Eyedolatry: orthokeratology

What's Happening to Our Vision? Peripheral Defocus and Myopia Progression

We're exploring the most talked about theory of why near sighted people keep getting more and more near sighted throughout childhood today -- Peripheral Defocus theory. If you want to read about other theories, check out the first post in this series discussing the Nearpoint Stress model. Keep in mind that none of these theories purports to stand alone -- we know that genetics plays a large role in why we become near sighted, but because the rates of myopia are increasing so rapidly worldwide (the US alone has seen an increase of myopia of 66% since the 1970s), there has to be environmental factors at work too.

The Basics:

Let's start from the true basics of what's going on in the eye, because if you try to read scientific journal articles about peripheral defocus it can get complicated really quickly. The key to understanding this theory is to have a god idea of how light focuses on the retinal photoreceptors in the back of the eye. Light enters through the pupil, is refracted by the lens, and is focused onto the retina to send a clear image to the brain. In emmetropes (people with "perfect" eyes or no prescription) light from a distant object will fall perfectly onto the retina without any need for the eye muscles to work and reshape the lens to change the eye's focus. In hyperopes (farsighted people), light would actually focus behind the retina, but the eye muscles move to reshape the lens to bring light onto the retina and into clear focus. That's why farsighted people tend to see very well in the distance unless they have a really high or unequal prescription -- their eye muscles are strong enough to bring things into focus and clear the world around them without any additional help. This muscle and lens action to bring light into focus on the retina is called accommodation. In myopes (near sighted people), light focuses in front of the retina. Accommodation can't shift this image backwards, so near sighted people can't see clearly at a distance no matter what they do.

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If you feel comfortable with things so far, we're ready for the next step. When we correct vision with glasses or contact lenses, they help you see better because they move the image to the retina, making your eye as if it was an emmetrope (without correction). That seems like it should solve the problem, right, because emmetropes stay emmetropes, they don't tend to get worse and worse? So why do corrected myopes, wearing the correct glasses prescription, keep getting more myopic?

Now Look a Little Further Out

The answer in peripheral defocus theory is because we need to look away from what's going on at the central retina (where the image you are viewing is being focused) and pay attention to what's occurring peripherally in the retina. Why don't corrected myopes act exactly like emmetropes? Because they don't have the same ocular anatomy -- that's why light is focusing incorrectly in myopes in the first place. Myopes have a longer eyeball (axial length is the techinical term) which is why light falls short of the retina when it enters the eye. Correcting a myope's vision with glasses or contact lenses can make them like an emmetrope centrally where light is focused for clear visual processing, but it can't negate the fact that the eye is too long, so light is still being focused incorrectly in the peripheral retina in a myope but not in the perfect length eyeball of the emmetrope.

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This image above is a favorite because it illustrates that a myope doesn't have a perfect circle for an eye -- notice how the illustration points out the move oval or prolate shape of the myopic retina -- it's stretched horizontally longer. So when light focuses clearly on that elongated central retina, it is now by default being focused behind the retina in the periphery. This is called peripheral hyperopic defocus, and it's our culprit for myopes worsening and worsening. The concept is that when these light rays fall behind the retina in the periphery, our body chemistry changes eliciting a response to make the eye increase it's axial length. The eye is programmed to want to get those peripheral light rays into focus ideally in front of the retina (called myopic peripheral defocus) where they would naturally fall in an emmetropic eye. But the more the eye grows to try to bring those peripheral retinal images into focus, the higher the myopic prescription gets, and we enter a constant feedback loop of the eye getting more and more nearsighted. We know from studies performed on chicks that myopic defocus induces choroidal thinning and axial length growth within a very short amount of time (within just 5 hours of a chick wearing a minus lens, researchers were seeing changes). Interestingly these same studies show that it doesn't matter the amount of hyperopic defocus induced, just the fact that it's there. Whether the chicks were wearing a -5.00D lens or a -15.00D lens, their eye grew by about the same length.

What Can We Do To Counteract Periperhal Defocus?

