EFFECTS OF CONTINUOUS LIGHT ON EXPERIMENTAL REFRACTIVE ERRORS IN CHICKS

It is possible to induce ametropias in young chicks either by deprivin g the developing eye of clear form vision with a translucent goggle or by defocusing the retinal image with convex or concave lenses, The re fractive properties of the developing chick eye are also altered by ra ising young birds in a continuous light environment. The effects of su perimposing form deprivation or defocus treatments on chicks raised in continuous light are unclear. Newly hatched (n = 31) chicks were rais ed for 2 weeks under continuous light while wearing either translucent goggles or + 10 or - 10 diopter (D) lenses over one eye. Refractive s lates, corneal curvature and intraocular dimensions were measured peri odically by retinoscopy, keratometry and A-scan ultrasound. The birds were sacrificed after 2 weeks and the eyes removed and measured with c alipers, Under continuous light, all eyes treated with translucent gog gle and - 10 D lens developed moderate myopia (- 2.6 +/- 0.5 D and -1. 4 +/- 0.3 D, respectively) by day 4. The eyes treated with a + 10D len s developed moderate hyperopia (+ 4.8 +/- 0.5 D) at day 4. Corneal cur vatures of all treated eyes were slightly, but significantly, larger t han contralateral control eyes by day 4. After 2 weeks of goggle or le ns application, all the treated eyes were hyperopic due to corneal fla ttening. But the eyes treated with a goggle or a - 10D lens still show ed relative myopia compared to the fellow eyes (treated minus untreate d = - 3.8 +/- 0.4 D and -2.8 +/- 0.4 D, respectively), and the eyes tr eated with a + 10 D lens showed more hyperopia than fellow eyes (treat ed minus untreated = + 5.1 +/- 0.6 D). Compared with the control eyes, the axial length (mainly vitreous chamber depth) was slightly, but si gnificantly, increased in the eyes treated with a goggle or a - 10D le ns, and the axial length decreased slightly in the eyes treated with 10D lens. The results suggest that form deprivation and retinal defoc us (induced by +/- 10D lenses) could still induce experimental refract ive errors (myopia and hyperopia) in chicks kept under continuous ligh t, but the effects of form deprivation and retinal defocus were partia lly suppressed by continuous light. Copyright (C) 1996 The College of Optometrists.