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Study links low cone photoreceptor density and high myopia
Invest Ophthalmol Vis Sci. 2008;49(10):4679-4687.
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Cone photoreceptor packing density was found to be lower in highly myopic eyes than in emmetropic eyes. A long axis and low cone density in myopic eyes may diminish visual function.
The in vivo study included 11 patients with a mean age of 26.6 years who were assigned to one of three groups based on refractive error: Five subjects were emmetropes, four were low to moderate myopes and two were high myopes. Only right eyes were studied. Patients underwent axial length measurement and high-resolution imaging of cone photoreceptors. Four sets of cone images were obtained from each eye.
Data showed a statistically significant link between axial length and refractive error (P<.0001).
“One possible explanation linking structure and function in myopic eyes is that retinal stretching caused by expansion of the posterior pole may lead to a reduction in neural sampling density,” the study authors said. “Therefore, our study provides further evidence in support of the hypothesis that retinal stretching in myopic eyes reduces retinal sampling density.”
Myopic eyes are larger in general, consistent with axial elongation, and this study shows that the “stretching” spreads the cellular mosaic as one might expect. However, cell density on an angular basis (ie, cells per degree squared rather than mm squared) is relatively constant. Thus, myopes have similar resolving capacity when images are properly focused on the more distant retina. This affects clinical practice little at present, but it is important to understand how the retina develops and how the actual mosaic of cone cells relates to our visual capabilities.
There is always a need for more research. The enormous power of adaptive-optics imaging is just beginning to be realized in ophthalmology. We can actually see photoreceptors now, and this will over time let us look at many normal and disease states from a cellular perspective. An example was my own study with Choi et al showing that an absent foveal pit (“fovea plana”) does not necessarily mean that the cones themselves were abnormal. Sharp vision is not only a function of the cone mosaic density in the fovea, but also of the efficiency of retinal connections and integration through the other retinal layers. Some patients with dystrophies are known to have a lot of cone loss and yet can read well, although they will have reduced contrast sensitivity and other capabilities. We are just beginning to sort out the relevant factors.
– Michael F. Marmor, MD
Professor of ophthalmology, Stanford University School of Medicine, Stanford, Calif.