Article

TURN UP YOUR TORIC LENS FITTING

Astigmatism-correcting designs are available in soft, GP, and scleral lens options.

Eye care is anything but monotonous—every patient interaction is different; every eye is different; every day is different! These changes keep us on our toes, ready for whatever comes our way. Of course, there are always a few comments that we have almost come to expect to hear during a new patient examination. When we ask about family ocular history, our patients inevitably respond “I think my mom and dad had cataracts.” And when we ask whether the patients themselves have been told that they have any eye diseases (such as glaucoma, macular degeneration, or a history of retinal problems), we often hear “No, but I think I do have ‘stigma.’” Generally, the patients are correct, and they do have some amount of astigmatism; statistically, astigmatism is the most common refractive error worldwide, according to a 2018 meta-analysis.1

After completing the examination and refraction, we discuss any pertinent ocular health information and review various methods of visual correction available. We emphasize the importance of having a pair of spectacles, but we always—always—offer contact lenses as an option. “Have you ever considered contact lenses? Because you would be an excellent candidate for them.”

Many patients are surprised by this, saying that their previous eyecare practitioner told them they could not wear contact lenses because they have astigmatism; some contact lens fitters are still skeptical of the idea that contact lenses are available for almost everyone. However, with the incredible variability of lenses on the market, the only true barrier to lens wear for most patients is their ability to safely apply, remove, and care for the lenses.

Some fitters are not advocates for fitting astigmatic patients in contact lenses. Granted, toric lenses are more involved compared to a simple spherical lens, but avoiding toric lenses limits positive patient experiences, negatively impacts your practice’s bottom line, and limits your impact as an eyecare provider. It is time to embrace the cylinder and turn up your toric lens fits.

During training, many of us are taught that we don’t need to fit astigmatism-correcting contact lenses until the cylinder reaches a value of –0.75 diopters cylinder (DC), because many patients are satisfied with a spherical correction when we use the spherical equivalent. For every 0.50D of refractive cylinder, we must add 0.25D more minus to the spherical power to derive the spherical contact lens power. For example, if a patient’s refraction is –1.00 –0.50 x 180, the spherical equivalent for this refraction would be –1.25 diopters sphere (DS). For many patients, this provides adequate vision and constitutes an easy spherical fit for both fitter and patient.

However, there are patients, especially those who have type-A personalities, who demand sharper, clearer vision. In these cases, there are a few options: 1) Several non-traditional lenses are available in –0.50DC powers, or 2) slightly over-correcting (with an additional –0.25DC over their refractive cylinder) and providing a toric contact lens instead of the spherical equivalent.

For the aforementioned patient example, we recommend comparing vision with the spherical equivalent (–1.25DS) versus slight astigmatic overcorrection (–1.00 –0.75 x 180). For those patients who do not note a significant difference between the two, the spherical lens would suffice. However, for those patients who demand perfection and appreciate the clarity of the astigmatism correction, starting the fit with a toric lens may be best. Another option for this fit would be to start patients in a spherical contact lens correction; if the patients report blur after trialing, they can be switched to a toric contact lens. Prior to the fitting, it is important to outline what a patient should expect while wearing toric lenses; this is especially important if a patient is transitioning from a spherical contact lens into an astigmatism-correcting design.

In a well-fitting spherical contact lens, the lenses are very stable on the eye, and there is generally no variability to vision following a blink. With a toric lens, some small vision fluctuation with a blink is expected. This occurs because, during a blink, the lid causes mild torsion of the lens on the eye. As the eye opens, the lens will typically quickly stabilize. In lower amounts of astigmatism correction, this variability is minimal. However, as the amount of astigmatism increases, even a small variability after a blink can result in residual astigmatic error.2,3 Despite the vision fluctuation, this option usually offers sharper, clearer, and more comfortable vision for patients compared to a spherical contact lens as a result of providing astigmatic correction.

CORRECTION OPTIONS

There are many different contact lens modalities into which we can choose to fit our patients. Our choices include premium toric soft lenses (daily disposable, two-week, monthly), custom soft lenses, corneal GP lenses, hybrid lenses, and even scleral lenses (Figure 1). Our choice is typically driven by a patient’s prescription and the amount of astigmatism that is present.

