HIGHER-ORDER ABERRATIONS (HOAs) have been known to affect optical systems since Kepler and Galileo developed their telescopes in the early 1600s.¹ Over the next few centuries, the understanding of how light courses through optical systems was continuously refined; however, aberration detection and quantification were not available.

In the early 1900s, Hartmann developed a plate that was used to help eliminate aberrations from telescopes. Then, in the 1970s, Roland Shack enhanced Hartmann’s original design by using lenslets to measure the deviation of a wavefront.² This led to the development of the Shack-Hartmann wavefront sensor, which has become common technology in ophthalmic aberrometers.²

Fast-forward to today, and modern aberrometers can be found in many clinics as either standalone devices or incorporated in multifunction devices. The commercialization of aberrometers has enhanced clinicians’ ability to detect ocular aberrations and customize ablative laser treatments to minimize postoperative HOAs, thereby improving diagnostic accuracy and surgical outcomes.³

Normative HOA data was published by Salmon and van del Pol in 2006 that allows practitioners to determine how high a patient’s HOAs are relative to the population norm.⁴ Common sources of elevated HOAs often include irregularities of the crystalline lens and both the anterior and posterior corneal surfaces, especially with keratoconus.⁵ In keratoconus, both the anterior and posterior corneal surfaces have an irregular shape, and the surface profiles are often incongruous.⁶

HOAs will often be present on both corneal surfaces, and total HOAs have been found to be 6 times higher than in nonkeratoconic corneas.^7,8 With the advent of corneal tomography and wavefront aberrometry, cases of subtle or subclinical keratoconus can be objectively determined prior to the overt manifestation of slit lamp signs or anterior corneal topographical changes.^6,9

The ability of these technologies to detect early keratoconus has led to epidemiological updates in the prevalence of the disease, with the disease being more prevalent than previously thought.¹⁰ With better detection of the disease, intervention with corneal cross-linking can be considered earlier, and laser refractive surgery can be avoided in suspicious corneas.^10,11

Eyecare professionals managing keratoconus frequently encounter patients who have significant HOAs. In moderate to severe disease, most of these aberrations arise from the anterior corneal surface.⁵ Corneal and scleral GP lenses can provide effective optical rehabilitation by creating a regular refractive surface.⁷ In these cases, the tear lens formed between the posterior lens surface and the anterior cornea neutralizes surface irregularities, which corrects a substantial proportion of anterior corneal HOAs.^5,12 However, in keratoconus, the posterior corneal surface is also irregular, and standard GP optics are unable to correct visually significant posterior corneal aberrations.^12,13

This article will discuss the HOAs associated with keratoconus, how contact lenses can be used to correct aberrations, the efficacy of this correction, and the concept of neuroadaptation; it will conclude with cases of successful HOA reduction with scleral lenses.

Ocular HOAs Associated With Keratoconus

The most prevalent HOA associated with keratoconus is coma, with vertical coma differentiating keratoconic corneas the most from normal eyes.^5,14 With both the anterior and posterior corneal surfaces affected in keratoconus, the total comatic aberration is typically negative with most of the coma coming from the air-corneal interface of the anterior corneal surface.¹³ The posterior cornea will typically be associated with positive coma due to a combination of the minus lens shape and the corneal-aqueous humor interface, which partially offsets the negative coma of the anterior surface by approximately 14% to 24%, depending on the severity of the condition.¹³ Figure 1 demonstrates total eye coma, the negative coma from the anterior corneal surface, and the positive coma from the posterior corneal surface of an eye with keratoconus.

