Fitting ortho-k contact lenses

Orthokeratology (ortho-k), or corneal refractive therapy, is a specialty contact lens fitting in which gas permeable (GP) contact lenses are worn overnight to change the corneal curvature, thus modifying the cornea’s refractive power.

It is a temporary, reversible, and alternative option to refractive surgery procedures, making it a good option for a wide range of patients.

Ortho-k was once approved only for refractive errors up to 4.00 D of myopia and up to 1.50D of corneal toricity, but newer designs allow for hyperopic, moderate astigmatic, and higher myopic correction.

The high Dk GP lenses used have a reverse geometry design and, when placed on the eye, work with the tear layer to flatten and reshape the contour of the central cornea. There is very thin tear layer in the center of the corneal surface compared to the mid-periphery, creating a negative pressure gradient, resulting in central cornea flattening and mid-peripheral steepening.¹

Along with eyelid pressure of the closed eye, these fluid forces under the lens cause cellular compression and redistribution of the corneal epithelium. These changes allow for an alteration in the refractive properties of the cornea, both centrally and peripherally.

The lenses are worn while the patient sleeps, molding the cornea so the patient has less dependence on glasses or contact lenses throughout the day.

It is important to achieve the right fit because it will allow the cornea to preserve this flattened shape when the lenses are removed and provide the patient with stable, uncorrected vision throughout the course of the day.

Most refractive errors should reach full correction within two weeks. Initially the patient may experience slight regression toward the end of the day, but this should diminish over time.

Fitting ortho-k lenses

Identifying potential candidates is a simple but important step to ensure success and patient compliance.

A discussion with the patient about occupation, hobbies, visual demands, and day-to-day activities should take place in addition to evaluating the patient’s refractive status and ocular health. Both children and adults can benefit from this type of vision correction.

Other data to be collected in the initial examination includes refraction, baseline topography, tear film and anterior segment evaluation, and pupil size measurement.

A topographer is necessary for observing corneal curvature characteristics, guiding the practitioner in choosing the initial lens and monitoring the treatment effects. On subsequent visits, topography is used to monitor corneal curvature changes, ensure proper centration of the lens, and observe the treatment zone.

There are a few ways to help the practitioner choose the initial lens to trial-for example, one method uses flat K values and the spherical portion of the manifest refraction. Using a diagnostic fitting set, empirically ordering the lenses through consultation or designing a lens using topography software are other options when fitting ortho-k lenses (Figure 1).

Ortho-k lenses have a reverse geometry design consisting of three curves (Figure 2). The central portion of the lens is the treatment zone or base curve (BC). This portion of the lens is chosen based on the refractive error of the patient and fit flatter than K in the amount of myopic correction needed.

The next curve is the return zone depth (RZD). It is the fitting curve, controls sagittal depth, and aids in centration of the lens. The RZD can be observed in the slit lamp as the area of mid-peripheral pooling.

The most peripheral curve- the landing zone angle (LZA)-controls edge clearance and alignment, facilitates tear and debris exchange, and must be fit correctly to prevent seal-off, ensure good comfort, and ease of lens removal. Each of these three curves can be changed separately in order to customize the best lens for your patient.

After lens parameters have been selected, the practitioner must evaluate the lens fit on eye. Using a Wratten filter with cobalt blue light, a bullseye fluorescein pattern of central bearing, mid-peripheral pooling, and adequate edge lift should be observed (Figure 3).

The central pattern of the bullseye should ideally be a 4 mm zone of bearing with approximately 0.5 mm of edge lift. As with all GP lenses, centration and movement should also be assessed. Minimal movement of ≤1 mm is ideal with centration both vertically and horizontally.

Once a good fit has been determined, the patient needs GP lens application and removal training. Patient education on proper hygiene, handling, treatment expectations, and care of lenses should take place prior to dispensing.

My preference on lens care is a hydrogen peroxide system due to its compatibility with GP lenses and the extended wear nature of this lens modality. You may contact the lens manufacturer for a list of recommended and compatible solutions. Suggest instilling a rewetting drop before lens removal because the lens may adhere to the eye upon waking.

Be sure to review signs of problems, such as prolonged redness, pain, tearing or decrease in vision, and what to do if such an event occurs.

Follow-up evaluations

The patient needs to be seen the morning after the first night of wear to verify centration was achieved. This is an important visit to determine

if the patient can continue with the dispensed lenses or if changes need to be made. An early appointment slot is imperative to get the most accurate measurement on topography and refraction-in the initial stages of fitting, the effect will quickly regress throughout the day.

At the one-day follow-up visit, unaided vision, refraction, and slit lamp exams are normally performed in addition to topography.

Topography scans must be analyzed for size of the treatment zone and centration, seen as a bullseye pattern of central flattening and paracentral steepening (Figure 4). Evidence of a “smiley” or “frown” pattern indicates lens decentration (superiorly or inferiorly, respectively), and an alteration in lens parameters is needed. Lens centration along the visual axis during the night is crucial for optimal treatment effect and to limit aberrations or distortion.

If no changes are indicated, the patient can be given a few pairs of daily soft contact lenses in lower minus parameters to allow her to have maximum vision during the initial treatment process.

