Myopia management is rapidly gaining popularity among the optometric community as well as with patients. Whether fitting a soft multifocal lens, figuring out when and how to prescribe atropine, or gaining confidence and success with orthokeratology (ortho-k) lenses, the quest for learning my passion.
In my 15 years as a progressive myopia manager, I have seen multiple changes in treatment strategies in the fight against progressive myopia. As technology improves, ODs can more easily advance treatments to a wider range of patients.
One area of hesitation I see from my colleagues is the treatment of myopic children who also suffer from corneal astigmatism. About 33.6% of Asian children have astigmatism,1 and in the United States, astigmatism is –2.00 D or more in 20% of children. Although soft toric multifocals and atropine are available for patients with astigmatism, many practitioners will choose ortho-k as a treatment option.2
For some of these patients, practitioners may find that lenses of spherical design provide unsatisfactory results. As such, I have seen neglected patients with astigmatism as potential candidates for ortho-k as a method of managing their myopia. This must not be the case. Understanding corneal elevation changes and how they affect centering and ultimately design choice can make a rewarding addition to a myopia management practice.
As clinicians, DOs have been trained to focus their efforts on the refractive error of their patients. Similarly, many ODs have focused solely on the myopic correction component of FDA-approved ortho-k designs from -5.00D to -6.00D. I have seen several patients with corneal astigmatism go for a treatment with ortho-k which could have been highly successful. However, for proper centering and effective corneal molding on those asymmetrical corneas exhibiting changes in elevation (toricity), understanding the available toric designs is essential for proper fit and treatment success.
ODs are familiar with the classic “bow tie” pattern of rule-compliant astigmatism when evaluating corneal topographic maps. In the case of ortho-k, the size of the bow tie can be as important as the amount of astigmatism. When the bow tie is wide and spans the entire meridian of the topography, or the entire cornea, it is called limbo-to-limbo astigmatism. Conversely, the bow tie may be small and contained in the central (greater than 7mm) portion of the cornea, and the peripheral cornea retains its more spherical shape. This is called apical astigmatism.
The distinction between limbo-to-limbo corneal astigmatism and apical corneal astigmatism is critical to the success of ortho-k. Patients with apical astigmatism can often be fitted with spherical or standard designs, such as Euclid Emerald, which will fit properly along the periphery of the cornea and produce the 360° “sealed” ring pattern topography of an inverted geometry ortho-k lens design. This constant ring is necessary for the patient to achieve a good reduction in myopia. When a patient presents with limbus-limbus astigmatism, a considerable difference in elevation exists between the steep (deep) and flat (shallow) meridians. Therefore, a lens of spherical design will fit differently at these elevations even though the amount of refractive cylinder is the same.
Lens shift on the eye and poor visual results will be observed if the ring area moves away from the primary meridians, particularly if the upper and lower areas are the same. In these cases, an asymmetrical toric design is required to achieve the best centering and consistent reduction in myopia.
The best way to examine corneal astigmatism before treatment is with a topographer, using the apical or elevation map setting. This setting allows the user to scroll the cursor across the corneal map and measure the highest and lowest elevation readings relative to the baseline. Most surveyors today offer an elevation estimate feature, but users may need to contact their individual instrument guide to see exactly how to perform this feature on a specific machine.
Practitioners may have different starting points for the amount of elevation difference that determines the chosen toric design. Clinically, I have determined that when the elevation difference is less than 25 μm between the primary meridians, I start with a standard spherical design. If the difference is more than 25μm or I see off-center with a spherical fit, I will consider using a toroidal design. If the amount of corneal lift is greater than 50 μm, I will start with an empirically ordered toric design to properly align with that patient’s cornea.
When a properly designed ortho-k lens is worn overnight, the topography should show mid-peripheral alignment. This provides stabilization of the lens on the eye and creates the ideal hydrodynamic force under the lens to gently move the epithelial tissue into position for optimized ortho-k effects. The positive “push” or downward force of the tears behind the center of the lens (treatment area) combines with the negative “up pull” along the mid-peripheral areas of the cornea to create the effects of reduction of ortho-k myopia. If a patient has a large difference in elevation between the meridians, they may have an insufficient seal of the fluidic force zone with resulting decentration and poor ring formation. For this reason, the patient with limbus-limbus astigmatism requires a toric periphery design to properly align the fluid forces in each meridian.
Toric designs have proven themselves. In 1 study of patients ages 9-16, children wearing the toric design showed a 92.8% success rate.3 I have seen great results in my practice with improved centering and visual performance for patients who were initially fitted in a spherical design and then transitioned to a toric design.
Here are the measures taken at the clinic for ortho-k patients:
– Screen all patients with cycloplegic refraction, short binocular vision assessment, and baseline axial topography.
– Discuss with parent and patient what
discoveries have revealed and what treatment options are available to them. When clinically relevant, we strongly recommend ortho-k. If the patient has corneal astigmatism, we discuss this specific finding and its success when choosing the right lens design.
– When all questions have been answered, assess the overall plan and come to an agreement and understanding of the process for returning child and parent visits.
– Gather all topographic elevation data and work directly with contact lens lab consultants to place an order for the patient-specific lens design, shape, toricity, and myopic correction target. An important learning point is to underestimate the amount of lens toricity, as too much will result in a tight fitting lens and uneven visual results or superficial corneal staining.
– When the lens order arrives, training in application and removal is scheduled. During this visit, we review with the patient the steps for proper cleaning, disinfection and storage of ortho-k lenses. We emphasize careful handling and teach them to rub the lens well in their palm and to pick up a lens whose back surface has fallen on a table or counter. The next visit is scheduled for the next day or in several days, depending on the specifics of the patient.
– At the first follow-up visit, we answer questions that arose during the patient’s first night of lens wear and determine if they followed the appropriate care steps. At this early stage, the topographic tangential view will show the best evaluation of the treatment and rings beginning to form on the cornea. They may not be finished yet, but it is possible to assess the general positioning overnight and the early formation of the rings for the breaks. Another option is to use sodium fluorescein and the lens on the eye to assess fit and ring formation to gain additional information to share with the lab consultant to make design changes . Remember that the rate of corneal tissue change is patient specific, so avoid rapid changes. I usually withhold changes until the 2 week follow-up visit, if possible.
Patients with corneal astigmatism should be offered the same opportunities for myopia management as their spherical counterparts. These patients are capable of similar ortho-k success. Consider evaluating patients for topographic elevation differences and choosing appropriate toric designs. Toric ortho-k designs work. Consider trying them.
1. Kleinstein RN, Jones LA, Hullett S, et al; Collaborative Longitudinal Assessment of Ethnicity and Refractive Error Study Group. Refractive error and ethnicity in children. Arch Ophthalmol. 2003;121(8):1141-1147. doi:10.1001/archopht.121.8.1141
2. Harvey EM, Dobson V, Clifford-Donaldson CE, Green TK, Messer DH, Miller JM. Prevalence of astigmatism in Native American infants and children. Optom Vis Sci. 2010;87(6):400-405. doi:10.1097/OPX.0b013e3181d95b23
3. Fadel D. Ortho-k remodeling. Examine contact lenses. 2017: 26-28. Accessed July 1, 2021. https://www. reviewofcontactlenses.com/CMSDocuments/2017/2/rcl0217i.pdf