Figure 1. Common lenses used to fit astigmatic corneas, including GP lenses, soft toric lenses, and scleral lenses (left to right, respectively).

Soft Toric Lenses For patients who have low-to-moderate astigmatism (–0.50DC to –1.75DC), soft toric contact lenses are often the lens of choice. Soft toric lenses are widely available in many parameters and modalities, including daily disposable, two-week, and monthly replacement. We have found daily disposables to be the safest modality in our practice, and we find that these patients are much more compliant. There are many soft lens manufacturers that offer daily disposable astigmatic correction of –1.75DC in around-the-clock axes in 10º steps; a few brands also have up to –2.25DC available in these parameters. There are also soft contact lens options available for individuals who have astigmatism higher than –2.25DC. Monthly replacement, high-astigmatism-correcting soft lenses are available in around-the clock axes in 5º steps from –2.75DC up to –5.75DC in –0.50D steps, if the spherical component is between ±10.00D.

With such a wide array of parameters, there are few patients who have healthy corneas who cannot be fit into a soft toric contact lens. Further, custom soft contact lenses can be designed from multiple laboratories for increased precision. As previously mentioned, the higher the cylinder correction, the more precise the fit needs to be to minimize post-blink blur.

Soft torics have a small hash mark on the lens to evaluate on-eye rotation and stability after settling. Although most soft toric lenses manufactured today are very stable and settle as expected, sometimes the hash mark shows excessive movement after a blink, or the lens does not settle directly inferiorly and instead is rotated left or right.

If there is excessive movement or rotation post-blink, this typically indicates a flat-fitting lens, and the base curve should be steepened if possible. The most accurate method for assessing and correcting rotation of the hash mark is to perform an over-refraction and enter this information into one of the many available online lens calculators to determine the new powers. Alternatively, an important mnemonic to remember is “LARS” (Left Add, Right Subtract). The LARS principle allows fitters to easily assess the optics of the lens and to correct for rotation by selecting a different axis.

To assess rotation, narrow the slit beam and increase the aperture height. Rotate the slit beam to precisely align with the angle to which the hash mark is rotated. Measure this angle as degrees left or right from 90º. For example, consider a contact lens with a prescription of –1.00 –0.75 x 180 that is placed on the eye and evaluated for stability and rotation after settling. The lens appears stable after each blink. However, the hash mark is not at 90º but instead is rotated 20º to the left. Because the lens is rotated left, adding 20º to the axis will correct the optical rotation, resulting in a new axis of 020; the new trial lens should be –1.00 –0.75 x 020. If the lens were rotated to the right 20º, the new lens power should be –1.00 –0.75 x 160, as 20º must be subtracted from the original axis of 180º.

A key detail that often affects new fitters is that after correcting the axis with the LARS principle, the hash mark should still be in the original position (rotated 20º left in the original example); we are changing the optical orientation of the lens, not the fit of the lens on the eye! As all slit lamps today have degree markings on them, you can also tilt the optical section to align with the axis markings and directly determine the exact number of degrees that the lens is rotating.

Until recently, patients who have astigmatism and presbyopia were quite restricted with frequent replacement soft contact lens options. However, in the last few years, there have been a couple new additions in the soft toric multifocal space (with the newest design launched in March). At this time, there is a vast range of parameters that can successfully accommodate even those highly astigmatic presbyopes; one manufacturer has available toric multifocal lenses from –20.00DS to +20.00DS, with up to –5.75DC in axes in 5º steps. Should these lenses still not provide optimal vision, fit, or comfort, custom soft toric multifocals can be designed and ordered from various laboratories.

GP Lenses These lenses can offer superior clarity and visual stability for patients who have 2.00D or more of corneal cylinder or for those who are not successful in soft toric lenses. As mentioned before, as refractive astigmatism increases, any rotation of a soft toric lens can induce cylinder, with the amount of cylinder increasing as the refractive cylinder correction increases for a specific amount of rotation.2,3 Therefore, GP lenses can result in better stability of vision for these highly astigmatic patients. Ideal corneal GP spherical lens candidates include those whose corneal cylinder is approximately equal to their spectacle cylinder—in which all or most of the refractive astigmatism is due to the cornea. When the cylinder powers do not align, patients may need to be fit into a GP lens that is toric rather than spherical.

