Case Report: When Two Is Worse Than One—Stereo Imbalance in a Case of Wavefront-guided Scleral Lenses

Abstract

SIGNIFICANCE 

Wavefront-guided scleral lenses (WGSLs) reduce visually debilitating residual higher-order aberrations. Although reduced higher-order aberrations lead to improvement in monocular high-contrast visual acuity (VA), the success of the lenses in everyday life depends on additional factors such as retinal contrast, binocular balance, and stereoacuity.

PURPOSE 

This report describes a case where WGSLs provided improved monocular vision compared with scleral lenses (SLs) but reduced binocularity and stereoacuity.

CASE REPORT 

A 48-year-old woman with moderate keratoconus right eye (OD) and severe left eye (OS) was fitted with SLs and WGSLs. Visual acuity with best SLs was 20/20⁻² OD and 20/25⁻² OS. Residual higher-order root-mean-square (HORMS) wavefront error (6 mm pupil) was 0.56 μm OD and 1.38 μm OS. Visual acuity with WGSLs was 20/16⁻² OD and 20/25⁺² OS, and residual HORMS was 0.41 μm OD and 0.98 μm OS. Monocularly, WGSLs were reported to provide better VA. However, binocularly, the patient reported an “imbalanced feeling” and preferred the SLs over WGSLs. Binocular VA at distance was 20/25 with SLs and 20/25⁻² with WGSL. To investigate, the Worth Four-Dot test was performed, and the outcomes reported fusion with SLs but suppression OS at distance with WGSLs. Stereoacuity was 160 arc seconds at near and 120 arc seconds at distance with SLs and 400 arc seconds at near and >1200 arc seconds at distance with WGSLs. Dichoptic contrast balancing showed a balance point of 0.48 with SLs and 0.17 with WGSLs, indicating a strong preference toward OD. Simulation of the patient's retinal image revealed a greater difference in image contrast between the two eyes with WGSLs.

CONCLUSIONS 

Wavefront-guided scleral lenses reduced HORMS and improved VA compared with SLs. However, in this case, it inadvertently caused binocular imbalance. As WGSLs become more widely available, future work should include methods to optimize binocular balance to maximize overall patient satisfaction.

Wavefront-guided scleral lenses are an emerging clinical optical treatment for patients with highly aberrated eyes (keratoconus, post-surgical ectasia, etc.) and have been demonstrated to reduce visually debilitating residual higher-order aberrations that are not correctable with traditional scleral lenses, which have been shown to mask higher-order aberrations between 60 and 65%.^1,2 By specifically targeting residual higher-order aberrations with wavefront-guided scleral lenses, visual symptoms such as smearing and ghosting are diminished, leading to an improvement in the individual's perceived visual quality.³ Although reduced higher-order aberrations lead to improvement in high-contrast visual acuity, the success of lenses in everyday life depends on additional factors such as retinal contrast, interaction of residual aberrations, binocular balance, and stereoacuity.⁴

This case report focuses on an individual whose best-corrected monocular visual acuity, after 8 weeks of adaptation for each scleral lens type, was reported to be qualitatively superior for both eyes with wavefront-guided scleral lenses compared with scleral lenses. Objectively, the residual higher-order root-mean-square wavefront error over a 6-mm pupil was lower with wavefront-guided scleral lenses in both eyes than with scleral lenses. However, the patient reported an “imbalanced feeling” during binocular viewing and preferred scleral lenses over wavefront-guided scleral lenses for everyday tasks. Although wavefront-guided scleral lenses have been found to allow more highly aberrated eyes to reach normal levels of optical performance, visual performance, and quality of life,^1,3,5 this case demonstrates that the success of wavefront-guided scleral lenses also relies on other factors, such as binocularity. The clinical timeline indicating key events relevant to this case presentation, intervention, and resolution is shown in Fig. 1.

