Influence of selected variables on monocular, interocular, and binocular visual acuity

The accurate determination of a patient's monocular and binocular visual acuity (VA) is a key feature of any ophthalmic examination. The work reported here was undertaken to inform the acuity assessment procedure by establishing the influence of test sequence, patient gender, and sensori-motor laterality preference on monocular, interocular, and binocular VA values.

The test protocol required the determination of independent monocular acuities (testing right and left eyes in a randomized sequence), followed by the binocular value, in each of 100 prepresbyopic subjects conforming to specific selection criteria. A set of Landolt-ring logMAR charts for use at 20 ft (6 m) was produced for this task. In addition, the sighting eye and preferred hand of each subject were established using recognized techniques.

The order of monocular vision testing was found to have no statistically significant influence on the VA level recorded. The group mean acuity of the left eye exceeded that of the right by 0.005 logMAR units, a difference of no statistical significance. Compared with female subjects, males revealed a consistently, statistically significant higher group mean acuity of about 0.02 logMAR units (> or =1 logMAR chart symbol). However, the clinical significance of this outcome and the relatively superior left acuity might both be regarded as doubtful. Binocular visual resolution was 13% greater than the mean monocular value. Associations between combinations of preferred eye, writing hand, and better-sighted eye were at levels no greater than chance would predict.

Informed by these results obtained upon visually-normal subjects, the importance of technique and the need to test a patient's VA to threshold is stressed, not only in the context of appropriate clinical case handling but also with regard to accurate record keeping.