Abstract
Purpose :
Clinical stereopsis tests are known to give different thresholds but the underlying mechanism is not fully understood. Specifically why does the TNO threshold differ from other tests (e.g. Titmus)? The global versus local stereo theory has been disproved. The effect of illumination levels and filter impact has been partially explored but is inconclusive. Also, differences between compensated and decompensating individuals have not been fully explored. This preliminary study investigates to what extent clinically used anaglyphic (red/green) and vectographic cross-polarised filters impact on clinical stereo results in the presence and absence of an induced decompensating near exophoria.
Methods :
Stereoacuities (SA) were measured using TNO and Titmus in 41 binocularly normal (i.e. fully compensated) participants (ages 19–32, mean age: 21.1 ± 2.5) both conventionally with their appropriate red/green or cross-polarised filters, and then with both filter options in place. This procedure was repeated in the presence of a base-out-prism-induced near exophoria (NXOP). The individual prism values required were calculated by manipulation of Sheard’s criterion to give a base-out prism value that theoretically leaves the subject still fusing but with a consistently compromised state of compensation.
Results :
Combining filters only: There was no significant reduction in the TNO SA (p = 0.246) but there was a significant reduction in the Titmus (p = 0.001)
Inducing decompensation (NXOP) with single filters: There was a significant reduction in SA for both tests, more so for the Titmus (TNO p = 0.044, Titmus p < 0.001)
Combining filters and inducing decompensation (NXOP): There was a reduction in SA in both tests (TNO p = 0.001, Titmus p < 0.001). This reduction was greater than in either of the above single-manipulation scenarios.
Conclusions :
The results of this pilot study suggest that the filters used to measure stereopsis may have a detrimental impact on stereopsis performance, especially in the presence of decompensating heterophoria: specifically the significant impact of red/green anaglyph filters on Titmus results suggest they may be more “invasive” than polarised filters and hence have a greater impact on the recorded threshold. Further research is needed to quantify this phenomenon and its relation to clinical data gathering and decision making.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.