October 11, 2006
One of the more common conditions requiring eyeglasses is astigmatism. First, a quick grammar lesson: It's not one of many "stigmatisms" (Patients come in saying, "Yeah, I was told I have one of those"!), it's just astigmatism!
Sorry, I just find it somewhat amusing.
A perfectly focusing lens is curved equally in every direction. If you took a glass sphere and sliced through a section of it, you would create such a lens. Now, if you took a glass football and sliced through a section (not the pointed part, I'll get to that in a moment), you'd get a lens that has unequal curves all around it, resulting in uneven multiple focuses. This is astigmatism. Sometimes astigmatism is on the front of the eye, the cornea. Sometimes it is in the lens of the eye, which is located just behind the pupil. Because human tissue is rarely perfectly symmetrical, astigmatism is extremely common. Often the astigmatism is not enough to be significant . . . what amazes me is how often the eye comes so close to perfection! When it is significant, eyeglass or contact lens correction is necessary.
What are some symptoms of astigmatism? Of ten, but not always, there is blurriness. Sometimes, the complaint is of visual discomfort with detail discrimination, either close or for both. Headaches are very commonly caused by astigmatism. A caveat for children: large amounts of astigmatism in either one eye or both can result in permanently reduced vision if they don't use their glasses for at least a few hours per day. This is because astigmatism results in imperfect vision at all distances. If a child never gets to see a clear image, the nerves in the retina will not develop fully. The old adage, "If you don't use it, you'll lose it", definitely applies to vision development in children.
How do we correct astigmatism? Let's go back to the glass football. If you take an identical section and position it at exactly right angles over the original section, you will neutralize the astigmatism, and create even focus from all directions. This is what eyeglasses do. Soft contact lenses can also be designed to do this. However, the lens cannot be allowed to rotate. If it does, vision will constantly be changing. Since, unlike with eyeglasses, there is no frame to maintain a constant position, soft lenses must be designed to eliminate rotation. Now, if the astigmatism is on the front of the eye, the cornea – a rigid or "hard" lens will act as a perfectly spherical surface and neutralize the astigmatism. In this case rotation of the contact will have no effect on vision. The drawback to rigid lenses is that they take longer to adjust to, and can pop out easier during sports.
Now, let's go back to our football one last time and take a section that includes one of its points. This will create irregular astigmatism that glasses will not correct well. The most common cause of this is a condition called keratoconus. This is best corrected with a rigid contact lens which acts as a spherical surface. This sounds straightforward; however, the fit of the proper lens shape is critical and often difficult to keep stable.
I hope you now have an understanding of astigmatism, and also will never again call them "stigmas"!