The 4 most common vision problems corrected by glasses and contacts: Your Eye Define memo of the month for January
posted: Jan. 14, 2016
Myopia. The most common. It simply means that you don’t see as well far away as you do up close. You are NEARsighted or you see NEAR BETTER. This visual condition is also the easiest to explain. The light is focused in front of the retina, so a minus lens is used to alter the light so that your eye focuses the light on the retina.
Hyperopia. Often referred to as the opposite. Light lands behind the retina and is brought incident to the retina with a plus lens instead of a minus lens. You see FAR BETTER because you are FARsighted, seems simple enough. But did you know that if you aren’t one or the other you are emmetropic, and emmetropia is as common as flipping a coin and having it land on it’s edge? So why doesn’t everyone wear glasses? Because of a thing called accommodation. Inside the eye there is a natural lens that can change its shape to focus for near and back to far. If we have a small amount of hyperopia (we need more plus), say around 1/2 unit deficient, our eye can internally compensate by adding +0.50 unit of power to our normal tonic state, flexing that muscle a little bit all the time. Hyperopes that need glasses can’t compensate because the focusing system doesn’t work as well as it once did or because the amount of hyperopia exceeds what they can do comfortably (eyestrain, headaches, blur at near, focusing delay…). This is one reason why a thorough history of symptoms is critical at any eye examination.
Presbyopia. When our focusing power is no longer able to bring things up close into focus we suffer from Presbyopia, a fancy word that really means “I need help focusing at near with multifocal lenses”. When we are young we have about 18 units of focusing power, by the time we are 45 we are left with only about 3 units. This is the result of several anatomical changes including fatigue of the muscle, changes in the crystalline lens, space available for flexure and attachment of the zonules to the lens. There are researchers tying to find a cure for presbyopia, but in the meantime treatment is done in the form of multifocal lenses: either glasses or contacts. If you suffer from presbyopia and you do not use a multifocal lens, you are not treating the focusing problem.
Astigmatism. This is the most often confused condition. It sounds scary, but nearly everyone has some degree of astigmatism. It simply means that the amount of correction needed in one direction, say horizontal, is different from the other, vertical. So if you have a prescription of +1.00-0.25 x 180, at the horizontal your power is +1.00 but if we were to move across the lens the power would change by -0.25 from horizontal to vertical making the correction in the lens’s vertical axis +0.75. Astigmatism distorts at all distances and should be accounted for whenever significant. Advanced equipment like the OPD III detect the location and amount of the astigmatism with extreme accuracy. Refinement is sometimes necessary by comparing two images and much of the “which is better one or two” is spent twisting between more vertical and more horizontal axes to find the one location that works best for you. Extreme amounts astigmatism can be an indication of corneal or other eye disease and should be monitored by your eye doctor. If you have high astigmatism, check with your doctor to see if topography is an appropriate test for you.