Predictors of visual acuity and the relative afferent pupillary defect in optic neuropathy

The relationships among visual acuity (log MAR), diagnostic category, age, the magnitude of a relative afferent pupillary defect (RAPD) in log units, photopic foveal thresholds to white and colored light (dB), and the mean de viation on the Humphrey visual field (dB) were studied in patients with var ious optic neuropathies. All acuity and dB values were expressed as interoc ular differences, the majority of cases having normal acuity in the fellow eye. In multiple regression analyses, acuity and RAPD were alternately chos en as the dependent or response variable with all remaining variables servi ng as the predictors or independent variables. The main finding was that th e only significant predictor of a RAPD was the interocular mean deviation d ifference on the Humphrey field and the only significant predictor of acuit y was the foveal threshold to white light. Redundant and insignificant vari ables were therefore identified with multiple regression analysis. Subsidia ry findings include: (a) although diagnostic group was not a significant pr edictor in the above, simple linear regression line slopes relating RAPD ma gnitude to the Humphrey mean deviation were significantly different between optic neuritis and compression categories; (b) for a given level of acuity , foveal thresholds were substantially worse in these cases with neuronal d amage than in strabismic amblyopia, refractive error, or corneal damage; an d (c) sensitivity losses for red vs. blue light were similar.