WE HAVE seen how the integration of orthokeratology (ortho-k) for myopia control can expand an eye care practitioner’s practice exponentially, but why stop there? By fully embracing all that ortho-k offers patients, and with appropriate patient selection, your practice can thrive.
Here, I discuss ideal candidates, increasing the likelihood of patient success and learning curve to facilitate fitting.
Patients who have specific refractive errors that fall under myopia, hyperopia, presbyopia or astigmatism may be ideal candidates for ortho-k.
Most studies report that myopic patients, regardless of age, are successful with ortho-k when their refractive errors are between 0.75D to 5.00D, with or without low amounts of accompanying with-the-rule astigmatism (up to 1.50D) and against-the-rule astigmatism (up to 0.75D). Patients with a myopic correction of >5.00D may also be fit, as new lens technology can now correct for higher amounts of myopia. However, be aware that these patients may experience more visual distortion, as reported by a June 2013 Optometry & Vision Science study. With higher amounts of refractive error, more treatment must be achieved, and an increase in corneal staining and/or lens decentration may be observed.
It should be noted that some adult presbyopic patients do quite well with having a portion of their vision corrected through ortho-k and the remaining vision corrected with spectacles. In addition, highly myopic patients who are also presbyopic often do quite well with intermediate and near tasks when slightly undercorrected with ortho-k.
For ortho-k to be effective at slowing the rate of myopia progression in children, treatment must be initiated between the ages of 5 to 7, as reported by several studies. The reason: Images come to a focal point in front of the central retina but behind the peripheral retina for uncorrected myopes. These patients have relative peripheral hyperopia, and this hyperopic defocus could stimulate the progression of myopia through axial elongation. Myopic progression slows through the introduction of peripheral myopic defocus. Peripheral rays focus in front of the retina, eliminating the stimulus to elongate. As this occurs, spherical aberrations often increase. That said, young children and adolescents usually do not complain of glare and halos, as they adapt to these aberrations easily.
While most ortho-k lens designs focus on the correction of myopia, we also see hyperopic applications. This occurs when the lens causes central corneal steepening and paracentral corneal flattening. However, a December 2008 Optometry & Vision Science study demonstrates no change in central corneal thickness compared with baseline (1.50D to 3.50D of hyperopia). This suggests that the primary corneal change that occurs in hyperopic ortho-k is mid-peripheral corneal thinning rather than central corneal thickening. The study also demonstrates that both refractive and topographic changes for hyperopic eyes followed the same time course as myopic eyes; with most change occurring on day one. Several ortho-k designs are available to now effectively correct up to 3.00D of hyperopia.
Additionally, patients who need presbyopic correction should not be dismissed from ortho-k candidacy. In terms of candidacy, those who struggle with ocular dryness during daily contact lens wear do well with ortho-k. When considering ortho-K for any form of refractive error, but especially for presbyopia, it is important to control patient expectations. It is valuable to determine the patient’s dominant eye, as he or she may need to be corrected with monovision. To have the patient’s vision best corrected for distance, he or she may need to wear reading glasses for near correction. Those presbyopic patients with +2.00D to -6.00D of distance refractive error are most successful.
Regarding astigmatic refractive error, patients with a range of < 1.50D with-the-rule astigmatism, or patients with ≤ 0.75D against-the-rule astigmatism are ideal candidates. These patients should also have low amounts of residual cylinder. Lenses tend to decenter on corneas that have high amounts of against-the-rule astigmatism and limbus-to-limbus astigmatism. However, this has decreased with the introduction of toric lens designs. A toric ortho-k lens design is ideal for patients who have greater than 1.50D of astigmatism, against-the-rule astigmatism or limbus to limbus astigmatism. A toric ortho-k lens may also prove beneficial when a spherical lens induces astigmatism, which is often due to lens decentration. The lens differs from a spherical design in that it has toricity across all zones except for the central base curve, which remains spherical.
By incorporating toricity into the lens design, centration improves, and improvements in vision follow. Toric ortho-k lenses have been able to correct up to 3.00D of corneal astigmatism, reports several studies. Patients with higher amounts of corneal astigmatism, may still have some residual astigmatism even with the use of toric lenses, so controlling their expectations is critical.
To increase the likelihood of patient success with ortho-k, it is imperative you:
- Collect all pertinent data. Not only is it imperative to collect all pertinent data, but also good quality data. This will increase the likelihood of successful lens fitting. The information needed to determine your patient’s initial lens parameters is manifest refraction, vertical and horizontal visible iris diameter, pupil size (best if <5 mm in normal room illumination), slit lamp examination and corneal topography. During slit lamp examination, it is important to closely examine the anterior segment, and rule out corneal staining and swelling as well as blepharitis, giant papillary conjunctivitis and MGD. Effectively managing these before ortho-k treatment can help prevent complications with lens wear. When looking at corneal topography, patients with moderate K readings of 41.00D to 46.00D and those with horizontal visible iris diameters (HVID) of 11 mm to 12 mm are ideal candidates. The ortho-k lens diameter tends to be approximately 90% of the HVID, and when patients have large (>12 mm), spherical corneas, the lens tends to decenter. Also, look at eccentricity values, which are mathematical interpretations of corneal shape. Eccentricity measures the rate of corneal flattening from the apex of the cornea to the periphery. Positive eccentricity values are ideal (0.3 to 0.7). Patients who have eccentricity values and who fall outside this range will typically need lens design adjustments. With young children, it is valuable to collect multiple topographic images to ensure that you have reliable data.
