There is a global myopia epidemic, and preventing children from developing high myopia is of paramount importance. The consequences of high myopia extend beyond the inconvenience of thick spectacle lenses and daytime contact lens wear. High myopia is associated with comorbidities such as early onset cataract, glaucoma, retinal detachment, and myopic macular degeneration.1,2
Orthokeratology (ortho-k) uses GP lenses to reshape the cornea for correction of myopia. Traditionally, ortho-k allowed adults to be void of daytime corrective lenses. While an off-label use, this technology is also now recognized as a safe and effective method for decreasing myopia progression in both children and adolescents. Ortho-k creates a peripheral defocus retinal cue that slows myopia progression.3-5 Myopia management with ortho-k reduces myopia progression by 40% to 60% compared to spectacle correction alone.6-14
Understanding how to properly fit and troubleshoot ortho-k lens designs is critical to success in effective ortho-k myopia management. Regarding instrumentation, corneal topography is required to fit and manage ortho-k patients. After patients are appropriately fitted with ortho-k lenses, myopia progression may be monitored through cycloplegic refraction, corneal topography, and axial length measurements. The following will discuss candidates for myopia management with ortho-k, basic lens design for myopic ortho-k, characteristics of a good fit, and how to troubleshoot common topographical corneal responses after ortho-k fitting.
CANDIDACY FOR ORTHO-K MYOPIA MANAGEMENT
In the United States, overnight ortho-k received U.S. Food and Drug Administration (FDA) clearance to correct myopia in 2002. As stated previously, at this time, prescribing ortho-k for myopia management is considered an off-label use. In some lens designs, refractive correction of up to 6.00D of myopia and 1.75D of corneal astigmatism is currently approved. Ideal candidates would be within this range. There are other ortho-k lens designs that can correct higher amounts of myopia; however, this, in itself, would also be considered an off-label usage.
Candidates for myopia management include children and adolescents who are progressive refractive and/or axial myopes or who have significant risk factors or genetic background with no signs of corneal ectasia or disease. To ensure that patients have healthy ocular anatomy for ortho-k, perform careful slit lamp evaluation of the lids and ocular surface in addition to corneal topography and pachymetry.
After patients have been fitted with ortho-k lenses, periodic observation of ocular and corneal health is required. Because the epithelial thickness is approximately 50 microns, and most underlying changes during ortho-k involve temporary remodeling of this part of the cornea, any changes beyond 50 microns should raise suspicion for ectasia.
MODERN LENS DESIGN FOR MYOPIC ORTHO-K
To effectively troubleshoot ortho-k for myopia management, eyecare practitioners must thoroughly understand the lens design. Modern ortho-k lenses have a reverse geometry design. The primary measurements needed to empirically order these lenses include a manifest refraction, horizontal visible iris diameter (HVID), and corneal topography. Some lenses are designed from flat keratometry (flat K) measurements, while custom lens design software may require eccentricity and apical radius of curvature measurements. These lenses are larger in overall diameter—often 90% to 95% of the total HVID.
The back surface of an ortho-k lens has a minimum of four curves; however, some designs that correct higher amounts of myopia can have up to six curves. The next section describes each of the major curves in detail, going from the center of the lens toward the periphery.
Base Curve/Back Optic Zone Radius (BOZR) The base curve is the central-most curve, and the corresponding anatomical diameter is typically between 5.0mm to 6.8mm. This curve creates the treatment zone for the ortho-k lens. The radius of this central curve is typically determined by the amount of corneal flattening needed to correct the myopic refractive error. Many designs incorporate a Jessen factor that ranges from 0.50D to 3.00D flatter than K. This additional correction to the targeted amount of myopia ensures that the desired treatment amount is achieved and that patients maintain satisfactory uncorrected vision throughout the entire day.15-17
The central base curve is typically spherical; however, an aspheric radius can be used to increase the amount of midperipheral clearance. This creates more peripheral myopic defocus to slow the progression of myopia even further. The desired amount of apical clearance under this curve is commonly 5 to 10 microns;15,16 this is worth noting, as it is less than the 20 microns needed to perceive fluorescein behind a contact lens. When observing the BOZR, therefore, it can falsely appear as an apical bearing fluorescein pattern.18
Lenses designed to correct higher degrees of myopia will have a smaller treatment zone in comparison to designs that target lower amounts of myopia. As designs that aim to treat lower degrees of myopia have wider treatment zones, there is concern for inducing an adequate amount of peripheral defocus within the patients’ visual axis to slow myopia progression. In designs that target lower degrees of myopia, an aspheric central radius can be used to increase the amount of midperipheral clearance, thereby creating more peripheral myopic defocus for effective myopia management.15,19
Reverse Zone The reverse zone connects the base curve and the relief/alignment zones. This zone is steeper compared to its surrounding curves and is typically 0.5mm to 1.0mm wide. The amount of clearance over the cornea depends on the amount of myopia being corrected. Lower amounts of myopia correction will result in a shallower reservoir in comparison to higher myopia correction.15,16
One of the primary functions of the reverse zone is to raise or lower the base curve to create apical clearance. A reverse zone that is too steep will cause excessive apical clearance, and a central island may result in the treatment zone. Likewise, if the reverse zone is too flat, the lens will land on the corneal apex and will create misalignment in the peripheral cornea, resulting in lens decentration.15,16 Increasing or decreasing the sagittal depth of the lens through steepening or flattening the reverse curve may be necessary to achieve proper treatment.
