Are things ever as they seem?

Abstract

Traditional teaching is that convergence resulted from accommodation, and this relationship could be used to help manage patients with exotropia by prescribing overcorrecting minus lenses to stimulate accommodative convergence. The relationship between accommodation, convergence, and its role in managing exotropic deviations always seemed straightforward. In this issue of the Journal of AAPOS, Brodsky and colleagues present a creative and thoughtful hypothesis that turns our prior understanding of the relationship between accommodation and convergence on its head. It is based on prior work by Horwood and her coinvestigators. All strabismologists and orthoptists are intimately familiar with how to calculate the accommodation to accommodative convergence ratio (AC/A). Most are less familiar with the concept of the convergence accommodation to convergence ratio (CA/C)—specifically the accommodation that is driven by convergence. Although the common methods for calculating AC/A ratios have proven clinically useful, they are also somewhat flawed, because they are “stimulus” AC/A ratios. This means that the calculation assumes the amount of accommodation occurring is equal to the stimulus to accommodate. So for a gradient AC/A calculation, it is assumed that if one introduces a lens of 1D or 2D power, the patient in fact accommodates 1 D or 2 Ds more, respectively; for a heterophoria calculation based on comparing the change of alignment when fixation changes from 6 M to 1/3 M, it is assumed that the patient is accommodating 3 D more for the latter measurement. This contrasts with “response” AC/ A calculations, which involve measuring how much accommodation actually occurs. Such calculations require equipment not routinely found in the clinic. Similarly, determination of the CA/C requires special equipment which is probably why most clinicians are not accustomed to calculating it. Using excellent methodology, Horwood and Riddell and Horwood and colleagues published some fascinating observations