Ocular Volume and Blood Flow in Human Anisomyopia

Abstract

Abstract: : Purpose: Measurements of ocular pulse amplitude and associated extrapolations to ocular blood flow are affected by refractive error (RE), axial length and total ocular volume. The study investigates, with reference to an isomyopic control group, the extent to which inter-eye differences between each of these parameters can account for inter-eye differences in estimates of pulsatile ocular blood flow in anisomyopia. A new technique for measuring total ocular volume is presented based on three separate measurements of anterior, intermediate and posterior sectors of the globe. Methods: Subjects, all Caucasians aged 18 - 26 years, 13 male, 15 female, comprised 14 anisomyopes and 14 isomyopes. The inter-eye difference in RE was for the anisomyopic group ≥ 2D ≤ 4D and for the isomyopic group ≤ 0.5D. Both groups were matched for age, degree and range of myopia (based on mean sphere of both eyes). Refractive error was measured objectively with an autorefractor (open-field IR autorefractor, Canon) and axial length with ultrasound (Storz Omega Compu-Scan Biometric Ruler). For each subject, rotational volumes for the anterior chamber, posterior segment (± 35°) and intermediate segment of the globe were generated with Mathcad software (Mathsoft) using data on longitudinal axial length and polynomial curve fitting for, respectively, the cornea, retina and scleral shell. Pulsatile ocular blood flow was measured with a pneumatonometer, the Blood Flow Analyser (Paradigm Inc.), which does not incorporate a correction factor for RE. Results: Statistically significant relationships were evident for inter-eye differences in: RE and axial length (r₂=0.54, p<0.01); RE and ocular volume (r₂=0.273, p<0.01); RE and ocular blood flow (r₂=0.184, p<0.05); ocular volume and ocular blood flow (r₂=0.19, p<0.05). Interestingly no significant correlation was found for inter-eye differences in axial length and ocular volume for myopia ≤ -7DS (r₂=0.12, p=0.08). Conclusion: We suggest that inter-eye differences in ocular volume do not account for the inter-eye differences in ocular blood flow for low to moderate levels of myopia. Animal models of myopia have shown choroidal blood flow to be reduced as a consequence of myopia development. We propose then that in anisomyopia actual differences in choroidal blood flow may account for the inter-eye differences in ocular blood flow.

Keywords: 481 myopia • 543 refractive error development • 542 refraction