In its reanalysis of population-based refraction data of 1,985 individuals from the 1958 British Birth Cohort study, a recent study in London has found that even small changes in the threshold definition of myopia (±0.25D) can significantly affect the conclusions of epidemiological studies, creating both false-positive and false-negative associations for specific risk factors. Any study of myopia can render meaningfully different findings, necessitating different interpretations, depending on the minimum threshold of spherical equivalent used to define myopia.

Researchers investigated the impact of reclassification of individuals when using different threshold values of spherical equivalent to define myopia, on estimates of frequency, distribution and associations with risk factors. Using less stringent thresholds, prevalence of mild myopia nearly doubled (from 28% to 47%) and could have been amplified further if the study had not used the mean spherical equivalent values.

Alongside this, there was a significant increase in the effect size of the positive association of mild myopia prevalence with higher social class and a significant decrease in the effect size of the positive association with higher educational attainment, with changes in risk ratios of approximately 20%.

To improve translational value and minimize the risk of spurious findings, researchers suggest an international classification for refractive error that is empirically evidenced and cognizant of the question(s) being addressed and the population(s) being studied.

Cumberland PM, Bountziouka V, Rahi JS. Impact of varying the definition of myopia on estimates of prevalence and associations with risk factors: time for an approach that serves research, practice and policy. Br J of Ophthalmol. 2018;102:1407-1412.