Vitamin D level in patients with pterygium

Pterygium is a chronic ocular surface disease characterized by invasion of the cornea by fibrovascular, wing-shaped conjunctival tissue that leads to impairment of visual acuity and cosmetic appearance. Risk of pterygium is increased by inflammation, viral infection, age, and genetic factors. Epidemiological studies have implicated ultraviolet light exposure as having a significant causative association with pterygia. Interleukin (IL)-6 and IL-8 are released by pterygium epithelial cells after ultraviolet irradiation, and accumulation of these proinflammatory cytokines in tears may induce chronic inflammation, fibrovascular proliferation, and pterygium formation. Vitamin D is a circulating steroid hormone with antiangiogenic, anti-inflammatory, and antifibrotic activity. Several ocular conditions including age-related macular degeneration, myopia, and allergic conjunctivitis have been associated with vitamin D, but there is limited information on vitamin D level in patients with pterygium. The primary study hypothesis was based on a possible relationship between pterygium and vitamin-D level. Pterygium is caused by chronic inflammation and fibrovascular ingrowth after ultraviolet exposure. Because vitamin D has antiangiogenic, anti-inflammatory, and antifibrotic activity, an inverse relationship between vitamin D level and pterygium would be expected. Second, pterygium is one of the most common UV-induced ocular diseases, and outdoor activity has been reported to be significantly associated with pterygium formation. Because the primary natural source of vitamin D is in synthesis in the skin secondary to sun exposure, vitamin D levels may be increased in patients with pterygium, especially outdoor workers. In this study, we investigated blood vitamin D level in patients with pterygium.


INTRODUCTION
Pterygium is a chronic ocular surface disease characterized by invasion of the cornea by fibrovascular, wing-shaped conjunctival tissue that leads to impairment of visual acuity and cosmetic appearance.Risk of pterygium is increased by inflammation, viral infection, age, and genetic factors.Epidemiological studies have implicated ultraviolet light exposure as having a significant causative association with pterygia (1)(2)(3) .Interleukin (IL)-6 and IL-8 are released by pterygium epithelial cells after ultraviolet irradiation (4) , and accumulation of these proinflammatory cytokines in tears may induce chronic inflammation, fibrovascular proliferation, and pterygium formation.
Vitamin D is a circulating steroid hormone with antiangiogenic, anti-inflammatory, and antifibrotic activity (5)(6)(7) .Several ocular conditions including age-related macular degeneration, myopia, and allergic conjunctivitis have been associated with vitamin D, but there is limited information on vitamin D level in patients with pterygium (8)(9)(10) .The primary study hypothesis was based on a possible relationship between pterygium and vitamin-D level.Pterygium is caused by chronic inflammation and fibrovascular ingrowth after ultraviolet ex-posure (4) .Because vitamin D has antiangiogenic, anti-inflammatory, and antifibrotic activity, an inverse relationship between vitamin D level and pterygium would be expected (5,6) .Second, pterygium is one of the most common UV-induced ocular diseases, and outdoor activity has been reported to be significantly associated with pterygium formation (11) .Because the primary natural source of vitamin D is in synthesis in the skin secondary to sun exposure, vitamin D levels may be increased in patients with pterygium, especially outdoor workers.In this study, we investigated blood vitamin D level in patients with pterygium.The study group included patients with pterygium, and the control group consisted of age-and sex-matched healthy volunteers without pterygium.All subjects were of the same race and had lived in the same region for at least 5 years.The participants were stratified by the amount of time spent indoors or outdoors by asking questions about their job and routine daily activities including walking, playing outdoor sports, and others.The outdoor group included those who spent >75% of the daytime hours in outdoor activities.The indoor group spent >75% of the daytime hours in indoor activities.

RESUMO
Participants with ocular and systemic diseases that may be associated with altered vitamin D levels, including myopia, age-related macular degeneration, diabetic retinopathy, glaucoma, diabetes, Crohn's disease, Alzheimer disease, and others were excluded.To avoid a possible confounding influence on activity we also excluded adult subjects with a body mass index above 30 kg/m 2 .evaluation Clinical evaluation included visual acuity using an Snellen chart, intraocular pressure measurement, slit-lamp biomicroscopy, and a fundus examination.The pterygium diagnosis was made by a single ophthalmologist (NK).Pterygium size was measured using a slit lamp, a horizontal slit beam of light from the limbus to the head of the pterygium and a vertical beam to the highest vertical extent of pterygium tissue on the cornea.

vitamin d meaSurement
Blood samples were collected from both study and control participants during the same autumn season and assayed at the Gaziantep University School of Medicine clinical laboratory.Serum 25-hydroxyvitamin D (25OHD, nmol/L) is the primary indicator of vitamin D status and is the form that was assayed.

StatiStical analySiS
Statistical analysis was performed with SPSS (version 16.0, Chicago, IL).The normality of all data distributions was confirmed with the Shapiro-Wilk test.Between-group differences in continuous variables were compared by the independent-sample t-test.The chi-squared test was used to compare differences in categorical variables.P-values <0.05 were considered statistically significant.

RESULTS
The demographic and clinical characteristics of the study and control groups are shown in table 1.There were no significant between-group differences in age and sex ratio.None of the participants routinely used sunglasses or hats regularly.Overall mean vitamin D levels (nmol/L) in patients with pterygium and controls were not significantly different (p=0.731,Table 1).In women, vitamin D levels in the study and control groups were not significantly different (P=0.86).In men, the vitamin D level was significantly higher in those with pterygium than in those without it (P=0.020,Table 2).In the pterygium group, vitamin D levels were significantly increased in men (P=0.000) and in the outdoor activity group (P=0.010,Table 3).In the control group, vitamin D levels in men and women (P=0.928) and the indoor and outdoor activity groups (P=0.126) were not significantly different (Table 4).Vitamin D level in the pterygium group was not significantly correlated with either age or horizontal and vertical pterygium length (P>0.05,Table 5).