Although comatic aberration is frequently pronounced with keratoconus, other HOAs such as spherical aberration and trefoil can be elevated as well.⁵ Figure 1 quantifies the magnitude and orientation of individual HOAs, but it is often more valuable for the clinician to review the higher-order root mean square (HORMS) as this value indicates the overall deviation of the wavefront. The HORMS values are a sum of all the HOAs added together, are reported in microns, and can range from very low values such as 0.1 µm to well over 3.0 µm.^5,8 It is important to note that HOAs are highly dependent on pupil size, with larger pupils having higher HORMS than small pupils.^8,15

So how can a practitioner detect the presence of residual posterior corneal HOAs? The first clue is suboptimal visual acuity with a GP lens with vision that does not improve with a spherocylindrical over-refraction. Aberrometry can then be performed over the GP lens to verify the presence, determine the type, and quantify the amount of HOAs.⁷ If visually significant HOAs are present, then wavefront-guided (WFG) surfaces can be applied to various types of contact lenses.⁷   

Contact Lens Correction of HOAs

Correction of HOAs can be achieved with soft lenses, corneal GP lenses, and scleral GPs.⁷ While spherical or aspheric contact lenses are rotationally symmetric, and toric lenses have cylindrical symmetry, HOA lenses require very specific orientation.⁷ Therefore, in order for a contact lens to successfully mitigate HOAs, it must be well centered and rotationally stable, as well as have minimal translation.⁷

Soft contact lenses that can be used for HOA correction can be standard inferiorly ballasted soft toric lenses, custom lenses that are very thick, or wavefront-correcting lenses.⁷ The base-down prism associated with soft toric contact lenses has been shown to induce positive coma, which can offset a portion of the negative coma commonly found in keratoconic eyes.⁷ A significant reduction in vertical coma has been found with a standard toric soft lens, though GP optics remain superior to soft lenses.¹⁶

Custom soft lenses designed for keratoconus are thicker than standard soft lenses, and although they perform better than spectacles, visual performance is inferior to GPs.¹⁷ Wavefront-corrected soft lenses have been independently tested by several researchers.^18,19 Each investigative team found reductions in HOAs and improved visual acuity with the WFG soft lenses, though GP lenses were superior.^18,19

When considering total ocular HOAs, the tear lens of a GP will correct the majority of the HOAs emanating from the anterior corneal surface, leaving only internal aberrations uncorrected.⁷ Due to the lack of a tear lens, a soft lens is responsible for correcting both the anterior and internal HOAs.⁷ This results in a lens that has a very high amount of HOA correction, which puts further significance on lens stability, as even a few degrees of rotation will negatively impact vision quality.⁷

Corneal GPs and scleral lenses have nearly identical optical properties in regard to correcting irregular astigmatism and refractive error. With keratoconus, both corneal and scleral lenses will greatly correct anterior corneal surface irregularity, though any significant HOAs from the posterior cornea will not be corrected.^12,13 In situations with significant residual HOAs, scleral lenses offer an optimal platform for HOA correction due to their superior lens stability, with minimal lens rotation and translation.^7,20 Contact lens practitioners can, therefore, take advantage of scleral lens stability when considering HOA correction for their patients.

Implementing HOAs Into Scleral Lens Fitting

Since rotational stability is the most critical step in the implementation of HOA optics, it is most important to perfect the lens fit before designing the front-surface optics.^20,21 Stability can be obtained using toric, quadrant-specific, or freeform landing zones as well as dual sagittal depth lenses.^7,20,23 The more asymmetric the eye is, the better the probability of lens fit stability.

Once a well-fitted scleral lens is obtained, aberrometry can be performed over the scleral lens to screen for residual bothersome HOAs.²³ This can be obtained as a routine part of the fitting process to provide an opportunity to optimize vision in all specialty lens patients or used as a troubleshooting tool when patients are not able to obtain 20/20 vision or have complaints of excessive glare, halos, and starbursts. Current scleral lens patients are great candidates for this technology, especially if they are already in a well-fitted lens and want to improve their vision further.

If HOA correction has been determined to be appropriate for the patient, a measurement lens with several black dots or fiducials is ordered (Figure 2). The dotted lens needs to settle for a few hours prior to aberrometry measurements,²³ so a tip would be to direct-ship the lens to the patient to wear to their appointment to reduce time in the office.

