The Patient's Guide to Refractive Error Part 1: Myopia

8:08 PM

Myopia, or "near-sightedness" is a very common ocular condition typically first found in school-age children.  These individuals have good near vision, but blurry distance vision.  They will typically need glasses for full time wear, especially for activities like driving.  Let's do a little break down of what causes myopes to have this blurry distance vision and how glasses work to correct the issue:

--Myopia is a result of one (or more) structural variations of the shape of the eye.  Each element of the eye has a certain size and shape to be "within normal limits," but when these measurements fall  slightly outside of normal, a refractive condition results.  Items that specifically affect the power of the eye are:

  • The Cornea (a thicker than normal cornea will result in myopia)  Note: this is why surgeries like PRK and LASIK work to correct a myope's vision; these techniques reduce corneal thickness!
  • The Lens (typically the lens thickness has minimal impact, but as it changes in conditions like cataracts, myopia may result)
  • The Axial Length (this is the length of the eye; if the eye is too long, myopia will result)
--To understand how these elements work together we can use the below diagram.  Light passes through the clear dome of the cornea and is refracted through the lens to the back of the eye, where the retinal cells (orange lining in the diagram below) respond to the light stimulus, send a signal to the brain, and the information is used to process the image you are seeing.  In myopia, the image that passes into the eye falls short of the retina, causing a defocused or blurry image to be captured and processed by the retinal cells and brain.  

Courtesy the National Eye Institute

How do glasses help myopia?  Optometrists use lenses in front of the eye to change how much light is bent (or refracted) as it travels through the eye so that a clear and distinct image falls on the retinal cells.  For myopia, "minus" lenses are used, which are thicker on the outer edges of the lens and thinner in the center.  With today's lens cutting technology, this difference in thickness if usually very subtle, so you may not notice it if you are a myope and just took off your glasses to test me out, haha.  Lenses have varying thickness and are measured in "Diopters (D)."  The dioptric value of the lens you need is the exact match to the amout of myopia you have (or the amount the image needs to be adjusted in order to fall clearly on the retina).  For example, I wear a -2.00 D lens, which corrects my 2.00 diopters of myopia.

Now what does this measurement of myopia mean?  To continue my example, a 2.00D myope means that I can't see clearly at any distance past 50 cm in front of my face.  Hold on -- how did I figure that out?  It is luckily a very simple equation.  If you divide 100 cm (1 meter) by 2.00D, you get 50 cm as the far point at which I can still see clearly.  This works for any amount of prescription.  If you are a 4.00D myope, then you can't see clearly past 25 cm in front of your face.  If you are a 20.00 D myope, well you get the picture, you have a hard time seeing much of anything.

Minus glasses not only clear the image to their wearers, they can cause a minification effect (especially in the higher powers).  This is why when you see someone wearing glasses for myopia it may look like their eyes are very small within their glasses (see picture below).
Note how the side of Buddy Holly's face seems smaller inside of his glasses.   Today, we can use high index material lenses to reduce the thickness of the lens and this minification effect.

Symptoms and signs of myopia include blurry distance vision, squinting, and having to move closer to an item to see it clearly.  If you have been diagnosed with myopia, make sure that you are wearing correction (glasses or contact lenses) for activities like driving.  If your vision is worse than 20/40 without glasses, you are not legal to drive!  Your local eye care provider will be happy to investigate if you feel you are having any symptoms of myopia (or other vision conditions for that matter).  In most cases, vision is clearly corrected with glasses or contact lenses.  Some people may be born with a condition called pathological myopia where the globe (eye ball) is so long that it has distorted the optic nerve and stretched the retinal cells too thin; permanent vision loss occurs in this condition.  A dilated examination would detect these changes.

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