HISTORICALLY, the focus of refractive correction has been on lower-order aberrations (LOAs) like myopia, hyperopia, and astigmatism. However, the advent of sophisticated technologies such as aberrometers has unveiled the previously overlooked impact of higher-order aberrations (HOAs) on visual perception.
Most visually significant HOAs are caused by irregularities in the shape of the cornea and/or the lens of the eye, and they can significantly degrade visual quality—particularly under mesopic and scotopic conditions (as pupil size increases).1 These aberrations are represented by a Zernike polynomial.2 Some common visually significant HOAs include:
• Coma This aberration causes light to be spread out in a comet-like shape, resulting in blurred vision and halos around lights.
• Trefoil This aberration causes light to be spread out in a three-lobed pattern, resulting in distorted vision and difficulty seeing fine details.
• Spherical Aberration This aberration causes light to be focused at different points on the retina, resulting in blurred vision and difficulty seeing objects at different distances.
These aberrations cannot be well corrected with traditional LOA optics; wavefront-guided optics are needed. Wavefront-guided optics are designed to correct the specific HOAs that are present in the patient’s eye, resulting in improved visual quality and reduced symptoms such as halos, starbursts, and difficulty seeing in low light.3
In addition to the visual benefits, there is also evidence that correcting HOAs can improve the quality of life for patients. Studies have shown that correcting HOAs can reduce symptoms such as eyestrain, headaches, and difficulty driving at night.3,4
Overall, the correction of HOAs is a significant advance in the field of optometry.
HOAs AND VISUAL FUNCTION
Patients who have significant HOAs often report symptoms such as glare, halos, starbursts, and decreased contrast sensitivity. These visual disturbances can be particularly bothersome in low-light environments, such as when driving at night or in foggy conditions (including the dreaded twilight that patients so often have difficulty with).
Glare, which is the scattering of light by the irregular corneal surface, differs from halos and starbursts, which are caused by the diffraction of light around the edges of corneal irregularities and can appear as bright rings or “streaks” around lights or stars (sometimes also referred to as “starburst.”) Decreased contrast sensitivity, which is the ability to distinguish between objects of different brightness or color, can also make it challenging to perform everyday activities such as reading or working on a computer.3,4,5
In addition to these visual disturbances, HOAs can impact overall quality of life.6 People who have significant HOAs may have trouble with activities such as driving, reading, and working on a computer.6 They may also be more sensitive to light and have difficulty adjusting to changes in lighting conditions. These visual issues can lead to frustration, anxiety, and a reduced ability to participate in social and recreational activities.5,7
HOAs play a significant role in various ocular diseases and conditions. For example, corneal ectasias—progressive thinning and steepening of the cornea—are associated with elevated levels of HOAs.8 These aberrations, such as coma, contribute to the irregular astigmatism and distorted vision experienced by corneal ectasia patients.3 Cataracts can also introduce HOAs, further compromising visual quality. By understanding the specific HOA patterns associated with these conditions, practitioners can attempt to mitigate the visual impact of HOAs and improve patient outcomes.9
Fortunately, there are a few treatment options available for HOAs. These most notably include contact lenses and refractive surgery. Contact lenses can help to correct HOAs and improve visual acuity. Refractive surgery procedures, such as laser-assisted in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK), can physically reduce HOAs and provide lasting improvements in visual function.
CLINICAL APPLICATIONS
The integration of HOA analysis into clinical practice is revolutionizing the way we approach patient care. Wavefront aberrometry, a non-invasive diagnostic tool, allows for precise measurement of the eye’s entire optical system, including both LOAs and HOAs. This comprehensive assessment provides valuable information about the quality of vision and enables the practitioner to target the specific visual needs of each individual. There is still ongoing research about determining whether a patient is a good or bad candidate for HOA correction.
The following sections will provide some insights into how HOAs are impacting various aspects of eye care.
REFRACTIVE SURGERY
In refractive surgery, HOA analysis plays a crucial role in determining the candidacy of patients for different procedures. For example, patients who have large pupils and minimal corneal irregularities are generally good candidates for HOA correction. LASIK and PRK can effectively reduce HOAs and improve visual quality in these patients.
Conversely, patients who have corneal opacifications, such as scars, sutures, or corneal ring segments, may not be ideal candidates for HOA correction, as these irregularities can affect the accuracy of the wavefront measurement and the outcome of surgery.1,10
CONTACT LENS FITTING
In contact lens practice, HOA correction is possible with very stable lenses—typically scleral contact lenses. Scleral contact lenses are large-diameter GP lenses that are often used to correct complex vision problems, such as keratoconus and corneal ectasia (conditions that correlate with notable increases in HOAs). There is active research on HOA-corrected corneal GP lenses, along with soft lenses, although these types of lenses are not yet widely available.11
When it comes to scleral lenses, practitioners know that traditional LOA scleral lenses can innately correct approximately 50% of HOAs (whereas some studies have shown wavefront-guided lenses can correct 85% of residual HOAs).12,13 This could explain why some patients experience subjectively good/poor vision despite objectively improving on our in-office Snellen charts.
