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Nimesh Bhikhu Patel, Ronald S Harwerth; Macula Structure Function Relationships Incorporating the Stimulus Area. Invest. Ophthalmol. Vis. Sci. 2016;57(12):No Pagination Specified. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
It has been demonstrated that the OCT derived ganglion cell inner plexiform layer (GCIPL) in the macula region of the retina reflects early neuropathy in glaucoma. However, reported relationships between GCIPL thickness (structure) and perimetry (function) have considerable variability, likely because perimetry measurements have relied on a stimulus area (Goldmann Size III) that exceeds the limits of spatial integration for central retinal locations. The purpose of this study was to determine whether incorporating the area of the GCIPL at the retinal region sampled by perimetry improves the clinical structure-function relationship.
Data were collected from two rhesus macaques that had undergone unilateral argon laser application. Both animals were behaviorally trained to perform clinical perimetry (central 10-2 fields) using stimulus sizes I-V and measures for perimetry and OCT were obtained over the time course of experimental glaucoma. After accounting for ocular magnification, RGC displacement, and ocular torsion, the GCIPL volume (stimulus size x [GCIPL thickness – residual]) and the corresponding increment threshold for each test location were used to determine the structure function relationship.
After transformation to logarithmic measures, the relationship between differential luminance and GCIPL volume was described using a two line function following spatial summation. For GCIPL volume measures of less than 1.99x10-4 mm3, the relationship was steeper (slope=-1, R2=0.65, p<0.01) compared to larger volume measures (slope=-0.35, R2=0.46, p<0.01). Measures corresponding to standard size III stimuli from the control eyes were located on the shallow slope, while those with experimental glaucoma were either at the breakpoint or on the steeper portion of the function, depending on the stage of neuropathy.
When GCIPL volume, which is directly related to the number of RGCs, is used the structure-function relation follows that predicted by spatial summation (Ricco’s law). Importantly, if the stimulus exceeded the critical summation area, as occurs with the standard size III stimuli, then significant losses of GCIPL thickness occurred prior to a significant losses in visual sensitivity. In addition, the results suggest that the critical area for spatial summation is dependent on the corresponding number of RGCs as the severity of neuropathy increases with experimental glaucoma.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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