It happens to people every day, all over the world. They notice their distance vision worsening. They notice their children’s vision worsening. They sit by helplessly as the optometrist reads them the new, higher prescription numbers and grimace when it’s higher than they expected. They ask the Doctor “how high will it go?” hoping for an answer yet knowing there is no way to predict. “What causes it, Doctor?” not getting any clear answer, just speculation regarding genetic factors interacting with the environment and vice versa. “What can we do, Doctor?” finding the answer to that question varies doctor to doctor. Historically several methods have been used to attempt to address what we call “myopic progression”, or “myopic shift”. They include (1) under prescribing, or writing a prescription weaker than the full prescription, (2) Prescribing “Plus” for near – the equivalent of having a distance prescription and a near prescription, often achieved with bifocal or multifocal eyeglass lenses, (3) using cycloplegic eye drops, usually Atropine in a process known as “atropinization” which paralyzes the muscles that overfocus some eyes causing myopic creep, (4) prescribing hard or gas-permeable contact lenses which were suspected to help reduce change and (5) prescribing orthokeratology, understood best as “vision braces”. I will attempt to provide a basic understanding of the physiologic mechanisms believed to cause progressive myopia and what scientific evidence has shown that proves or disproves the effectiveness of the various methods offered by eye care professionals.
Myopia is defined by the Merriam-Webster dictionary as a condition in which visual images come to a focus in front of the retina of the eye resulting especially in defective vision of distant objects. The cornea of the eye, or clear front dome has a power that enables it to bend light. The “perfect” eye ( an eye with no prescription needed for perfect distance vision) has a cornea that bends light to focus precisely on the retina, or nerve layer on the back of the eye. A myopic eye can be thought of an eye where the cornea’s “power” is too weak for its length; it doesn’t allow the light to bend in concert with the retina – in essence, a myopic eye is an eye that is too long axially. A myopic eye in which the negative, or “minus” prescription is increasing (say a -2.00 to a -2.75) is actually getting longer in its axial dimension. The longer the axial dimension, the higher the prescription. The most common causes of the increase in axial length include genetic predisposition1, sustaining focused vision at distances closer than arms length for periods of time2 and decreased use of peripheral vision3.
Let’s take a look at the aforementioned methods used to manage myopia and use study data to demonstrate promise or debunk them.
This is undoubtedly the most common method attempted to manage myopia. Historically, under prescribing has been performed by doctors in the belief that under correction slows down the worsening of myopia. This was done in the hopes of reducing near focusing strain that has been suggested as a cause of progressive myopia. Unfortunately this practice went on for a long time without any longitudinal study data to confirm that it was or was not effective. That means that tens, maybe hundreds of thousands of people have functioned with blur without knowing whether or not it was an effective method of managing their myopia. I frequently encounter parents who request I under prescribe for their children. Some people believe that this method works, others believe that it is helpful for the eye to attempt to adapt to a lower prescription. A recent study 4 failed to support this idea, finding no statistically significant difference between those who received full correction and those who received under correction. Two other studies5 found that under correcting nearsightedness actually increased the rate of its progression. Under correcting myopia is therefore not a proven strategy for slowing the progression of nearsightedness in children. It also has the disadvantage of causing blurred distance vision if the treatment is performed with single vision lenses.
PRESCRIPTION OF BIFOCAL OR MULTIFOCAL EYEGLASSES
We have stated that doctors believed that under prescribing works by reducing near eyestrain. While many doctors and parents attempted to under prescribe the entire prescription, others felt under prescribing distance vision puts the patient at a disadvantage yet desire the patient have less prescription when viewing near objects. To achieve this, they prescribe adult “bifocals” or “multifocals” – lenses that provide the proper distance prescription when viewing distance targets and a lower near prescription when looking lower to read or use a computer. Two studies 6,7 have shown a small, but statistically significant decrease in progression of myopia in the first year of wearing the multifocals which remained similar and significant for the next 2 years. This method is more successful in people with certain types of eyes; those who “over accommodate” and/or manifest a slight eye deviation inwards, known as esophoria. In our practice we often offer this as an option for parents with children who are slightly esophoric and manifesting prescription change greater than 2 steps (.50 diopters) per year.
Drops known as Cycloplegics are used to paralyze the muscles that focus to provide near vision through the process known as accommodation. The most common cycloplegic agent used is Atropine, and the process of using Atropine is termed “Atropinization”. The theory behind cycloplegics is that by minimizing accommodation, the continued growth of the axial length of the eye is controlled. Several studies have consistently shown a beneficial effect for proper atropinization. In one study8, sixty-two children were treated with atropine in one eye for one year; the fellow eye was the control. The eyes were switched the second year. Twenty-eight patients were treated for four years on the same basis. Control eyes showed significant increases in myopia compared to treated eyes. Some treated eyes showed decreases in myopia; no decreases were seen in control eyes. Post treatment data analysis indicated the effects are long-term. In other studies 9,10, after stopping treatment, eyes treated with atropine demonstrated higher rates of myopia progression compared with eyes treated with placebo. However, the absolute myopia progression after 3 years was significantly lower in the atropine group compared with placebo. Still another study showed similar benefits using a different cycloplegic agent Pirenzepine11. Atropinization is rather severe and can limits a child’s function at near as Atropine paralyzes their ability to focus up close – they must use reading glasses for near work. While we offer this option to patients, it is not the most popular option.
