I’m a clinical optometrist in a two-OD practice. I love all things myopia, in addition to the challenge and fun of providing full-scope eyecare. Encountering myopia in kids used to be mundane, but over the last decade, proactively treating myopia is the new standard to slow down excessive growth of the eye.
Orthokeratology (often referred to as Ortho-k) is an excellent choice when considering myopia control options. Lens design has come a long way since George Jessen pioneered the concept of altering corneal curvature with lenses.1 Note: Some practitioners prefer the terms Ortho-k mold or Ortho-k retainer instead of Ortho-k lens.
Improvements in lens materials, corneal topography, and manufacturing techniques have changed our ability to optimize orthokeratology for vision improvement and myopia control. Read on to get a little myopic about myopia management. You’ll learn to put power in the pupil to power up your myopia management results.
Power to the patient
First, I’ll give some context for myopia control, communicating with the patient, and a simple overview of corneal topography. Then, we’ll review a case study that illustrates some of the theories behind the myopia control effect—by which I mean the slowing of axial elongation—of Ortho-k. After the case study, we’ll review recent research that helps us understand how to enhance orthokeratology for myopia control.
I am very grateful for well-done research: like all of you, I take the information from these studies and apply it to clinical patients who often don’t fit into the exact parameters of a clinical study.
Research in myopia control usually focuses on the efficacy of slowing axial elongation, reducing refractive error increases, and the safety of the treatment method. Along with efficacy and safety concerns, patients and parents are also interested in ease of adaptation and the ability of the patient to maintain the prescribed myopia control program. Achieving these objectives is the challenge and reward of prescribing Ortho-k for myopia control.
Educating parents and patients on myopia management
For any myopia management case, the most effective treatment is the treatment the patient is willing to start and comply with every day. Even with parental supervision and encouragement, if a patient (i.e., child) is not on board with the treatment, they are less likely to have success.
While it is easy to spend the majority of the exam time talking with the parent, it is critical to build rapport with the child. I challenge you to build a connection to the child first and then the parent. By doing that, the parents will appreciate your care for their child more and the doctor-parent relationship will be even stronger.
After establishing that connection with the child and parent, then establish the “why” of myopia control, and lastly, the “how.” If that order is reversed, fewer kids will start myopia management.
For more information on how eyecare providers can educate parents and patients on myopia management, check out The Myopia Talk: How To Craft Patient Communication with Parents/Guardians.
Ortho-k is an effective method for the “how” of myopia control, and thankfully it also is a great way to provide clear vision.2-6 Many lenses claim a high first-fit success rate, but is that the right metric for myopia management?
As a clinician, I understand the value of first-fit success for both the provider and the patient. While the first fit almost always achieves good visual acuity, it may not be the optimal design for myopia control. In a myopia control treatment plan, it’s not power to the people, it’s power to the pupil.
Power to the pupil
We are familiar with looking at the map view of a corneal topography. In normal corneas, the peak power is near the center of the cornea, which should be centered near the visual axis. We can also look at a profile view to understand how, in a normal, prolate cornea, the dioptric power reduces with distance away from the corneal apex.
Corneal topography and orthokeratology
Below are two images of a normal cornea prior to any contact lens wear. The first is an axial map, and the second is the same eye with a profile view across the 180 meridian. This eye has minimal astigmatism and a consistent corneal power around 42D across the central 4mm.
Figure 1: Axial view, absolute power scale numbers shown are dioptric values in that zone.
Figure 1: Courtesy of Chad Anderson, OD.
Figure 2: Profile view, the x-axis is millimeters from the corneal apex.
Figure 2: Courtesy of Chad Anderson, OD.
In orthokeratology for myopia control, the lens design is made to reduce the central dioptric power and increase the dioptric power in the mid-periphery. Creating relative plus power, or myopic defocus, in the mid-periphery is believed to be the reason Ortho-k works for myopia control.
The basic method for designing Ortho-k lenses is outside of the scope of this article, but in brief summary, Ortho-k lenses can be adjusted for optic zone diameter and radius, reverse curves, alignment curves, and overall diameter. Lens designs create corneal shape changes, which can be viewed in traditional topographic map form and profile view. The central zone is usually called the treatment zone, and the mid-peripheral plus is referred to as the red ring.
Orthokeratology case report
Now, let’s review corneal maps and profile views from a recent case in my office. All three sets of images are from the right eye of the same patient after wearing an Ortho-k lens.
Note: The profile views are across the 180 meridian of the cornea. The visual acuity from all three lenses was 20/25 or better. The first lens (Figure 3) could be called a first-fit success but may not optimize myopia control.
Figure 3: Axial profile 180 meridian (top) and tangential map lens 1 (bottom). These images are from a lens with a 6mm back optic zone diameter (BOZD).
Figure 3: Courtesy of Chad Anderson, OD.
Figure 4: Axial profile 180 meridian (top) and tangential map lens 2 (bottom). These images are from a lens with a 5mm BOZD.
