The Patient's Guide to Refractive Error Part 3: Astigmatism9:08 PM
Astigmatism may very well be the most poorly understood medical terminology that optometrists use. With the apprehension that is often associated with this word, it's hard to believe that astigmatism is one of the most common refractive errors that exists! Here is an example of how astigmatism often gets brought up during my exam:
Me: "Have you ever been diagnosed with any eye diseases like glaucoma or macular degeneration?"
Patient: "Yes-- I had a doctor tell me that I have that 'stig....stigmatism"
Patient: "So what is wrong with my vision?"
Me: "Well today we have found some changes due to cataracts, but we can improve your vision by increasing your astigmatism correction in your glasses."
Patient: "Oh no! I have astigmatism!!?"
Rest assured world, it is not a medical emergency to have astigmatism! It is a big and scary word to be sure (I think the inherent fear it evokes comes from the similarity to that horror film, Stigmata). Astigmatism is really no different from myopia or hyperopia; it is a refractive error that we can correct with glasses.
Breaking down the word astigmatism into its Latin roots, you find that it means the absence ("a") of a single point of clarity ("stigma"). This means that 2 images are being produced for every point of light, creating constant blur. A person with low amounts of astigmatism can often compensate for this by squinting, moving one image towards the other to forcibly create a clearer single image. Needless to say this is tiring to the eye, and in some cases even the most skilled squinter can't overcome the disparity between the two images.
|Multiple points of light fall onto the retina in Astigmatism.|
Why do we get astigmatism? It is most commonly the result of differing curvature of the cornea. The cornea is the clear dome over the iris (the colored part of the eye), and it is responsible for bending light to focus onto the retina. If your cornea is not a perfect circle, astigmatism in some degree will result from steeper or flatter areas. This is why many doctors explain astigmatism as "football shaped" eyes. The other common cause of astigmatism is a result of the lens of the eye, and your astigmatism prescription may change as the lens changes shape or cataracts form.
How do we correct astigmatism? With glasses or contacts just like any other refractive error. Astigmatism creates 2 images, so you will need glasses that have 2 separate powers. These powers are called sphere and cylinder (but I won't get too carried away here). The two powers also must be specifically oriented (like a clock-dial position) to properly create clear vision. Otherwise the world will appear twisted and distorted to the wearer. The degree mark is called "the axis" which specifies where the astigmatism powers sit. The most common position is horizontal or "180 degrees." Because this is the most common orientation it is called "with the rule". A 90 degree (or vertical) axis orientation is "against the rule," but it is actually more common that "with the rule" when we are young children and again when we become older adults.
|The inner circle of the phoropter is used to refine the axis|
When people get their astigmatism glasses for the first time, I always like to warn them that it can be VERY hard to get used to. Why? Well for one, these people have been compensating for seeing 2 blurry images for many, many years, and it has become second nature for them to squint. They do it without even noticing in most cases. The other problem (that is also very hard to make an astigmatic person believe) is that they see their world twisted or stretched all of the time, they just don't realize it. Astigmatism stretches the image horizontally or vertically (depending on the axis). Your brain is smart enough to know that things aren't supposed to look like a fun house mirror, so the mind corrects for it. If I put a pair of glasses on that fixes the problem, the brain has to learn it doesn't need to work so hard correcting things anymore. At first, the glasses may give the wearer headaches or that "fun house mirror effect" we were trying to fix! Rest assured, this is normal and as you continue to wear the glasses your brain and focusing system will adapt and appreciate not having to work so hard!
Astigmatism Trivia and Quick Facts (for those who are still interested)
1) Astigmatism is not a medical condition, but it can be caused by medical conditions. For example, conditions like keratoconus cause significant change in the shape of the cornea, resulting in high amounts of astigmatism.
|Keratoconus causes large amounts of astigmatism and usually requires specialty contact lenses or even a corneal transplant|
2) Patients with high enough amounts of astigmatism need special "toric" contact lenses. These lenses will properly orient into one place so that the lens is fully correcting the refractive error. If you wear toric contact lenses, you may note a little dash mark on the lens. That is normal and is not a manufacturing defect! It allows your eye care provider to make sure the lens is properly oriented on your eye.
3) With high amounts of astigmatism in children, it is possible for the visual system to fail to develop the capability to see 20/20 vision, even with the best possible glasses. The world looks so distorted to these patients that the important eye-brain connections fail to reach their full developmental potential. This condition (refractive or meridional amblyopia) is just another reason why vision exams are important for children!
4) Either an optometrist or an ophthalmologist can prescribe glasses or contact lenses for astigmatism. Your do not have to see a specialist! Of course, I only bring this up because I know a fellow colleague who was told by their patient that they couldn't see an optometrist because of their astigmatism. But I guess this is just another example of how people are so afraid of the the stigma associated with that astigmastism word!