Understanding and Managing Astigmatism: Your Top Questions Answered

Over the previous sessions, we learned that at least 90% of astigmats have errors of 1.75 D and below—something that enables them to take advantage of both the most popular and emerging correction options. As Dr. Moody explains, the statistic comes from data collated by Philip B. Morgan. “Dr. Morgan runs a data collection survey every year that looks at the contact lens prescribing patterns,” he explains.¹ “However, this survey doesn’t just look at the global patterns of prescribing but breaks the data down further to evaluate the prescribing patterns by country. We have a lot of patients that come into our practices every day, and references like this are really great sources of data for us.”

Dr. Morgan’s work sheds light on other areas of optometric practice—such as his research into the reasons why patients discontinue contact lens wear.² Although it’s long been thought that ocular dryness or discomfort were the primary reasons that would lead a patient to stop wearing contact lenses, regardless of the length of time they’d been in use, here, Dr. Morgan and his team found that new and long-term contact lens wearers’ dropouts diverged in their reasoning for doing so. Unlike established contact lens wearers, the majority (44%) of whom cited the expected ‘dryness’ or ‘discomfort,’ most (57%) new contact lens wearers stopped wearing them due to issues concerning vision—with over a quarter dropping out within the first 60 days.

“This paper shone a light on many under-acknowledged reasons for contact lens discontinuation, doing so in a particularly vulnerable population,” highlights Dr. Rueff. “It should be really compelling to us as clinicians in spurring us to really consider how we can refine our patients’ vision when fitting contact lenses.”

So, how can you maximize vision in your patients? Dr. Rueff advises that you carefully consider the refractive data you collect and use, including getting a fresh and accurate refraction at the fitting, taking into account how you’re refracting in a real-world environment with ambient light, and ensuring that you balance your patients. “One thing that I often hear from students I work with is that they think there’s no need to balance presbyopes because they erroneously think that these patients can’t accommodate, but that isn’t true. Such a patient may take 0.25 D more in one eye compared to the other, so performing a red-green balance or a binocular blur can be really helpful.”

Other parts of maximizing vision in astigmats include correcting all existing astigmatic errors, especially in lower hyperopes or myopes who may be sensitive to even low amounts of astigmatic error; vertex correcting both sphere and cylinder; and communicating effectively with patients—and following up afterward. “The paper also talked about the silent sufferer,’ anywhere between 70 or 80 percent of whom would just find a solution on their own—which we know is going to be to drop out and discontinue lens wear,” Dr. Rueff highlights. “We really need to think about keeping our lines of communication open to these patients, especially during the first two months before patients fall off the wagon.”

The design of the lens that you choose may also impact vision—and the likelihood of a new contact lens wearer desisting. “Soft toric lenses are stabilized either using a vertical prism, which weighs the lens down (prism-ballast), or by thinning out the top and the bottom of the lens to utilize the natural forces around the eye to tuck the lens under the eyelids (double thin zone/slab off),” explains Dr. Rueff. “The problem with prism-ballasted lenses is that while the weight of the prism is used to stabilize the lens, patients are still subject to the visual impacts of vertical prism, which will seep into the optic zone.”

This can mean, especially if the patient is only wearing a toric prism-ballasted lens in one eye, that they may experience vertical prism imbalance of ~0.3–0.8 D, which can cause visual discomfort and fatigue. “Especially with monocular patients, you might want to think about selecting non-prism-ballasted lenses—all Acuvue lenses in particular use a double think zone design over prism, meaning that when offering these lenses to your patients, you don’t need to worry about any vertical prism imbalance,” Dr. Rueff offers.

In patients with against-the-rule astigmatism, because the horizontal meridian—which alters the vertical aspect of the optical image—lies in front of the retina when looking at a distant object, the image that these patients will see will be vertically blurred. This can be especially problematic when looking at distant numerals, which tend to be very vertically oriented. However, as Dr. Moody demonstrates, when an object moves closer to the with-the-rule astigmatic eye and the image shifts towards the retina, these patients will experience a boost in their amplitude of accommodation, leading to a slight bump in acuity.

“If a patient is using a multifocal lens, they’re not going to have to use much of the lenticular optics to see at near if they have some degree of with-the-rule astigmatism—this is pseudoaccommodation,” explains Dr. Moody. But how and when can you leverage this knowledge? Uncorrected with-the-rule astigmatism may provide better distance vision, but it can still result in poor near acuity in patients. Therefore, you should consider fully correcting the astigmatic error in these cases. But Dr. Moody advises changing your approach if the patient you’re correcting has against-the-rule astigmatism in their non-dominant eye. “In these cases, to try to leverage the pseudoaccommodation to my advantage, depending on the astigmatic error (0.5D up to ~1.0 D), I may be okay with leaving a bit of this error uncorrected.” Doing so can improve patients’ near vision while also having minimal effect on their binocular distance vision.

Ultimately, as Drs. Moody and Rueff highlight, such refinements may be the difference-makers that take you from an unsuccessful patient who drops out to a successful, happy, and long-term patient.