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8th Scientific Meeting in Optometry

The aim of this study was to investigate the role of scleral creep in the process of axial eye elongation in experimentally induced myopia in the chick and the tree shrew. Myopia was induced with a diffusing occluder worn over one eye for several days. In the in vitro experiments, scleral samples were cut from myopic and contralateral (control) eyes and loaded with a tensile force of 5 g for 20 minutes, while creep strain was monitored. In the in vivo experiments, the intraocular pressure (IOP) of anaesthetised animals was raised to 100 mmHg for one hour via an intra-vitreal cannula, while ocular component dimensions were monitored with A-scan ultrasound. Results of in vitro experiments showed that in both chick and tree shrew, scleral samples from myopic eyes had significantly (P < 0.05) greater creep strains than samples from control and normal eyes (n = 10, per group). Among individual tree shrews the difference in creep strain of samples from the myopic and control eyes correlated with vitreous chamber elongation (r = 0.746) and myopia (r = 0.792) of the deprived eye. No such association was found in the data from chicks. In the in vim experiments, the axial eye length of both species initially increased on raising IOP. Thereafter, axial length and vitreous chamber depth progressively increased in chick eyes (n = 4/4) but decreased in tree shrew eyes (n = 8/8). In chick and tree shrew, form deprivation myopia is associated with increased scleral creep in vitro. In tree shrew, the correlation between increased scleral creep and induced myopia suggests that scleral creep is a potential mechanism for eye enlargement in myopia. However, the in vivo experiments imply that other choroidal or scleral mechanisms can actively reduce axial eye dimensions in tree shrews. In this study, the relationship between the Stiles-Crawford effect and myopia was investigated. It is well established that the receptors across the retina are oriented in such a way that they align themselves toward the centre of the exit pupil, but in myopes there is a unique yet consistent characteristic in receptor alignment in the nasal retina, specifically between the fovea and the optic nerve head. The common finding among myopes was that the receptors in the temporal retina were pointing towards the centre of the exit pupil as in normal eyes (emnietropes) whereas those receptors in the nasal retina were aligning more nasally toward the optic nerve head. The degree of such nasal bias in receptor alignment was found to correlate with the axial length of eyeball.