Outdoor activity and myopia progression in 4-year follow-up of Chinese primary school children: The Beijing Children Eye Study.

To investigate factors associated with ocular axial elongation and myopia progression during a 4-year follow-up in primary school children in Beijing.This school-based study included 382 grade-1 children at baseline in 2011 (age:6.3±0.4 years) with 305 (79.8%) returning for the follow-up examination in 2015. At baseline and in yearly follow-up examinations, the children underwent a comprehensive eye examination including auto-refractometry, ocular biometry with measurement of axial length, and fundus photography. The parents underwent a standardized interview.During the study period, the mean axial length elongated by 1.15±0.56mm in boys and 1.10±0.63mm in girls. At baseline and at the end of follow-up, axial length was significantly (P