Uncorrected astigmatism in the presbyopic patient has been a challenge for many optometrists.
A good number of these patients may have been successful soft toric lens wearers for years, and once becoming presbyopic, have found that the options of monovision, distance correction with readers, or possibly dropping out and reverting to spectacles have not been attractive to them.
With new multifocal options and the knowledge of how to leverage uncorrected astigmatism, the optometrist will be better equipped to satisfy these patients, plus attract new patients.
This article will address how uncorrected astigmatism could benefit the presbyope.
Astigmatism and aging
In my last article, The Dynamics of Astigmatism, I mentioned the fact that I enjoyed good uncorrected vision into my early 60s.1
I attributed this benefit to the fact that my spherical equivalent continued to be near zero while my astigmatic component had increased and switched from with-the-rule (WTR) to against-the-rule (ATR).
Pre-presbyopic Rx:
- OD: +0.25 -0.50 x 10
- OS: +0.25 -0.50 x 170
Presbyopic Rx:
- OD: +1.00 -1.50 x 105
- OS: +0.50 -0.50 x 75
It's been well documented that refractive astigmatism increases with age and shifts from WTR to ATR.1 Leube et al. noted in their paper that due to this change in refractive astigmatism, plus changes in ocular aberrations and pupil diameter, the natural depth of focus of the eye increases by 0.03D per year between the ages of 20 and 50.2
Considerations for correcting refractive errors in astigmatic presbyopes
The most commonly used method to correct presbyopia is the use of progressive spectacles. The result of this correction is minimal loss in visual acuity across distances.2 Most multifocal soft contact lenses use spherical aberration to create a “stretch focus” to increase depth of focus to allow for good visual acuity at multiple distances.3
The use of spherical aberration in these lenses could increase the depth of focus by up to 2.0D, but doing so decreases the contrast of the image. The greater the amount of spherical aberration in a lens, the greater the reduction in contrast.2,3
Lower order aberration primary astigmatism has also been called out as being useful in near vision.2,4 Most of the published work on leveraging uncorrected astigmatism to improve near vision in presbyopes has been associated with intraocular lens (IOL) correction.2,4,5,6
It is established that astigmatism increases with age. As optometrists prescribing soft contact lenses, we should expect that our presbyopic patients will encounter uncorrected astigmatism during their presbyopic years.1 It's critical we understand the effect this may have on their outcomes and how we could manage these changes; thus, the work published on the IOL patient could be very useful.
Understanding the optics behind what has been called “pseudoaccommodation” in the uncorrected or under-corrected astigmatic presbyope could be useful when managing these patients.
Astigmatism correction
We know that the astigmatic eye optically creates two line images at different distances.7 What might be unknown is that the distance between these images is known as the “Conoid of Sturm.”8
Figure 1: Illustration of the Conoid of Sturm, noting that each meridian will create a separate line image, with the circle of least confusion being between these points. The smaller the circle of least confusion, the better the focus.
Ideally, we aim to reduce the size of the circle of least confusion by correcting each meridian. Some doctors are still leveraging the practice of “masking” uncorrected astigmatism by prescribing the spherical equivalent. When uncorrected astigmatism is corrected in this manner, the result is an increase in the size of the circle of least confusion compared to when the full spherical and cylindrical errors are fully corrected.
As the circle of least confusion is increased in size, objective and subjective vision decrease.
A look at pseudoaccommodation
Most people in the US read and write using the Roman alphabet. The Roman alphabet has a high prevalence of vertical detail and a very low incidence of oblique detail.9 This point is important when we think about the effect uncorrected astigmatism can have on the retinal image when viewing letters in the distance and near. Think about the ATR patient noted in Figure 1; Plano -1.00x 90.
When viewing letters in the distance, the higher prevalence vertical lines of the Roman alphabet will be blurrier than the horizontal lines. As we bring the image closer, though, now the vertical lines will come into focus due to the myopic meridian being focused on the retinal plane. It is this optical characteristic that is considered “pseudoaccommodation.”6
Figure 2: Illustration of pseudoaccommodation in an astigmatic eye.
In summary, the uncorrected WTR astigmatic patient will be least impacted in the distance, but the uncorrected ATR astigmatic patient can actually gain some “pseudoaccommodation” at near. This point was noted in the work by Serra et al., where they noted that distance vision was most impacted for the oblique astigmatic patient, and WTR had the least degradation.9
This would explain my experience; my -1.50D of uncorrected ATR astigmatism in my non-dominant eye had minimal negative effect on my distance vision (because it was in my non-dominant eye), but helped me at near due to this “pseudoaccommodation” phenomenon being addicted to the small amount of amplitude of accommodation I still had.
Published data on pseudoaccommodation
If we look at the published data on this, which is mostly from IOL patients, it seems to be consistent, reporting that 0.50D to 1.0D of uncorrected astigmatism can benefit near acuity.2,3,4,5 Wolffsohn specifically calls out his opinion that 0.50D of ATR astigmatism will cause marginal loss of binocular distance acuity but enhance near vision in the pseudophakic patient.4
Further evidence of this can be noted in the work of Nagpal, who looked at post-IOL patients in India; his results pointed out that uncorrected ATR astigmatism could be a desirable goal after cataract surgery, specifically because of this “pseudoaccomodation.”6
Understanding optics leads to optimal outcomes
So, how could this information be useful for the optometrist encountering patients in practice? When dealing with multifocal soft contact lens patients, our aim should be to provide the best retinal image with the lowest add possible to maximize binocular acuity.3
Ideally, I always strive to address 100% of the refractive error, but with limitations to some parameters, we may consider how we could make those limitations work to our advantage. Astigmatic correction is a prime example of this. Unlike the spectacle correction, we can not correct every quarter diopter of astigmatism for our soft toric patients.
Correcting astigmatism in presbyopia patients
Having a game plan for that uncorrected astigmatism can take an unsuccessful patient and turn them into a success and a great source of referral. Understanding the optics of the uncorrected or undercorrected presbyope is the key. Consider the unique refractive condition of your patient and whether you could turn a limitation into an advantage.
Knowing that many presbyopic patients may be exhibiting an increase in ATR astigmatism, consider if you could leverage this fact. I’ve found in my personal experience that leaving a small amount of ATR astigmatism uncorrected in the non-dominant eye could give me that extra bonus to improve near acuity with minimal effect on distance vision.
In my opinion, up to 0.75D of ATR astigmatism could be tolerable in the non-dominant eye for many patients and allow me to minimize the add and achieve good near binocular acuity. As experts in the optics of the human eye, we can offer our patients options that may seem a bit “out of the box” and illustrate the value we bring to their care. With the advent of soft toric multifocal lenses, being the optics expert can set you and your practice apart.
Key takeaways
- Strive to minimize the circle of least confusion by correcting all refractive errors when possible.
- In cases of presbyopia correction with toric soft contact lenses or IOLs, understand the ramifications of uncorrected astigmatism and consider leveraging low amounts of uncorrected ATR astigmatism (-0.75D and less) to improve near acuity with minimal negative effect on distance acuity.
