Disparity-driven vs blur-driven models of accommodation and convergence in binocular vision and intermittent strabismus

Abstract

PurposeTo propose an alternative and practical model to conceptualize clinical patterns of concomitant intermittent strabismus, heterophoria, and convergence and accommodation anomalies.MethodsDespite identical ratios, there can be a disparity- or blur-biased “style” in three hypothetical scenarios: normal; high ratio of accommodative convergence to accommodation (AC/A) and low ratio of convergence accommodation to convergence (CA/C); low AC/A and high CA/C. We calculated disparity bias indices (DBI) to reflect these biases and provide early objective data from small illustrative clinical groups that fit these styles.ResultsNormal adults (n = 56) and children (n = 24) showed disparity bias (adult DBI 0.43 [95% CI, 0.50-0.36], child DBI 0.20 [95% CI, 0.31-0.07]; P = 0.001). Accommodative esotropia (n = 3) showed less disparity-bias (DBI 0.03). In the high AC/A–low CA/C scenario, early presbyopia (n = 22) showed mean DBI of 0.17 (95% CI, 0.28-0.06), compared to DBI of −0.31 in convergence excess esotropia (n=8). In the low AC/A–high CA/C scenario near exotropia (n = 17) showed mean DBI of 0.27. DBI ranged between 1.25 and −1.67.ConclusionsEstablishing disparity or blur bias adds to AC/A and CA/C ratios to explain clinical patterns. Excessive bias or inflexibility in near-cue use increases risk of clinical problems.

Determining the relative contribution of retinal disparity and blur cues to ocular accommodation in Down syndrome

“…When investigating the use of cues in adults (both naïve and expert observers) and children using experimental procedures similar to those of the present study, Horwood and Riddell2152122 found that retinal disparity was the primary cue to both the accommodative and vergence systems followed by retinal blur. This finding was replicated for both typically developing control participants and those with DS in the present study.…”

Convergence accommodation in orthoptic practice

“…11 We suggest that the more even weighting of cue use in middle infancy may provide a mechanism for the development of these differences in style between individuals and between clinical diagnoses. 11 It would allow different styles to emerge during the critical periods of visual development, based on the best cues an infant has available at the time. If stereopsis emerges normally, the most accurate and efficient disparity fiCfiCA 'normal' pattern develops.…”

“…We have evidence that there are different blur-and disparity-biased styles which we can detect in our laboratory and which correlate with a wide range of specific clinical diagnoses in heterophoria, intermittent strabismus, refractive error and accommodation/convergence anomalies. 11 We hypothesise that there may also be 'proximity people', for example in non-binocular strabismus where proximal cues could remain influential beyond early infancy. It appears that it may be just as, or more, important to know an individual's (or clinical group's) style as to measure any 'ratios', and our model goes much further to explain clinical characteristics that we meet.…”

“…It may be slightly easier if it is re-phrased as 'if someone drives their vergence from blur, they don't drive accommodation as much from disparity'. 11 How does this work? Table 1 illustrates responses we could typically find in the laboratory or on clinical tests.…”

Intermittent Exotropia and Accommodative Esotropia: Distinct Disorders or Two Ends of a Spectrum?

“…27e30 To the surprise of many, these investigators found that disparity cues provide the primary drive for both convergence and accommodation in normal subjects. 27,30 This observation may explain why monocular accommodation responses (which eliminate disparity cues) so often are worse than binocular ones. 27 These investigators have found that patients with intermittent exotropia underaccommodate in the exodeviated state during distance fixation and overaccommodate in the orthotropic state during near fixation.…”