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4. Ortho-K vision and residual refraction expectations

6. Orthokeratology TroubleshootingCharl Laas

It is important to bear in mind both the patient and practitioner expectations prior to beginning orthokeratology. The patient’s expectation is to see clearly and comfortably without visual aids, and the eye care practitioners aim is to achieve good unaided vision through minimising optical aberration while maintaining the integrity of the cornea.

For practitioners, there may be an unrealistic expectation of achieving a plano sphero-cylindrical residual-refraction as we need to look at the way we approach minimising refractive error to achieve good unaided vision. There are a number of reasons why aiming to achieve a plano over-refraction is flawed. This can be due to the magnitude of the ametropia and refractive versus corneal astigmatism. Either of these can lead to a result other than plano following orthokeratology. These should be assessed and discussed for each case. Another reason that may cause reduced vision is Higher Order Aberration (HOA). Higher order aberrations can have advantages in cases where we are attempting myopia control and multifocal presbyopia management. They have a negative impact on a patient’s vision in low light.

Two key points to keep in mind with all myopic orthokeratology patients are:

  1. We are reshaping the cornea from a prolate shape to a more oblate shape, and this affects the aberrometry of the eye
  2. We need to look more at the goal of providing good unaided vision when determining the need for a remake following review of the residual refraction

Myopic orthokeratology improves unaided visual acuity during the day but also increases high order aberration and decreases contrast sensitivity. Although contrast sensitivity function is reduced, it has been shown to be within that of the normal range when compared to a healthy adult population.

Orthokeratology visual outcomes are comparable to refractive surgery. Factors that can affect visual acuity are the target power of optical correction and the treatment zone size. The influence of optical zone size on vision quality is well documented in refractive surgery. The smaller the optic zone size, the poorer the optical quality. The relationship is more significant when the pupil size is larger than the ablation zone diameter (treatment zone diameter in orthokeratology). The resultant increase in higher order aberrations in orthokeratology is due to an increase in coma and a positive shift in spherical aberration. As such a sphero-cylindrical overcorrection may not be the most appropriate measure to determine whether a change to an orthokeratology lens is required.

We suggest that orthokeratology practitioners aim to use a similar approach that refractive surgeons use when assessing the success of an orthokeratology fit. If the residual prescription yields good vision, ie 6/6 (20/20) or better and the residual prescription falls within a low degree of ametropia (+0.75 to -0.50 spherical equivalent). The fit should be considered a success.


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