Managing the Astigmat with Contact Lenses

Where there's a will, there's a way. Fitting astigmatic patients is no exception. Learn how to use the various options available to make your astigmat a happy contact lens wearer.

RONALD K. WATANABE, O.D., F.A.A.O.
AUGUST 1999

We all know that astigmats can wear many types of contact lenses, from rigid spheres to soft torics to specialty hybrid and scleral designs. We also know that astigmatism takes on several forms, and depending on the type of astigmatism a patient has, the management may differ.

Despite the many options currently available to correct astigmatism, relatively few astigmats actually wear contact lens types that correct their astigmatism. It's understandable why astigmatic patients were previously told that they couldn't wear contact lenses, but with today's improved designs and parameter reproducibility, there's no reason why more astigmats can't wear them. In this article, I will discuss the various types of astigmatism and the contact lens choices that are most suitable for each type. I will also touch on the psychological and communication aspects of contact lens fitting that can influence success.

Regular astigmatism is the most common type of astigmatism. The major meridians of the corneal and refractive astigmatism are 90 degrees away from each other and the patient generally has normal corneal topography and good best corrected visual acuity. The astigmatism can be with-the-rule (WTR), where the minus cylinder axis is at or near 180 degree, or against-the-rule (ATR), where the minus cylinder axis is at or near 90 degrees. It can also be oblique, with the cylinder axis around 45 or 135 degrees. The magnitude of regular astigmatism can be small or large, but, with the appropriate cylindrical spectacle or contact lens correction in place, the patient can achieve good visual acuity, assuming that there is no refractive amblyopia or organic source of reduced acuity.

About 40 percent of spectacle-corrected patients have cylindrical corrections of 0.75D or greater. A great disparity exists between the number of suitable contact lens candidates and the number of patients who are actually wearing astigmatic contact lens options. Most regular astigmats don't wear contact lenses even though they may be good candidates. Many are not even aware that they are candidates for contact lens wear. About half of contact lens wearing astigmats wear spherical soft lenses, which do not correct any of their astigmatism. The other half is split between rigid and soft toric lens wearers. However, renewed interest in toric options from practitioners and manufacturers is rapidly increasing the number of contact lens wearing astigmats.

Soft Torics: What's New?

Over the years, soft torics have received mixed emotions from practitioners due to their inconsistencies and idiosyncrasies. It was not uncommon for a doctor to order a replacement for a successful patient, only to find that the replacement was nothing like the previous lens. Two or three reorders later, and he was ready to give up torics altogether. This attitude has changed remarkably in the past few years. A niche that was largely custom conventional lenses has blossomed into a mainstream lens type that offers everything from 2-week disposables to custom quarterly replacements, with daily disposables on the horizon. How has the soft toric industry changed so much?

First, the demand for soft toric lenses is high. In the United States alone, there exists an estimated 80 million potential astigmatic contact lens wearers who are still wearing spectacles or spherical soft lenses. With such a huge potential market, manufacturers have worked hard to develop toric lenses that are more reproducible, more comfortable, less costly and easier to fit. Automation and more precise tools have taken much of the variability out of lens manufacturing and have reduced production costs significantly -- a fact which has given practitioners greater confidence that future lenses will perform just like the previous ones. They have also helped bring the disposable modality to astigmats who never had the option before.

In addition to improved reproducibility, the availability of greater soft toric parameter ranges and free trial lenses has taken much of the guesswork out of fitting. By placing a trial lens that is similar to the patient's refractive error on the eye, you can quickly assess the fitting characteristics so that you can make appropriate adjustments in lens axis and base curve. You can also determine whether the lens will work or if you need to try a different lens type. Telephone consultants have also helped practitioners to troubleshoot the more difficult cases.

