Visual Psychophysics and Physiological Optics Shape of the Posterior Vitreous Chamber in Human Emmetropia and Myopia

PURPOSE To compare posterior vitreous chamber shape in myopia to that in emmetropia. METHODS Both eyes of 55 adult subjects were studied, 27 with emmetropia (mean spherical error [MSE] ≥ -0.55; <+0.75 D; mean +0.09 ± 0.36 D) and 28 with myopia (MSE -5.87 ± 2.31 D). Cycloplegic refraction was measured with a Shin Nippon autorefractor and anterior chamber depth and axial length with a Zeiss IOLMaster. Posterior vitreous chamber shapes were determined from T2-weighted magnetic resonance imaging (3.0-T) using procedures previously reported by our laboratory. Three-dimensional surface model coordinates were assigned to nasal, temporal, superior, and inferior quadrants and plotted in two dimensions to illustrate the composite shape of respective quadrants posterior to the second nodal point. Spherical analogues of chamber shape were constructed to compare relative sphericity between refractive groups and quadrants. RESULTS Differences in shape occurred in the region posterior to points of maximum globe width and were thus in general accord with an equatorial model of myopic expansion. Shape in emmetropia is categorized distinctly as that of an oblate ellipse and in myopia as an oblate ellipse of significantly less degree such that it approximates to a sphere. There was concordance between shape and retinotopic projection of respective quadrants into right, left, superior, and inferior visual fields. CONCLUSIONS Prolate ellipse posterior chamber shapes were rarely found in myopia, and we propose that spherical shape in myopia may constitute a biomechanical limitation on further axial elongation. Synchronization of quadrant shapes with retinotopic projection suggests that binocular growth is coordinated by processes that operate beyond the optic chiasm.