Evaluation of an Objective Visual Image Quality-Based Method to Identify Spectacle Corrections for Individuals with Keratoconus



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Purpose: Clinical best practice recommends providing every contact lens patient with a pair of spectacles for times when contact lenses cannot be worn. Individuals with keratoconus are often thought of as poor candidates for spectacle correction (with many not receiving a spectacle prescription at all) due to a lack of efficacy of spectacles with this disease. It is our conviction that while spectacles will never be the primary correction modality for the majority of individuals with keratoconus, they may be made more efficacious by evolving the method by which the spectacle prescription is identified. Therefore, this work is designed to examine a novel method by which spectacle prescriptions are identified for individuals with keratoconus. In arriving at a sphero-cylindrical correction for an eye with keratoconus, this alternate method is objective in nature and leverages 1) a quantitative measure of the optical performance of the eye; 2) an approximation of the neural performance of the visual system; and 3) an objective method to search the refractive space in the phoropter. Methods: This work was conducted in two parts. First, a direct comparison was made between this objective method of identifying a spectacle prescription, which is based on visual image quality, and subjective refraction, which is the current clinical standard of care for most patients. Both the objectively and subjectively derived corrections were tested on subjects using standard sphero-cylindrical trial lenses. These particular methods were chosen based on previous work that had shown a visual image quality based objective refraction to be predictive of visual acuity in typical subjects.1,2 The subjective refraction was determined using current best clinical practices (refraction with a phoropter, adjusting step size based on visual acuity, using a Jackson cross cylinder to determine cylinder axis and power when appropriate) beginning from an autorefractor measurement when possible. To assess the relative performance of the two methods of identifying refraction, visual acuity and subjective preference were assessed under varied viewing conditions. Second, having examined the performance of both objective and subjective refractions on individuals with keratoconus, and with an eye towards clinical translation of this method, the performance of the objective sphero-cylindrical refraction was modeled in the presence of a dynamically changing pupil diameter, which would naturally occur under real world lighting conditions. Results were compared to values measured on 100 typical eyes recorded as part of the TINCO study for comparison to a control group. The goal of this second phase of the work was to quantify the predicted change in visual image quality as measured by the visual Strehl ratio (VSX) as the dynamic pupil varied behind a static correction. Results: Data from the first part of this study found that a wavefront-derived objective sphero-cylindrical refraction optimized for the VSX provided equivalent quantitative levels of high contrast visual acuity (HCVA) as a subjective refraction. This is in agreement with previous work on typical eyes.3 However, when asked to identify which form of correction they preferred, individuals chose the objective refraction 76% of the time over a subjective refraction. Having demonstrated a baseline level of utility in the objective method, the second part of the study demonstrated that when applying a correction that was optimized for a 4mm pupil to the same eye when the pupil increased in diameter to 5mm, visual image quality (quantified as logVSX) was reduced by 13.9% for keratoconic wavefronts compared to a 1.8% decrease for a typical eye. Conclusions: In aggregate, the two experiments show the promise of the objective method in identifying a correction that is preferred by the individual, while identifying challenges (a potential variation in visual image quality) associated with the wear of spectacles under real-world conditions. Future work could focus on determining the role adaptation plays in perceived visual preference, determining the optimal phoropter search space and development of a method to increase the speed of calculation necessary to identify the objective refraction.



Keratoconus, Visual Strehl Ratio, Visual image quality, Wavefront Aberrometry