Because many parameters of a GP lens can be modified, even the highest levels of astigmatism can be corrected with corneal lenses. Various corneal GP lens designs for astigmatism include spherical, back-surface toric, front-surface toric, or bitoric; the latter has toricity on both surfaces of the lens.

Spherical GP lenses can often be utilized on patients for whom the corneal cylinder is generally within the central cornea because the post-lens tear layer can compensate for areas in which the lens is not fit exactly “on K” (i.e., the base curve radius is equal to the flatter keratometry reading). If the corneal astigmatism is limbus-to-limbus, a back-surface or bitoric GP lens may be needed to accurately fit the toric cornea.

During GP lens fits, we can calculate the post-lens tear layer and estimated residual astigmatism to know whether a spherical lens will provide optimal vision or whether a back-surface or bitoric design is needed. Use a back-surface toric design when the residual astigmatism is approximately one-half of the back-surface toricity and has an axis equal to the flat K reading.4 This should also be considered when a patient’s refractive cylinder is approximately 1.5 times the corneal cylinder.4 Otherwise, for the great majority of these patients, a bitoric design is recommended, as it compensates for the residual cylinder induced by the toric back surface of the lens.

If you do not feel comfortable performing the calculations to determine the optimal lens type for a patient, there are multiple GP toric calculators available, or you can ask your GP laboratory consultant to help you design the lens. With these designs, we typically try to fit the horizontal meridian “on K” and the vertical meridian 0.50D flatter than K. In essence, this allows the lens to move freely in the vertical meridian during a blink but limits left-to-right motion. We find that this offers optimal and stable vision for patients. It is ideal for corneas in which the axis of astigmatism is with-the-rule or against-the-rule; however, it is more challenging with oblique axes.

There are two methods to GP lens fitting: diagnostic and empirical. In diagnostic fits, lenses are placed on the eye in-office. The lens-to-cornea fitting relationship is evaluated, and an over-refraction is performed, prior to ordering lenses. Empirical fits are ordered by taking various measurements—often including corneal topography (Figure 2), refraction, corneal diameter, and, in some cases, pupil size—and sending them to a lab consultant to help design the lens.

Figure 2. A topographical map of asymmetric irregular astigmatism, a common characteristic of keratoconus.

We prefer empirical fits for multiple reasons but mostly because we have higher success rates, especially among first-time GP lens wearers. By ordering empirically, a patient’s first experience with a GP lens is typically a near-perfect fit with (usually) great vision, which may partially offset any subjective concerns about initial awareness. We also use a topical anesthetic to numb the eye prior to applying the lens for the first time. According to a study by Bennett et al, the use of a topical anesthetic at the fitting and dispensing resulted in fewer dropouts, improved initial comfort, and enhanced perception of the adaptation process.5

Another advantage that corneal GP lenses have over soft lenses is that multifocal capabilities can easily be integrated into the lenses, whether it is a spherical, front- or back-surface toric, or a bitoric lens design. The rigid lens optics, as well as the distance-center optics, are potential benefits visually.

There are multiple types of multifocal designs for corneal GP lenses, including aspheric-gradient multifocals, simultaneous multifocals (similar to many soft contact lens designs), and even segmented or blended segment translating bifocals and multifocals. The customization of multifocals in GP materials and designs gives them a significant advantage over soft lenses; similarly, a multifocal GP lens still requires the “right” person and an understanding of the give-and-take relationship that comes with balancing the distance and near vision common in a multifocal lens.

Corneal GP toric lenses should not be overlooked. They are an incredibly versatile option that allows many patients the opportunity to wear contact lenses when other options are not ideal and when they may have been previously told that contact lenses were not an option. Many lens fitters have begun to shy away from fitting corneal GP lenses because they are fearful of how patients will respond to the initial feeling of “hard contact lenses.” Sometimes the difference between a successful GP corneal lens fit or a failed fit can be as simple as the verbiage that you (as the fitter) and your staff choose to use to describe and promote these lenses. For example:

  • Say “GP” lenses instead of “hard contact lenses.”
  • Say “You will be more aware of these lenses for the first few days until your eyes become accustomed to them,” rather than “These lenses are much less comfortable compared to soft lenses.”
  • Use positive phrases such as “These lenses can offer the sharpest and clearest vision compared to other astigmatism-correcting lenses.”

