We thank Firth and associates for their interest in our article. They suggest that the mechanism behind the change in accommodation during binocular viewing among the patients with intermittent exotropia is the result of convergence accommodation, rather than accommodative convergence. We appreciate this opportunity to differentiate accommodative convergence and convergence accommodation for the mechanism associated with our finding.
As pointed out by Firth and associates, several reports in the literature, including our report, refer to accommodative convergence for the mechanism to maintain ocular alignment in intermittent exotropia. Both accommodative convergence and convergence accommodation refer to convergence occurring with accommodation, and the 2 are used without discrimination in these reports. However, the 2 concepts are slightly different with regard to the initial driving mechanisms. Accommodative convergence is defined as convergence that occurs in response to accommodation. Therefore, it initially is driven by accommodation. However, in patients with intermittent exotropia, convergence mechanisms have been postulated as the initial controlling mechanism of exodeviation. In the context of convergence mechanisms, fusional convergence and accommodative convergence have been suggested as the mechanisms to maintain ocular alignment. Our report showed that accommodative responses change under convergence stress that occurs during binocular viewing in patients with intermittent exotropia.
In our study, accommodative responses during binocular vision were measured by subtracting the spherical equivalent of the refractive errors in the better eye during binocular viewing at a distant object from the value obtained during monocular vision. Thus, accommodation was induced in response to convergence stress during binocular viewing in intermittent exotropia, and our results indicate that the mechanism behind the change in accommodation is driven by vergence. We agree with Firth and associates’ argument that the increased convergence required to control the exodeviation drives overaccommodation, which results in blur in some patients with intermittent exotropia. Whether convergence drives accommodation or accommodation drives convergence in intermittent exotropia also was addressed by the recent report by Horwood and Riddell. They showed that increased vergence to control exodeviation induces more accommodation, which agrees with our report, and there is no evidence that convergence was stimulated by accommodation driven by blur cues in our setting. Because the convergence accommodation-to-convergence ratio varies in patients with intermittent exotropia, the ratio may explain the different accommodative responses compensating for ocular misalignment during distant fixation. We also believe that both the convergence accommodation-to-convergence ratio and the accommodative convergence-to-accommodation ratio may explain the roles of convergence accommodation and accommodative convergence in intermittent exotropia.
In conclusion, regarding the driving mechanisms of accommodation and convergence, accommodative convergence and convergence accommodation should be discriminated for the controlling mechanisms of exodeviation. In our setting, we emphasize the role of convergence accommodation in changing accommodation and, in turn, binocular visual acuity.