[Objective assessment of minimum visual acuity by suppression of optokinetic nystagmus]

To develop and to validate an objective method which allows an estimation of the visual acuity (VA) of adults.

A horizontal optokinetic nystagmus (OKN) was elicited by a vertical rectangular grating presented on a PC-screen and was recorded by an infrared reflection method. Superimposition of three stationary black detection stimuli was used to suppress the OKN. The size of these stimuli was increased every two seconds, in ten steps. Suppression of the OKN was taken as an indication that the stimuli were detected. The relation between the smallest effective stimulus and logVA was evaluated in 65 healthy volunteers (130 eyes) with artificially reduced VA and 425 cooperative patients (842 eyes) whose VA was reduced due to different etiologies. The tolerance intervals of logVA related to the discrete detection stimuli were calculated.

The method allows an estimation of the least expectable VA in steps of 0.8, 0.32, 0.25, 0.12, 0.1, 0.06, 0.03 und 0.025 due to the smallest value of three trials. The least VA of strabismic amblyopic eyes can only be estimated in steps of 0.16, 0.06, and 0.02. A VA > 0.02 can be proven with high reliability.

The method allows a clinically useful, objective estimation of the least VA without observer bias in non-amblyopic eyes.