 2/112 (1.8 ± 3.0%) 1/89 (1.1 ± 2.9%) 3/201 (1.5 ± 2.1%) 6/112 (5.4 ± 2.2%) 4/89 (4.5 ± 2.2%) 10/201 (5.0 ± 1.5%)
Total 45/1103 (4.1 ± 0.8%) 56/1525 (3.7 ± 0.7%) 101/2628 (3.8 ± 0.5%) 11/1103 (1.0 ± 0.8%) 17/1525 (1.1 ± 0.7%) 28/2628 (1.1 ± 0.5%) 56/1103 (5.1 ± 0.7%) 73/1525 (4.8 ± 0.5%) 129/2628 (4.9 ± 0.4%)
The 10-year incidence of pterygia did not differ significantly (P = 0.74) between males (5.1 ± 0.7%) and females (4.8 ± 0.5%) (Table 3). The 10-year incidence of pterygium in the subjects with an age of 40 to 49, 50 to 59, 60 to 69, and 70+ years was 5.4 ± 0.7%, 4.7 ± 0.7%, 4.5 ± 0.8%, and 5.0 ± 1.5%, respectively. 
Table 3. 
 
Associations of 10-Year Incidence of Pterygium in the Beijing Eye Study 2001/2011 (Univariate Analysis)
Table 3. 
 
Associations of 10-Year Incidence of Pterygium in the Beijing Eye Study 2001/2011 (Univariate Analysis)
Parameter Incident
Pterygium
No Incident Pterygium P Value
Sex, males/females 56/73 1047/1452 0.73
Age, y 64.1 ± 9.7 64.7 ± 9.7 0.48
Rural/urban 101/28 1157/1342 <0.001
Education level, <high school/≥high school 111/15 1516/853 <0.001
Occupation, peasant/no peasant 85/41 856/1514 <0.001
Income, <3000 RMB/mo; 3000+ RMB/mo 104/22 1183/1187 <0.001
Smoke, ever smoking/never smoking 40/86 680/1688 0.47
Alcohol consumption, ever alcohol/never alcohol 38/91 589/1910 0.13
Height, cm 160.8 ± 8.0 161.9 ± 8.1 0.19
Weight, kg 68.4 ± 11.2 66.8 ± 11.5 0.09
Cognitive function score 25.4 ± 4.0 26.5 ± 3.6 0.001
Diastolic blood pressure, mm Hg 72.9 ± 12.1 69.8 ± 12.4 0.005
Systolic blood pressure, mm Hg 136.8 ± 22.3 130 ± 20.4 <0.001
Fasting blood glucose level, mM 4.99 ± 1.30 5.5 ± 1.7 0.002
HbA1C 3.83 ± 0.73 3.85 ± 0.98 0.79
High-density lipoprotein, mM 1.66 ± 0.97 1.57 ± 1.78 0.63
Low-density lipoprotein, mM 3.30 ± 0.98 4.37 ± 43.3 0.79
C-reactive protein, mM 2.27 ± 2.95 1.99 ± 3.75 0.44
Cholestorin, mM 4.83 ± 1.03 4.96 ± 1.33 0.34
Triglyceride, mM 1.60 ± 1.22 1.84 ± 2.87 0.40
Creatinine, mM 65.76 ± 16.16 68.11 ± 30.78 0.60
Astigmatism, D 0.46 ± 1.04 0.36 ± 0.91 0.51
Best corrected visual acuity 0.9 ± 0.2 0.9 ± 0.2 0.52
Refractive error, spherical equivalent, D 0.15 ± 1.28 −0.23 ± 2.25 0.13
Intraocular pressure, mm Hg 14.2 ± 2.7 14.4 ± 2.73 0.055
Of 157 incident pterygia detected in 2011, 80 (50.9%) pterygia were detected in right eyes and 77 (49.1%) pterygia in left eyes. The length of pterygium measured as the ratio of pterygium diameter on the cornea divided by the total diameter of the cornea was 30.4 ± 14.7% of the corneal diameter (median: 28.5%; range: 2%, 88.4%). The limbal width of the pterygium as measured by the circumference of the limbus covered by the pterygium was less than 30° in 108 (68.8%) eyes, 30° to 60° in 41 (26.1%) eyes, 60° to 90° in six (3.8%) eyes, and larger than 90° in two (1.3%) eyes. Defining congestion as visible dilated blood vessels in the pterygium, 66 (42%) pterygia were congested. 
In univariate analysis, the 10-year incidence of pterygia was significantly associated with the rural region of habitation (P < 0.001; OR = 4.18; 95% CI: 2.73, 6.41) (Fig. 1), lower level of education (no high school versus high school or higher; P < 0.001; OR = 4.16; 95% CI: 2.41, 7.19) (Fig. 2), occupation (peasant versus nonpeasant; P < 0.001; OR = 3.67, 95% CI = 2.50, 5.37), lower self-reported average income (less than 3000 renminbi [RMB] versus 3000+ RMB per month; P < 0.001; OR = 4.74; 95% CI: 2.97, 7.57), higher systolic blood pressure (P < 0.001; OR = 1.02; 95% CI: 1.01, 1.02), higher diastolic blood pressure (P = 0.005; OR = 1.02; 95% CI: 1.01, 1.03), lower fasting blood glucose level (P = 0.001; OR = 0.80; 95% CI: 0.69, 0.91), and lower cognitive function score (P = 0.001; OR = 0.92; 95% CI: 0.88, 0.97). It was not significantly associated with age (P = 0.48), sex (P = 0.73), intraocular pressure (P = 0.46), best corrected visual acuity (P = 0.17), and refractive error (P = 0.13) (Table 4). 
Figure 1. 
 
