Interocular Contrast Balancing May Improve Stereoacuity in Keratoconus

Intraocular contrast balancing may improve stereoacuity among patients with bilateral asymmetric keratoconus, according to research published in Optometry and Vision Science.

Researchers included patients with bilaterally asymmetric keratoconus (n=43; median age, 21 years; 26 boys or men) and age-matched control group participants (n=10) recruited from a single center in the investigation. Participants underwent random-dot stereoacuity and binocular rivalry paradigm tasks. The team repeated stereoacuity measurements in a subset of patients (n=33) which started with 100% contrast presented to each eye, with 6 different contrast levels (2.5%, 5%, 10%, 20%, 40%, 80%) randomly induced by attenuating contrast to the stronger eye while maintaining contrast at 100% for the weaker eye. The contrast balance point was defined as the percentage contrast in the weaker eye when the dwell time (percentage of time each eye’s orientation was perceived) was equal in both eyes, and served as the point to evaluate changes in stereoacuity.

Among participants with keratoconus, median [IQR] visual acuity in the stronger and weaker eyes were 0.04 [0.00-0.08] and 0.3 [0.2-0.4] logarithm of the minimum angle of resolution (logMAR) and spherical equivalents were -2.3 [-3.1 to -1.3] and -3.6 [-5.9 to -1.6] diopters (D), respectively. Overall, interocular D-index (unitless corneal distortion topographic measurement) differences ranged from 0.25 to 14.74 and average keratometry from 0.05 to 21 D.

Stereoacuity ranged from 24.4 to 2093.8 arcsec and correlated with the contrast balance point (r, -0.47; P =.002).

Among the subset of participants who were retested at the determined contrast balance point and on either side of that point, improvements were noted in 28 compared with baseline. The remaining 5 experienced either no improvement or a slight deterioration in stereoacuity.

In a secondary analysis, a larger interocular difference in visual acuity correlated negatively with contrast balance point (r, -0.54; P <.001) and positively with stereoacuity (r, 0.37; P =.01).

Among control group participants, stereoacuity remained constant at a median 29 arcsec at 10% stimulus contrast — a point beyond which stereoacuity deteriorated (85.6 and 97.2 arcsec at 5% and 2.5%, respectively). When contrast was reduced differentially in the eyes, stereoacuity was stable until an 80% interocular differential, beyond which stereoacuity deteriorated (P <.001).

“[S]tereoacuity losses in bilaterally asymmetric keratoconus may be driven by contrast imbalances in the two monocular inputs,” according to the researchers. “Stereoacuity may be partially restored in these patients by minimizing the contrast difference between the two eyes.”

Study limitations include a small sample size and single center design.