Stability of fixation in healthy subjects during automated perimetry

The stability of fixation of 12 healthy young subjects was assessed with a high-resolution eye tracker during automated perimetry. All the subjects exhibited eye movements and fixation shifts during the examination. The average shift in eye position between successive 1-minute intervals ranged from 0.41 degrees to 2.11 degrees, and the average standard deviation of eye position around the average of each interval ranged from 0.56 degrees to 2.55 degrees. In nine of the subjects the eye remained within 3 degrees of centre for 96% or more of the stimuli. The remaining three subjects experienced difficulty maintaining central fixation: the eye was more than 3 degrees away from centre for 28% or more of the stimuli. In the latter subjects the ability to repeatedly stimulate the intended retinal test locations was substantially reduced. The authors discuss the effects of unstable fixation on short-term fluctuation and on fixation loss.