Imbalanced adaptation of accommodation and vergence produces opposite extremes of the AC/A and CA/C ratios.

Abstract

It is customary to describe abnormal interactions between accommodation and convergence according to the Duane-White classification of convergence excess and insufficiency or divergence excess and insufficiency. However, recent studies indicate that these extremely high and low categories of accommodative vergence may result from adaptive disorders of accommodation and convergence. The AC/A ratio is shown in this paper to vary inversely with adaptability of accommodation and to vary directly with adaptability of vergence. Extremely high AC/A ratios can result from an imbalance in which vergence adapts readily to prism, whereas accommodation does not adapt to minus lenses. Similarly, extremely high values for convergence accommodation (the CA/C ratio) are observed when there is robust adaptation of accommodation to lenses and little adaptation of vergence to prism. Furthermore, in these extreme cases the AC/A and CA/C ratios tend to be inversely related. For example, abnormally high AC/A ratios are often accompanied by extremely low CA/C ratios and vice versa. These observations suggest that equalizing the adaptability of accommodation and vergence with orthoptics may prove to be an effective means of restoring the AC/A and CA/C ratios to normal values.