Graph showing the distribution of the 10-year incidence of prevalence in the Beijing Eye Study 2001/2011, stratified by region of habitation.
Figure 1. 
 
Graph showing the distribution of the 10-year incidence of prevalence in the Beijing Eye Study 2001/2011, stratified by region of habitation.
Figure 2. 
 
Graph showing the distribution of the 10-year incidence of prevalence in the Beijing Eye Study 2001/2011, stratified by the level of education.
Figure 2. 
 
Graph showing the distribution of the 10-year incidence of prevalence in the Beijing Eye Study 2001/2011, stratified by the level of education.
Table 4. 
 
Associations of 10-Year Incidence of Pterygium in the Beijing Eye Study 2011
Table 4. 
 
Associations of 10-Year Incidence of Pterygium in the Beijing Eye Study 2011
Parameter P Value Odds Ratio 95% Confidential Interval
Univariate analysis
 Rural/urban <0.001 0.24 0.16 0.37
 Education level, <high school/≥high school <0.001 0.24 0.14 0.41
 Cognitive function score 0.001 0.94 0.90 0.98
 Occupation, peasant/no peasant <0.001 0.27 0.19 0.40
 Income, <3000 RMB/mo; 3000+ RMB/mo <0.001 0.21 0.13 0.34
 Moderate outdoor activity, d/wk <0.001 1.13 1.07 1.20
 Diastolic blood pressure, mm Hg 0.005 1.02 1.01 1.03
 Systolic blood pressure, mm Hg <0.001 1.01 1.00 1.02
 Fasting blood glucose level, mM 0.002 0.80 1.69 0.91
Multivariate analysis
 Rural/urban <0.001 0.24 0.14 0.42
 Fasting blood glucose level, mM 0.009 0.82 0.71 0.95
In the multivariate analysis, we included as independent variables all parameters that were significantly associated with incident pterygia in the univariate analysis. From this full model, nonsignificant terms were removed step by step. It revealed that the 10-year incidence of pterygium remained to be significantly associated with the rural region of habitation (P < 0.001; OR: 0.24; 95% CI: 0.14, 0.42) and lower fasting blood concentration of glucose (P = 0.009; OR: 0.82; 95% CI: 0.71, 0.95), whereas all other parameters (level of education [P = 0.90], cognitive score [P = 0.64], occupation as farmer [P = 0.47], diastolic blood pressure [P = 0.11], and systolic blood pressure [P = 0.10]) were no longer significantly associated with the 10-year incidence of pterygia (Table 4). 
Discussion
In our population-based study on adult Chinese living in Greater Beijing, the 10-year incidence of pterygia was 2.9 ± 0.3% per eye and 4.9 ± 0.4% per subject. In 78% of the subjects, the incident pterygia had occurred unilaterally, and in 99% of the eyes, the incident pterygium was located nasally. Of 44 participants with unilateral pterygium in 2001, six (13.6%) subjects developed an incident pterygium in the fellow eye during the 10-year follow up. The 10-year incidence of pterygia was associated with the rural region of habitation (P < 0.001; OR: 0.24) and lower fasting blood concentration of glucose (P = 0.009; OR: 0.82). 
The 9-year incidence of pterygia reported in the Barbados Eye Study on an African Caribbean population was 12% or 1.32% per year. 14 Compared with the Barbados Eye Study, the incidence of pterygia in our study was considerably lower. The reason for the difference between both studies may have been the prominent differences in climate, with a tropical sunny climate with high ultraviolet radiation exposure in the Caribbean region and cold winters and dusty summers in Beijing in North China. Correspondingly, the main factor associated with the incidence of pterygia in the Beijing Eye Study was the rural region of habitation. There have been no other major longitudinal studies on the incidence of pterygia, the results of which could be compared with the findings in our investigation. 
One of the factors that have so far been considered to be associated with pterygia was age. In our longitudinal study, however, age was not significantly associated with the incidence of pterygia. A similar finding was reported in the Barbados Eye Study. 14 The paradox may be explained by an age-independent constant increase in the prevalence of pterygia, so that the prevalence, however, not the incidence, increases with age. Correspondingly, cross-sectional studies have clearly shown an age-related increase in the prevalence of pterygia. The association between pterygia and the rural region of habitation as found in our study on the incidence of the disorder is parallel to the association between the prevalence of pterygia and the rural region as found in Victoria, Australia, where the prevalence of pterygia was 5-fold that in rural residents than that in urban residents. 5  
