Abstract
Purpose :
To determine if the area of complete spatial summation (Ricco’s area, RA) enlarges in response to retinal stretching in non-pathological axial myopia.
Methods :
Achromatic contrast thresholds were measured for six circular stimuli of different area (0.01–2.07 deg2) but fixed duration (200ms) at four visual field locations (10° eccentricity, along 90°, 180°, 270° and 360° meridians) in 24 participants with axial myopia (mean -4.14DS, range -0.50DS to -9.75DS) and 20 age-similar non-myopic controls (mean +0.71DS, range +1.75DS to -0.25DS). Stimuli were presented on a gamma-corrected CRT display (refresh rate 75 Hz) with a background luminance of 10 cd/m2. To explore the effect of refractive error induced variations in retinal image size (RIS) on the measurement of RA, refractive error was separately corrected with (i) full aperture trial lenses placed at the anterior focal point (constant RIS in mm), and (ii) contact lenses (variable RIS). Spatial summation functions were generated using two-phase regression analysis from which the size of RA was estimated. Co-localised off-axis measures of ocular length were obtained using a customized IOL-Master (Carl-Zeiss Meditec, USA).
Results :
With spectacle correction, RA was significantly larger (p=0.02) in the myopes (median -0.92 log deg2, IQR -1.10 to -0.78) compared to the non-myopic controls (median -1.14 log deg2, IQR -1.29 to -1.07). For contact lens correction, there was no significant difference in RA (p=0.44) between the myopic (median -1.19 log deg2, IQR -1.28 to -0.96) and non-myopic groups (median -1.14 log deg2, IQR -1.24 to -0.87). RA was significantly smaller with contact lens than spectacle correction in the myopic group only (p=0.01). A statistically significant positive correlation was observed between the peripheral spectacle-corrected RA and peripheral ocular length (Kendall’s tau=0.24, p=0.03). This relationship was not found with contact lens correction (Kendall’s tau=-0.06, p=0.58).
Conclusions :
RA increases in myopia when differences in projected RIS are accounted for. Such changes are likely a compensation for reduced retinal ganglion cell density secondary to axial elongation in myopia. Further research is required to determine whether changes in RA could be used as a functional biomarker for progressive axial myopia and the implications such findings have for perimetric strategies that seek to probe changes in RA in glaucoma.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.