The remaining follow-up schedule should include visits at one week, one month, and three to six months. At each follow-up visit, unaided visual acuity, refraction, anterior segment evaluations, and topography need to be repeated (Figure 5).

When fitting ortho-k lenses, it is important to aim for a slightly hyperopic end point refraction or overcorrection of about +0.50 D to compensate for any regression that may occur at the end of the day.

On slit lamp examination, the practitioner must evaluate for corneal staining, paying close attention centrally and peripherally, monitoring for mechanical irritation from a lens fit too flat or from poor peripheral edge design. The lenses should be inspected for defects and deposits at later visits to avoid complications.

Myopia progression and ortho-k

Although myopia is a common refractive error, there is still no consensus about the etiology of myopia. Environment, genetics, increased near work, outdoor activities, age, gender, ethnicity, education, geography, a myopic parent, personality, and socioeconomic status are all associations widely considered to be risk factors for developing myopia.^2-4

Myopia is most likely a result of a combination of genetics and environment.⁴ While a common and relatively benign condition of the eye, myopia comes with public health concerns due to its impact on medical, social, and economic factors.

Some statistics have shown that more than 40 percent of people in the U.S. are myopic-over the last 50 years, the number of people with this refractive disorder has almost doubled.⁵ At least half of those who develop distance vision problems start becoming myopic in their first decade of life.

As many as 4 million children and teenagers younger than 18 years are contact lens wearers.^6,7 Fitting young patients in orthokeratology lenses is gaining attention due to recent studies focusing on promising evidence that this treatment modality may slow the progression of myopia.⁸

A multitude of past studies have looked at the use of treatments such as progressive addition lenses, atropine, and multifocal contact lenses to provide answers on how to halt myopia, but recent attention has turned to orthokeratology.

The key seems to lie in providing peripheral defocus to the retina. Ortho-k provides this defocus by steepening the mid-peripheral cornea, thus reversing the normal peripheral refractive properties, allowing light to focus in front of this section of the retina. It has been theorized that this cuts off the signal for the eye to elongate as it would normally in an attempt for emmetropization.

While there is evidence to demonstrate that this may be a successful treatment for myopia progression, many questions remain. It is important to remember that reverse geometry contact lenses are currently not a treatment approved by the U.S. Food and Drug Administration (FDA) for myopia control.

Lenses should not be fit on young patients solely based on this reasoning, but it can be discussed with the parents as a potential additional benefit from wearing ortho-k lenses.

Incorporating ortho-k into your practice

Because wearing ortho-k lenses is an extended wear modality, the risk of adverse events when fitting these lenses needs to be addressed with the patient. The FDA recently reviewed the occurrence of microbial keratitis with CRT lenses in children vs. adults.

Researchers were not able to find a difference in the rate of infectious keratitis between children and adults.⁹

With new focus on orthokeratology as a possible treatment for slowing myopia in children, its safety in the pediatric population is especially important.

Researchers involved in the Myopia Control with Orthokeratology Contact Lens Study (MCOS) examined the incidence of adverse events in children wearing these lenses.

They concluded that this treatment modality was a safe option for children due to a low incidence of adverse events (13.3 percent per year), which were comparable to problems that arose with other types of contact lenses.¹⁰  Proper education, including both verbal and written informed consent, must be given to the patient concerning the risks involved and signs of a negative event.

Eyecare practitioners must continue to monitor advances in new research concerning myopia progression and contact lenses, specifically orthokeratology, because there is potential to change how we manage our patients.

References

1. Bennett ES, Henry VA. Clinical Manual of Contact Lenses.3rd ed. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins; 2009.

2. Benjamin WJ. Borish's Clinical Refraction. 2nd ed. Oxford: Butterworth-Heinemann; 2006.

3. Pan CW, Ramamurthy D, Sam SW. Worldwide prevalence and risk factors for myopia. Ophthalmic Physiol Opt. 2012 Jan;32(1):3-16.

Foster PJ, Jiang Y. Epidemiology of myopia. Eye (Lond). 2014 Feb;28(2):202-8.

4. Vitale S, Sperduto RD, Ferris FL 3rd. Increased prevalence of myopia in the United States between 1971-1972 and 1999-2004. Arch Ophthalmol. 2009 Dec;127(12):1632-9.

Efron N, Morgan PB, Woods CA. Survey of contact lens prescribing to infants, children, and teenagers. Optom Vis Sci. 2011 Apr;88(4):461-8.

5. Swanson MW. A cross-sectional analysis of U.S. contact lens user demographics. Optom Vis Sci. 2012 Jun;89(6):839-48.

6. Cho P, Cheung SW. Retardation of Myopia in Orthokeratology (ROMIO) study: a 2-year randomized clinical trial. Invest Ophthalmol Vis Sci. 2012 Oct 11; 53(11): 7077-85.

7. Bullimore MA, Sinnott LT, Jones-Jordan LA. The risk of microbial keratitis with overnight corneal reshaping lenses. Optom Vis Sci. 2013 Sep;90(9):937-44.

8. Santodomingo-Rubido J, Villa-Collar C, Gilmartin B, Gutiérrez-Ortega R. Orthokeratology vs. spectacles: adverse events and discontinuations. Optom Vis Sci. 2012 Aug;89(8):1133-9.