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 deviation on the Humphrey visual field (dB) were studied in patients with various optic neuropathies. All acuity and dB values were expressed as interocular differences, the majority of cases having normal acuity in the fellow eye. In multiple regression analyses, acuity and RAPD were alternately chosen as the dependent or response variable with all remaining variables serving as the predictors or independent variables. The main finding was that the only significant predictor of a RAPD was the interocular mean deviation difference on the Humphrey field and the only significant predictor of acuity was the foveal threshold to white light. Redundant and insignificant variables were therefore identified with multiple regression analysis. Subsidiary findings include: (a) although diagnostic group was not a significant predictor in the above, simple linear regression line slopes relating RAPD magnitude 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 damage than in strabismic amblyopia, refractive error, or corneal damage; and (c) sensitivity losses for red vs. blue light were similar.