The Dynamics of Astigmatism in Presbyopia

Astigmatism is defined by the the Dictionary of Visual Science as, “A condition of refraction in which rays emanating from a single luminous point are not focused at a single point by an optical system, but instead are focused as two line images at different distances from the system, generally at right angles to each other.”¹

As optometrists, we learned this in our first year of professional education and have addressed this with our patients almost daily. Once a patient has been told they have the “dreaded” astigmatism, they frequently ask, “Will it increase or change over time?”

This has been a common question heard over and over throughout my 40+ year career as an OD. My typical answer: “For most patients who have regular astigmatism, this component of the prescription stays the same, meaning what we measure today will basically not change.”

With the exception of the keratoconus or pellucid patient, I believed this was true—that was until I personally experienced the dynamic nature of astigmatism.

Personal perspective from an emmetrope

As a functional emmetrope, I was always blessed with very good vision. My distance OU correction exhibited very minor with-the-rule (WTR) astigmatism of 0.50D and a spherical equivalent of near zero (OD: +0.25 - 0.50 x 10; OS: +0.25 - 0.50 x 170). As I aged, I found it interesting that my uncorrected distance and near vision continued to be very good into my early 60s.

This compelled me to obtain a fresh refraction and to my amazement I discovered I now had 0.50D of against-the-rule (ATR) astigmatism in my dominant eye and 1.50D of ATR astigmatism in my non-dominant eye, but my spherical equivalent continued to be near zero (OD: +1.00 - 1.50 x 105; OS: +0.50 - 0.50 x 75).

This explained my acceptable functional distance, intermediate, and near acuity into my 60s (benefits of uncorrected astigmatism for the presbyope will be addressed in the next article). This led me to do some research into the dynamics of astigmatism with age, which I will summarize here.

Distribution of astigmatism magnitude

A 2015 paper reviewed the distribution of astigmatism in a large group of Australian participants. Specifically, they looked at 3,841 participants with non-diseased and non-surgically altered eyes with astigmatism between 0 and -5.50D of astigmatism.

They noted the amounts of refractive astigmatism (RA) and corneal astigmatism (CA), as well as the type of astigmatism being either WTR, ATR, or oblique.²

This data set revealed interesting points regarding the prevalence of RA:²

The right eye and the left eye were statistically the same, meaning astigmatism was essentially symmetrical between the two eyes.
Prevalence of RA in this cohort of < 0.50D was 51.9%, having between 0.50 to 1.75D was 44.8%, and > 1.75D was 3.3%.
- Therefore, the vast majority of subjects (96.7%) had 1.75D or less of refractive astigmatism.

Graph 1: The distribution of refractive astigmatism from the study.²

$Distribution of refractive astigmatism$

^{Graph 1: Courtesy of Sanfilippo et al.}

Distribution of the type of astigmatism

The axis of the astigmatic error could be from 1 to 180°. Arbitrarily, astigmatism is broken into three groupings: WTR or horizontal, ATR or vertical, and oblique or orthogonal.

This directionality can have ramifications on how we correct the error as well as the visual effect. An extensive review paper on the visual and functional impacts of astigmatism highlighted the distribution of the type of astigmatism and also how the type changes over time.³

Graph 2: Demonstration of how the type of astigmatism changes during the aging of the patient.³

^{Graph 2: Courtesy of Read et al.}

3 key points from the study:³

Early in life, WTR astigmatism is more prevalent.
Between 40 and 50 years, WTR and ATR are similar.
From 50 and beyond, ATR is more prevalent.

This shift in the type of astigmatism is of note and important as we manage patients who are maturing. Updating glasses is straightforward, but it may mean having to move a spherical soft contact patient into toric lenses to maintain good vision.

Effect of age on the magnitude of astigmatism

The study referenced above on the large Australian population of 3,841 participants aged 5 to 90 brings to light what occurs to astigmatism as we age.²

5 key points from this work:²

RA magnitude remained stable until about the age of 50.
The magnitude of RA increased with age.
Age was a significant predictor of RA in adulthood.
There was a directional shift in the magnitude and type of RA with age; such that children tend to have lower levels of WTR RA, while adults often require ATR RA correction of higher powers.
CA remained fairly constant until later in life.

Graphs 3 and 4: Refractive astigmatism and corneal astigmatism as a function of age, respectively.²

$Refractive Astigmatism$

^{Graph 3: Courtesy of Sanfilippo et al.}

^{Graph 4: Courtesy of Sanfilippo et al.}

Perspective and pearls on correcting astigmatism

This research has been very interesting in validating my personal experience of a shift in both astigmatism type and magnitude. It's very clear that regular astigmatic error is not stable but dynamic over time.

In early childhood, we should expect to see WTR astigmatism that typically declines by the age of 5 and then remains fairly constant and low until about the age of 50, where we should expect to see a shift from WTR to ATR and an increase in the magnitude.³

The question you may have is why this occurs—and, to date, there is not a definitive answer. However, most agree it is most likely due to a change in the corneal curvature with age.

This change is believed to occur from a combination of intrinsic and extrinsic factors. Intrinsic changes include changes in collagen orientation in the cornea. Extrinsic changes consist of biomechanical changes brought on by eyelid changes with age might be the cause but more research is needed.²

Key takeaways

In general, we should expect the vast majority of our patients to have 1.75D of astigmatism or less. Those patients with greater amounts of astigmatism should be rare and may be worth considering additional testing to rule out certain corneal conditions.
There are a considerable number of options for us to correct this large group of low to moderate amounts of astigmatism, including glasses and soft and rigid contact lenses, both spherical and multifocal.
As our patients approach the age of 50 years, closely monitor their type and magnitude of astigmatism so you are always offering them the ideal correction.
The fact that patients in this age group are exhibiting changes in their accommodation status and now also their astigmatism allows a perfect opportunity to educate them and offer new corrective strategies and/or options.
1. Patient education and new offerings are always ideal opportunities for our patients to experience the importance of their optometrist in their eyecare.

Final thoughts

Understanding the dynamics of astigmatism is just part one of optimizing your patient's vision. In addition to these dynamic changes in astigmatism, it's important to understand the visual impact these changes create.

Understanding these changes can offer the optometrist the ability to leverage the visual impact of these changes to enhance the patient's outcome. Next month, I will delve into how these astigmatic changes can, at times, improve the visual experience.

Since it is typically the presbyopic patient having these astigmatic changes, I will explain how to optimize the astigmatic presbyope with the newest soft multifocal contact lenses.