This nested case–control study used the Japanese administrative claims database to investigate the association between statin use and onset of OAG. Since studies using databases have several limitations, we constructed a total of four patterns in the two-model design to increase the robustness of our results. Consequently, our findings revealed that there was no significant association between short-term statin use (< 2 years) and onset of OAG in Japanese working-age population with dyslipidemia, a finding that was consistent across all models used in this study.
The National Health and Nutrition Survey in Japan (2018), reported by the Ministry of Health, Labour and Welfare, showed that the frequency of high low-density lipoprotein cholesterol levels decreased after peaking in the 40s for men and in the 50s for women [13]. The proportion of males in our study was approximately 60%, with a median age of 52 years, indicating that the JMDC database can reflect the portion of the Japanese population with dyslipidemia. Approximately half of the patients diagnosed with dyslipidemia were prescribed statins once or more in both cases and controls. The number of patients prescribed statins once or more times was higher in model 2 (≥ 24 months of observation) compared to model 1 (≥ 12 months of observation), suggesting that a long-term prevalence of dyslipidemia may lead to an increased number of statin users.
One of the advantages of our study is that it confirms the use of a nested case–control design for estimating the hazard ratio. Our results can be interpreted in that a 12- or 24-month period administration of statins to young patients with dyslipidemia does not make a significant difference in the hazard ratio of OAG compared to patients without administration of statins. A nested case–control study targeting elderly Australians also showed that statin use was not associated with glaucoma onset [14]. However, their study showed an increased risk of glaucoma onset in participants with a longer duration (> 3 years) of statin use compared with a shorter duration (< 1 year). No consistent conclusions have been extracted regarding the relationship between the duration of statin administration and the risk of OAG, although some reports have suggested that long-term or short-term statin use can reduce the respective risk [7, 15]. Another large-scale study with a long follow-up duration of five or more years also showed that longer statin use was not associated with risk of OAG [8]. The twenty-four months period of exposure to statins was considered in our study; however, longer-term observations may need to be considered. Racial differences may influence the relationship between statin use and OAG. A cross-sectional study using big data in the United Kingdom showed that no evidence of a protective association between statin use and glaucoma was found [16]. In a study of the Korean population, dyslipidemia treatment was not found to be significantly associated with OAG [17]. Regardless of the racial or ethnic composition, statins are presumably not involved in the onset of OAG. No statin was found to be significantly associated with the onset of OAG in our results for the individual statin investigation. For simvastatin, the odds ratios in all the analyses were below 1.0, although not statistically significant. Simvastatin has been reported for its possible role in visual field stabilization in glaucoma patients [18]. Consideration with a larger sample size of simvastatin is desired.
Furthermore, several reports have examined the relationship between serum cholesterol levels and glaucoma/intraocular pressure [19, 20]. Serum HDL3 cholesterol level, which is involved in the promotion of cholesterol efflux and has a protective effect on vascular endothelium, was reported to be associated with glaucoma [9]. These reports indicate that dyslipidemia and blood lipid level are associated with glaucoma and high intraocular pressure. Regardless of the use or nonuse of statins in dyslipidemia, poor control of dyslipidemia may increase the risk of glaucoma depending on serum cholesterol level.
Comorbidities, such as diabetes, myopia, and the use of steroids, were identified as significant risk factors for OAG in our models, a finding which is consistent with current literature. Diabetes mellitus has already been suggested to increase the risk of OAG [21, 22], because higher serum glucose levels are associated with high intraocular pressure, which is thought to result in optic nerve damage [23]. Myopia, especially high myopia, has been reported as a major risk factor for OAG [24, 25]. This is further supported by the Tajimi study conducted in Japan, which also showed an association between myopia and glaucoma [4]. Finally, steroid use has been long known to increase intraocular pressure, resulting in steroid-induced glaucoma [26–28]. Patients with these factors are at a particularly increased risk of developing OAG, thus regular examinations are extremely essential.
Nonetheless, our study has several limitations. First, since the JMDC administrative claims database is based on social insurance enrollees in the Japanese working-age population, most of the people registered is under 65 years of age, and thus data for patients over 75 years old are not included. This is especially important considering that the incidence of glaucoma increases with age [29]. Therefore, this study may be underestimating events of OAG in patients over 65 years old. Second, OAG is associated with ocular features, such as intraocular pressure and family history. However, this information was not included in the JMDC claims database. Serum cholesterol level was also not included. Therefore, these factors could not be considered in our analysis. Furthermore, our definition might not accurately identify primary open angle glaucoma. Glaucoma is likely heterogenous (open-angle glaucoma, exfoliative glaucoma, some secondary glaucoma and some ocular hypertension). Therefore, there would have been misclassification of the outcome in this study. Third, the number of prescriptions for statins was used as an indicator of exposure. However, factors such as adherence to statins, daily dose, or dose intensity were not taken into consideration. Furthermore, modifiable environmental factors, such as lifestyle, exercise, and nutrition [30], could not be investigated.