Modern lens design -- All soft toric lenses have spherocylindrical powers that are kept at the proper axis by rotational stabilizers. The most common stabilizer is the prism ballast, and truncation is not used in modern toric soft lenses. By integrating prism into a contact lens, a thickness gradient is created, which interacts with the upper eyelid to force the thick edge downward. Gravity also plays a minor role in maintaining this orientation. The greater the prism ballast, the more stable the rotation. However, with greater ballasting comes increased lens thickness, which impacts ocular health and patient comfort. In addition, the lens induces a small amount of base down optical prism, which is not noticeable for most patients but can be bothersome for those with fragile binocular systems. Peri-ballast is a variation of the prism which incorporates the thickness gradient in the peripheral portions of the lens only. This has the advantage of not creating any optical prismatic effects.

Thin zones incorporate thin top and bottom edges that sit underneath the upper and lower eyelids. The thicker center would be pushed interpalpebrally. Though more comfortable than prism ballasted lenses, thin zones are not quite as stable.

Some lens brands also utilize back surface toricity to align with toric corneas in a similar, though less effective manner to back toric RGP lenses. Though back surface toricity alone is not enough to prevent lens rotation, highly toric corneas may best be fit with soft torics that also have a toric back surface to add greater rotational stability.

Traditionally, the thickness profile of soft torics varied with changes in cylinder axis. The thicker portions of the lens are pushed more by the upper eyelid such that lens rotation sometimes results. In fact, ATR powers tend to be most stable because the horizontal meridian of the lens is thickest and tends to orient between the lids. This is enhanced further by prism ballasting or thin zones so that the lenses can maintain a stable orientation. WTR powers, in contrast, are thickest in the vertical meridian. This causes the upper eyelid to push the top portion of the lens downward. As a result, the lens may simply move in a vertical direction without rotating, but it could also rotate clockwise or counterclockwise, depending on the direction of force exerted by the upper lid. The addition of prism ballasts and thin zones helps, but the WTR cylinder lenses tend not to be as stable as ATR powers. Oblique axes are torqued by the upper lid such that minus axis 45 lenses are rotated counterclockwise and minus axis 135 lenses are rotated clockwise. Though undesirable, this is a more predictable rotation on the blink.

Newer soft toric lens designs incorporate a combination of stabilization features, such as prism ballast with thin zones or eccentric lenticulation, to make the thickness profile of the lens more consistent among the different cylinder axes. This minimizes torque effects of the upper lid on the front surface of the lens, thereby minimizing rotation on the blink. This provides more consistent orientation of the lens, which results in more consistent visual acuity.

Patient selection -- Although soft toric lenses have improved, it is still wise to carefully select these patients. The goal is to select patients who exhibit minimal rotation of toric lenses or who are tolerant to small degrees of cylinder rotation. Attempting to preselect patients who will have no lens rotation is impossible, but you can increase your chances of success by incorporating the principles of lens mechanics described above. First, determine the type of astigmatism your patient has. ATR cylinders are the best and can very likely be fit with any toric brand. WTR patients are more variable, so you may need to try two to three lens brands to find the one that is most stable. Consider lenses that have moderate to large amounts of prism and lenses that are larger in diameter to provide an extra margin of stability. In addition, if the patient has a significant amount of corneal toricity, use a back surface toric design. Oblique cylinders are the most difficult to fit, and they usually require a large diameter, highly prism ballasted lens brand.

Another aspect to consider is the significance of the patient's cylindrical component. If the patient is a moderate to high myope or hyperope with only a modest amount of astigmatism, you are more likely to have success. For example, a patient with a refraction of -8.00 -1.25 x 180 is less likely to notice that the cylinder is not corrected optimally. In contrast, a patient with a spherical component near plano will be more sensitive to the astigmatic correction and must be fit more carefully. For example, a refraction of plano -1.25 x 90 is more difficult to correct precisely with a soft toric lens. These patients may be better advised to consider rigid lenses or to simply stay with spectacles.

Perhaps more important than refractive error is the patient's tolerance to cylinder axis rotation. The Twist Test, attributed to Doug Becherer, O.D., can be used to determine a patient's sensitivity to axis rotation (Table 1). With their refraction in the phoropter or trial frame, simply rotate the cylinder axis slowly until the patient first notices a definite blur of the chart. The higher the tolerance, the greater the success rate.