FIGURE 1: Clinical timeline: a 48-year-old woman with keratoconus was initially intervened with SL and then WGSL because of elevated residual HORMS WFE. The patient then underwent evaluation for binocular vision. HORMS WFE = higher-order root-mean-square wavefront error; ORx = over refraction; RGP = rigid gas permeable; SL = scleral lens; unit for Randot test = arc second; VA = visual acuity; WGSL = wavefront-guided scleral lens.

CASE REPORT

A 48-year-old woman with bilateral keratoconus (moderate disease in the right, severe disease in the left; disease severity was determined using the ABCD keratoconus grading system⁶) was enrolled in a research study examining wavefront-guided scleral lenses, which was approved by the Institutional Review Board of the University of Houston and adhered to the Declaration of Helsinki. Written informed consent was obtained for identifiable health information; however, no identifiable health information was included in this case report. After completion of conventional scleral lens fitting, refinement of the prescription, and 8-week adaptation, the Snellen visual acuity was 20/20⁻² in the right eye and 20/25⁻² in the left eye. As displayed in Table 1 and the timeline in Study 1, the residual higher-order root-mean-square wavefront error through the scleral lenses (3rd through 10th Zernike radial orders) measured with a COAS HD wavefront sensor (Johnson & Johnson Vision, Santa Ana, CA) over a 6-mm pupil was 0.56 μm in the right eye and 1.38 μm in the left eye, both of which were higher than the typical values for the subjects age at this pupil diameter.⁷ Following the study protocol,^1,5 the patient was dilated with one drop of 1% tropicamide and one drop of 2.5% phenylephrine (Neo-Synephrine). Conventional scleral lenses were applied, and the residual wavefront error and lens' offsets with respect to the patient's dilated pupils were measured. Wavefront-guided scleral lenses targeting aberrations in the second to fifth Zernike radial orders were manufactured using a DAC 2X-ALM OTT ophthalmic lens lathe (DAC International, Carpinteria, CA) in Boston XO material (Bausch + Lomb, Rochester, NY). The wavefront-guided correction was designed with an offset from the geometric center of the lens such that the correction was coaxial to the center of the dilated pupil and rotationally aligned to correct the underlying wavefront error. After refinement of the prescription and 8-week adaptation period wearing the wavefront-guided scleral lenses, the patient's Snellen visual acuity with wavefront-guided scleral lenses was 20/16⁻² in the right eye and 20/25⁺² in the left eye, and residual higher-order root mean square was 0.41 μm in the right eye and 0.98 μm in the left eye. When judged monocularly, wavefront-guided scleral lenses were reported to be qualitatively superior to scleral lenses for both eyes. However, the patient reported an “imbalanced feeling” during binocular viewing and preferred scleral lenses over wavefront-guided scleral lenses for everyday tasks. Binocular visual acuity at a distance was 20/25 with scleral lenses and slightly reduced to 20/25⁻² with wavefront-guided scleral lenses. Furthermore, simulation of the patient's retinal image by importing the residual wavefront error into Visual Optics Laboratory software (Saver and Associates, Inc., Cookeville, TN), as well as convolving the corresponding point-spread function with a logMAR visual acuity chart, suggested a greater difference in retinal image contrast between the two eyes with wavefront-guided scleral lenses as compared with scleral lenses. The image from the left eye wearing the wavefront-guided scleral lenses appeared to have clearer edges and greater contrast but also doubled (Fig. 2).

TABLE 1 - Acuity and aberrations data (Study 1) and binocular assessment data (Study 2) for scleral lenses and wavefront-guided scleral lenses

		Scleral lens		Wavefront-guided scleral lens
		Right eye	Left eye	Right eye	Left eye
Study 1 Acuity and aberrations	Visual acuity	20/20−2	20/25−2	20/16−2	20/25+2
	Residual HORMS (6-mm pupil)	0.56 μm	1.38 μm	0.41 μm	0.98 μm
	Binocular visual acuity	20/25		20/25−2
Study 2 Binocular assessment	Worth Four-Dot test	Fusion		Suppression left eye
	Randot stereoacuity at near	160 arc seconds		400 arc seconds
	Sloan RDS stereoacuity at distance	120 arc seconds		>1200 arc seconds
	Dichoptic contrast-balancing test at near	0.42		0.41
	Dichoptic contrast-balancing test at distance	0.48		0.17

HORMS = higher-order root mean square; RDS = random dot stereograms.

