Pterygium is often considered to be a primarily cosmetic concern; however, visual reduction may ensue due to induced astigmatism and eventually obscuration of the visual axis. Sunlight, in particular, ultraviolet-B radiation, is a major risk factor; and population-based studies1^–10 demonstrate region-dependent prevalence rates, ranging from 1.2% in urban, temperate Caucasian populations,6 to 36.6% in adult Indians of the Brazilian rainforest.1 But even in studies conducted in tropical zones1–5 7 9 visual impairment (acuity<6/18) due to pterygium is rare, and to our knowledge, pterygium blindness has never been reported in a population-based study. The Meiktila District in central Myanmar is predominantly a rural region, where, anecdotally, pterygium remains a cause of preventable visual impairment. Robust epidemiological data from this region regarding the prevalence, severity, impact on vision, and risk factors for pterygium would assist in the development of prevention and treatment strategies using the limited available healthcare resources.

MATERIAL AND METHODS

The Meiktila Eye Study was a population-based, cross-sectional ophthalmic survey of the inhabitants of rural villages in the Meiktila District of central Myanmar. The township of Meiktila lies centrally within this District and is located at 20°53′N, 95°53′E. Meiktila has a tropical climate with an average sun transit time of 12 h, and the people are predominantly involved in outdoor, agricultural activities. Participants were randomly selected using a stratified, cluster sampling process. A sampling frame consisting of the list of all villages in the Meiktila District with their populations was obtained from the Ministry of Health. Villages were arbitrarily stratified as large (population of 826 or more) or small (population of 825 or less), with small villages in each of the six zones within the Meiktila District constituting six separate strata. (This design was chosen to optimise the precision of the visual impairment prevalence estimates.) Detailed methodology has been published elsewhere.11

Study population

In brief, all persons 40 years and over within each selected village were eligible for inclusion. There was a total sample population of 2481 people, all belonging to the Burman ethnic group.

Data collection

Data collection occurred at the end of the rainy season in November 2005. A single survey team conducted the entire study. All equipment and personnel were transported to each village, and the data collection occurred on site. A medical and ophthalmic history was obtained from each patient in their own language by qualified healthcare workers. The interview obtained information about smoking, occupation and the use of ophthalmic drops (most commonly gentamicin). Each participant then received a comprehensive vision and eye examination, which included: presenting, and pin-hole visual acuity (VA) using a consistently lit, non-linear, front-illuminated, illiterate, E Snellen chart at 6 m; slit lamp (Model SL-3C, Topcon, Tokyo) examinations of ocular surface, anterior segment, and lens; and stereoscopic fundus examination. If the VA was <6/18 in either eye, based on the comprehensive assessment, an experienced ophthalmologist assigned a principal cause of visual impairment.

Definitions

A pterygium was defined as a radially oriented fibrovascular lesion crossing the nasal or temporal limbus. Grading was based on the visibility of the underlying episcleral blood vessels. This has been previously described and validated as a marker of severity.12 T1 (“atrophic”) is defined as episcleral vessels clearly visible, T2 (“intermediate”) as vessels partially visible, and T3 (“fleshy, opaque”) as vessels wholly obscured. Size was measured as the greatest distance from the limbus to the apex of the lesion. Two experienced ophthalmologists conducted all of the pterygium examinations with good agreement for the grading (kappa = 0.71).

Blindness and low vision were defined according to WHO criteria as corrected VA <3/60 in the better eye and corrected VA <6/18 but ⩾3/60 in the better eye, respectively. Visual impairment was defined as the combined set of low vision and blindness. Field defects were not taken into consideration. Pinhole vision was considered best-corrected vision for the purposes of the study (aphakic patients used a +10 dioptre (D) lens and pinhole).

Ethics

The study was approved by the Ministry of Health in Myanmar and had ethical approval from the Royal Adelaide Hospital Ethics Committee. Consent for participation was obtained from the head of each village prior to commencement of the survey, and written, informed consent, in the participants’ own language, was obtained from all willing participants. The study was conducted in accordance with the Declaration of Helsinki. Treatment of minor ailments was provided free of charge at the examination sites, and participants blind from cataract or other treatable cause, including pterygium, were offered referral to Meiktila Eye Hospital.

