Association between pupil size and response to atropine therapy in the Atropine Treatment of Myopia study (ATOM2).

At baseline, the mean photopic and mesopic pupil size was 3.96+/-0.63 mm and 4.48+/-0.66 mm, respectively.

Pupils on higher doses were more dilated, with eyes with smaller pupil size at baseline having smaller pupil size on treatment.

So while a larger change in pupil size (e.g. ≥2.5 mm in 6.0-8.9year old children, or ≥2 mm in older children) was more likely to be associated with a better result; having a smaller pupil size change did not necessarily suggest a poor result.