Aberrometry scans are retaken over the base measurement lens using the darkest environment possible to allow for large pupil size correction.²³ It should be noted that pharmaceutical dilation is not required or recommended.²³

Once a successful scan has been captured, that data can be forwarded on to the lens manufacturer to produce the WFG lens. The last steps in the process are assessment of HOAs over the WFG lens and then lens dispensing.²⁰ Often, there may need to be minor base power adjustments once all HOAs are neutralized to provide optimal vision. Figure 3 demonstrates a keratoconus patient’s HOAs from the naked eye, base lens, and HOA-correcting lens.

Efficacy of HOA-Correcting Lenses

WFG scleral lenses can offer superior HOA correction compared with adjusting front-surface eccentricity of lenses.^21,22 In 2013, Sabesan and colleagues evaluated the efficacy of WFG scleral lenses on 11 eyes of 6 advanced keratoconic patients.²⁰ The WFG lenses provided a 3-fold decrease in HORMS, nearly a 2-line improvement in visual acuity, and improved contrast sensitivity when compared with lenses that had conventional spherical optics.²⁰ Marsack and coworkers²¹ performed a similar study evaluating WFG scleral lens correction on 14 eyes of 7 keratoconic patients. Ten of the 14 participants achieved HORMS levels within 1 standard deviation of the normative levels, and those 10 eyes gained 1.5 lines of visual acuity as compared with habitual lenses.²¹

In 2019, Hastings and colleagues²² performed a randomized crossover study on 20 eyes of 10 participants. When compared with normative data of normal eyes, 85% of the eyes achieved normal levels of HORMS, visual acuity, and 90% achieved normal contrast sensitivity with WFG scleral lenses.²² Gelles and coworkers²³ performed a similar crossover study comparing WFG scleral lenses and traditional scleral lenses with spherocylindrical optics on 31 eyes of 18 participants. When compared with traditional scleral lenses, the WFG lenses improved HORMS by an average of 56%, and the mean improvement in logMAR visual acuity was 0.12 (just over a line of acuity).²³ In the Gelles study, when asked which lens the participants preferred, 94% selected the WFG scleral lenses.²³ Figures 4 and 5 summarize the HORMS and visual acuity findings of habitual or traditional scleral lenses against WFG scleral lenses.

Successful WFG scleral lenses often reduce HORMS values to levels between 0.2 µm and 0.3 µm.⁷ So patients in conventional scleral lenses that have HORMS values less than 0.4 µm may not significantly benefit from WFG lenses. A guarded prognosis is also necessary for patients with large or deep corneal opacities.

Neuroadaptation

Neuroadaptation is the process by which the visual system—primarily the brain and visual cortex—adjusts to changes in optical input over time. In ophthalmic optics, neuroadaptation reflects the brain’s ability to reinterpret and normalize altered retinal image patterns following changes in the eye’s optical system. This phenomenon is frequently encountered in clinical practice when managing changes in spectacle prescriptions, refractive surgical interventions, or advanced contact lens designs.²⁴

The mechanisms and clinical implications of neuroadaptation have been investigated across multiple areas of ophthalmic research, providing insight into how the visual system responds to substantial alterations in optical quality. Hastings and colleagues²² assessed the visual performance of habitual scleral lenses and compared conventional and WFG scleral lens performance in a randomized crossover study.

Mean visual acuity was improved with WFG scleral lenses; however, this investigation demonstrated that improvements in visual performance following significant optical changes are not always immediate.²² Rather, the evidence suggests that neuroadaptation is a gradual process that requires sufficient time and consistent visual exposure before the full benefit of advanced optical correction can be realized.²²

With this understanding, practitioners can better appreciate the critical role of educating patients about their initial visual experience and setting realistic expectations. By adhering to the principle of “underpromise, overdeliver,” clinicians can help ensure that patients are pleasantly surprised as their vision improves during the neuroadaptation period.

