To investigate the association between smartphone use and axial length and refractive error in teenagers using the Myopia app.

Cross-sectional population-based study.

A total of 525 teenagers aged 12 to 16 year old from six secondary schools and from the birth cohort study Generation R participated in this study.

A smartphone application (Myopia app) was designed to objectively measure smartphone use and face to screen distance, and to pose questions about outdoor exposure at regular intervals. Participants underwent cycloplegic refractive error and ocular biometry measurements. Mean daily smartphone use was calculated in hours per day; continuous use in the number of episodes of 20 minutes on screen without breaks. Linear mixed models were conducted with smartphone use, continuous use, and face to screen distance as determinants, and spherical equivalent (SER) and the ratio of axial length and corneal radius (AL/CR) as outcome measures stratified by median outdoor exposure.

Main outcome measures SER in dioptres and AL/CR ratio.

The teenagers were on average 13.7 (0.85) years old, 54% of them were girls. Myopia prevalence was 18.9%. During schooldays, total smartphone use was on average 3.71 (1.70) hr/day, and was only borderline significantly associated with AL/CR (β=0.008, 95%CI=-0.001, 0.017) and not with SER. Continuous use was on average 6.42 (4.36) episodes of 20 minutes use without breaks/day, and was significantly associated with SER and AL/CR (β=-0.07, 95%CI=-0.13, -0.01; β=0.004, 95%CI=0.001-0.008, respectively). When stratifying for outdoor exposure, continuous use remained only significant for teenagers with low exposure (β=-0.10, 95%CI=-0.20, -0.01 and β=0.007, 95%CI=0.001-0.013 for SER and AL/CR, respectively). Smartphone use during weekends was not significantly associated with SER and AL/CR, nor was face to screen distance.

Dutch teenagers spent almost 4 hours per day on their smartphones. Episodes of 20 minutes continuous use was associated with more myopic refractive errors particularly in those with low outdoor exposure. This study suggests that frequent breaks should become a recommendation for smartphone use in teenagers. Future large longitudinal studies will allow more detailed information on safe screen use in youth.