Vision is a critical player in posture control, especially in the upright position, and studies show elderly patients’ posture control deteriorates with refractive blur, blurred vision and cataracts, which can increase the risk of falls. A team of researchers from South Korea found prism-induced convergence and vertical vergence to maintain binocular fusion caused posture instability to a greater extent than diplopia, which could have important implications for posture control in patients with large exophoria or vertical phoria.

The investigation, published in Optometry and Vision Science, enrolled 42 normal subjects who were approximately 24 years old and examined the influences of prism-induced vergence (convergence, divergence and vertical vergence) and diplopia in horizontal and vertical directions on the subjects’ posture stability in the upright position.

Researchers noted vergence signals play an important role in the control of static posture. Fusional vergence is stimulated by the presence of retinal disparity, while convergence and divergence drive the eyes inward and outward, respectively. Vertical vergence can elevate or depress one eye relative to the other to attain binocular single vision.

To simulate convergence and divergence, base-in and base-out prisms of 4D and 8D were added, respectively. To simulate vertical vergence, a vertical prism of 2D was added in a trial frame to correct the subject’s refractive error during far distance (6m) gaze. The investigators determined the prism power needed to break fusion in each subject and applied it to induce diplopia in horizontal and vertical directions.

The study reported postural instability was increased significantly when convergence induced by base-in and base-out four and eight prism diopters and vertical vergence induced by two prism diopters were stimulated. Additionally, the correlation coefficient between ranges of base-in and base-out break point and indices of general instability was −0.308 in the four prism diopter and −0.306 in the eight prism diopter conditions.

 “Although binocular input is recognized as an important factor in postural stability, binocular input with excessively stimulated convergence and vertical vergence during a far distance gaze is a latent factor affecting postural stability. On the other hand, diplopia did not influence postural stability,” the researchers wrote in their paper.

Kim SY, Cho HG, Moon BY, Yu DS. Fusional single vision with prism-induced vergence has more influence than diplopia on postural stability. Optom Vis Sci. 2020;97(3):218-26.