We read with interest the article by Ma and associates on spectacle correction and myopia.
As the authors pointed out, near work and peripheral retina defocus are identified factors in myopia progression. Wear of conventional spectacles might contribute to both hyperopic defocus in near vision (in presence of high accommodative lag) and peripheral defocus. However, as indicated by the authors, 2 prospective studies with follow-ups of 18–24 months failed to show any beneficial effect of undercorrection. More recently, in a retrospective study researchers found that the greater the undercorrection, the greater the myopic progression. In a case-control study published in 2015, the authors found no significant differences in myopia progression between the group of children undercorrected and those fully corrected: −0.64 diopter (D)/year and −0.68 D/year, respectively. Mean axial elongation was also similar (0.31 mm/year for both groups). However, using the regression analysis the authors found that progression decreased slightly with increasing amounts of undercorrection, although there was a lot of variability among the subjects. The full correction group had a progression very similar to that of children with undercorrection up to 0.50 D (−0.68 D vs −0.69 D), but in children with an undercorrection higher that 1.50 D it was smaller (−0.42 D), without reaching statistical significance.
Ma and associates acknowledged that lack of data regarding final refractive status, making it impossible to compare the spherical equivalent change between groups over the study period, was a weakness of their study. Visual acuity might reflect refractive changes, but it is also influenced by many factors like children’s cooperation, environmental light conditions (which might vary from one examination site to another), and performance of examiners (in this case, trained nurses and staff assistants). Children randomized to receive glasses had around a quarter line of vision chart lines better than those randomized to the control group. Taking into account that the ETDRS charts display 5 letters per line, we are talking about just 1 letter of difference. We are not sure that this small difference might be clinically significant.
Additionally, since the mean amount of progression in myopic children aged 6–12 years from many studies is around 0.6 D/year, we think that another significant weakness of the study was the short follow-up time. A period of 8 months seems not long enough to draw a robust conclusion on myopia progression (a maximum difference of around 0.3 or 0.4 D in refraction was expected to occur in the effect would be maximum).
According to these results, it seems to us that although the balance is leaning toward the possibility that the undercorrection of myopia has no beneficial effect on slowing the progression, it is still an unsolved question. More studies are needed, like the one the authors performed but with longer follow-up and preferably including refraction and axial length data.