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So how can we correct a young myope's vision so that they can see clearly, but prevent that correction from causing hyperopic periperhal retina defocus and thus increasing their axial length growth? The STAMP study split myopic children into two groups; one wearing single vision glasses and the other wearing progressive lenses, and looked at the rate of myopia progression as well as the change each group's central and peripheral retinal defocus. As we expect, the kids wearing single vision distance glasses had hyperopic peripheral defocus in all quadrants of their retina. The kids in the progressive addition group however had myopic peripheral defocus induced in their superior retina due to the bifocal addition in the bottom of the glasses (see this concept depicted in the diagram above). Having that myopic peripheral defocus did seem to help -- kids with superior myopic defocus only progressed by an average of -0.38D during that year of wear, while kids with hyperopic defocus in all quadrants progressed by -0.65D on average. But in both groups, the children still progressed, so we know that in order to truly control myopia we will likely need to induce myopic peripheral defocus in all quadrants, not just superiorly like we do with bifocal or progressive glasses. This is where multifocal contact lenses and orthokeratology lenses come into play.

Diagramatic of Biofinity Multifocal soft contact lens design -- note the concentric rings of progressive addition around the more peripheral aspect of the lens! via

An orthokeratology lens via

What do the two types of contact lenses that studies show can help slow or prevent myopia progression have in common? Well from the pictures above you can spot the big similarity -- they both have ring designs that change the way light focuses on the retina between the central and peripheral retinal areas. Peripheral defocus theory suggests that these designs work at controlling myopia because they create that all important peripheral myopic defocus, interrupting the feedback loop for the eye to continue lengthening that is our bane in glasses and single vision contact lens wear. Orthokeratology has been studied at length with great results for myopia control -- reducing progression by anywhere from 50-90%. Soft multifocal lens wear for myopia control is just emerging as an option, but early studies look promising. The one year CONTROL study showed 50% reduction in myopia progression in children wearing distance center soft multifocal contact lenses compared to children in distance only soft contact lenses. More studies are underway but if their results are also promising, expect a future where contact lens companies specifically market a soft multifocal design for myopia control in kids.

But besides wearing specialty contact lenses, what else can we do to slow down myopia progression? Our next post in this series will explore how the light in our every day environment may be related to our current myopia epidemic.

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Troubleshooting Your Orthokeratology Fit

basic design orthoK contact lenses 7:00

The goal of orthokeratology is to correct vision overnight by reshaping the cornea with enough pressure to have lasting clarity the next day. Besides having the correct amount of treatment pressure to negate your prescription, the lens has to be well centered on the eye all night long, and still allow appropriate tear exchange underneath the edge of the contact lens to allow for good corneal health. If the lens edge is too tight or too flat, the result is reduced comfort as well as a higher risk for mechanical damage to the eye like superficial punctate keratitis, inflammation, and even corneal abrasion. If you think of it in the most simplistic terms, your orthoK fit needs to achieve these 3 things: Correction, Centration, and Comfort. Here's how to acheive your three keys to success:

1) Patient Selection

Selecting the right base curve is your key to correcting your patient's prescription. Popular lenses like Paragon CRT have a nomogram tool to help calculate your proper base curve dimension based on the patient's spherical Rx and their flat K reading. The best bet for success also lies in patient selection. Orthokeratology won't work on all prescriptions, so know your best patient candidate to ensure the highest chance for success before you get started.

The ideal orthokeratology patient has up to -4.00D myopic prescription and no more than -1.00D of corneal astigmatism. Their K values will ideal be between 42 and 45. It is possible to do orthokeratology on patients outside of this range, but the lens selection will be more difficult, and there is a good chance the patient won't have perfectly corrected vision the next day.

Difficult patients for orthokeratology include: against the rule astigmatism (the lens tends to decenter!), very flat corneas (post-LASIK patients would be very difficult), and patients whose astigmatism is greater than their spherical prescription. Also, patients who have lenticular astigmatism will not be expected to have that improve with orthokeratology, so make sure you check your Rx with your K's to verify their astigmatism is corneal.

2) Breaking Down the Ideal Lens Fit:

The ideal lens is well centered on the eye with a central treatment zone of about 3-4 mm diameter. You would want alignment staining in the return zone area 360 degrees around, with no bubbles. And on the edge of the lens you would ideally want 1-2 mm of clearance to prevent lens seal off. Read this post for more details and a deeper explanation of each zone.

3) Troubleshooting Fit Issues -- change one of these parameters at a time. Paragon recommends starting with option #1 in the lists provided when changing fit, and then if the issue persists, changing option #2 next.

An inferiorly decentered lens. Read below to learn how to fix this! via

Poor Centration:

Lens Decenters Left, Right, or Superior?

