Who are ideal candidates for orthokeratology?

The most important step in successful orthokeratology (ortho-k) fitting starts with identifying suitable candidates. From there, obtaining high-quality corneal topography maps, lens fitting, and adhering to an appropriate follow-up schedule define the process. If you're getting started in orthokeratology, it's a good idea to begin with a straightforward patient, such as those described below. This article will discuss who the ideal candidates are for ortho-k.

Ortho-k is well known for its robust evidence-base for myopia control,¹ and is a viable choice for patients that require correction of mild to moderate myopia. Ortho-k reliably corrects up to : beyond this, refractive and visual outcomes are less predictable. It is important to note that certain levels of refractive (myopia and astigmatism) correction will be on-label, or regulatory approved, for different ortho-k lens designs. Verifying the on-label range of an ortho-k lens design supports safe and effective practice - read more in our article on Understanding on- and off-label prescribing.

Refractive correction caused by overnight ortho-k can be attributed to the changes in corneal thickness it induces. The reverse-geometry design flattens the central cornea, inducing central corneal epithelial thinning. With high prescriptions, a greater amount of thinning is required in order to achieve full correction; hence, central corneal epithelial thickness limits the range of refractive error that is correctable via ortho-k.² The majority of studies on ortho-k in children set -0.50D to -0.75D of myopia as the lower limit of treatment.¹ 

Although there have been some reported cases of fully corrected high myopia with ortho-k,³ high myopes are not ideal candidates for ortho-k. The ideal levels of myopia range from -0.50D to -4.50D.^4-5 It is important to bear in mind, though, that every patient is unique and may not achieve full correction despite residing within this prescription range due to their own individual corneal anatomy and biomechanics.

Ortho-k has been shown to correct up to -3.50DC of astigmatism.⁶ Spherical designs of ortho-k lenses can typically correct up to -1.50D of corneal astigmatism, so the ideal candidate would sit within this range for a straightforward ortho-k fit. Attempting to fit a spherical lens for patients with astigmatism upwards of -1.50DC will likely result in lens decentration and poorer visual outcomes.⁷ With greater amounts of astigmatism through to -3.50DC, this would require a toric design for optimal fitting and refractive outcomes.

Selecting spherical versus toric ortho-k lens designs based on the 1.50DC limit comes with caveats: spherical ortho-k lenses typically fit and treat more effectively in cases of with-the-rule versus against-the-rule astigmatism.⁸ It is also important to ensure that corneal astigmatism closely matches the amount of astigmatism measured during refraction. In cases of a mismatch, this may indicate the presence of lenticular astigmatism which ortho-k will not correct. This would result in residual astigmatism being present after ortho-k treatment.

One study revealed that adults older than 36 years exhibit a diminished anatomical and refractive response to ortho-k in comparison to younger adults ranging from 17 to 35 years, and children aged between 5 to 16 years.⁹ While individuals over the age of 40 may still be suitable candidates for ortho-k, it is important to note that their response is likely to be reduced or delayed when compared to younger individuals using ortho-k.⁹ The ideal candidate for ortho-k would be below 35 years old.

Regarding the youngest potential age for ortho-k, several studies have included children as young as 6 years old.¹ While ortho-k provides an excellent option for children, it is important to assess whether or not each individual child is suitable: are they able to handle the lenses? Are they comfortable being fit with ortho-k? Involving parents or caregivers is crucial, as they can monitor their child's lens use and hygiene practices.¹

In addition to corneal astigmatism, corneal eccentricity and apical corneal power are essential topography measures to assess at the initial lens fit.^10-11 These readings can tell you how effective ortho-k treatment will be in correcting myopia.

• Corneal eccentricity: corneal eccentricity (e-value) refers to the rate of flattening between the central and the peripheral cornea. A higher baseline corneal eccentricity is ideal as it means there is a greater difference between the central and peripheral corneal curvature, and a greater potential for change to the corneal profile. An eccentricity value of e=0.50 has been shown to correspond to an ortho-k treatment effect of approximately 2.50D; hence, ideal candidates would have an e-value of 0.50 or higher.¹⁰ A lower e-value limits the treatment potential.
• Apical corneal power: Higher baseline apical corneal power is associated with a greater refractive change achieved through ortho-k lens usage. When the cornea is steeper, it possesses a greater ability to undergo flattening^10-11 which is ideal in ortho-k. The corneal changes in ortho-k are more complex than just central flattening, though, so ideal baseline values for this metric are not defined in the literature and fits can be possible with a variety of values.^10,11

Ortho-k is the only non-surgical treatment option that provides freedom from daytime spectacle and soft contact lens wear. It is ideal for patients who are physically active, and especially in those that engage in water sports, which is a key risk factor for acanthamoeba infection in soft contact lens wearers.¹² Those that work in industrial occupations or dusty environments also benefit from having unaided clear vision, and so would be good candidates for ortho-k.

A patient who suffers from dry eye syndrome but does not want to wear spectacles may find that ortho-k is a viable solution. Soft contact lenses can cause or exacerbate pre-existing dry eye symptoms.¹³ Ortho-k eliminates the need for daytime contact lens wear, allowing the eyes to rest from potential irritants and discomfort associated with soft contact lenses. A recent study found that overnight wear of ortho-k has minimal effects on meibomian gland function and tear film stability in children,¹⁴ indicating that it would be a good option for those that experience soft contact lens discomfort.

Another study refit adults aged 18-45 years who had suffered ocular dryness and discomfort in soft contact lens wear into ortho-k. Around three-quarters were successfully refit, and showed improved symptoms and less conjunctival staining after 3 months of ortho-k wear.¹⁵

Considering your patient's refractive, corneal and lifestyle factors will allow you to assess whether they are an ideal candidate for ortho-k, and streamline the fitting journey. Here is a summary of the candidate factors described above, describing the ideal candidate for ortho-k. Along with having good ocular health suitable for contact lens fitting,¹ this patient has:

• Up to 4.50D of myopia
• Up to 1.50D of astigmatism for a spherical ortho-k lens; up to 3.50D could be corrected with toric ortho-k.
• Aged between 6 and 35 years. Adults older than this can wear ortho-k but treatment response may be reduced.
• A higher corneal eccentricity (e-value) and higher apical corneal power can denote more treatment potential.
• An active lifestyle for which the benefits of ortho-k are recognized, including water sports.
• Those who are motivated to wear contact lenses but have suffered soft contact lens related dry eye symptoms.

While there are software fitting systems available for ortho-k that can help you to assess topographical suitability of your patient's eyes and support lens design,¹⁶ it is important to also assess a patient's individual, lifestyle and motivational factors to ensure success in ortho-k fitting.