HARD AND RIGID GAS PERMEABLE CONTACT LENSES
In the late 1950’s the first plastic hard lenses were mainstreamed. Prior to plastic hard lenses, glasses had been the only mainstream vision correction option. Doctors noted after a few years a significant percentage of the patients who were wearing hard lenses seemed to manifest a reduction in the amount of myopic “creep” year to year. Early studies produced intriguing results for scientists and clinicians, but contain many problems that challenge the significance of the studies’ findings; studies failed to provide proper attention to many important variables. In more recent and better controlled studies, researchers found that rigid gas permeable contact lenses slow the progression of myopia (nearsightedness) in young children12,13,14. The difference in myopia progression between the rigid gas permeable contact lens wearers and the soft contact lens wearers was 0.63 diopters (D). Although the myopia of children assigned to wear rigid gas permeable contact lenses progressed less than the myopia of children assigned to wear soft contact lenses, the difference is not enough to warrant prescribing rigid gas permeable contact lenses solely for the purpose of slowing the progression of nearsightedness. Researchers also found that rigid gas permeable contact lenses do not slow the growth of the eye, which is responsible for the majority of myopia in children. Instead of slowing the growth of the eye, rigid gas permeable contact lenses kept the cornea from changing shape more than soft contact lenses. The change in the cornea is not likely to be a permanent change, so the effect of rigid gas permeable contact lenses on myopia progression may not be permanent.
Another interesting finding around the time that hard lenses were mainstreamed was that some patients reported being able to see clearly, or clearer, at distance without glasses after they removed their hard or gas permeable contact lenses. It didn’t take long for doctors to realize that lenses that were fit flatter than the curve of the cornea were responsible for this finding. The art and science of prescribing lenses with a curve flatter than that of the cornea is known as Orthokeratology. Now, Orthokeratology is purposefully prescribed to provide patients a refractive option that allows people to see without glasses or contacts during their waking hours. Since orthokeratology is performed with hard but preferably gas permeable contact lenses, doctors believed that it also may help limit the progression of nearsightedness. Recently it has been shown15 that Ortho-k can have both a corrective and preventive/control effect in childhood myopia. However, there are substantial variations in changes in eye length among children and there is no way to predict the effect for individual subjects. A new study, called the Corneal Reshaping and Yearly Observation of Nearsightedness (CRAYON) study, is now underway and has confirmed that corneal reshaping with specially designed gas permeable contact lenses does indeed slow eye growth in myopic children at one year of treatment. We have a very lage orthokeratology practice. OrthoK seems to be the method of choice due to it’sconvenience, ease of use and positive study data. Parents who choose OrthoK for their children need to be aware of the benefits and risks of sleeping in contact lenses. They should be educated as to the signs and symptoms of an infection or inflammation. They should also know not to use tap water or distilled water at any time to clean their lenses or lens cases. Lens cases should be cleaned every night as they have been identified as the source of most infections in overnight lens wear.
Every day in our practice we talk with parents concerned about their childs worsening vision. While myopia “creep” is a reality, the big picture regarding what the “creep” ultimately means has changed since the advent of laser vision correction. When parents were young, the perception was that whatever change occurred did so permanently, relegating the child to strong eyeglasses or contact lenses the rest of their life. The concept of eventually reversing the change with laser refractive surgery didn’t exist. Now there is a somewhat permanent solution to higher levels of nearsightedness at the end of the road for most people with myopia and astigmatism – light at the end of the tunnel, pun intended. Myopia no longer has to mean a lifetime of wearing glasses or contact lenses. If viewed from this perspective parents may take solace in knowing that even though their child will deal with a period in his/her life where they are dependent on refractive correction, there is light at the end of the tunnel. Their child is likely to have the opportunity to experience life without contacts and glasses again. This is a generational shift in thought brought on by technological advances. A relevant long term strategy for many parents is manage as best as one can by using available myopia control methods that demonstrate efficacy according to solidly designed scientific studies with the long term view that whatever happens, options exist to improve the childs quality of life with surgery as an adult.
Copyright 2009 – Dr. Alan N. Glazier, Optometrist, PA – All Rights Reserved
Courtesy of the Doctors at Shady Grove Eye and Vision Care; Optometrists, Ophthalmologists and Opticians working together to help you see better. Serving the Rockville, Potomac and Gaithersburg Maryland suburbs of Washington, DC for over 40 years. For more information visit youreyesite.com or call (301) 670-1212 begin_of_the_skype_highlighting (301) 670-1212 end_of_the_skype_highlighting
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