DISCUSSION
Vitamin D research has become increasingly popular in ophthalmology Dadaci et al. found decreased plasma vitamin D levels in patients with seasonal allergic conjunctivitis (9) .In a population-based study, Yoo et al. reported that the lowest serum vitamin D quintile had a significantly increased odds ratio of open angle glaucoma (12) .Galor et al. reported that increased vitamin D was associated with decreased dry eye syndrome symptoms (13) .There is also evidence that low-level outdoor sunlight exposure may be involved in myopia pathogenesis (14) .Grotting et al. found that decreased vitamin D was associated with increased risk of anterior uveitis (15) , and Jee et al. reported that increased serum vitamin D was associated with decrea sed risk of age-related cataracts in humans (16) .
In this study, vitamin D levels were significantly higher in male, but not female, patients with pterygium than in controls.In patients with pterygium, vitamin D levels were significantly higher in men than in women, possibly because men had more outdoor activity than women, which resulted in more UV exposure.Vitamin D level was also significantly higher in pterygium group participants with more outdoor activity than in those with more indoor activity.That is in line with a recent study by Jee et al. (17) who found a positive association between blood vitamin D level and pterygium even after adjusting for sun exposure.They also reported that the relative odds of pterygium were significantly increased in third, fourth, and fifth vitamin D quintiles versus the lowest quintile.
The pathogenesis of pterygium is poorly understood, but histopathological evidence suggests that genetic factors, cytokines, growth factors, antiapoptosis activity, extracellular matrix modeling, immunological responses, and viral activity may be involved (18) .It has been proposed that elevated vitamin D levels may increase cellular calcium, which in turn would activate S100 protein (19) .S100 is a calcium-activated signaling protein that interacts with other proteins to modulate diverse biological processes (20) .An increase in S100 protein has been seen in pterygium tissue compared with normal conjunctiva (19) .Although increase in vitamin D level has been associated with more time spent outdoors, Mutti et al. did not find a such a relationship (21) .Similarly, in this study, vitamin D level in healthy controls did not differ significantly with the extent of indoor or outdoor activity.Patients with pterygium with more outdoor time had higher vitamin D levels than patients who spent most of the day indoors.These findings are consistent with an association of vitamin D level and pterygium formation.
An alternative explanation is that pterygium formation may occur as a direct effect of focal UV exposure of conjunctival tissue independent of vitamin D level.Conjunctival damage from UV exposure may activate a complex series of oxidative reactions and cell-death pathways (22) .UV exposure activates epidermal growth factors, expression of proinflammatory cytokines, and matrix metalloproteinases    in pterygium cells.Focal UV radiation may also destroy limbal stem cells, which may then result in conjunctival invasion of the cornea and pterygium formation (23) .A previous case-control study reported a significant association of outdoor work and pterygium formation, possibly because sunlight exposure increased the risk compared with indoor workers (11) .The same investigators also confirmed an increased risk of pterygium formation in individuals who spent more time in the sun, especially with cumulative exposure over 5-10 years.Epidemiological studies have also reported an association between pterygium and increased ultraviolet radiation exposure in lower latitude regions (24) .Patients with Pterygium thus tend to live in regions with high UV indices ant to work outdoors.Jee et al. (17) reported an association of vitamin D and pterygium formation in both men and women, but we found it in men only.There is evidence of a significant gender effect on vitamin D status (25) .Low vitamin D levels appear to be common in postmenopausal women (26) , and Jee et al. found a negative correlation of vitamin D level and development of nonproliferative or proliferative diabetic retinopathy in men but not in women. (27)In observational studies of patients with type 2 diabetes mellitus, both Scragg et al. and Suzuki et al. concluded that mean vitamin D concentrations were significantly higher in men than in women (28,29) .Gender-spe cific association of vitamin D receptor polymorphism has an effect on susceptibility to diabetes (30) , but the relation has not been studied in pterygium.It is not clear why vitamin D is associated with pterygium in men but not in women, but the relationship may depend on outdoor activity.Further studies are needed to identify the factors responsible for this difference and to explain the nature of the sex-related biological mechanisms.
One of the study limitations was not estimating the food and supplement intake of the patients.The primary source of vitamin D is production in the skin secondary to sun exposure, but small amounts are obtained from foods such as fatty fish and milk.Another was not comparing vitamin D levels in women with primarily indoor and outdoor activity.This was because most women were homemakers who spent most of the day indoors.We did not perform histopathological evaluations, which would have confirmed conjunctival squamous metaplasia or primary acquired melanosis in pterygia samples.Also, because we did not conduct a genetic analysis we do not know whether any patients may have had a genetic predisposition for pterygium.In summary, in this series of patients with pterygium, vitamin D level was significantly increased in men and in participants with more outdoor activity.

Table 1 . Demographic and clinical characteristics of patients with pterygium and healthy controls
N= number; SD= standard deviation; mm= millimeter; n/a= non-applicable; *= independent sample t test.

Table 2 . Demographic and clinical characteristics of men and women with and without pterygium
N= number; SD= standard deviation; mm= millimeter, n/a= non-applicable; *= independent-t test.