Association between axial length elongation and spherical equivalent progression in Chinese children and adolescents

Background It is generally believed that a 1-mm axial length (AL) elongation of the eye corresponds to a -3.00 D spherical equivalent (SE) progression, but this is disputed. Purpose To investigate the association between AL elongation and SE progression among children and adolescents. Methods A prospective cohort study of 710 children and adolescents aged 6-16 years was included. Ophthalmic examinations, including cycloplegic SE, AL and corneal curvature, were performed at baseline and 1-year follow-up. The ratio of SE change (Delta SE) to AL change (Delta AL) (Delta SE/Delta AL) was calculated, and its association with age and refractive status was explored using a general linear model. Results Among all participants, 396 (55.77%) were male, with 265 (37.32%) myopes at baseline. The average 1-year Delta SE and Delta AL were 0.61 +/- 0.40 D and 0.33 +/- 0.22 mm, respectively. Both Delta SE and Delta AL gradually decreased with age (p < 0.001). In the general linear model analyses, age and refractive status were independently associated with Delta SE/Delta AL after adjustment for covariates (age: beta$$ \hat{\beta} $$ = 0.04, p < 0.05; myopia vs nonmyopia: beta$$ \hat{\beta} $$ = 0.28, p < 0.05). Based on the developed formula Delta SE/Delta AL = 1.74 + 0.05*age (for myopes), mean Delta SE/Delta AL in myopes increased from 2.06 D/mm in the 6-year-olds to 2.59 D/mm in the 16-year-olds. In nonmyopes, Delta SE/Delta AL = 1.33 + 0.05*age, and the ratio increased from 1.65 D/mm in the 6-year-olds to 2.18 D/mm in the 16-year-olds. Conclusions The ratio of Delta SE/Delta AL varied with age and refractive status in children and adolescents. The age-specific Delta SE/Delta AL could be used to estimate SE progression through the actual AL change.