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Education and myopia: assessing the direction of causality by mendelian randomisation

BMJ 2018; 361 doi: (Published 06 June 2018) Cite this as: BMJ 2018;361:k2022

Linked Editorial

Intense schooling linked to myopia

Linked Opinion

Education and myopia: assessing the direction of causality by mendelian randomisation

Re: Education and myopia: assessing the direction of causality by mendelian randomisation

Dear Editor,
With great interest, we read the mendelian randomisation study undertaken by Edward Mountjoy and colleagues that showed exposure to more years in education contributes to the higher myopic refractive error by −0.25 dioptres/y. The study proved a strong causal relationship between education and myopia. The relationship is important to the prevention and control of myopia. Meanwhile, we also notice the following important details and hope the authors of the manuscript could help to clarify them.
From Table 1 in the supplementary of the study, the average age of the sample population was 57.09 years old. Therefore, even those who have been educated for the longest time (university and above) should have about 30 years of work experience. The long term working experience could also influence the refractive status of the individuals. Some occupations have been reported to be associated with myopia [1-2], such as lawyer, physicians and editors. The potential mechanisms could be excessive near work required during the occupational life. From Figure 3 in the supplementary of the study, the SNPs used as the instrumental variables affects the individuals’ education level via IQ. However, some study has indicated that children’s IQ could also influence their occupations [3]. As a result, another potential pathway from the SNPs of IQ to the myopia occurrence via occupation also existed , that pathway could bias the effect estimation of education. For example, a lawyer’s myopia refraction may continue developing during his/her career due to extensive near work. However, if this myopia progression had not been considered, the effect of occupation on myopia might be absorbed into that of education, making the effect of education to be overestimated. Therefore, we would like to know how this study got the pure effect estimation of education and removed the mixed effect of occupation. We hope the author could provide some clarification.

1. Shimizu N, Nomura H, Ando F, et al. Refractive errors and factors associated with myopia in an adult Japanese population. Jpn J Ophthalmol. 2003 Jan-Feb;47(1):6-12.
2. Wong TY, Foster PJ, Hee J, et al. Prevalence and risk factors for refractive errors in adult Chinese in Singapore. Invest Ophthalmol Vis Sci. 2000 Aug;41(9):2486-94.
3. Ramos-Olazagasti MA, Castellanos FX, Mannuzza S, et al. Predicting the Adult Functional Outcomes of Boys With ADHD 33 Years Later. J Am Acad Child Adolesc Psychiatry. 2018 Aug;57(8):571-582.e1.

Competing interests: No competing interests

19 January 2019
Senlin Lin
Doctor of Public Health
Yingyan Ma, Haidong Zou
Shanghai Eye Disease Prevention and Treatment Center / Shanghai Eye Hospital
No. 380, Kangding Road, Shanghai, China