- Provide patient/parent education. Educate both patients and parents about the overall lens fitting process, including expectations and follow-up schedule (consultation, fitting examination, dispensing examination and one-day, one to two-week and one-month follow-ups.) No matter how great the candidate may be for ortho-k, if he or she is not mature and/or motivated for lens wear, the overall results may not be successful.
- Present a patient/parent contract. Have a contract for both the patient and parent to sign, stating that they fully understand all expectations, pricing, benefits and risks of lens wear.
Ortho-k lenses may be ordered empirically or by utilizing a diagnostic fitting set. Although fitting patients is not difficult, it is important to understand the specific function of each curve, or zone, of the lens, as this can help save tremendous time.
Ortho-k lens designs traditionally feature reverse geometry technology with four to six different curves or zones. These curves include the base curve — or back optic zone radius — the reverse curve, the alignment curve and the secondary — or peripheral curve(s). Some designs (those that treat higher amounts of myopia) have a relief zone, which assists the migration of the epithelial cells from the alignment zone toward the tear film reservoir. Some designs also have more than one peripheral curve.
The base curve of an ortho-k lens is determined by how much corneal flattening is desired from the patient’s prescription, corneal curvature and the Jessen formula, which assumes there is a linear relationship between the amount of myopia reduction and the base curve selected. For example, if a patient has a flat K reading of 42.00D and a prescription of -2.00D, the initial base curve needed to change the corneal curvature and correct the refraction would be 40.00D. Overall, approximately 6 μm of corneal thickness flattening results in 1.00D of vision change in myopia. The base curve creates the treatment zone, observed with topography, after lens removal. This zone varies in diameter — smaller for myopia control, larger for myopia reduction — depending on the patient’s vision correction. When examining the lens on the eye with sodium fluorescein, the amount of desired apical clearance under the base curve is 5 μm to 10 μm, which is difficult to observe. Secondary to the low amount of apical clearance, the base curve is the central area of “bearing” when observing the bull’s eye pattern of the lens.
The reverse curve is steeper than the base curve and is 0.5 mm to 1.0 mm wide. The goal of this zone is to create the amount of desired apical clearance or sagittal depth. The greater the amount of myopia to be corrected, the greater the depth of this zone.
Outside the reverse curve is the alignment curve, which is responsible for proper lens centration. The alignment curve, or zone, is also 0.5 mm to 1.0 mm wide, and the shape of this curve is dependent on the amount of the patient’s corneal eccentricity. The lower the eccentricity, the steeper the alignment curve must be to achieve proper alignment and landing on the peripheral cornea. This is the second area of “bearing” when observing the bull’s eye pattern of the lens from the inside out.
The secondary, or peripheral, curve is the outermost curve and is responsible for providing adequate edge lift and tear exchange between the cornea and lens.
When considering prescribing ortho-k for myopia progression, select a lens that has a smaller optic zone, or base curve, to make sure the peripheral optics are going through the pupil. Younger patients may, at least in the initial stages of wear, need assistance from their parents with insertion, removal and lens care.
For older myopic patients, the prescribing pattern is similar to that of myopia control. However, when correcting for myopia in an adult, larger optic zones should be chosen for several reasons. First, these patients are often more bothered by glare and halos that can result with smaller optic zones vs. children. Second, larger optic zones limit the amount of peripheral defocus that reaches the retina. This decreases the symptoms of glare and halos for adult patients, while for children who are using ortho-k for myopia control, we want peripheral defocus.
For presbyopic patients, the fitting approach is similar to that of myopia correction. Presbyopes may be fit with hyperopic/presbyopic, myopic/presbyopic or monovision ortho-k lenses. The two basic lens designs for presbyopes and hyperopes: a continuous aspheric curve design and a reverse geometry design. Presbyopes are often fit with monovision correction or have their distance vision corrected with ortho-k and use reading glasses for near tasks. In presbyopic patients who are fit with monovision, the non-dominant eye is not fully corrected for distance. When observing with fluorescein, the non-dominant eye will have less bearing centrally. Smaller treatment zones may also be used to enhance the near add.
When fitting hyperopic patients in ortho-k, the reverse geometry lens produces a steeper central treatment zone. The area next to the beginning of the reverse curve applies pressure inward to the paracentral treatment zone. The paracentral flattening causes the central steepening to produce the hyperopic correction. The hyperopic ortho-k design lens pattern has been described to have a steep-flat-steep-flat pattern. When observing the lenses with sodium fluorescein, an ideal pattern will show central pooling, touch paracentrally, fluorescein in the reverse curve, peripheral alignment and adequate edge lift. Centration of the lens is critical for all forms of correction, but especially for hyperopic correction.
Ortho-k is an exciting management option for practitioners to embrace to provide a full array of vision correction options for patients. Patient motivation and controlling patient expectations when deciding to prescribe ortho-k are important keys for success.
We will continue to see improvements in lens design, instrument technology and aberration control. As these advancements develop — embrace them — and watch your practice take off! OM