The findings of our study may have clinical implications, since a pterygium in its advanced stage needs to be operated to avoid a scar formation in the optical center of the cornea and corneal astigmatism and to prevent a restriction of the motility of the globe, usually into a direction opposite that of the pterygium. In agreement with the notion that exposure to ultraviolet light is a risk factor for the development of pterygia, the incidence in our study population in North China was considerably lower than that in the African Caribbean population. 14 Consequently, it may indicate a lower need for pterygium surgery in North China as compared with the Caribbean region. In a similar manner, pterygia may be a less frequent reason for corneal scars and visual impairment in North China than that in equatorial countries. The causes for the association between a higher incidence of pterygia and a lower fasting blood concentration of glucose have remained unclear. One of the causes could be a survival bias since patients with diabetes mellitus may not have survived the 10-year follow-up period in the same percentage as did nondiabetic participants. 
Potential limitations of our study should be mentioned. First, as in any population-based study, selection bias could have accentuated some estimates and masked others. The overall participation rate in our survey was 60.7% of the original cohort, or 66.4% of the survivors, so that it is possible that nonparticipation may have influenced the results of our study. In particular, one may have to take into account that there were statistically significant differences in age, region of habitation, profession as a peasant, and systolic blood pressure between participants of the Beijing Eye Study 2001 who were included in the present investigation and those who had died before 2011 and those who did not return for the follow-up examination in 2011 (Table 1). One may also have to consider, however, that the follow-up period of 10 years was relatively long, so that age-related mortality will have affected more markedly elderly people than young participants of 2001. It may explain the significant differences in age and age-related parameters between the study participants and nonparticipants. It has remained unclear whether these differences influenced the results, that is, led to an overestimation or underestimation of the incidence of pterygium. Second, previous pterygium surgery might have remained undetected if it was not reported in the questionnaire in 2011, so that the true incidence of pterygia may have been underestimated. However, since subjects may remember a previous eye surgery and since a previous surgery on an advanced pterygium usually leaves scars on the corneal surface, which can be detected by slit-lamp examination and on corneal photographs, it may be unlikely that this potential weakness of the study may have significantly influenced its results. If an eye showed a scar on the cornea in the typical location for a previous pterygium and if the subjects described the previous lesion typical for a pterygium, this eye was counted as incident pterygium (if there was no pterygium present in the 2001 baseline examination). Pterygia in their early stage, which usually do not leave a corneal scar after surgical removal, were unlikely operated during the study period since the medical infrastructure in the study period and in the study region was not so well developed that patients with a minor form of a pterygium went for surgery. Third, ultraviolet light exposure data were not collected, although ultraviolet light has been discussed to be a major risk factor for incident pterygia. 111 As a surrogate of ultraviolet light or outdoor activity, however, we used the profession as peasant. It revealed that in univariate analysis, the 10-year incidence of pterygia was significantly associated with the profession as a peasant (P < 0.001), whereas in multivariate analysis after adjustment for region of habitation and fasting blood concentration of glucose, occupation as farmer was no longer significantly (P = 0.47) associated with the incidence of pterygia. Strengths of our study are that it was the first longitudinal study from China to report on the incidence of pterygia and the second longitudinal study on the topic overall; that the follow-up period with 10 years was relatively long; and that the study included a relatively large population. 
In conclusion, the 10-year incidence of pterygium in adult Chinese was 4.9%, and it was significantly higher in the rural region of habitation and low fasting blood glucose concentration. 
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Footnotes
 Supported by National Natural Science Foundation of China Grant 81170890 and Beijing Nova Program Grant 2010B032.
Footnotes
 Disclosure: L. Zhao, None; Q.S. You, None; L. Xu, None; K. Ma, None; Y.X. Wang, None; H. Yang, None; J.B. Jonas, None
Figure 1. 
 
