Effect of retinal image defocus on the thickness of the human choroid

PurposeTo describe the time-course and amplitude of changes to sub-foveal choroidal thickness (SFCT) induced by imposed hyperopic and myopic retinal defocus and to compare the responses in emmetropic and myopic subjects. MethodsTwelve East Asian subjects (age: 18-34years; six were emmetropic and six had myopia between -2.00 and -5.00dioptres (D)) viewed a distant target (video movie at 6m) for 60min on two separate occasions while optical coherence tomography (OCT) images of the choroid were taken in both eyes every 5min to monitor SFCT. 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. ResultsBaseline SFCT in myopic eyes (mean S.D.): 256 +/- 42m was significantly less than in emmetropic eyes (423 +/- 62m; p<0.01) and was correlated with magnitude of myopia (-39m per dioptre of myopia, R-2 = 0.67: p<0.01). Repeated measures anova (General Linear Model) analysis revealed that in both subject groups, 2.00D of myopic defocus caused a rapid increase in SFCT in the defocussed eye (significant by 10min, increasing to approximately 20m within 60min: p<0.01), with little change in the control eye. In contrast, 2.00D of hyperopic defocus caused a decrease in SFCT in the experimental eye (significant by 20-35min. SFCT decreased by approximately 20m within 60min: p<0.01) with little change in the control eye. ConclusionsSmall but significant changes in SFCT (5-8%) were caused by retinal defocus. SFCT increased within 10min of exposure to 2.00D of monocular myopic defocus, but decreased more slowly in response to 2.00D of monocular hyperopic defocus. In our relatively small sample we could detect no difference in the magnitude of changes to SFCT caused by defocus in myopic eyes compared to emmetropic eyes.