Relief Zone Some ortho-k designs incorporate a relief zone to encourage epithelial changes from the alignment zone toward the tear film reservoir. This may allow for more effective treatment in higher degrees of myopia. When present, the relief zone is 0.5mm to 0.7mm wide, with a clearance of 10 to 20 microns with fluorescein.16
Alignment Zone The alignment zone is critical for proper centration of an ortho-k lens.15,20 This lens zone lands on the eye, and alignment beneath this area is desired. The alignment zone is typically 0.5mm to 1.0mm wide, and its shape is best determined by the amount of eccentricity present along the flat meridian of the midperipheral cornea. Individuals who have lower corneal eccentricity will have steeper alignment curves, and those who have higher corneal eccentricity will have flatter alignment curves.15,16
Secondary/Peripheral Zone(s) In the periphery of ortho-k lenses, two curves next to the alignment curve are typically incorporated to create edge lift at the peripheral cornea. The secondary zone has a width of 0.2mm to 0.5mm and a depth of about 20 microns. This curve is not included in all lens designs. The function of the secondary zone is to create a transition between the alignment and peripheral zones. The peripheral zone also has a width of 0.2mm to 0.5mm, and its depth is usually between 80 to 100 microns.15,16
THE IDEAL FIT
Ortho-k lenses for myopia management should center well, and patients should be able to achieve their best-corrected visual acuity out of the lens. Additionally, the lens should display an adequate fluorescein pattern of central alignment under the BOZR, clearance all throughout the reverse zone, and proper landing throughout the alignment zone. The lens should also move well and have adequate clearance under the peripheral zone to maintain good tear exchange.
Topographically, after the lens is removed, there should be a classic bull’s eye pattern of an even, flattened treatment zone and a steepened reverse curve zone centered within the pupillary circumference. This proper centration of the reverse zone allows for peripheral defocus to be generated inside the pupil, thus allowing for a more efficacious control of myopia progression. Adjacent to the steepened reverse zone, a more pronounced flattened area 360º around should be visible, which represents adequate alignment and applanation of the alignment curve within the peripheral cornea.
After lens removal, the cornea should be clear without signs of punctate epithelial staining, negative staining, or stromal opacities. Any of the aforementioned findings indicate mechanical contact of the lens with the corneal surface or corneal hypoxia. Additionally, patients should report good, functional vision throughout the day.
TORIC ORTHO-K LENSES
Spherical ortho-k lenses on highly astigmatic corneas usually result in poor centration, leading to induced irregular astigmatism, glare, and poor visual outcome. Limbus-to-limbus, against-the-rule, and larger astigmatic errors are not well corrected with spherical lens designs. The mechanisms behind ortho-k consist of a combination of central positive with midperipheral negative pressure forces.17 This requires peripheral landing 360º around between the lens and the cornea to prevent tear squeeze film forces from escaping along the steepest meridian in an astigmatic eye. Thus, a toric ortho-k lens is the only option for correction of myopes who have limbus-to-limbus astigmatism.16,21
Corneal topography is used to determine whether a toric design is indicated. A sagittal height difference between the flat and steep corneal meridians of 30 microns or greater at the landing chord length of the alignment curve suggests that the treatment may benefit from adding lens toricity.22
TROUBLESHOOTING TOPOGRAPHICAL RESPONSES
Centration of the treatment zone is the most crucial factor for success with ortho-k. Lenses that decenter cause various problems that can be observed both with corneal topography and on the ocular surface once the lens is removed. Common visual concerns that arise from a decentered lens include a non-uniform treatment zone that causes insufficient vision correction upon lens removal, early regression toward a more myopic refractive error before the day ends, and distortion in quality of vision from induced irregular astigmatism. Besides visual complaints, the ocular surface can also show insult from a decentered lens, such as positive epithelial staining on the corneal and conjunctival tissue or stromal edema (Figure 1). A decentered lens may also not generate sufficient peripheral defocus within the central pupil and therefore fail to offer efficacious myopia management.