Data analysis

Prevalence data for the various eye conditions were calculated as ratio estimates using appropriate weights for each of the sampled villages. Bootstrapping was used to overcome the problem of variance estimation in clusters where only the one primary sampling unit (village) was selected. The associations between pterygium and risk factors were assessed using logistic regression models, all of which took into account the sampling design. Confidence intervals (CI) were calculated to include 95% of potential values, and statistical significance was accepted where p values were ⩽0.05. All calculations were performed using SAS Version 9.1 (SAS Institute, Cary, NC).

RESULTS

A total of 2481 subjects were eligible, and 2076 (83.7%) participated. Adequate data about pterygium were obtained on all participants. Non-responders in each village were usually unable to attend due to occupation-related commitments. The mean age of those surveyed was 56.2 years (SD 11.6). There were 836 males (40.2%) and 1240 females (59.8%). (table 1)

The prevalence of pterygium in either eye was 19.6% (95% CI 16.9 to 22.2) and of bilateral pterygia 8.0% (95% CI 7.7 to 8.3). There were 84 people who were blind in both eyes, two of whom were blinded in both eyes from pterygium (1.7%; 95% CI 0.2 to 6.1). There were 230 blind eyes with pterygium accounting for 2.2% (95% CI 0.7 to 5.0) of blindness in at least one eye. No participants had low vision in both eyes due to pterygium, but pterygium led to 0.8% (95% CI 0.3 to 1.6) of low vision in at least one eye. Pterygium was therefore associated with 0.4% (95% CI 0.04 to 1.3) of binocular visual impairment and 1.0% (95% CI 0.6 to 1.8) of visual impairment in a least one eye. The age- and gender-specific prevalence of pterygium in either eye is presented in table 1.

There was a significant relationship between age and the prevalence of pterygium, where those people aged 40 to 49 had reduced prevalence of pterygium compared with those in any older age category (p<0.005). This effect did not persist on the multivariate analysis. There was no significant difference in the prevalence of pterygium between the genders.

The most prevalent clinical grade of pterygium was grade 2 (intermediate), affecting 8.7% of the population (table 2). Increasing age conferred a higher risk of higher-grade pterygium; however, this did not reach statistical significance (p = 0.06). This was also true for outdoor work, where those people who were predominantly employed in outdoor occupations were more likely to have higher-grade pterygium (p = 0.001; see fig 1).

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Figure 1 High-grade pterygium leading to blindness in this participant. Informed consent was obtained for publication of this figure.

There were pterygia on 537 eyes. The average extent from the limbus to the apex of these pterygia was 2.2 mm (SD 1.3). Grade 2 pterygia (2.3 mm) were slightly longer than grade 1 (2.1 mm) pterygia; however, grade 3 pterygia averaged only 2.1 mm in length, making the trend non-significant (p = 0.35). There was no significant difference in the grade of lesions in the right eye compared with the left eye (table 3).

The presence of pterygium was associated with astigmatism (defined as cylinder greater than or equal to 0.5 D; p = 0.01). This association increased with increasing grade of lesion (p for trend <0.01). For those with pterygium, cylinder increased with higher grade of lesion (p = 0.02) and the further the pterygium extended from the limbus (p<0.01).

Logistic regression analysis results are presented in table 4. There was no significant difference in pterygium prevalence according to gender, age, smoking history or the use of eye-drops. Those people who primarily worked outdoors were 1.5 times more likely than people who primarily worked indoors to have pterygium (p = 0.001).

DISCUSSION

These findings represent the first results relating to pterygium that have been derived from a population-based ophthalmic study in central Myanmar. The results demonstrate a high prevalence of pterygium among the people of this region, where it appears that one in five people over the age of 40 years suffer with pterygium in at least one eye. This represents a higher rate than has previously been reported in Indonesia for people over 40 years (16.8%)3 and is second only to subjects who were examined as part of the Barbados Eye Study (23.4%).4 Pterygium is often thought of as a relatively benign condition; however, this paper demonstrates that it is not without morbidity.

