When it comes to astigmatism, the research shows that approximately 40% of the initial astigmatism remains post treatment when using spherical OrthoK lenses.1 Following this research, -1.25DC is likely to end up at -0.50DC which, in most cases would be considered clinically acceptable. By the same conversion -1.50DC would give a predicted outcome of -0.60D, which, while not much different, is beginning to push towards clinically unacceptable levels of residual astigmatism. All of this also assumes that the astigmatism is corneal rather than lenticular in origin and that a standard (spherical) OrthoK design is being fit. The axis orientation and overall extent of corneal astigmatism also should be taken into consideration when considering which lens type to fit.
Corneal vs lenticular astigmatism
Astigmatism is caused by irregularity in any of the refractive surfaces of the eye but is typically subclassified into being either corneal where the anterior corneal surface is the main component or lenticular, which generally means any of the refractive surfaces within the eye. Following this classification, the astigmatic part of someone's refractive error is a factor of corneal astigmatism or lenticular astigmatism or the combined effect of both.
The ocular surfaces responsible for astigmatism matters, as in most cases only the corneal astigmatism component can be corrected from OrthoK. A spherical OrthoK lens will mold the corneal surface towards the spherical shape of its back-optic zone, in effect ironing out any corneal astigmatism. The same is not the case for lenticular astigmatism as OrthoK lenses, by virtue of only affecting the anterior corneal surface, cannot alter the profile of the refractive surfaces inside the eye. Any lenticular astigmatic, if present, will therefore persist.
Revisiting the 40% remainder rule stated at the beginning of this post it becomes apparent that it only applies if any astigmatism is mostly corneal rather than lenticular, which is generally the case and can readily be determined using a keratometer or from simulated keratometry values calculated from corneal topography. Corneal astigmatism is the difference between the flat and steep keratometry values when measured in diopters, and if subtracted from refractive astigmatism will give a reasonable approximation of any lenticular astigmatism.
The rule then becomes: Approximate residual astigmatism from regular (spherical) OrthoK = 40% x corneal astigmatism + lenticular astigmatism, which means even relatively small amounts of lenticular astigmatism can start becoming problematic.
Extent of corneal astigmatism
How extent of corneal astigmatism impacts likelihood of successful correction is covered in more detail in a later post that covers the different ways corneal astigmatism can influence OrthoK lens fit. The crux is that corneal astigmatism becomes more detrimental to standard (spherical) OrthoK lens fit the further it extends into the corneal periphery. In central corneal astigmatism, as shown in the images below, the mid-periphery of the cornea is mostly spherical and so follows close conformity to a standard (spherical) OrthoK lens design. In cases where corneal astigmatism extends out towards the periphery, this conformity is lost leading to an increased likelihood for a standard (spherical) OrthoK lens design to decenter during wear. Toric OrthoK designs, covered in a later post, offer a better solution in these cases.
Orientation of astigmatism
Spherical OrthoK lens fits tend to be more stable on astigmatic corneas that have with-the-rule astigmatism, meaning the axis is within 180 ± 30˚, which in real terms equates to 150˚ through 180˚ to 30˚. The vertical eyelid blink action causes the lens to rock across toric corneas exhibiting with-the-rule astigmatism. In doing so causing a tear pumping action that more easily replenishes the post-lens tear film.
In the opposing case of against-the-rule astigmatism (axes 90 ± 30˚), or oblique astigmatism (45˚/135˚ ± 15˚), a spherical (standard) OrthoK lens instead rocks from side to side during a blink and as a result can be pushed towards either side by the eyelid. Using a toric design in this situation will tend to offer a better solution, but can only be fit if there is sufficient difference in sag height between the principal flat and steep meridians regardless of astigmatism orientation.
Correcting astigmatism when starting out
To give you the best chance of achieving a well-centered lens fit, when starting out it I suggest you limit your first few patients to those with -1.25D or less with-the-rule astigmatism. Getting a few successful low astigmat fits under your belt will improve your understanding of how OrthoK corrects astigmatism and the limits of astigmatic correction that can be achieved from standard spherical designs. Having gained this experience you will then be ready to consider the suitability of toric OrthoK designs for correcting higher amounts of astigmatism.
- Soni S et al. Overnight orthokeratology. Eye & Contact Lens 2003;3:137-45