Evidence that convergence rather than accommodation controls intermittent distance exotropia

Abstract

. Purpose: 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. Methods: 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. Results: 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. Conclusions: 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.

A comparison of the conventional and modified push-up methods of measuring the near point of accommodation

“…Convergence, binocular disparity, blur and proximity cues have all been described as being responsible for driving accommodation. 5,20,21 The differing strength or influence of these cues may be another reason for the difference between the results of two methods.…”

Convergence accommodation in orthoptic practice

“…Instead, and perhaps more logically, our research has found that all the intermittent exotropes we studied converge to control. 28 Most intermittent exotropes appear to be 'disparity people' and are no more reliant on blur cues than the general population. They appear to converge to overcome their primary large exodeviation, which then may bring along additional accommodation, so they actually accommodate a bit more than typical children and some may even over-accommodate.…”

Survey of British and Irish orthoptists in the measurement of the AC/A ratio

“…Furthermore, there is more recently published evidence that challenges our basic assumptions about the relationship between accommodation and convergence in general and how it affects the behaviour of the visual axes in certain types of patients, in particular how it may not actually be accommodation but rather vergence that allows control of the deviation at near in IDEX. 13 If these findings are supported by further evidence, then we really do need to re-evaluate our basic understanding of the role the AC/A ratio plays in the diagnosis and management of strabismus.…”

“…This has been refuted 12,13 on the basis that unless the amount of actual accommodation is measured, with an autorefractor for example, only an assumption can be made about how much accommodation is physically exerted or relaxed through concave and convex lenses. Fig.…”