Evidence that convergence rather than accommodation controls intermittent distance exotropia

Abstract

This study considered whether vergence drives accommodation or accommodation drives vergence during the control of distance exotropia for near fixation. High accommodative convergence to accommodation (AC/A) ratios are often used to explain this control, but the role of convergence to drive accommodation (the CA/C relationship) is rarely considered. Atypical CA/C characteristics could equally, or better, explain common clinical findings. Nineteen distance exotropes, aged 4-11 years, were compared while controlling their deviation with 27 non-exotropic controls aged 5-9 years. Simultaneous vergence and accommodation responses were measured to a range of targets incorporating different combinations of blur, disparity and looming cues at four fixation distances between 2 m and 33 cm. Stimulus and response AC/A and CA/C ratios were calculated. Accommodation responses for near targets (p = 0.017) and response gains (p = 0.026) were greater in the exotropes than in the controls. Despite higher clinical stimulus AC/A ratios, the distance exotropes showed lower laboratory response AC/A ratios (p = 0.02), but significantly higher CA/C ratios (p = 0.02). All the exotropes, whether the angle changed most with lenses ('controlled by accommodation') or on occlusion ('controlled by fusion'), used binocular disparity not blur as their main cue to target distance. Increased vergence demand to control intermittent distance exotropia for near also drives significantly more accommodation. Minus lens therapy is more likely to act by correcting overaccommodation driven by controlling convergence, rather than by inducing blur-driven vergence. The use of convergence as a major drive to accommodation explains many clinical characteristics of distance exotropia, including apparently high near stimulus AC/A ratios.