Accordingly, patients fitted with HOA-correcting scleral lenses may experience limitations in visual performance during initial wear.^25-27 While some demonstrate immediate improvement, others may present with reduced visual acuity. In select cases, spherical over-refraction may improve acuity and can be incorporated into the lens design early in the fitting process. However, in patients who demonstrate limited improvement or reduced acuity that is not enhanced with over-refraction, dispensing the lens and allowing time for habituation is often critical to long-term success.

Troubleshooting

Optimizing the optical benefits of HOA scleral lenses is highly dependent on lens fit, as even small amounts of decentration, excessive movement, or uneven scleral alignment can degrade visual performance and limit treatment efficacy. As noted previously, the use of toric, quadrant-specific, profilometry-guided, or impression-based designs can enhance lens centration and rotational stability, thereby optimizing optical outcomes.^{7, 20,23}

Beyond lens fit, management of ocular surface disease is critical to the success of HOA-correcting scleral lenses. Midday fogging is a common complication among scleral lens wearers and can significantly degrade visual quality.²⁷ Effective management of underlying ocular surface disease, particularly meibomian gland dysfunction, is therefore essential to achieving consistent and reliable visual performance.²⁷

Some patients may continue to demonstrate reduced visual acuity and quality despite several weeks of lens wear.²⁶ In cases of nonadaptation to full HOA correction, partial HOA correction may provide a useful strategy, allowing the patient to adapt to a lower HOA adjustment first and potentially build up to more or full HOA correction over time.

Case Examples

Case 1—Post-Athens protocol: A 24-year-old male presented seeking HOA correction with scleral lenses. The patient had bilateral keratoconus, but the condition was more advanced in the right eye. In 2023, he underwent topography-guided photorefractive keratectomy (PRK) combined with corneal cross-linking. This combined procedure was introduced by Kanellopoulos in 2011 and is known as the Athens Protocol.²⁹ Postoperatively, the patient still had vertical asymmetry with tomography (Figure 6), best spectacle-corrected visual acuity of 20/25^-2, and corneal haze. The left eye was myopic with some regular astigmatism, but spectacle correction was 20/20.

Due to the good spectacle-corrected visual acuity in the left eye, the patient opted for a scleral lens fitting on the right eye only. With over-refraction, the patient read 20/25 and reported a subjective improvement in visual quality as compared with spectacles. Aberrometry over the diagnostic lens indicated a visually significant amount of HOAs. A base lens was then ordered, and aberrations were assessed again, resulting in a HORMS of 1.84 µm (Figure 7). Finally, an HOA-correcting scleral lens was ordered and reduced the HORMS to 0.55 µm, a 70% reduction. The patient achieved a visual acuity of 20/25⁺², which was only limited by the persistent corneal haze.

Case 2—Post-PRK ectasia: A 41-year-old male with post-PRK ectasia and scarring in his left eye presented complaining of starbursts with his current scleral lens. His best-corrected visual acuity (BCVA) improved from 20/200 in glasses to 20/25 in a traditional scleral lens, but he still experienced intolerable nighttime glare. Aberrometry over the initially ordered scleral lens showed a HORMS of 1.16 µm (Figure 8). After WFG HOA optics were applied to the anterior surface of the scleral lens, his HORMS value improved to 0.27 µm and BCVA improved to 20/10 with a significant subjective improvement in visual quality.

Conclusion

HOA-correcting scleral lenses are gaining popularity among practitioners and patients alike. Multiple manufacturers are implementing this technology, which is providing practitioners with the opportunity to improve vision for irregular corneal patients in ways that we have never been able to in the past. Instead of patients and practitioners being disappointed in those “20/unhappy” outcomes, our profession has the technology available to take these patients to the next level by correcting their most troublesome visual complaints of glare, halos, and starbursts.

References

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