To evaluate longitudinal changes in subfoveal choroidal thickness (SFChT) among children receiving 0.05%, 0.025%, 0.01% atropine over two-years and their associations with treatment outcomes in myopia control DESIGN: : Double-blinded randomized controlled trial METHODS: : SFChT was measured at 4-month intervals using spectral domain optical coherence tomography. Cycloplegic spherical equivalent (SE), axial length (AL), best-corrected visual acuity, parental SE, outdoor time, near work diopter hours, and treatment compliance were also measured.
Totally 314 children with qualified choroidal data were included. The 2-year changes in SFChT from baseline were 21.15±32.99 μm, 3.34±25.30 μm, and -0.30±27.15 μm for the 0.05%, 0.025%, and 0.01% atropine groups respectively (p<.001). A concentration-dependent response was observed, with thicker choroids at higher atropine concentrations (β=0.89, p<.001). Mean SFChT thickness significantly increased at 4 months in 0.025% and 0.05% atropine groups (0.05% atropine, p.05 for all groups). Over two years, increase in SFChT was associated with slower SE progression (β=0.074, p<.001) and reduced AL elongation (β=-0.045, p<.001). In the mediation analysis, 15.5% of effect on SE progression from 0.05% atropine was mediated via its choroidal thickening.
Low concentration atropine induced a choroidal thickening effect along a concentration-dependent response throughout the treatment period. The choroidal thickening was associated with a slower SE progression and AL elongation among all the treatment groups. Choroidal response can be used for assessment of long-term treatment outcomes and as a guide for concentration titrations of atropine.

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