FIGURE 2: Simulation of retinal image quality shows a greater difference in retinal image contrast between the two eyes with wavefront-guided scleral lens as compared with scleral lens. The simulated image in the left eye has greater contrast and clearer edge but also doubling with wavefront-guided scleral lens.

To further investigate the subject's perception, a follow-up study assessing binocular vision was conducted, which was approved by the Institutional Review Board of the University of Houston and adhered to the Declaration of Helsinki, and written informed consent was obtained. The Worth Four-Dot test was performed, and the outcomes suggested fusion at distance and near with scleral lenses and only at near with wavefront-guided scleral lenses; left-eye suppression was detected at distance with wavefront-guided scleral lenses. Using the Randot test (Stereo Optical, Inc., Chicago, IL) at near with presbyopia correction, the measured stereoacuity was 160 arc seconds with scleral lenses but reduced to 400 arc seconds with wavefront-guided scleral lenses. To test stereoacuity at distance, a computerized global stereopsis test developed in-house was used.⁸ Stereo presentation was achieved using the Nvidia active shutter glasses. The target size was scaled to 20/800 logMAR (0.75 cpd) at 4 m, and the task was to identify Sloan letters that “popped out” in depth from random dot stereograms. The stereo threshold was estimated using the staircase method with six reversal points, and the mean of the last four reversals was used to calculate the mean stereo threshold. The testing procedure was repeated for each condition, and the average value was calculated. With wavefront-guided scleral lenses, the stereoacuity was poorer compared with scleral lenses at near (400 vs. 160 arc seconds) and more significantly at far (>1200 vs. 120 arc seconds), suggesting poorer binocularity.

To determine whether stereoacuity loss is an indirect result of binocular contrast balancing, as in Marella et al. (IOVS 2022;63:E-Abstract 4307), binocular contrast balancing test was administered. Studies have suggested that abnormal binocular interaction may be affected differently at different spatial frequencies^9,10; hence, the employment of a dichoptic contrast-balancing test that consists of the letter E at different sizes scaled according to five different spatial frequencies (0.5, 1.5, 5, 9.52, and 15 cycles per degree). As illustrated in Fig. 3, two dichoptic Sloan letter E's with different contrast levels and orientations were presented simultaneously at the same location on a 3D monitor. While wearing a pair of active shutter goggles, the subject was asked to identify the orientation of the “more visible letter E.” The task was tested with multiple contrast combinations (90 to 10%, 80 to 20%, 60 to 40%, 40 to 60%, 20 to 80%, and 10 to 90%; the first value indicates right-eye contrast), four different sizes, at distance (4 m) and near (40 cm). For each condition, a psychometric sigmoidal curve was fitted, and the midpoint (50-50) response rate between the left and right eyes was estimated to determine the contrast balance point. A contrast balance point of 0.5 reflects an equal contribution between the two eyes, whereas a smaller value (closer to zero) suggests a bias toward the right eye, and a value closer to 1 indicates a preference for the left eye. The subject's balance point was 0.42 at near and 0.48 at distance with scleral lenses, which was within the reported normative data,¹¹ suggesting a balanced binocularity both at distance and near. In contrast, the subject's balance point was 0.41 at near and 0.17 at distance with wavefront-guided scleral lenses, suggesting a strong preference toward the right eye at distance while maintaining a more balanced binocularity at near (Fig. 4). The results of the binocular assessments are summarized in Table 1, Study 2.

