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Barbara Swiatczak, Marita P Feldkaemper, Frank Schaeffel; Measuring changes in fundal reflectance in myopic chickens with isotropic photorefraction.. Invest. Ophthalmol. Vis. Sci. 2018;59(9):697.
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We found previously that fundal reflectance in humans declines when they are exposed to bright outdoor illuminances for only a few minutes, in particular in the short wavelength part of the spectrum. The physical or biochemical basis of these changes is unknown. To find out whether such changes may also occur when myopia is induced, we measured fundal reflectance in normal-sighted and myopic chickens using a single ultraviolet (UV) LED (375 nm) that was centered in the camera lens. Pupil brightness, corrected for pupil area, was taken as a measure of fundal reflectance. However, since the optics of isotropic photorefraction may affect the brightness of the pupils in eyes with refractive errors, they were corrected with trial lenses during the measurements.
A monochrome ultraviolet camera (SONY XC- EU50-CE) with a UV transmitting lens (F=60mm, f/# 4) and a UV LED (LED375L,ThorLabs) was used. Software was developed in Visual C++ to flash the UV LED (100 msec duration) and to analyze the pixel brightness in the pupil that was back-illuminated after reflection of light from the fundus. Measurements were taken in a distance of 44cm. Myopia was induced in one eye in one week old male chickens (n=12) by placing frosted plastic diffusers in front of one eye for six days.
All chickens developed myopia (average -4.6D ± 2.0D) in eyes treated with diffusers. Pupil sizes did not change when myopia was induced. Interocular differences in pupil brightness between both eyes, myopic and control, increased with the amount of induced myopia, both with and without trial lens correction (p<0.004 and p<0.02 without). Differences were not significant in chicks with low myopia (less than 3D) but were conspicuous in the cases of high myopia.
Since myopic eyes have higher f/# (longer eyes but similar pupil sizes), darker pupils would have been expected. Brighter pupils therefore suggest an increased fundal reflectivity. A confounding factor in these measurements could have been that uncorrected refractive errors increase pupil brightness due to the optics of isotropic photorefraction. However, the differences in pupil brightness persisted after correction of the refractive errors by trial lenses, suggesting that they are based on an increase in fundus reflectivity. The causes for higher fundal reflectivity in myopic chick eyes are unknown and require further studies.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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