A well-centered lens can be best visualized with the tangential subtractive map. It will be illustrated on this map as a “red ring” that is concentric with the pupil. This bull’s eye pattern is the only acceptable topographical response after ortho-k (Figure 2). Common topographical patterns that result from poor centration or inadequate fit of the lens include smiley face, frowny face, lateral displacement, true central island, and false central island. The following will discuss how to troubleshoot the common topographical decentration patterns of myopic ortho-k lenses.
Smiley Face Response In the smiley face topographical response, the red ring decenters superior-temporally with respect to the pupil (Figure 3). This is caused by a flat-fitting lens. The flatter the fit, the more superior the red ring. A patient who exhibits this topography pattern may have less-than-expected myopia reduction, an increase in with-the-rule astigmatism, or associated ghosting, glare, and flare.
A flat, superior-riding lens has too little sagittal depth either from underestimating the corneal sagittal depth or overestimating the corneal eccentricity. To correct for this, increase the sagittal depth of the lens. Depending on the lens design, practitioners can accomplish this in a number of ways that include steepening the alignment curve, the BOZR, or the reverse curve zone. Additionally, widening these curves can also increase the sagittal depth of the lens and fix the problem of a smiley face topography.
Frowny Face Response With a frowny face response, the red ring decenters inferiorly with respect to the pupil (Figure 4). This is caused by a steep-fitting lens. The steeper the fit, the more inferior the red ring. A patient who presents with this topography pattern may have less myopia reduction than expected, an increasing amount of astigmatism, or associated ghosting, glare, and flare.
A frowny face topography is caused by the lens having too much sagittal depth from either overestimating the corneal sagittal depth or underestimating the corneal eccentricity. A smaller overall lens diameter may cause this as well.
Remedy this topographical response by reducing the lens sagittal depth. This can be accomplished by flattening the alignment curve, the reverse curve, or the BOZR. Decreasing the width of these curves will also decrease the overall sagittal depth of the lens. If a small diameter is causing decentration of the lens, increasing the diameter will help achieve a proper fit.
Lateral Displacement A topographic red ring that decenters nasally or temporally with respect to the pupil indicates lateral displacement of the lens (Figure 5). A patient whose ortho-k lens is laterally displaced may exhibit associated ghosting, glare, or flare; have induced astigmatism; or have vision that is reduced in the paracentral region of the pupil. Possible causes of lateral decentration are a lens that is too steep or too flat, a lens that is not large enough, an asymmetrical corneal curvature, or a decentered corneal apex.
Increasing or decreasing the sagittal depth of a lens can help remedy a flat- or steep-fitting lens, respectively. If a small lens is causing lateral decentration, increasing the lens diameter will help improve the fit. If an asymmetrical corneal curvature or decentered apex is causing decentration, altering the alignment curves or changing to a toric design lens may help with this concern.
True Central Island In a true central island, the red ring centers perfectly; however, there is also a central area of steepening on topography. This area of central corneal steepening relative to the rest of the treatment zone is caused by a steep-fitting lens. A true central island has no sign of epithelial damage upon lens removal, and the central peak is > 0.00D on corneal topography. The steeper the lens, the steeper the central island.
Patients who present with a true central island may have an over-refraction with no clear end-point, reduced best-corrected visual acuity of more than one line on the Snellen acuity chart, or unaided acuity that is worse compared to the pre-fit acuity if the cornea becomes steeper than baseline. Possible causes of a true central island include a lens sagittal depth that is too high because of overestimated corneal sagittal depth or underestimated corneal eccentricity. True central islands can be resolved by reducing the sagittal depth of the lens (similar to the changes needed to resolve a frowny face topography).
False Central Island A false central island appears as a central peak within the treatment zone that has a topographical value of < 0.00D (Figure 6). This is caused by a flat-fitting lens and often is accompanied by a smiley face topography pattern of superior lens displacement. Patients who have a false central island will often have corneal staining upon lens removal from epithelial damage. Additionally, distortion of the placido disc mires may appear with topography. In these cases, patients may have associated ghosting, glare, and flare or have poor best-corrected vision if the staining is central in location.
A false central island is caused by heavy bearing of the lens on the corneal surface; the topographer interprets the induced epithelial damage as an area of steepening. This may be remedied by increasing the lens sagittal depth (similar to the changes needed to resolve a smiley face topography response).
Myopia is reaching epidemic proportions, and the consequences of high myopia are destined to become more common in the future. Myopia management options such as ortho-k can limit the rate of myopia progression. Successful implementation of ortho-k necessitates a deep understanding of the lens design as well as advanced examination skills and technology such as corneal topography to assess the lens fit and visual outcome. Efficiently troubleshooting ortho-k fits is possible and will enhance your ability to restrain myopia progression in your patients. This will pay dividends for your practice and your patients alike. CLS
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