The quickest way to ensure that a patient will be unhappy in a lens is to plant doubt or negativity before the fit is initiated. To really turn up your toric fits, make sure that your staff is informed and ready to be supportive throughout the process. After all, they likely will spend more time with a patient during examination preparation and contact lens training than you will.

Scleral Lenses Another GP lens option for patients who have regular and irregular astigmatism is scleral GP lenses. Scleral lenses are large contact lenses that are fit to the contour of the sclera as opposed to the shape of the cornea. Scleral lenses completely vault the cornea and take advantage of the post-lens tear layer to effectively negate the corneal shape from being a factor in the optical system. Scleral contact lenses are generally used for patients who have highly irregular corneas, such as those who have keratoconus, pellucid marginal degeneration, epithelial basement membrane dystrophy, and post-penetrating keratoplasty, among many others. The post-lens tear layer fills in the irregular corneal contour so that the refractive component of the system occurs on the regular, smooth curvature of the scleral lens itself rather than on the irregular cornea. Scleral lenses, while a fantastic option for irregular corneas, are also being used for regular, healthy corneas, especially those that are highly astigmatic.

Scleral lenses, like corneal GP contact lenses, are extremely customizable and provide us with the capability to fit nearly all eyes. In our experience, most scleral fits result in spherical optical powers. However, in cases in which there is a sphero-cylindrical over-refraction (i.e., due to residual astigmatism), a toric front surface may be added easily.

Scleral toricity is evident in most patients.6 As we are fitting the lens to the scleral contour, we typically must use toric peripheral haptics on scleral lenses in an attempt to closely match the scleral curvature; this ensures maximum stability of the lens. This is especially useful if we do have to add a front-surface toric, because toric peripheral haptics greatly minimize lens rotation.

Like corneal GP contact lenses, sclerals have great multifocal capabilities, including the option to decenter the optic zone over the pupil for maximum success. Sclerals are often advantageous compared to corneal GP lenses with regard to adaptation. As a scleral lens is fit onto the sclera and, as with soft lenses, has very limited movement with the blink, the initial awareness is less.

Despite being utilized primarily for irregular corneas, many scleral manufacturers have lenses being marketed to fit regular corneas. Often, these lenses are smaller in diameter than those needed for diseased corneas. An optimum scleral fit should provide good comfort, good vision, and good corneal clearance.

SUMMARY

The correction of toricity does not have to be a complicated process. With the advantages that we have today from excellent daily disposables and frequent replacement designs, outstanding custom soft lenses, and highly refined, computer-controlled lathes, nearly any cornea is ideal for contact lenses. Are you ready for the challenge? CLS

REFERENCES

  1. Hashemi H, Fotouhi A, Yekta A, Pakzad R, Ostadimoghaddam H, Khabazkhoob M. Global and regional estimates of prevalence of refractive errors: Systematic review and meta-analysis. J Curr Ophthalmol. 2017 Sep 27;30:3-22.
  2. Myers RI, Jones DH, Meinell P. Using overrefraction for problem solving in soft toric fitting. Cont Lens Anterior Eye. 1990 Sep;17:232-235.
  3. Snyder C. A review and discussion of crossed cylinder effects and over-refractions with toric soft contact lenses. ICLC. 1989 Apr;16:113-117.
  4. Byrnes S, DeNaeyer G, Edrington T, et al. Contact Lens Clinical Pearls Pocket Guide. Available at https://www.gpli.info/pdf/pearl-fitting-guide.pdf . Accessed April 28, 2020.
  5. Bennett ES, Smythe J, Henry VA, et al. Effect of topical anesthetic use on initial patient satisfaction and overall success with rigid gas permeable contact lenses. Optom Vis Sci. 1998 Nov;75:800-805.
  6. DeNaeyer G, Sanders D, van der Worp E, Jedlicka J, Michaud L, Morrison S. Qualitative assessment of scleral shape patterns using a new wide field ocular surface elevation topographer. J Contact Lens Res Sci. 2017;1(1):12-22.