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

Horwood A, Riddell P. JAAPOS 2014: 18(6); 576-583 10.1016/j.jaapos.2014.08.009

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• Understanding whether disparity is used more than blur (or vice versa), and whether accommodation exceeds convergence (or vice versa) helps us understand why intermittent squint, heterophorias and convergence/accommodative problems behave as they do

Purpose

To propose an alternative and practical model to conceptualize clinical patterns of concomitant intermittent strabismus, heterophoria, and convergence and accommodation anomalies.

Methods

Despite 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.

Results

Normal 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.

Conclusions

Establishing 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.