The Clinical Near Gradient Stimulus AC/A ratio correlates better with the response CA/C ratio than with the response AC/A ratio

Horwood,A Riddell P.. Strabismus. June 2013, Vol. 21 (2),140-144 (doi:10.3109/09273972.2013.786741)

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• The “Near Gradient AC/A Ratio” (+ lenses for near) may actually reflect the CA/C ratio due to the dissociative effect of the occlusion used during the test. This may explain why it correlates so poorly with the Distance Gradient method (- lenses at distance)

Aim

To provide evidence that a near clinical gradient AC/A ratio could instead reflect the CA/C relationship (the accommodation driven by response to disparity)

Design

Case control study

Methodology

27 emmetropic participants with heterophoria <4PD, 19 with intermittent distance exotropia and 17 with near exophoria >6PD were tested. A remote haploscopic photorefractor which can measure simultaneous convergence and accommodation to a range of targets containing all combinations of presence or absence of binocular disparity, blur and proximal (looming) cues was used to assess response AC/A and CA/C relationships. These were compared with clinical gradient AC/A ratios at near and distance fixation using alternate prism cover test and plus or minus lenses

Results

Although the near and distance clinical AC/A ratios correlated weakly with each other (p=0.03), neither clinical method correlated with the more accurate response AC/A ratio from the laboratory method (p=0.88 & p=0.93). The laboratory CA/C ratio correlated strongly with the near clinical AC/A ratio (p=0.004) but only very weakly with the distance ratio (p=0.16).

Conclusions

The “near gradient AC/A ratio” may actually reflect the CA/C linkage as the dissociation of the prism cover test disrupts vergence accommodation. If the near deviation diverges more with plus lenses, it may be because the lenses allow clear near vision without needing to recruit convergence accommodation to achieve it.