FIGURE 3: Schematic of the dichoptic contrast-balancing test.

FIGURE 4: Contrast balance point as a function of spatial frequency. At near, the contrast balance points were almost the same for wavefront-guided scleral lenses and scleral lenses, with a slight bias toward the better eye (right eye). These two values were close to the equal contrast balance between the two eyes (dotted red line at 0.5). Data points closer to the red line indicate better binocular balance. However, there is a strong bias toward the right eye with the wavefront-guided scleral lenses at distance (dashed dark gray line), suggesting degraded binocularity.

DISCUSSION

Wavefront-guided corrections directly target the residual, uncorrected higher-order aberrations that are inevitable with conventional corrections, particularly for highly aberrated eyes.³ This technology is no longer a theory but a reality, as indicated by commercially available wavefront-guided scleral lenses and clinical trials.^11–13 Typically, in both laboratory and clinical settings, the improvement in visual quality with wavefront-guided scleral lenses is quantified by visual acuity, which is a high-contrast letter recognition task. As such, it does not reflect the quality of the letter, only whether the letter can be correctly identified, as illustrated by the simulation in Fig. 2. Importantly, aberrations are known to interact, and these interactions can lead to both an improvement and reduction in monocular visual performance that is not reflective of the quantity of the aberration present. Work on these interactions has focused on intraeye interactions. The current case suggests future study of intereye interactions.^14–16 In addition, visual acuity is typically measured monocularly, and clinicians would expect an improvement in stereopsis with better visual acuity. However, the visual scenes and stereo demands in the real world are far more complex and differ from the targets used in clinical tests. They vary in contrast, color, and size and, most importantly, are not static. This study highlights the discrepancy between clinical tests and visual performance in the real world from the perspective of the subject. Furthermore, in this study, a contrast-balancing paradigm was incorporated as part of the binocular assessment to assess the depth of interocular suppression as a result of monocular dominance and the potential to improve stereoacuity at the balance point. Similar methods have revealed analogous deficiencies in the amblyopic population.¹⁷ Marella et al. (IOVS 2021;62:E-Abstract 15) have also reported that, in the keratoconus population, the better eye dominates and suppresses the worse eye, leading to an imbalanced contrast balance point. This could potentially explain the loss of stereoacuity observed in our study subject. The subject had a lower contrast balance point with the wavefront-guided scleral lenses than with the scleral lenses. This suggests more binocular rivalry, stronger suppression, and, ultimately, degraded stereopsis.

Although the results were rather unexpected, given the slight improvement of monocular visual acuity following wavefront-guided scleral lenses, particularly in the right eye, this behavior has been reported in the stereopsis literature. Interocular differences in blur have been reported to affect stereoacuity more than binocular blur.^18,19 Although wavefront-guided scleral lenses improved visual quality by reducing (but not to age-and-pupil–matched typical levels) higher-order aberrations in this case, stereoacuity in the presence of residual higher-order aberrations during wavefront-guided scleral lenses wear was reduced compared with that with scleral lenses. Depth perception requires precise motor vergence.²⁰ Patients with longstanding keratoconus may have altered binocular fixation behaviors or individualized strategies to minimize the impact of higher-order aberrations on binocularity. Wavefront data reported in the current study were recorded monocularly and not in a manner in which ocular alignment or first-order prismatic effects can be assessed.

Although the future is bright for wavefront-guided optics, continuing education on wavefront-guided corrections, their strengths, limitations, and other requirements is essential in using wavefront-guided scleral lenses to meet the binocular visual needs of highly aberrated eyes. As wavefront-guided scleral lenses become more widely available, the next step should include binocular wavefront measurement and emphasis on binocularity to maximize overall patient satisfaction. This may include an algorithm to simulate and predict stereo thresholds with different combinations of higher-order aberration corrections that include different orders and magnitudes and a derivation of a metric to optimize retinal image similarity in both eyes.