1. Increase Return Zone Depth by 25 microns
2. Increase Landing Zone Angle by 1 degree
3. Go to larger diameter lens

Lens Decenters Inferior?

1. Decrease Landing Zone Angle by 1 degree
2. Decrease Return Zone Depth by 25 microns

Inadequate Central Treatment Zone:

1. Decrease Landing Zone Angle by 1 degree
2. Decrease Return Zone Depth by 25 microns
NOTE: If you aren't getting a good treatment zone, double check your over refraction. Ideally over top of the contact lens you should get a plano to +0.50 over refraction with no cyl. If there is cyl in your over-refraction over the contact lens, then it is untreatable. If you have > +0.50 over refraction, then Decrease the Base Curve by 0.1 mm for every 0.50 D of correction you need to reverse.

Fitting someone with more than -5.00D of myopia? It can be done, but the treatment will take longer, and there is a high chance they may have residual myopia even with the best fit lens. Paragon recommends increasing your Return Zone Depth by 25 microns if you need more myopic treatment, and then have the patient wear the lens for 1-2 weeks and reassess before making further changes.

3) When to Follow-Up

Initial Dispense: Dispense the lens, and have patient wear it overnight and return to clinic wearing the lens the next morning.

Assess fit and centration with NaFl staining

A 3-D topography of an eye post-orthoK lens wear. The central cornea is
depressed (the treated zone) and the mid peripheral cornea will be
elevated where the corneal tissue has been displaced. via

Over-refract over the lens (you want plano to +0.50)
Remove the lens and perform topography to assess treatment zone pattern
Refract to see the amount of treatment achieved

On average, expect about 2.00D of treatment on night one with a well centered orthoK lens. The patient may need to take home soft contact lenses at this visit if they have a higher prescription to correct,

One Week: Typically we have the patient come in after school, not wearing the lenses. Make sure they bring the contact lenses with them to the visit!

Perform topography and refraction (you are wanting close to full correction at this point unless they were a higher Rx in which case they may need a second week to achieve full correction)
Insert lens and assess fit and over-refraction

One Month: Same as 1 week visit, just following up to ensure continued comfort and performance

Six Months: Typically we are fitting these lenses on growing children, so if they were known to have a high speed of myopia development before their orthoK fit it isn't a bad idea to recheck in 6 months to make sure things are under control. Especially in brand new wearers the 6 month follow-up can help keep things on the right path and instill the concept that the eye is changing and we may need to change the lenses to keep up. The hope is that we have stabilized the eye's myopic growth rate, and the best way to achieve that success is to make sure the lens is properly fitting and fully correcting the issue.

One Year: Studies recommend for best chance of myopia control, the lenses should be changed yearly in growing children. The lenses themselves can warp with time, like any RGP, causing less controlled treatment of myopia and a greater chance of myopic slippage.

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Orthokeratology: The Basic Design

base curve ortho K 7:00

Orthokeratology contact lenses reshape the cornea overnight to provide clear vision during the day without the need for contact lenses or glasses during those daytime hours. These specialty contact lenses can be great for children, athletes, or those with jobs or lifestyles that make glasses or contact lens wear very difficult. As a practitioner, prescribing these contact lenses involves much more technical skill than the average contact lens, and adjusting the lens for better vision and fit requires a working knowledge of different parts of the design that are completely unique to orthokeratology lenses. These lenses employ a large reverse curve change, where the central part of the lens bears down on the cornea while the mid peripheral region of the lens vaults back up to align the contact lens properly on the eye. Today's post breaks down the basic design of orthokeratology contact lenses, and to help determine what parameters need to be changed to achieve that best fit for maximum lens performance and next day clarity!

A Perfect Fit with Paragon CRT lenses via

Fitting orthokeratology contact lenses requires assessment of 3 zones of the contact lens:

1) Base Curve

The central area of the contact lens (6mm in size in the popular Paragon CRT lens design) is the base curve region. This area is the main pressure point for vision correction -- specifically calculated to negate the patient's myopic prescription. The only reason to change this curve is to get more vision correction -- don't change this to change the fit of the contact lens! With Paragon CRT lenses, the appropriate base curve will be selected by the patient's spherical equivalent prescription and their flat K using a nomogram, so very little calculation will be needed by you in initial lens selection. If you need more prescription correction later on, you can flatten the base curve to achieve more correction.

