One of the most common questions that I receive is what is astigmatism and how did I develop it? Let’s delve into the issue.
The eye exam
There are two major components to an eye exam. The first is a complete health assessment of the eyes; the second is measuring how the eyes focus and prescribing appropriate glasses or contact lenses.
The eyes are examined for conditions such as cataracts, glaucoma, macular degeneration, retinal detachment and any other diseases.
Next, an autorefractor is used to measure how the eye focuses light. During the test, you look inside the autorefractor at a target, such as a balloon or barn. The autorefractor shines light into your eye while you are looking at the target and measures how the light reflects back to the instrument. This autorefractor gives us a best estimate of how your eyes are focusing.
The second part of measuring how the eyes focus is when you’re looking through the phoropter. The phoropter is the instrument that you look through and are asked to answer the question: “which lens makes the letters clearer, lens one or lens two?” This process in which we measure how the eyes focus is referred to as a refraction.
The refraction has two parts. The first is measuring how the eyes focus things in the distance and the second is measuring how the eyes focus things at near (16 inches away). During the refraction, we determine whether someone has myopia, hyperopia, astigmatism or a combination.
Myopia is commonly referred to as nearsighted. It is when you can see better at near and worse in the distance. The higher someone’s myopic prescription is, the worse someone’s distance vision is. As an example, someone who has -5.00 diopters of myopia will have a much more difficult time seeing in the distance than someone who has -1.00 diopter of myopia. Hyperopia, commonly referred to as farsighted, is when someone’s near vision is worse than their distance vision. This is not to be confused with a refractive error referred to as presbyopia. The higher someone’s hyperopic prescription is, the worse someone’s near vision is.
As an example, someone who has +5.00 diopters of hyperopia will have a much more difficult time seeing at near than someone who has +1.00 diopter of hyperopia. Often, these people will have an increasing dependency on their glasses to help their distance and near vision.
Astigmatism is a refractive error in which light that enters the eye focuses in two different meridians. People with astigmatism actually have a different prescription going up and down in their glasses than they do going left and right. Each line in the lens represents a different power. Although the lines in the figure represent different powers in the lens, there are no visible lines in lens.
Astigmatism will often times make somebody’s distance and near vision blurry if it’s uncorrected. The higher the level of someone’s uncorrected astigmatism, the more it will affect their vision. If you have astigmatism correction in a pair of glasses for the first time or you have an increase in the amount of astigmatism, your glasses may feel awkward when you first put them on. The vision will often times seem clear but they may feel like things are magnified or minified, it may feel like the floor is tipping away or towards you, it may feel like doorways are tilted, or you may feel like you are reaching for things that are not exactly where you would expect them to be.
The reason for the potential awkward feeling with astigmatism prescription is when you have two different powers in your glasses, they have different levels of magnification in each meridian. Whenever those powers are changed they can affect the way you perceive the world through the glasses.
The good news is that simply wearing the glasses and performing daily tasks in the glasses will help you get used to the new prescription. It’s important to not simply sit stagnant and watch TV or look at a computer screen with your new prescription but make sure that you get out and do active tasks with glasses on.
There are two forms of astigmatism: regular and irregular. Regular is the form of astigmatism that most people have. This is where the two different powers that we discussed previously are located 90° apart. Irregular astigmatism happens with certain conditions where the astigmatism is not located 90° apart.
Some examples of conditions that can cause irregular astigmatism include keratoconus, corneal injuries and epithelial basement membrane dystrophy. Presbyopia Presbyopia is where individuals in their early 40s lose the ability to focus things at near. This happens because the lens that’s located inside of the eye loses elasticity overtime. As the lens loses elasticity and hardens, it has less ability to focus things that are closer to the eye. It’s part of the reason why the first compensatory mechanism to presbyopia is to hold things further away from your face to help see it clearly. Presbyopia will worsen once someone begins to experience symptoms. This is the reason that near glasses power or the bifocal power in your glasses needs to be increased until it stabilizes at about 60 years old.
Getting back to astigmatism
Astigmatism is when there are different power requirements in two different meridians.
There are good ways to correct it with either glasses, contact lenses or, for certain individuals, a combination. It’s not terribly uncommon for astigmatism to change over time.
Remember that it’s simply a different way that your eye focuses light and not a medical condition.
Mile Brujic, OD, FAAO is a 2002 graduate of the New England College of Optometry. He is a partner of Premier Vision Group, an optometric practice with three locations in Northwest Ohio. He practices full scope optometry with an emphasis on ocular disease management of the anterior segment and specialty contact lenses. Brujic is on the editorial board for a number of optometric publications. He has published over 350 articles and has given over 1600 lectures, both nationally and internationally on contemporary topics in eye care.