10 March 2009

Nearsightedness (Myopia)

What is nearsightedness?

Nearsightedness (myopia) is a common cause of blurred vision. If you are nearsighted, objects in the distance appear blurry and out of focus. You might squint or frown when trying to see distant objects clearly. View a photo as seen through a normal and a nearsighted eye.

Nearsightedness is usually a variation from normal, not a disease. Less often, nearsightedness happens because of another disease or condition.

What causes nearsightedness?

Most nearsightedness is caused by a natural change in the shape of the eyeball that makes the eyeball oval (egg-shaped) rather than round. Less often, nearsightedness may be caused by a change in the cornea or the lens.

These problems cause light rays entering the eye to focus in front of the retina. Normally, light focuses directly on the retina. See a picture of the parts of the eye.

What are the symptoms?

The main symptom of nearsightedness is blurred vision when looking at distant objects. You may have trouble clearly seeing images or words on a blackboard, movie screen, or television. This can lead to poor school, athletic, or work performance.

You may think your child is nearsighted if he or she squints or frowns or holds books or other objects very close to his or her face. Children who are nearsighted may sit at the front of the classroom or very close to the TV or movie screen. They may not be interested in sports or other activities that require good distance vision.
How is nearsightedness diagnosed?

A routine eye exam can show whether you are nearsighted. The eye exam includes questions about your eyesight and a physical exam of your eyes. Ophthalmoscopy, tonometry, a slit lamp exam, and other vision tests are also part of a routine eye exam.

Eye exams should be done for new babies and at all well-child visits.1 Nearsightedness most commonly begins in childhood or in the early teens (between the ages of 8 and 14), so it is usually first discovered in children of grade-school age.

Nearsightedness can be mild, moderate, or high.

How is it treated?

Eyeglasses or contact lenses can help correct nearsightedness. Surgery can also be done to change the shape of the cornea or to implant artificial lenses in the eyes to reduce or fix nearsightedness.

Myopia, or nearsightedness, is a big problem for many children. This difficulty with distance vision interferes with many daily activities, including learning at school. And as they grow, many children experience progressive myopia, which means the problem gets worse over time.

Using Rigid Contact Lenses

Over the years, several studies seemed to indicate that myopia could be controlled by wearing rigid gas permeable or RGP contact lenses. (Now more eye care practitioners are calling them GP contact lenses.) The idea was that the rigid contact lens would act as a splint to fortify the front of the eye without affecting the overall corneal shape. The lens would reduce myopic progression, as compared with wearing eyeglasses or soft contact lenses.

This idea was controversial, and some eyecare practitioners scoffed. Since many of the studies were flawed because of inadequate controls of important variables, incomplete follow-up and poor selection of study participants, their results were inconclusive.

Finally, the Contact Lens and Myopia Progression (CLAMP) study published its findings in 2004. The CLAMP study, funded by the National Eye Institute, followed myopia progression in more than a hundred children aged 8 to 11 over a three-year period. Some wore rigid GP contact lenses, while others wore soft lenses. The researchers measured the participants' visual acuity as well as the physical growth of their eyes. In myopic people, the eyeball grows longer than normally, with a steeper cornea; this longer axial length is what causes the blurred distance vision.

The GP lens wearers did show less myopia progression, but it was only temporary. Their eyes continued to grow as long as the eyes of the soft lens wearers, and since the GP lenses were not able to slow or stop the growth, they could not provide permanent myopia reduction. A clinical trial conducted in Singapore reached a similar conclusion.

One difficulty in proving that wearing GP lenses definitely retards myopia lies in not knowing how nearsighted someone would be without such treatment. It's not an exact science: practitioners can't say that your child would have progressed to a prescription of -8.50 diopters if he hadn't worn GP lenses to control myopia. On the other hand, myopia does seem to run in families, and if most of the family members are very myopic, it's not unreasonable to suppose your child will eventually become very myopic as well.
Undercorrecting Myopia

Some eye doctors have tried undercorrecting nearsightedness, in hopes of reducing near focusing strain that has been suggested as a cause of progressive myopia. A recent study failed to support this idea, finding no statistically significant difference between those who received full correction and those who received undercorrection. Two other studies found that undercorrecting nearsightedness actually increased the rate of its progression.

Another study, the Correction of Myopia Evaluation Trial (COMET), has been testing the idea of using eyeglasses with bifocal lenses or progressive lenses to reduce the eye focusing needed for sustained near vision. So far it has found that progressive lenses, compared with regular single vision lenses, did slow myopia progression in children by a small, statistically significant amount during the first year. But the effect wasn't significant in the next two years.

Undercorrecting 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.

Atropine and Pirenzepine Drug Therapies

Several studies have shown that atropine eye drops can reduce myopia progression by temporarily paralyzing the ifocusing muscle inside the eye. (Atropine also causes the pupil to dilate widely.) One such study is the Atropine in the Treatment of Myopia (ATOM) study, which tested 400 children aged 6 to 12 over a two-year period.

So why isn't atropine a standard treatment for myopia? The focusing paralysis and pupil dilation caused by atropine cause light sensitivity and reduce children's ability to perform well at school and during sports. Plus, a constantly dilated pupil looks odd, a problem for kids because they tend to want to fit in, not stand out from the crowd.

Pirenzepine gel has also shown potential as a drug therapy for slowing myopia progression, but it is not FDA-approved, and, like atropine, it has unwanted side effects.
Corneal Reshaping with CRT

With Corneal Refractive Therapy (CRT), children (and adults) wear special contact lenses overnight to reshape the cornea and correct nearsightedness. Normally, you wear them every night to see clearly throughout the next day without them. But the effect is not permanent: if you stop wearing the lenses altogether, your eyes will gradually slide back into most, if not all, of your former nearsightedness.

The Longitudinal Orthokeratology Research in Children (LORIC) study, published in 2005, tested whether these contact lenses could slow myopia progression, even if they couldn't permanently correct all the myopia already in place. The authors of the two-year pilot study concluded that corneal reshaping can have both a corrective and a control effect in childhood nearsightedness.

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. Stay tuned for further results.

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