Axial length as a function of age, sex, and ethnicity: Results from the CLEERE study

Axial length is emerging as the primary outcome variable used for assessing myopia control efficacy, in both clinic and clinical trials. This report provides a model of axial length as a function of age, sex, and race/ethnicity, in addition to percentiles of axial length across age in childhood.

To model axial length in juvenile-onset myopia and children in general as a function of age, sex, race/ethnicity, parental history of myopia, diopter-hours of near work, and hours of outdoor/sports activities.

Axial length from the time of myopia onset was modeled using quadratic fits as a function of age, sex, race/ethnicity, and other covariates. Myopic participants were 590 children in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) study with at least three annual visits: one without myopia, an onset visit 1 year later (spherical equivalent at least -0.75 D), and another visit after myopia onset. Percentiles for axial length from the entire CLEERE sample were determined using 23,154 observations from 4877 children.

Axial elongation in myopic children was greatest at younger ages, slowing with age in a quadratic trajectory between 6 and 14 years. The average rate of elongation at a given age, however, was independent of the age of myopia onset. In the general sample of children, axial length percentiles at age 6 years were similar across racial/ethnic groups, but Asian American children had the steepest increases in axial length with age, followed by Native American and Hispanic children. The shallowest increases occurred in Black and White children. Females had shorter axial lengths than males by 0.4 to 0.5 mm, but a higher probability of being myopic for a given age and axial length percentile. Parental history of myopia, time spent reading, and time spent in outdoor/sports activity were not significant factors for axial length in multivariate models.

The models of axial length as a function of age, sex, and race/ethnicity, along with their percentiles, may prove useful in sample size planning for clinical trials, for judging efficacy of myopia control in individual children, and for comparison to more recent datasets.