Abstract
Purpose:
Recent studies (Read et al, IOVS 2010) have shown that positive defocus, imposed monocularly for up to one hour, can reduce axial length and increase choroidal thickness in human eyes (range of changes 10 µm). Since these small changes tend to reduce the imposed refractive errors, they could be considered as precursors of emmetropization. Furthermore, since emmetropization is also driven by peripheral refractive errors, it is of interest whether defocus imposed only in the periphery can also change central axial length and choroidal thickness.
Methods:
In the present study, axial length was measured in both eyes, before and after 30 and 60 minutes treatment with a +3D lens in the dominant eye in 13 young adult subjects, using the Lenstar LS 900® (Haag Streit AG, Koeniz, Switzerland). In 8 of the subjects, choroidal thickness could also be measured. Three conditions of blur were tested: (a) Full field myopic blur, (b) restricted peripheral myopic blur, induced by a lens with a central hole, leaving 13 deg of the central field unobstructed and (c) the control condition without induced blur. The viewing target consisted of a movie projected on a 4m wide screen seen at a distance of 5m. The stimulus was divided into one central screen (13 deg diagonal) and 4 peripheral ones.
Results:
Axial length tended to increase on average by 7.27 ± 11.29 µm after 30 minutes and 4.05 ± 10.66 µm after 60 minutes exposure to full field myopic blur but these changes did not achieve significance (P = 0.06 and P = 0.24). Providing only peripheral myopic blur also resulted in a trend of increased axial length (3.50 ± 9.02 µm and 3.38 ± 9.81 µm, respectively; P = 0.19 and P = 0.24). Choroidal thickness decreased after 60 minutes of myopic blur (full field blur -11.17 ± 15.57 µm; only peripheral blur -6.55 ± 13.59 µm), although this was, again, not significant.
Conclusions:
Different from previous observations, we could not induce shorter axial length and thicker choroids in human eyes by short term imposed positive defocus in the dominant eye. In contrast, a trend of a change in the opposite direction emerged which did not achieve significance, whatever the tested condition. As both studies used the same instrument for the measurements, other explanations must apply like (1) different luminances (2) different ethnicity of the subjects (3) different diurnal cycles or (4) different stimuli (movie and size).
Keywords: 511 emmetropization •
677 refractive error development •
406 adaptation: blur