There’s a lot to consider when managing and correcting astigmatism; however, you can only do so effectively if you have a solid foundation of understanding. In this session, Kurt Moody, OD, FAAO, FBCLA, Dipl. CCLRT, and Erin Rueff. OD, PhD, FAAO, Dipl. CCLRT break down astigmatism fundamentals.
Astigmatism defined
“When it comes to astigmatism, the Dictionary of Visual Sciences is a good starting point,” Dr. Moody says. “It defines astigmatism as a condition of refraction where rays from a single point aren’t focused at a single point, but instead as two line images at different points, generally at right angles of each other.”
There are also key optical features to be aware of, including: the Conoid of Sturm, the geometric configuration of rays that pass through a toric surface; the Interval of Sturm, the difference between a patient’s horizontal and vertical meridians, dictating the amount of astigmatic correction; and the circle of least confusion. “The circle of least confusion is the conoid area with the best focus,” explains Dr. Moody. “The larger it is, the poorer the image quality. Our goal is to make that circle of least confusion as small as possible by shortening the interval of Sturm or, ideally, eliminating it altogether.”
What are you seeing?
Astigmatic error can be broken down into two subgroups: regular and irregular. “Although it’s easy to convince ourselves of our familiarity with astigmatism, I’m sometimes met with blank stares when I ask my students about the difference between regular and irregular astigmatism,” Dr. Rueff highlights. “It’s never a bad idea to refresh your memory regarding the types of astigmatic refractive error that our patients experience.”
Regular astigmatism is when the two meridians are perpendicular to one another, while irregular astigmatism covers the cases in which this isn’t the case. “Irregular astigmatism is usually caused by some sort of corneal disease, like keratoconus or pellucid marginal corneal degeneration, or a traumatic event, such as trauma or surgery to the eye,” she explains. “It often involves corneal thinning, scarring, and other disease processes.”
Regular astigmatism can be further subcategorized into: with-the-rule astigmatism, in which the vertical meridian is steeper and the horizontal meridian is flatter; against-the-rule astigmatism, where the opposite is true; and oblique astigmatism, which encompasses cases in which patients are neither with- nor against-the-rule and don’t lie within 30 degrees of one major meridian. “Most astigmats have with-the-rule; however, oblique astigmatism can sometimes be the most disruptive, as oblique axes can be tougher to find parameters to correct for,” says Dr. Rueff.
Distribution breakdown
How common is astigmatism? “We have data from a large Australian chart assay assessing a little over 3800 non-diseased corneal patients,” explains Dr. Moody. “Although the astigmatic error ranged between 0.0 and 5.5 D, the majority (51.9%) of patients had either no astigmatism at all or 0.25 D.” In addition to this, a further 44.8% had between 0.5 and 1.75 D of astigmatic correction.
“Oftentimes we’re shy about prescribing the full amount of astigmatism, but considering the fact that most people in the populace, 96.7%, have relatively low-to-mild amounts of astigmatism, we should be correcting all of that to really give our patients the best vision they can get,” highlights Dr. Rueff. “1.75 D or below means that there’s a contact lens option for almost everyone; this magnitude in a pair of glasses won’t be particularly bothersome to patients either.”
However, things may vary depending on the region you practice in and the populations you tend to serve. “We typically see higher magnitudes of astigmatism in certain demographics, such as Hispanic or Native American populations—these are also populations we tend to see more ectatic corneal conditions in, something else to keep in mind,” she explains.
Change over time
Age can also influence the likelihood of a patient experiencing astigmatic error. “It’s easy to think what you're born with is your lot for life—but the data says otherwise,” says Dr. Moody. “Instead, our patients’ refractive astigmatic correction stays relatively stable up until they reach 40–50, when their likelihood of experiencing 0.5–1.75 D increases.” This timeframe also coincides with increases in the proportion of patients experiencing against-the-rule astigmatic error, compared to with-the-rule astigmatism, which is more common in younger patients.
But why do these age-related changes occur? “The leading hypothesis is that, as we age, we experience changes in the corneal collagen layout, altering its malleability,” Dr. Moody offers. “We also see decreases in eyelid elasticity, potentially altering the stress the cornea experiences. Together, these two factors may result in both the increase in astigmatism and the shift from with-the-rule to against-the-rule prevalence. However, we don’t yet have the data to say this with certainty.”
Three to one
An example of the importance of not only being clear regarding the fundamentals of astigmatism, but also how understanding has evolved, is the ‘3:1 ratio rule’—an outdated principle where correcting astigmatism with a toric lens was only considered worthwhile if the magnitude of astigmatic error was at least a third of the magnitude of the sphere. “This rule likely originated when toric lenses weren’t very stable or didn't have great parameter options,” Dr. Rueff explains. “However, this rule no longer has any importance—in fact, we have lots of data showing that any magnitude of astigmatism is valuable to correct.”
For patients, correction may be the difference between achieving satisfactory outcomes and missing out. “Correcting smaller amounts of astigmatism can impact both visual quality and satisfaction with vision,” she says. “Although patients with 0.75 D against-the-rule astigmatism may still be able to read 20/20 on a chart when corrected with just a spherical contact lens, we know that the quality of vision is poorer compared to correction of the full error with a toric lens.”1,2
Dr. Moody agrees, “In the 1990s, all toric lenses started at 1.0 D. I was part of a study that demonstrated patients were sensitive to 0.75 D of uncorrected astigmatism—ultimately resulting in the first 0.75 D toric lens, which is now well-established industry-wide.3 Now, if a patient is 0.75 D or greater, especially if non-presbyopic, we should generally be correcting that.”
Keeping these fundamentals in mind is key as we delve deeper into the current-day best options and strategies available for effectively managing the range of astigmats that you’ll come across in your practice.