Pterygium accounted for 2.2% of blindness among those people who were blind in at least one eye. This is relatively high, given that previously no percentage of worldwide blindness has been attributed to pterygium.13 Similarly, pterygium was responsible for 1.0% of the visual impairment among those who had visual impairment in at least one eye, which demonstrates the visually significant sequelae that untreated pterygium may have. The contribution of pterygium to visual impairment is primarily through the induction of astigmatism; however, advanced pterygium may cause corneal opacity over the visual axis. Two participants had perception of light vision only due to large bilateral pterygia.

Gazzard et al3 previously demonstrated the association of pterygium with astigmatism. The results of this paper support these findings. The presence of pterygium was associated with cylinder greater than or equal to 0.5 dioptres (D; p = 0.01). This association increased with increasing grade of lesion (p for trend <0.01) and the further the pterygium extended from the limbus (p<0.01). It has previously been suggested that pterygium induces astigmatism by at least two mechanisms. These include contraction and scarring with subsequent flattening of the cornea, and/or tear-film effects.14

This study has shown a variable prevalence of pterygium according to age on multivariate analysis (table 4). Table 1, however, demonstrates a significant increase in the prevalence of pterygium with age (p = 0.005), but when gender is considered the significant trend does not persist in males (p = 0.78) but is present among females (p = <0.001). This may be because men begin to work outdoors earlier than women, and so the pterygium occurs earlier, making the difference in the prevalence of disease more even across the age groups. The proportion of women in this sample approached 60%, and this is more likely to reflect a higher rate of male non-responders than the population gender distribution, which may also impact on these findings. Previously, studies have demonstrated an increased prevalence of pterygium with increasing age.3 7 8 It is likely that advancing age provides a greater exposure to ultraviolet radiation, which is known to induce mutations in the p53 gene,15 and has been epidemiologically associated with the prevalence of pterygium.16^–19 There was no gender association demonstrated in the multivariate analysis; however, men have previously been reported to suffer pterygium more frequently than women.6 8 This has predominantly occurred in Western cultures where men are perhaps more likely to spend time working outdoors, but in Myanmar both men and women are involved in outdoor activities, particularly in farming districts such as Meiktila.

The multivariate risk factor model constructed for this population demonstrated an independent increase in the risk of pterygium among those who primarily work outdoors. This would support the hypothesis that increased exposure to ultraviolet radiation leads to an increased risk of pterygium. Table 1 demonstrates the significantly increased odds of pterygium in those who primarily work outdoors (p = 0.002), and it is interesting to note that this effect does not persist among men (p = 0.18), whereas it continues to be significant among women (p = 0.002). This may represent the fact that nearly 80% of men work primarily outdoors (compared with only 60% of women), or are involved in other outdoor activities that may balance the prevalence of pterygium across the two groups.

Unlike the Indonesian population previously described by Gazzard et al,3 there appeared to be no significant difference in the length of pterygium from the limbus as the grade increased (table 3). This does not discount the assertion that the clinical grade of the lesion has an impact on pterygium severity (in terms of astigmatism, fleshiness or recurrence), as we did find a significant association between astigmatism and pterygium severity.

Although the participation rate was reasonable (83%), we have no knowledge about the demographics or visual status of the non-participants. Anecdotally, (according to the village chiefs), the principal reasons for non-participation were occupation-related. As most of the non-responders had an outdoor occupation (farming), it is possible that the prevalence of pterygium is under-reported in this study. This effect may have also reduced the proportion of men in the survey. Furthermore, although individuals with pterygium-related low vision may not have participated, it is less likely that any individuals blinded by pterygium were missed.

Our survey of the people of central Myanmar demonstrates a high prevalence of pterygium and highlights its potential to induce visual impairment. Pterygium can be unsightly, irritating and potentially blinding, leading to economic, social and personal cost.20 It is therefore important that pterygium prevention programmes become an integral part of public healthcare and community health services in order to further diminish the prevalence of this condition. Future work may focus on the modifiable risk factors identified by this paper, such as educational intervention promoting the use of protective measures such as sun avoidance, sunglasses and hats, especially for those with outdoor occupations. Further epidemiological work should be undertaken to monitor the prevalence of the condition and determine the most cost-effective and sustainable prevention and treatment options that are culturally appropriate for the people of central Myanmar.