Ganglion cell losses underlying visual field defects from experimental glaucoma

PURPOSE. TO investigate the relationship between ganglion cell losses and v isual field defects caused by glaucoma. METHODS. Behavioral perimetry and histology, data were obtained from 10 rhe sus monkeys with unilateral experimental glaucoma that was induced by argon laser treatments to their trabecular meshwork. After significant visual fi eld defects had developed, the retinas were collected for histologic analys is. The ganglion cells were counted by light microscopy in cresyl violet-st ained retina sections, and the percentage of ganglion cell loss (treated to control eye counts) was compared with the depth of visual field defect (tr eated to control eye thresholds) at corresponding retinal and perimetry tes t locations. Sensitivity losses as a function of ganglion cell losses were analyzed for Goldmann III, white and Goldmann V, and short- and long-wavele ngth perimetry test stimuli. RESULTS. The relationship between the proportional losses of ganglion cells :Ind visual sensitivity, measured with either white or colored stimuli, wa s nonlinear. With white stimuli, the visual sensitivity losses were relativ ely constant (approximately 6 dB) for ganglion cell losses of less than 30% to 50%, and then with greater amounts of cell loss the visual defects were more systematically related to ganglion cell loss (approximately 0.42 dB/p ercent cell loss). The farms of the neural-sensitivity relationships for vi sual defects measured with short- or long-wavelength perimetry stimuli were similar when the visual thresholds were normalized to compensate for diffe rences in expected normal thresholds for white and colored perimetry stimul i. CONCLUSIONS. Current perimetry regimens with either white or monochromatic stimuli do not provide a useful estimate of ganglion cell loss until a subs tantial proportion have died. The variance in ganglion cell loss is large f or mild defects that would be diagnostic of early glaucoma and for visual f ield locations near the fovea where sensitivity losses occur relatively lat e in the disease process. The neural-sensitivity relationships were essenti ally identical for both white and monochromatic test stimuli, and it theref ore seems unlikely that the higher sensitivity for detecting glaucoma with monochromatic stimuli is based on the size-dependent susceptibility of gang lion cells to injury from glaucoma.