Choroidal expansion alone cannot account for recovery from form deprivation myopia in the chick

Purpose: Retinal (20-30%) and choroidal (60-80%) shrinkage are seen in both translucent and opaque occlusion form deprivation myopia (FDM) in chick. Also the rod outer segments are elongated and are seen in very close proximity to the basal membrane of the retinal pigment epithelium (RPE). Swelling of the choroid following occluder removal has been implicated in refractive recovery, however the temporal sequence of ultrastructuial changes in the outer retina/choroid over the first 72 hours following occluder removal has not been described. Methods: 36 hatchling chickens were occluded from day 1 to day 15. Retinoscopic measurements of refractive state and ultrasonography were carried out under light ketamine/rompun anaesthesia at 0, 2, 8, 24, 48 and 72 hours post-occluder removal. Animals were then sacrificed immediately. Results: Choroidal expansion (25-45μm) of the large vessels, and photoreceptor withdrawal from the RPE is apparent within the first 2 hours. By 8 hours post occluder removal, much of the retina is oedematous with small retinal detachments around the subretinal space. 24 hours post occluder removal refraction has changed by ∼2D, smaller vessels are apparent but abnormal and severe retinal oedema is present in 50% of animals. Photoreceptor outer segments have withdrawn from 1 5 to 8μm from the basal membrane of the RPE and their length is reduced by one third. Little change was seen in choroidal thickness over the next 24 hours but refraction changed by a further 4D. By 72 hours post-occluder removal the choriocapillaris appears normal, all evidence of oedema is gone, the choroid is significantly thicker (248μm) and refractive state shows a further shift of 2.5D towards emmetropia. There is no significant change in axial length over this 72 hour period. Conclusions: These results suggest that there is a discrepancy between the time course of choroidal thickness expansion and the refractive changes towards emmetropia. Refractive recovery appears to "wait" for the recovery of the choriocapillans.