Interocular differences in visual function in normal subjects.

A commonly used clinical technique for determining whether visual function is abnormal, particularly when a patient is suspected of having unilateral or asymmetric disease, is to compare the visual function of the 'suspect' eye to that of the normal eye. This assumes that in normal patients visual function in each eye is equal and that this symmetry is maintained with age. To determine the extent of normal symmetry for clinical measures of visual function, we assessed high and low contrast visual acuity, letter contrast sensitivity and motion sensitivity (Dmin) in both eyes of a group of 91 subjects spanning an age range of 21 to 82 years. For the whole group the interocular differences in sensitivity for each of the tests approximated a Gaussian distribution. The results demonstrate that interocular differences should exceed more than 0.16 and 0.17 logMAR (approximately 2 lines) for high and low contrast visual acuity respectively, before patients can be considered to be outside the range of normality. For the Pelli-Robson Chart, interocular differences must exceed 0.23 log CS (1.5 triplets, 4.5 letters) and for Dmin, more than 0.35 log minarc for the differences to be considered abnormal. For low contrast letter visual acuity, there was a trend for the absolute interocular differences to increase significantly with age, but asymmetry remained relatively constant for the other visual measures.