Myopia Classification

The screening for myopia and other refractive errors, which was conducted in a single military draft center, was performed in the following manner: a medical history was obtained, and the candidate’s unaided visual acuity was evaluated using a standard Snellen chart at 6 m. All candidates with unaided visual acuity lower than 6/6 in either eye underwent noncycloplegic refraction by using an automated refractometer (Speedy K; Nikon Corp; KR-8000, KR7000S, and earlier models; Topcon), and the results were recorded. This procedure was followed by a complement subjective refraction for validation using a Snellen chart. For each participant, refractive error was calculated as the spherical equivalent (SEQ = sphere + [cylinder/2]). Myopia was defined as an SEQ of −0.50 D or less.¹⁹ Low myopia was defined as an SEQ between −0.50 and −2.99 D, moderate myopia was defined as an SEQ between −3.00 and −5.99 D, and high myopia was defined as an SEQ of −6.00 D or more.¹⁹ The classification for each individual was made based on the worse SEQ between both eyes. Worse SEQ was chosen to be used for myopia definitions after preliminary analysis that demonstrated a satisfactory correlation between both eyes (Pearson correlation coefficient, 0.92).