Increasing the Base Curve will flatten the contact lens (or create more treatment).

Increase the Base Curve (flatten) by 0.1 mm = 0.50D more myopic treatment

2) Return Zone or Reverse Curve

The Return Zone area is just outside of the central base curve region where the contact lens lifts back up off of the cornea. In the Paragon design it is ideally about 1 mm in size. The Return Zone is measured in microns of sagittal depth (much like when you fit scleral contact lenses!) so you are measuring the amount of lift off of vault away from the cornea. The Return Zone is essential to aligning the contact lens for centration, so this is an area that you may need to adjust to achieve best fit!

Higher Return Zone Depth = More Lift Off of Cornea

Typically change this by 25 micron units to achieve visible results

The key to Return Zone measurements is that you want alignment fit with the cornea underneath. If there are bubbles in this area, you have too much sag depth and you need to lower the Return Zone. If you don't have a well defined central pressure zone, you may need to increase the Return Zone to get more treatment and better alignment.

3) Landing Zone or Alignment Curve

The very edge of the contact lens is the landing zone, allowing the lens to fit comfortably on the cornea. The lower the landing zone angle, the more edge lift you will achieve as the edge of the lens lifts up and away from the cornea. Ideally you want 1-2 mm of tear film between the edge of the contact lens and the cornea. If you have too little tear film underneath, the edge would be too tight and will be increasingly uncomfortable with wear, not to mention that underneath your closed eyelid the contact lens may not be centered and you would awake with poor vision the next day. If the edge lift is excessive, again comfort will be a major issue as you blink, and centration will also be an issue.

Lower the Landing Zone Angle by 1 Degree to Increase Edge Lift

Increase the Landing Zone Angle by 1 Degree to Decrease Edge Lift

Here's another tricky part, if you change the Landing Zone angle, you will also change the Return Zone depth: Decreasing the Landing Zone by 1 degree will Decrease the Return Zone sag depth by around 15 microns. Typically if you want to make changes to contact lens alignment in small increments, you would start by changing the Landing Zone before you change the Return Zone. That's because changing this area will impact your Return Zone anyway.

Stay Tuned for the next post in this series: trouble shooting orthokeratology fits.

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Controlling Myopia Progression

atropine 21:15

Is your child near-sighted (or myopic)? Chances are, their prescription is only going to increase as they grow! Myopia progression is something that I discuss with worried parents daily. Many parents have high prescriptions themselves, and would love to save their child from the difficulties involved with highly myopic prescriptions. Unfortunately trying to prevent this from happening is a difficult battle.

On average, myopia progresses by 0.50D every single year in a growing child. That means if your child is a -1.00D myope at age 6, they will be wearing a -6.00D by age 16! Myopia is a condition that results from the curvature of the eye and the length of the eyeball. Often, as we go through normal growth and puberty changes, that length can increase, inducing more myopia. Eventually, these changes will slow down, and normally stop progressing by our 20s. But in this critical time period, a prescription can develop that permanently changes that individual's lifestyle forever.

How do we prevent myopia progression?

That's a question scientists still don't have complete answers to. Today's best research suggests that 3 options are giving the best results in slowing myopic progression:

Atropine: Warning--it burns when it
is inserted into the eye!

1) Atropine

-this involves daily insertion of a potent dilating drop, and having to wear bifocals to read

2) Corneal Reshaping with specialty hard contact lenses

3) Soft multifocal contact lenses

-the premise is the cause reduced near strain and peripheral blur, giving a negative feedback signal to the eye's myopia development process

None of these options are easy. Atropine is an eye drop that causes the pupil to dilate and the accommodative muscles in the eye to lose the ability to focus. As a side effect, the child will need to wear bifocals in order to see to read. They may experience headaches, and will complain of intense burning when the drop goes in. Great for kids, right?

Corneal reshaping is performed by wearing rigid (hard) contact lenses overnight, almost like a retainer that molds the eye into a different shape. Believe it or not, it's not as uncomfortable as it sounds.

The third option involves soft contact lenses, which is another extra addition to the child's daily routine, for cleaning and caring for these lenses.

Even with these techniques, myopia will likely still progress. Studies have shown that the rate of progression with these options can be reduced anywhere from 30-50% however, which can make a real difference in your child's final prescription. If you are interested in any of these options, talk to your doctor to see if your child would be a candidate!

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