The effect of retinal image defocus on the thickness of the human choroid

Abstract

PURPOSE. The aim was to describe the amplitude and time-course of choroidal thicknesschanges induced by imposed hyperopic and myopic retinal defocus and to compare theresponses in emmetropic and myopic subjects. METHODS. Six emmetropic and 6 myopic East Asian subjects aged 18 to 34 years, viewed a distant target (video at 6 metres) for 60 mins on two separate occasions while optical coherence tomography (OCT) images of the choroid were taken in both eyes every 5 mins to monitor choroidal thickness. On each occasion, one eye was optimally corrected for distance with a contact lens while the other eye wore a contact lens imposing either 2.00D hyperopic or 2.00D myopic retinal defocus. Three of the emmetropic subjects repeated the experiment,viewing the target for 60 mins with OCT every 5 mins, but without monocular defocus.RESULTS. Choroidal thickness in myopic eyes (mean ± SD: 256.30μm ± 41.24μm) wassignificantly less than in emmetropic eyes (423.09μm ± 60.68μm; p<0.01) and was inverselycorrelated with myopia magnitude, reducing by 39μm per dioptre of myopia (R2 = 0.68).Repeated measures ANOVA (GLM) analysis revealed that in both subject groups, 2.00D ofmyopic defocus caused a rapid increase in choroidal thickness (significant by 10 mins,increasing to ~20μm within 60 mins: p<0.01) whereas 2.00D of hyperopic defocus caused aslower choroidal thinning (significant by 20-35 mins; ~20μm within 60 mins: p<0.01). In the 3emmetropic subjects viewing the DVD binocularly with no defocus, the choroid thickened inboth eyes by ~15μm, an effect which may be explained by myopic defocus from a lead ofaccommodation. CONCLUSIONS. Small but significant choroidal thickness changes were caused by retinal defocus. Choroidal thickness increased within 10 minutes of exposure to monocular myopic defocus, but thinned more slowly in response to monocular hyperopic defocus. In our smallsample we could detect no difference in the magnitude of thickness changes in myopic eyescompared to emmetropic eyes. Binocular viewing at distance with no imposed defocuscaused choroidal thickening, suggesting that future studies should concentrate on the effectsof binocular rather than monocular defocus.