In these cases of subjectively poor vision, aberrometry and adding wavefront-guided optics may rectify the issue.1,3,4, 12, 14 Here are a few tips to improve success when utilizing HOA correction on scleral lenses:
• Stable Platform Minimizing lens translational movement and rotation is critical. Technology such as scleral topographers or impression-based lenses can help improve the fit and maximize stability.
• Large Optical Zone The larger size of scleral lenses allows for a wider area of correction, encompassing more of the visual field and potentially addressing a broader range of HOAs.
• Patient ExpectationsI always try to tell patients that this technology attempts to minimize glare and scatter, but that some of those artifacts will likely stay present (as some HOAs are induced by outside factors, such as tear film).
LOW VISION REHABILITATION
HOA correction has applications in low vision rehabilitation as well. Practitioners know that HOAs can have a significant effect on contrast sensitivity function. They can work with their vision rehabilitation colleagues by identifying and addressing the specific HOA components that contribute to visual impairment and correct them using the means outlined above. Additionally, the low vision specialists can design customized optical devices and rehabilitation strategies to improve contrast sensitivity, and thus visual function.3,14
PHARMACOLOGICS AND MYOPIA CONTROL
It has been documented that some pharmacologics used in myopia control can influence HOAs.15 Some mydriatic agents, like atropine, showed an increase in spherical aberration in animal models, while others, like tropicamide, showed little to no effect. However, it must be noted that increased pupil size is highly contributory to an increase in HOAs and could impact the results, as it was not addressed in the data.1,4,15
Overall, the integration of HOA analysis into clinical practice has led to significant advancements in the fields of optometry and ophthalmology. By providing a detailed understanding of the eye’s optical aberrations, HOA analysis enables clinicians to tailor treatments and interventions to the individual needs of each patient, resulting in improved visual outcomes and patient satisfaction.
FUTURE DIRECTION
As technology continues to advance, practitioner understanding of HOAs and their clinical implications will undoubtedly increase. Several future directions hold great promise for improving visual outcomes in patients who have HOAs.
The first of these is diagnostic advancements. Development of more sensitive and precise diagnostic tools, such as advanced wavefront aberrometers and adaptive optics, will enable a more comprehensive evaluation of HOAs. Artificial intelligence (AI) and machine learning algorithms can analyze large datasets of HOA measurements, leading to improved diagnostic accuracy and personalized assessments.
For example, one study showed AI ray-tracing for combined corneal cross-linking and PRK procedures on a keratoconus patient.16 The patient was monitored for 12 months after the procedure and showed stabilization in keratoconus and improvement in VA from 20/80 to 20/20 in the right eye and 20/40 to 20/25 in the left eye.16
Actually applying these optics is a different challenge than measuring them, but options are steadily growing. Customized contact lenses and intraocular lenses with HOA-correcting designs can further enhance visual quality and reduce visual distortions. Wavefront-guided laser refractive surgery techniques, such as wavefront-guided LASIK and PRK, can provide precise correction of HOAs, offering patients clearer and sharper vision. Novel therapeutic approaches, including corneal collagen cross-linking and topography-guided corneal reshaping, may hold promise for managing HOAs.10,16
Each patient’s HOA profile is unique, although common trends can be found among specific populations. Personalized medicine will be a key focus in HOA management. This approach will involve a combination of diagnostic tools, treatment modalities, and lifestyle modifications to achieve the best possible visual outcomes.4,10
Lastly, patient education and awareness about HOAs is essential. Many patients are unaware of this condition and may not seek appropriate medical attention. By raising awareness, practitioners can encourage individuals to seek early diagnosis and treatment, leading to better visual outcomes and improved quality of life.
By embracing these future directions, eyecare practitioners can work toward a world in which patients who have HOAs have access to state-of-the-art diagnostic tools, innovative treatments, and personalized interventions, ultimately improving their quality of life and visual experience.
CONCLUSION
The advent of wavefront aberrometry and the subsequent understanding of HOAs has revolutionized the field of optometry. By recognizing and addressing these subtle yet impactful optical imperfections, practitioners can now offer patients a new level of visual clarity and comfort, even in challenging lighting conditions.
The integration of HOA analysis into clinical practice, spanning refractive surgery, contact lens fitting, and even a role in low vision rehabilitation, underscores its transformative potential. As technology continues to evolve, the future holds greater promise for refined diagnostics, innovative treatments, and personalized interventions, ultimately enhancing the visual experiences and overall well-being of individuals with visual impairment due to HOAs.
REFERENCES
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