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From the Discoveries in Sight, Devers Eye Institute, Portland, Oregon.
PURPOSE. To assess the structural and functional consequences of local ganglion cell (GC) loss in an experimental model of a retinal nerve fiber layer (NFL) bundle defect. To evaluate and compare three commonly used multifocal electroretinogram (mfERG) stimuli, as well as the standard transient pattern-reversal ERG (pERG) and the photopic full-field ERG, for detection of local GC damage.
METHODS. Intraretinal axotomy was achieved by multiple treatments with a diode laser adapted to a slit lamp biomicroscope. Retinal laser burns were applied along an arc, subtending approximately 60°, about one disc diameter superotemporal to the optic nerve. Functional measures were acquired before laser application and at numerous time points thereafter. These included mfERGs for three different stimuli: a standard fast m-sequence flicker, a global-field flash paradigm (MOFO), and a slowed m-sequence (with seven dark frames inserted to each m-step [7F]). pERGs were measured for a 24° x 32° checkerboard stimulus (0.56 cyc/deg, 90% contrast, 75 cd/m2, 5 reversals/s). Photopic full-field ERGs were measured for red flashes (0.42 log photopic cd-s/m2) on a blue rod-saturating background (30 scotopic cd/m2). Retinal photography, fluorescein angiography and postmortem histologic evaluation of the optic nerve, NFL, and retinal tissues were performed.
RESULTS. After six laser sessions, the NFL bundle defect appeared to be complete and contiguous and was visible both proximal to and distal to the site of the photoablation by clinical examination of the fundus and stereoscopic photographs. Histologic evaluation demonstrated localized loss of GC axons, confirmed at the level of the retrobulbar optic nerve. Retinal cross sections in the temporal retina (distal to the axotomy) showed loss of GC soma and NFL degeneration, whereas all other layers appeared intact. mfERGs showed loss of high-frequency components (HFCs) for responses located within the arcuate region corresponding to the NFL defect. Local GC damage was most easily detected using the slowed 7F m-sequence stimulus. This stimulus elicited relatively large HFCs that were significantly reduced from local responses after axotomy and that were tetrodotoxin (TTX)-sensitive in a control experiment. Low-frequency component loss with the 7F stimulus did not reach statistical significance. The photopic full-field ERG was not significantly affected. pERG amplitudes declined significantly from baseline but remained within normal limits.
CONCLUSIONS. Focal loss of GC function in the macaque retina is most easily detected using the slowed-sequence mfERG. Local 7F HFCs depend on intact GC function.
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