In a previous post, I have covered who should and shouldn't be fit with OrthoK. When starting out I suggest further refining this criterion to improve the likelihood of successful outcome with the first lens you fit. You will have to be very choosy with your first few patients if you follow my suggestions to the letter, but doing so will help you get to grips with the basics of Orthok fitting so that you can quickly expand your scope from a firm footing. In the same way as learning to ride a bike, it's safe to say you would be happier starting on a gentle obstruction free slope than an alpine class downhill slalom!
The easiest way to approach this problem is to consider what characteristics typically define potential difficulty with fitting Orthok, and avoid patients with these characteristics, at least until you feel comfortable in removing your training wheels. Our main considerations here are prescription, age, and corneal shape. But before we even get to this you need to have a pool of interested candidates from which to select your first patient, and this can get tricky if you don’t want to saddle yourself with a difficult patient
Who are the best candidates?
The best patients to fit when you are starting out are those with the most motivation to wear OrthoK, which will help them through the extra appointments that you might need. The largest motivated group currently seems to be parents wanting their children fitted with OrthoK for myopia control. However, while kids are not generally any harder to fit than adults, until you are ready to remove your training wheels I suggest you stick to fitting adults.
When it comes to adults, the two main motivated groups are those that desperately don’t want to wear glasses but find contact lenses too uncomfortable (basically good candidates for laser surgery) and those actively playing sports where contact lenses could be a hindrance, such as physical contact and water sports. Maybe hanging out at a sports club can be made tax-deductible after all, if you can convince your accountant that you are purely doing so only to recruit new patients to your OrthoK fitting apprenticeship cause!
Where to find them
How you approach this problem will depend on how comfortable you are in sticking your head above the parapet. If you only want to tentatively stick your toes in the water, it is easiest to probably just mention OrthoK as a vision correction option to those that fit the typical OrthoK wearing demographic, prescription and topography limits indicated below. If you want to be a bit more brazen, then you could create an information leaflet to distribute amongst your colleagues.
Choose an easy prescription
Working from the theoretical limits of refraction change, my suggestion while you are starting out is to limit the range further and only look for candidates with spherical refraction between -1.00D and -2.50D inclusive, with up to -1.25D of with-the-rule astigmatism, purely to stack the odds of achieving a successful outcome in your favour. What we are looking for here is for you to find candidates where you are pretty much guaranteed to achieve a successful outcome and to not get yourself into deep water by inadvertently choosing a patient that just happens to be a poor responder. Setting a ‘starting out’ upper limit of -2.50D pushes into the range where even the most poorly responding patient is still likely to achieve a good effect. Be patient - you’ll quickly get the hang of things within your first few patients and capable of paddling out into deeper water.
Choose the best corneal shape
There are a few things we need to consider here, including curvature (which defines the corneas refractive power), the eccentricity (rate of corneal flattening towards the periphery), and degree of corneal toricity. Each of these impacts your outcomes in different ways, with toricity affecting how the lens fits, and refractive power and eccentricity influencing the maximum outcome that you are likely to achieve. Each of these topics are covered in separate posts, but the basic beginners limits are as follows: Corneal curvature no flatter than 42.00D (8mm); Corneal eccentricity of 0.5 or greater; Corneal toricity of no greater than 50µm difference between the flat and steep corneal meridians at the bearing chord of the lens you are using.
Age
In a nutshell, for your first few fits stick to young adults. Thanks to the research from Jayakumar and Swarbrick,1 we know that for some reason younger eyes respond to OrthoK lenses to a greater degree and at a faster rate than older eyes. The older eyes being in the group aged 36 and above, which in this study was 43.9 ± 6.1 years. Children between 5 and 16 years were included in the study and found to respond in a similar way to the younger adults aged 17 to 35 years. However, while children have been shown to be good candidates for contact lenses, and not require any greater chair time other than a slight increase in time for teaching insertion and removal, fitting them does create a greater deal of complexity through having to include their parents in the consultation. I get that your primary reason for fitting OrthoK might be for myopia control, but honestly, you will thank me for suggesting you get a few adults under your belt first. Your learning curve is going to be steep, so it won’t take long for you to be fitting kids if this is your main purpose for wanting to learn OrthoK lens fitting.
Pushy eyelids
It seems that the prime role of the eyelids in OrthoK is to push the lens around leading to a decentered treatment zone, which makes them a pain in the rear end, though of course we really need eyelids to prevent the eye drying out overnight on top of many other benefits that is offered. When it comes to differentiating them regarding unacceptable OrthoK lens bashing there really is no easy way, other than to recognize that Asian eyelids are more likely to cause lenses to decenter than Caucasian eyelids.
I personally don’t tend to consider eyelids as a major factor in my decision making on whether to suggest OrthoK. I instead tell all patients that we won’t know how successful OrthoK will be until they have undergone an overnight trial. Again, recognizing that this is a guide for beginners, if Caucasians are part of your patient demographics, then to simplify what you might have to correct at the follow-up visit consider limiting your first few fits to Caucasian eyes. But it really isn’t a problem if this is not the case, as if you follow the other patient selection conditions this will limit the influence eyelids are likely to have. In general, it is when you are trying to achieve higher corrections that eyelids become more problematic.
Pupil diameter
A final consideration, although I would probably argue as not quite so important as the others when you are starting out is pupil diameter. Don’t get me wrong when I say that, because in general pupil diameter is an important factor, just that if you follow the rules I have suggested and stick to a patient with:
- Between -1.00 and -2.50D of myopia
- Less than -1.50D of with the rule astigmatism
- Cornea curvature of 42D or steeper
- Cornea eccentricity of 0.5 or greater
then you are likely to end up with a pretty large treatment zone diameter. OrthoK lenses typically induce flattening over a larger area in lower compared to higher degrees of refractive correction. In the case where pupil diameter is larger than the treatment size, the patient is more likely to see haloes in their vision, which are more likely to have a greater impact at night when their pupil diameter will be larger. So, if you follow my ‘beginners’ rules in most cases you should end up with a treatment zone that is larger than most pupils, though you should be aware that this might not be the case and advise your patient of this possibility accordingly.
In writing this guide, I have in mind that you already have enough to worry about without adding further considerations, but don’t let me stop you measuring pupil diameter if you want to, just make sure you measure it in dark conditions with the eye adapted to ensure maximum pupil diameter is reached. In its effect on success with OrthoK fitting, I have experienced too many real-life cases where a patient’s pupil size defies logical reason. I have had people with the smallest of all small pupils complain about halos, and those with dinner plate sized pupils not even notice them.
So instead my general practice is to advise all patients that they might notice halos, especially at night, and that this might be bothersome to the point that they don’t like OrthoK. With this forewarning, they can then decide if they want to proceed. Where this changes, however, is when pushing the limits of refraction change where final treatment zone is therefore likely to be small and the patient has massive pupils. But, if you are following this guide you will already be exceedingly conservative in your refraction pickings, so your warning on the possibility of haloes is more likely than not to be met with this being unproblematic at the follow-up visit.
References
- Jayakumar J and Swarbrick HA. The effect of age on short-term orthokeratology. Optom Vis Sci 2005;82:505-11.
About Paul
Dr Paul Gifford is a co-founder of Eyefit, an information resource to assist contact lens practitioners in all modes of practice. Learn more about him here.
