|Author:||Chu, Chin Hung Geoffrey|
|Title:||Astigmatism and ocular structural correlates in chicks|
Eye -- Growth.
Chickens as laboratory animals.
Hong Kong Polytechnic University -- Dissertations
|Pages:||xxvi, 165 pages : illustrations ; 30 cm|
|Abstract:||Purpose: To study: 1) the effects of hemiretinal form deprivation on eye growth; 2) the effects of experimentally induced astigmatism on corneal accommodation; and 3) the effects of optically imposed astigmatism on eye growth. Methods: White Leghorn chicks were used. The treatment started from 5 days of age and usually lasted for 1 to 3 weeks. Refractive status was measured by using a Hartinger refractometer. Eye shape profiles and corneal topography were measured by using an eye shape imaging system and a videokeratography, respectively. Right eye served as the treated eye while left eye served as untreated control; a separate group of birds received no treatment served as the control group. In Exp.1 (Chapter 2), hemiretinal form deprivation was used to cover the visual fields corresponding to four retinal regions: superior (SRD), inferior (IRD), temporal (TRD) and nasal (NRD). Refractive changes over three weeks were recorded and the eye shape profiles along four meridians were captured at the end of the experiment. In Exp.2 (Chapter 3), the characteristics of corneal accommodation in normal chicks and chicks with experimentally induced astigmatism were studied. The videokeratography provided a continuous recording of the changes in the corneal profile over time, allowing further characterization of corneal accommodation. In Exp.3 (Chapter 4), crossed-cylinder lenses were used for optically imposing astigmatism. The effects of orientation (45, 90, 135 and 180) and magnitude (+4.00DS/-8.00DC and +2.00DS/-4.00DC) on corneal topography and eye shape profiles were studied. Results: Exp.1. Differences in refractive status and eye shape profiles were found when different retinal regions were form-deprived. SRD group exhibited the highest magnitudes of spherical-equivalent (M) among the four treatment groups. Astigmatism was also induced, but only subtle differences were found across the treatment groups. Exp.2. Bi-directional changes in corneal accommodation were found in normal and astigmatic chicks. The magnitudes of positive accommodation were associated with those of the induced astigmatism. Exp.3. Both the orientation and magnitude of optically imposed astigmatism influenced the characteristics of induced astigmatism. Chicks treated with WTR astigmatism (minus cylinder axis 90) developed the highest magnitude of induced astigmatism, whereas those treated with ATR astigmatism (axis 180) developed the lowest magnitude of astigmatism. Both corneal and internal astigmatism contributed about 50% of the refractive astigmatism. Conclusions: These studies extended our current knowledge about the role of visual error signals on the genesis of astigmatism. In particular, the changes in ocular biometric parameters from the anterior (i.e., corneal curvature) to the posterior segment (i.e., eye shape profile) should be considered during astigmatic eye growth.|
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