Figure 4: Courtesy of Chad Anderson, OD.
Figure 5: Axial profile 180 meridian (top) and tangential map lens 3 (bottom). These images are from a lens with a 5mm BOZD.
Figure 5: Courtesy of Chad Anderson, OD.
Achieving myopic defocus with Ortho-k lenses
Ortho-k for myopia control creates myopic defocus in the peripheral retina. The maximum myopic defocus in Figures 3, 4, and 5 is similar, but the pattern is clearly different between the lens designs. The peak peripheral diopter value is around 42.5D temporally and 43D nasally, but the slope and central treatment zone width differ.
The peak diopter value is reached more quickly as the curvature moves away from the corneal apex. With adjustments to lens design, the Ortho-k practitioner can achieve a larger area of myopic defocus in the pupil.
Design changes are made to ensure good vision, maintain corneal health, and ideally maximize myopia control. What design change happened to create the topography in Figures 4 and 5? In this case, Figure 4 was the result of a change to a 5mm BOZD with fitting parameters following the lens manufacturer’s algorithm, other than adding 25μm to the return zone.
Figure 5 was an additional adjustment that should further enhance the slowing of axial elongation. The central base curve was flattened by 0.1mm, and the return zone depth increased by another 25 microns. These changes created a smaller treatment zone with more power in the pupil. The myopic defocus, or the plus power, moved closer to the center of the pupil, and the slope of the mid-peripheral plus increased.
What makes the profile in Figure 5 more effective? It is widely believed that more myopic defocus in the peripheral retina achieves slower axial elongation. In Figure 5, the relative plus power begins and peaks closer to the corneal apex, which has been shown to slow axial length growth.7-9
Maintaining corneal health
Just like soft, gas-permeable, or scleral lenses, it’s critical to ensure the cornea stays healthy while patients wear Ortho-k lenses. The practitioner must frequently monitor to ensure there is no corneal staining, haze, infiltrate, or epithelial defect created by the lens design. The vision quality and needs of the patient must also be considered to ensure acceptable vision is maintained.
More myopic defocus within the pupil border can be achieved by creating a smaller treatment zone and/or creating a more aspheric treatment zone. The amount of plus in the pupil has been called the plus power ratio (PPR).
A study group at the University of Montréal showed that higher PPR leads to slower axial elongation.10 With PPR, the area of defocus within the pupil matters. In their study, all lens designs created similar peak plus values but different ratios of plus power within the pupil. The lenses with more plus power in the pupil demonstrated slower axial elongation.10
When evaluating corneal topography patterns, carefully evaluate how much of the mid-peripheral plus falls within the pupil. When I look at patterns, I consider both photopic and mesopic pupil sizes.11
Consider treatment zones
In addition to looking at the peak plus value, it may be more important to evaluate the asphericity of the profile. In other words, how quickly does the relative plus power increase?
In the images above, Figure 5 is a more aspheric pattern than Figures 3 or 4. Zhang et al. concluded, “The spatial distribution of corneal relative power rather than the cumulative amount of corneal power change may be more important in explaining the variance in myopia control effect.”9
In other words, when reviewing the corneal profile, look at the height and the width of the relative plus power, and not just the height. Then, evaluate how much of the relative plus power falls within the pupil.
Several peer-reviewed studies have shown slower axial elongation with smaller treatment zones.12-14 The VOLTZ study compared 6mm BOZD lenses to 5mm BOZD lenses and followed participants for 24 months. Over the length of the study, the 5mm BOZD group showed less axial elongation.13
Comparing 5mm and 6mm BOZD lenses
An Ortho-k lens with a smaller BOZD may not always create a small, aspheric treatment zone. For optimal myopia control, aspheric and small treatment zones are the goal.7,9-10 Regardless of the lens optic zone diameter, evaluate the pattern in relation to the pupil size. Bring more power to the pupil to enhance your myopia management.
I’ve made the case for smaller treatment zones to enhance myopia control, but does that mean a small BOZD for everyone? I don’t think that is the case for a few reasons. First, while I often utilize the smaller BOZD lenses, it sometimes requires more fitting visits to create the optimal profile. This might be challenging for patients who travel from farther distances.
Second, in the VOLTZ study data, there were a few more adverse events in the 5mm BOZD group than in the traditional 6mm BOZD lenses.13 Third, if a patient has large pupils, a 6mm BOZD lens may provide good myopia control. Fourth, I have more success fitting highly toric corneas or higher levels of myopia in larger BOZD lens designs.
One final tip: In my experience, lenses with smaller BOZD often require more sagittal depth than initially thought.
Power to your practice
It will take all of us to make myopia management the mainstream option for young patients with myopia. I believe myopia management is doable for each eyecare professional.
If we listen to our patients, it’s what they already are asking for; I’ve never had a patient come in hoping their vision got worse. Power up your topographer and bring power to the pupil to slow down the worsening of myopia.
