The fixation target influences the near deviation and AC/A ratio in intermittent exotropia

Divergence excess intermittent exotropia is thought to comprise 2 types of deviation, simulated and true. It is believed that simulated divergence excess is akin to a basic deviation but is controlled at near through accommodative and adaptive fusional vergences. While fixation target detail is known to influence accommodation and subsequently deviation size, no previous study has investigated the effect of the fixation target on the AC/A ratio in intermittent exotropia and its influence on the deviation's classification.

Twenty-five participants with intermittent exotropia underwent near and distance measurement before and after 45 minutes of occlusion. The near angle was also measured through +3.00 D lenses, and using 2 different sized targets, an N60-equivalent "butterfly" picture and N5 print. The gradient AC/A ratio was calculated for each target.

There was a significant difference between the measurements using the 2 targets, t(24) = 8.3, p < or = <0.001. On average, the near angle was 8.8(Delta) greater using the N5 print. This also resulted in a significant difference for the AC/A ratio, t(24) = -8.4, p < 0.001, the mean with the target being 3.6(Delta):1D, as compared to 6.6(Delta):1D with the N5 print.

Through careful control of accommodation by ensuring relaxation with plus lenses to clear N5 print, we revealed increases in the AC/A ratio and unmasked deviations that would otherwise have been considered to be characteristic of true divergence excess. Simulated and true deviations are possibly part of a continuum and clinical delineation may be influenced by testing artefact.