If you are in the world of optometry, you’re going to encounter astigmatism on a daily (if not hourly) basis. Although it is an extremely common refractive error, it is a wildly misunderstood condition by the general public.
For most patients with astigmatism, their understanding typically ranges from “my eye is just shaped like a football” to “I have the ‘stigma’ and it is the cause of every issue in my life, visually and otherwise.”
It is important to be able to truly understand and be comfortable both prescribing for and explaining astigmatism to patients. By providing a simple yet thorough explanation of this condition, we can help eradicate the fear and confusion that unnecessarily surrounds astigmatism.
I’ve been in practice for a few years so I’m no stranger to astigmatism, but sometimes the specifics get lost in the humdrum of churning out prescriptions and fitting toric contacts. This article will help guide you through all the basics of astigmatism, whether you’re a well-polished practitioner in need of a refresher or a new optometry student looking trying to make sense of astigmatism!
What is astigmatism . . . really?
In its basic form, astigmatism is a refractive error caused by an irregularly shaped cornea (corneal astigmatism) or lens (lenticular astigmatism). When light enters the eye through this irregular surface, it converges to more than one focal point, causing blurred or distorted vision, headaches, squinting, and eyestrain.
Are you “with” me?
A spherical eye is shaped like a ping pong ball. In astigmats, however, that shape isn’t perfectly round: rather, it is shaped more like an egg (hence the football simile). In this case, one meridian is more curved than the meridian perpendicular to it, and the steepest and flattest ones are called the principal meridians.
When the steepest (more myopic power) meridian is vertical, this is called “with the rule astigmatism” and it is mostly found in children (e.g., 0.00-0.50x180). When the steepest meridian is horizontal (more myopic power in horizontal meridian), this is called “against the rule astigmatism” and is mostly found in older patients, especially after surgery (e.g., 0.00-0.50x090).
So what if you’re not at or closely around 090 or 180? If the steepest meridian is around 30-60 or 120-150, this is called oblique astigmatism. If the steepest meridian and flattest meridian are NOT perpendicular to each other (90 degrees apart), this is called irregular astigmatism (such as in keratoconus). Irregular astigmatism requires more complex forms of treatment than spectacle or soft contact lens correction. These patients require either specialty contact lenses to alter the shape of their cornea or surgical intervention.
Types of astigmatism
As we break this definition down even further, depending on who you ask, there are either three or five types of astigmatism. The three main types are myopic, hyperopic and mixed. If we expand upon this, there are also simple and compound versions of both myopic and hyperopic astigmatism. In all of these cases, the light will enter the eye and focus on two focal points, the difference lies in where those focal points are located in relation to the retina.
Simple myopic astigmatism
One focal point is on the retina, another is in front of it.
Simple hyperopic astigmatism
One focal point is on the retina, another is behind it.
Compound myopic astigmatism
Both focal points are in front of the retina but at different locations.
Compound hyperopic astigmatism
Both focal points are behind the retina but at different locations.
One focal point is in front of the retina and another is behind it.
Who gets astigmatism?
Now that we’ve brushed up on what astigmatism is, let’s talk about who develops this condition. Astigmatism is most commonly present from birth and can be hereditary, but it can also develop from eye injuries, surgeries, or diseases, such as keratoconus or pellucid degeneration.
Astigmatism is typically discovered in childhood or young adulthood, and around 15-28% of children have it. Children six to twelve months old are three times as likely to have it than children ages five to six. Once a child has developed astigmatism, they will commonly progress to a higher amount as they mature. Also, those born to a mother who smoked while pregnant are 1.46 times as likely to develop it than those with non-smoking mothers.
Other risk factors include a family history of astigmatism, premature birth, and being of Hispanic or African American descent. Having additional refractive error is also a risk factor, as patients who have myopia or hyperopia are more likely to be an astigmat as well (4.6 times and 1.6 times, respectively).
How do you test for astigmatism?
Testing for astigmatism typically begins during pre-testing, where auto-refraction (or retinoscopy results) exposes astigmatic power. This power can then be refined during refraction to locate the patient’s true cylinder power and axis. If the patient is unable to perform refraction due to any reason (such as young age or poor comprehension), then a cycloplegic auto-refraction and/or retinoscopy may be necessary to finalize the prescription.
Additional helpful tests for testing for astigmatism are keratometry and topography. Keratometry helps determine the amount of corneal astigmatism and defines the location of the principal meridians. Topography helps determine refractive error as well as monitor for any elevations or irregularities. These results are helpful for monitoring for changes in corneal astigmatism, assessing for pathology, and assisting in the fitting of soft, RGP and corneal reshaping contact lenses.
How do you treat astigmatism?
Treating regular astigmatism
Regular astigmatism can easily be treated with spectacle wear, contact lens wear, or surgery (such as LASIK or PRK).
There are a variety of soft contact lenses that are now available in toric designs to better accommodate the correction of astigmatic patients. Becoming familiar and comfortable with these lenses is a pivotal part of practicing primary care optometry.
If you’re looking for a simple tool to help vertex your toric spectacle prescriptions into appropriate contact lens prescriptions, check out this calculator tool from Coopervision!
Treating irregular astigmatism
Due to the abnormal corneal surface in irregular astigmatism (such as keratoconus), it is poorly improved by spectacle wear. A specialty contact lens fitting or surgical intervention would be appropriate in this case.
Other treatment considerations
Occasionally, high or asymmetric astigmatism can result in refractive amblyopia, requiring treatment as deemed necessary by the severity of the condition and age of the patient. This may include amblyopia treatment with patching or drops, or contact lens wear to help with symptoms from aniseikonia.
Although treating astigmatism will become second nature throughout years of practice, recognizing the various types and staying comfortable with treatment options is pertinent for properly caring for astigmatic patients.
It is equally important to be able to pass this knowledge onto your patients in a clear and easy-to-understand manner. By providing excellent patient education and thoroughly discussing potential treatment options, we can work to remove the “stigma” from astigmatism one office visit at a time!