Is the fovea the apposite location for investigating the effect of accommodation on posterior eye conformation?

Purpose : Previous studies have investigated transient changes to axial eye length during accommodation, with the presumption that maximum eye elongation occurs along the visual axis. Yet, it is unknown whether the visual axis is the most appropriate datum to measure these changes, and whether axial elongation is consistent across the posterior eye. Consequently, this study addresses three questions: 1) does peripheral eye length alter with accommodation; 2) is the effect of accommodation on eye length homogenous across the retina; and 3) does the maximum change in eye length occur at the fovea or at another eccentric location(s). Methods : Eye length measurements were obtained from the right eye of 29 participants (age 20.9 years (SD 2.10), MSE -1.34 D (SD 1.52)), consisting of 13 myopes (MSE -2.81 D (SD 0.98)) and 16 emmetropes (MSE -0.14 D (SD 0.38)) with the IOLMaster. Data for the axial length and six eccentric fixation points (5°, 15°, 25°) in both the horizontal (nasal and temporal) and vertical (superior and inferior) planes, corresponding to the central 50° (14 mm) of the posterior pole, were collected in random order, whilst subjects viewed a Maltese cross target for one minute at three accommodative demand levels (0 D, 4 D, 8 D). Corneal topography was also measured. Eye length data were converted back to optical path length using the equation OPL = (GL + 1.367)/0.7711. The data were used alongside a semi-customised eye model, based on the individual corneal topography and Navarro eye model, to compute the retinal intercept coordinates. Subsequently, the retinal sagittal height was calculated and a best conic curve was fitted. Results : The sagittal height increased with accommodative effort indicating retinal stretch, which was homogenous across the posterior pole. The maximum increase to the sagittal height occurred between 7 mm nasal retina and 4.5 mm temporal retina as well as 1.5 mm in the inferior retina, rather than solely at the visual axis. The retinal sagittal height was invariant with accommodation for the whole cohort, along both meridians, although, there was a significant effect of refractive error on the axial length (F_7.961, p= 0.009), with the greatest elongation occurring within the myopic cohort. Conclusions : The findings suggest that, during accommodation, the vitreous may exert pressure on the posterior pole producing the observed uniform eye elongation.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.