Graph showing the distribution of the 10-year incidence of prevalence in the Beijing Eye Study 2001/2011, stratified by region of habitation.
Figure 1. 
 
Graph showing the distribution of the 10-year incidence of prevalence in the Beijing Eye Study 2001/2011, stratified by region of habitation.
Figure 2. 
 
Graph showing the distribution of the 10-year incidence of prevalence in the Beijing Eye Study 2001/2011, stratified by the level of education.
Figure 2. 
 
Graph showing the distribution of the 10-year incidence of prevalence in the Beijing Eye Study 2001/2011, stratified by the level of education.
Table 1. 
 
Baseline Data of the Participants of the Beijing Eye Study 2001, Stratified by the Follow-Up Characteristics of 2011
Table 1. 
 
Baseline Data of the Participants of the Beijing Eye Study 2001, Stratified by the Follow-Up Characteristics of 2011
Seen in 2011
(n = 2695)
Did Not Participate in 2011 (n = 1744) P Value (1) Died before 2011
(n = 379)
Did Not Return for 10-y Examination (n = 1365) P Value (2)
Age in 2001, y 54.6 ± 9.8 58.7 ± 11.3 <0.001 66.6 ± 10.6 56.5 ± 10.5 <0.001
Males/females 1139/1556 789/955 0.051 202/177 587/778 <0.001
Rural/urban region 1319/1376 654/1090 <0.001 247/132 407/958 <0.001
Level of education 3.92 ± 1.08 3.83 ± 1.24 0.12 3.02 ± 1.39 4.06 ± 1.10 <0.001
Refractive error, diopters −0.33 ± 2.21 −0.43 ± 2.48 0.53 −0.09 ± 2.54 −0.51 ± 2.45 0.006
Profession peasant/no peasant 1293 (48.0%) 631 (36.2%) <0.001 232 (61.2%) 399 (29.2%) <0.001
Blood glucose concentration, mmol/mL (measured in 2006) 5.50 ± 1.60 5.60 ± 1.83 0.53 5.86 ± 2.17 5.55 ± 1.76 0.06
Systolic blood pressure,
mm Hg (measured in 2006)
133.5 ± 11.1 134.8 ± 11.8 0.005 139.4 ± 13.5 134.0 ± 11.3 <0.001
Diastolic blood pressure,
mm Hg (measured in 2006)
79.0 ± 5.9 79.0 ± 6.1 0.93 78.7 ± 7.3 79.0 ± 5.9 0.87
Table 2. 
 
The 10-Year Incidence (Mean ± SE) of Pterygia, Stratified by Age and Sex in the Beijing Eye Study 2001/2011
Table 2. 
 
The 10-Year Incidence (Mean ± SE) of Pterygia, Stratified by Age and Sex in the Beijing Eye Study 2001/2011
Age Group, y Unilateral Pterygium Bilateral Pterygium Total
Males Females Total Males Females Total Males Females Total
40 to 49 10/370 (2.7 ± 1.1%) 32/600 (5.3 ± 1.3%) 42/930 (4.3 ± 0.9%) 2/370 (0.5 ± 1.1%) 8/600 (1.3 ± 1.3%) 10/970 (1.0 ± 0.9%) 12/370 (3.2 ± 0.9%) 40/600 (6.7 ± 1%) 52/970 (5.4 ± 0.7%)
50 to 59 12/293 (4.1 ± 1.6%) 16/481 (3.3 ± 1.2%) 28/774 (3.6 ± 1.0%) 3/293 (1.0 ± 1.6%) 5/481 (1.0 ± 1.2%) 8/774 (1.0 ± 1.0%) 15/293 (5.1 ± 0.3%) 21/481 (4.4 ± 0.9%) 36/774 (4.7 ± 0.7%)
60 to 69 19/328 (5.8 ± 1.7%) 5/355 (1.4 ± 1.2%) 24/683 (3.5 ± 1.0%) 4/328 (1.2 ± 1.7%) 3/355 (0.8 ± 1.2%) 7/683 (1.0 ± 1.0%) 23/328 (7.0 ± 1.4%) 8/355 (2.3 ± 0.8%) 31/683 (4.5 ± 0.8%)
>70 4/112 (3.6 ± 3.0%) 3/89 (3.4 ± 2.9%) 7/201 (3.5 ± 2.1%) 2/112 (1.8 ± 3.0%) 1/89 (1.1 ± 2.9%) 3/201 (1.5 ± 2.1%) 6/112 (5.4 ± 2.2%) 4/89 (4.5 ± 2.2%) 10/201 (5.0 ± 1.5%)
Total 45/1103 (4.1 ± 0.8%) 56/1525 (3.7 ± 0.7%) 101/2628 (3.8 ± 0.5%) 11/1103 (1.0 ± 0.8%) 17/1525 (1.1 ± 0.7%) 28/2628 (1.1 ± 0.5%) 56/1103 (5.1 ± 0.7%) 73/1525 (4.8 ± 0.5%) 129/2628 (4.9 ± 0.4%)
Table 3. 
 
