Monday, April 10, 2017

myopia control

What is myopia?
Myopia is the fancy name for near-sightedness. Myopia causes vision to be blurry in the distance. It is the result of light rays focusing in front of the light-sensitive tissue that lines the back of the eye (the retina) rather than on the retina. This can happen when the power of the front of the eye is too strong (refractive myopia) or when the eyeball is too long (axial myopia). Below is an illustration. A minus (concave) lens is used to focus the light rays on the retina so that images are clear. The most common type of myopia begins between the ages of 6 and 12.

A myopic eye focuses the image IN FRONT of the retina, producing a blurry image. Minus lenses are used to correct myopic eyes, putting the image ON the retina so you can see clearly. 

The prevalence of myopia has been increasing over the past decades. In 2000, 23% of the world's population was myopic, and researchers are predicting that 50% of the world's population will be myopic by 2050 (1). THAT'S HALF OF THE WORLD!! Higher rates of myopia, nearing 90%, occur in some Asian populations. The increase in myopia prevalence suggests that environmental factors play a role in its development, though there is certainly a genetic component as well. People with higher amounts of myopia are also at greater risk for eye health issues such as retinal detachment and myopic maculopathy (2), so the myopia "epidemic" is a public health concern.

How does myopia progress? 
The exact mechanism of myopia development is still unclear. The thought is that myopia progression is caused in large part by the elongation of the eye. Research suggests that peripheral hyperopic defocus causes elongation of the eye. The periphery can be blurred even if the center is clear, and this peripheral blur is not really something we notice. At near, the periphery is more out of focus than it is when looking in the distance, so near work may be implicated in the progression of myopia (3). Some more recent studies have shown a greater association between myopia development and the time spent outdoors than the time spent doing near work. Translation: increasing myopia is more closely related to how little time people spend outdoors than how much time people spend reading. So minimal time spent outdoors could be a risk factor for myopia development. Very recent research has identified a cell in the retina that may cause myopia when it dysfunctions. The dysfunction may be linked to the amount of time spent indoors/away from natural light (4).
image: Review of Optometry

Options that have been shown to slow the progression of myopia:
  1. Orthokeratology (aka ortho-K, corneal reshaping technology, CRT). These are customized contact lens worn only at night, temporarily changing the shape and power of the front part of your eye (the cornea).  When you wake up, you take the contacts out and you can see! Ortho-K lenses involve reverse geometry, meaning the curves of the lens are structured so that the tear film beneath the lens essentially flattens the center of the cornea while steepening the midperipheral cornea. This creates peripheral myopic defocus that negates the peripheral hyperopic defocus that is linked to eyeball growth (5). Orthokeratology is FDA approved for myopia up to -6.00D and mild amounts of astigmatism (up to 1.75D). There is no minimum age; the child just has to be able to put the contacts in, take them out, and maintain them on their own. It all depends on the child's maturity level. As with any contact lens wear, there is a small risk for infection. Studies have found that ortho-K treatment produced an average of 30-50% reduction in the progression of myopia.(6). Ortho-K appears to be more effective for those with higher amounts of myopia and larger pupils. Both the LORIC and later the CRAYON study showed that ortho-K slowed the axial growth of the eye, thus reducing myopia progression (7). The SMART study is yet another recent study that supports the theory that ortho-K reduces myopia progression. At the conclusion of this 3-year study, the ortho-K group saw an average increase in myopia of 0.12D while the soft lens control group increased by an average of 1.01D (8).
  2. Soft multifocal contact lenses. Multifocal contact lenses are those that correct your distance while also giving you plus power to help see up close when needed. The specific design of multifocal lenses that are used for myopia control are center distance design (see image below). Center distance means more plus power in the periphery of the lens, which decreases peripheral hyperopic defocus and induces peripheral myopic defocus, reducing axial elongation. Several study results have supported the use of distance-center soft multifocal contact lenses for myopia progression, averaging a 40% reduction in myopia progression (91011)The CONTROL study found a whopping 72% reduction in progression of myopia over a one year period when compared to wearing single vision soft contact lenses (12), though this study involved myopic children with a specific focusing/postural issue (eso-fixation at near).

  3. A multifocal lens with a center-distance design
    image: Review of Optometry

  4. Atropine. This method is different from the above two because it is not about changing the stimulus that contributes to eye elongation, but rather interfering with a biochemical pathway.  How does it work? We don't really know for sure. But it is thought to act on the white part of the eye (the sclera) or the tissue that lines the back of the eye (the retina) to prevent the sclera from thinning or stretching (13). If you have had your eyes dilated during an eye exam, the doctor likely used tropicamide.  This drop made your pupils big and your vision up close was blurry for 3 or 4 hours. Atropine is a similar drop from the same family of drugs, but the effects last for much longer. Atropine 1% has been shown to produce a 90% average reduction in myopic progression, from 0.5 D/yr to 0.05 D/yr (14). The downside: blurry near vision, light sensitivity, and large pupils. Other studies have shown comparable results using lower concentrations of atropine, which produce less side effects. Atropine 0.01% has been shown to slow myopia progression by 50%(1516) Pirenzepine has also been tested, showing slightly less efficacy (44%), but with fewer side effects. Unfortunately, it's not commercially available as an eyedrop or gel in the US. More research is being done on the long-term effects of atropine therapy, and whether or not its effects are permanent.

Options that are NOT GREAT for slowing the progression of myopia:
  1. Undercorrection of myopia. I've had some parents specifically ask me to give their child less powerful glasses in hopes that that would make the child need glasses less. There is no validity to this claim; actually, the undercorrection of myopia has been shown to INCREASE its progression (171819).
  2. Progressive addition lenses (PALs). Commonly called "no-line bifocals," PALs are lenses that have your distance prescription at the top and gradually become more plus-powered as you go vertically down the lens. Many people over the age of 40-ish wear these to help them see clearly at all distances. For the purposes of myopia control, they're not a top option. The COMET study found a small, statistically significant decrease in myopia progression in children wearing PALs vs children wearing regular, single-vision lenses, but only in the first year. So the conclusion was yes, it produces a little decrease in progression, but not enough to warrant a change in how we prescribe for myopic children (20). Also, other aspects of the child's binocular vision should be taken into consideration. 
  3. Spherical soft contact lenses or rigid gas-permeable (RGP) contact lenses (aka regular distance vision contacts). These two choices are excellent forms of vision correction, but they have shown little value in terms of controlling myopia progression. The CLAMP study showed some reduction in myopia with RGPs in year 1 in comparison to soft contact lenses; however, it was not clinically significant because it didn't change axial length and was likely due to the flattening of the cornea, which is not permanent (21).

CliffsNotes: Glasses and regular contact lenses don't help slow myopia progression. Undercorrection of myopia doesn't help either; it actually makes it worse. Atropine therapy, orthokeratology, and soft multifocal contact lenses have been shown to be effective in controlling myopia progression. Ask your optometrist for more information!

Additional recommended resources: