The interaction between homatropine and optical blur on choroidal thickness

To analyse the short-term interaction between a short period of myopic and hyperopic defocus and the muscarinic antagonist homatropine upon the choroidal thickness and ocular biometrics of healthy subjects.

Thirty young adults (15 myopes and 15 emmetropes) aged 18-35 years had subfoveal choroidal thickness (ChT) and ocular biometry measurements taken before, 30 min, and 60 min following the introduction of monocular optical blur (0.00 D, +3.00 D and -3.00 D) combined with administration of either 2% homatropine or placebo (total of six conditions). Each combination of optical blur and drug was tested on different days, 2 days apart, in randomised order. For choroidal thickness, we captured three SD OCT images (5 mm, cross scans centred at the fovea with 999 A-scans and 50 B-scans) with the Copernicus SOCT HR instrument (www.optopol.com). A masked observer manually segmented the average B-scan images to derive subfoveal choroidal thickness measurements from each measurement session.

The choroid exhibited significant thinning after imposing hyperopic defocus (-3.00 D) combined with placebo (-11 ± 3 μm, p < 0.001). Homatropine prevented the significant choroidal thinning response with hyperopic defocus (+3 ± 2 μm), and the magnitude of ChT change was significantly different to placebo and hyperopic defocus (p < 0.001). There was a significant increase in ChT after the introduction of myopic defocus (+3.00 D) with placebo (+12 ± 3 μm, p < 0.0001) and homatropine combined with myopic defocus also caused a similar increase in ChT (+11 ± 3 μm; p < 0.001). Eyes treated with homatropine alone exhibited a significant increase in ChT (+14 ± 3 μm, p < 0.0001). There was no evidence of differences in choroidal response between refractive groups. Axial length also underwent small but significant changes (all p < 0.01 except homatropine/hyperopic blur and placebo) that were of similar magnitude, but of opposite direction to the changes in choroidal thickness.

Homatropine appears to block the thinning effect of hyperopic defocus on choroidal thickness but did not enhance the thickening effect of myopic defocus. The changes in the choroid may relate to the different pathways in the eye's response to myopic and hyperopic blur or reflect an upper limit on the capacity of the choroid to thicken in the short-term.