Associations of 10-Year Incidence of Pterygium in the Beijing Eye Study 2001/2011 (Univariate Analysis)
Table 3. 
 
Associations of 10-Year Incidence of Pterygium in the Beijing Eye Study 2001/2011 (Univariate Analysis)
Parameter Incident
Pterygium
No Incident Pterygium P Value
Sex, males/females 56/73 1047/1452 0.73
Age, y 64.1 ± 9.7 64.7 ± 9.7 0.48
Rural/urban 101/28 1157/1342 <0.001
Education level, <high school/≥high school 111/15 1516/853 <0.001
Occupation, peasant/no peasant 85/41 856/1514 <0.001
Income, <3000 RMB/mo; 3000+ RMB/mo 104/22 1183/1187 <0.001
Smoke, ever smoking/never smoking 40/86 680/1688 0.47
Alcohol consumption, ever alcohol/never alcohol 38/91 589/1910 0.13
Height, cm 160.8 ± 8.0 161.9 ± 8.1 0.19
Weight, kg 68.4 ± 11.2 66.8 ± 11.5 0.09
Cognitive function score 25.4 ± 4.0 26.5 ± 3.6 0.001
Diastolic blood pressure, mm Hg 72.9 ± 12.1 69.8 ± 12.4 0.005
Systolic blood pressure, mm Hg 136.8 ± 22.3 130 ± 20.4 <0.001
Fasting blood glucose level, mM 4.99 ± 1.30 5.5 ± 1.7 0.002
HbA1C 3.83 ± 0.73 3.85 ± 0.98 0.79
High-density lipoprotein, mM 1.66 ± 0.97 1.57 ± 1.78 0.63
Low-density lipoprotein, mM 3.30 ± 0.98 4.37 ± 43.3 0.79
C-reactive protein, mM 2.27 ± 2.95 1.99 ± 3.75 0.44
Cholestorin, mM 4.83 ± 1.03 4.96 ± 1.33 0.34
Triglyceride, mM 1.60 ± 1.22 1.84 ± 2.87 0.40
Creatinine, mM 65.76 ± 16.16 68.11 ± 30.78 0.60
Astigmatism, D 0.46 ± 1.04 0.36 ± 0.91 0.51
Best corrected visual acuity 0.9 ± 0.2 0.9 ± 0.2 0.52
Refractive error, spherical equivalent, D 0.15 ± 1.28 −0.23 ± 2.25 0.13
Intraocular pressure, mm Hg 14.2 ± 2.7 14.4 ± 2.73 0.055
Table 4. 
 
Associations of 10-Year Incidence of Pterygium in the Beijing Eye Study 2011
Table 4. 
 
Associations of 10-Year Incidence of Pterygium in the Beijing Eye Study 2011
Parameter P Value Odds Ratio 95% Confidential Interval
Univariate analysis
 Rural/urban <0.001 0.24 0.16 0.37
 Education level, <high school/≥high school <0.001 0.24 0.14 0.41
 Cognitive function score 0.001 0.94 0.90 0.98
 Occupation, peasant/no peasant <0.001 0.27 0.19 0.40
 Income, <3000 RMB/mo; 3000+ RMB/mo <0.001 0.21 0.13 0.34
 Moderate outdoor activity, d/wk <0.001 1.13 1.07 1.20
 Diastolic blood pressure, mm Hg 0.005 1.02 1.01 1.03
 Systolic blood pressure, mm Hg <0.001 1.01 1.00 1.02
 Fasting blood glucose level, mM 0.002 0.80 1.69 0.91
Multivariate analysis
 Rural/urban <0.001 0.24 0.14 0.42
 Fasting blood glucose level, mM 0.009 0.82 0.71 0.95
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