For most patients, higher order aberrations (HOAs) are small subclinical visual imperfections that have a minimal impact on vision. However, for some, particularly those with corneal irregularities, HOAs can be significant and visually disruptive. Historically, we had no means to address them in these instances and were just thankful we could correct the vision to an acceptable functional level, often 20/25 to 20/40. With the advent of scleral lens technology that can provide a stable, unmoving optic surface, we now have a platform for placing HOA correction on an optical surface that can further correct vision to unprecedented levels.
There are various HOAs that can impact vision in our patients, the most disruptive of which are vertical and horizontal coma, spherical aberrations, trefoil, and secondary astigmatism. These are more common in patients with prior refractive or corneal surgery and corneal irregularities such as keratoconus (see Figure 1).
Figure 1: Significant vertical coma over a scleral lens in a patient with keratoconus (left), a perfect candidate for an HOA-correcting lens (right).
When patients with these HOAs presented in the past, we might offer them GP lens correction to try to improve the eye optically. In many cases, taking an eye that was 20/100 best corrected with glasses and getting them to 20/30 was a minor miracle. However, that is no longer the standard for success any more than correcting them to 20/100 with glasses would be. Applied to a scleral lens, HOA correction is much like custom LASIK correction, which for many years has been an attempt to improve vision beyond 20/20.
The first step is to detect HOAs, either with an aberrometer or retinoscopy. Patients with residual, visually significant HOAs will often have specific visual complaints, such as ghosting or streaking of images, halos, and glare. Inevitably, most keratoconus patients will have HOAs that could benefit from correction. Once you determine HOAs are present to a level that the patient is symptomatic, you can address correcting them.
When correcting HOAs with scleral lenses, it’s necessary to achieve a proper and stable fit. Stability in this case is defined as lack of movement with the blink as well as lack of rotation. These fit criteria must be achieved as completely as possible, otherwise HOA correction will not work properly. This means that the back surface toricity is properly placed in the lens, apportioning it into the vaulting chamber and the haptic to match eye shape as much as possible and ensuring full contact between the haptic and the sclera.
Once you have achieved that centered, stable fit, you are ready to add the HOA correction to the front surface of the lens. This is done using an aberrometer that can provide the data to the lab for fabrication. Once finished, HOAs can be remeasured over the lens to check for residual HOAs and further fine tuning of the correction.
HOA correction can be just as life-changing for patients as GP or scleral lens correction have been in the past. Recognizing those ideal patients—particularly those with symptoms of ghosting and smudging of images, glare, and halos—can lead you to the discussion of how HOA correction can benefit. By utilizing a custom-fitted scleral lens, you can achieve an exceptional fit and ensure the HOA correction works as effectively as possible, delivering the best quality of vision possible for your patients.
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