Fitting methods: empirical vs. diagnostic -- Most patients can be fit the first time by ordering the lens power empirically from the vertexed subjective refraction. The advantage of doing this is that the patient's first experience will be a relatively clear and comfortable one, since the lens is brand new and the lens power will be close to, if not right on, their refractive error. The disadvantage of ordering empirically is that the fit characteristics of the lens are unknown until the lens is placed on the eye. Therefore, if the empirically ordered lens does not fit well, the patient will have to wait an additional few days while a second lens is reordered.

Diagnostic fitting with trial lenses allows the practitioner to determine that the lens fit is acceptable prior to ordering the first lens. However, because it is rare to have a lens in your diagnostic set with a close enough power to your patient's refractive error, their first experience with toric lenses will most often be a fuzzy one, and the patient may walk away from the fitting with negative impressions.

Free diagnostic lenses for disposable and frequent replacement lenses offer an immediate assessment of vision and fit. For patients with refractive errors within the parameter ranges of these lenses, it may be the most efficient fitting method.

Assessing rotation -- All toric lenses have some sort of marking to allow the practitioner to measure lens rotation. When the markings are ideally aligned, the cylinder axis is located as marked on the lens package. If the markings are rotated, so is the cylinder axis. For example, if the lens markings are rotated clockwise 10 degrees, we know that a lens with a cylinder axis of 180 degrees now has its axis at 170 degrees (Fig. 1). We use this measurement to compensate for lens rotation via the LARS rule: Left Add Right Subtract. Clockwise rotation means the bottom portion of the lens has rotated to the left. Therefore, we add 10 degrees to the refractive cylinder axis (not the diagnostic lens axis). For a refraction of -3.00 -1.50 x 165, we would order a lens with axis 175, assuming that when the new lens is placed on the eye, the markings will again rotate 10 degrees clockwise, indicating that the cylinder axis is now at axis 165.

Assessing the amount of rotation takes some practice. Most practitioners visualize the rotation as compared to clock hours (each clock hour = 30 degrees). Alternatively, a slit beam can be aligned with the markings (Fig. 2), and the amount of rotation can be read off the protractor on the slit lamp. Lenses that have multiple markings at a known spacing are even more useful in helping to estimate the rotation. In the end, we want the lens rotation to be minimal and stable with the blink and throughout the day.

Determining the lens power -- Most toric lens powers can be ordered the same as the vertexed refractive error. Some lenses are designed such that some "lacrimal lens" effect occurs (the result is that less minus, more plus power is needed in the contact lens itself.)

The cylinder axis can usually be ordered the same as the refractive cylinder axis, but if there is significant lens rotation, or if the patient is very sensitive to axis rotation, you may have to use LARS to compensate.

Cross cylinders can also be used to determine the final lens power. This is usually done with difficult cases where the patient has already had two to three lenses reordered. First, a spherocylindrical overrefraction (SCOR) is performed. If the patient achieves sharp acuity through this SCOR, it is combined with the contact lens power to yield the new power. The simplest way to do this is to place spectacle trial lenses equal to both the lens power and the SCOR into a trial frame and then read the combination on a lensometer. Programmed calculators are available from some manufacturers, as are online versions of the program. Consultants can also be called to do the calculations for you. If there is significant lens rotation, then the amount of rotation must be compensated before and after performing these calculations.

It is important to remember to allow the lens to stabilize for at least 20 minutes before assessing vision and performing an overrefraction. Allowing a patient to leave with a lens that is 20/25 or 20/30 is not a bad idea since the lens will usually perform differently in the office as compared to in the "real world." Visual assessments at follow-up visits after several hours of wear are also more accurate than initial assessments, because the lens has had time to stabilize. Patience during the initial visits will save time and frustration later; rushing through the fit assessment may result in chasing the cylinder axis.

Retinoscopy and corneal topography -- Performing retinoscopy or corneal topography over soft toric lenses may provide a clearer picture of why a patient does not have sharp acuity with a soft toric that centers and moves well. In many cases, the lens does not drape the cornea smoothly, resulting in irregularities in the optic zone. This translates into unstable and blurry vision. A dark area in the center of the retinoscopy reflex indicates that a lens is too steep. An irregular corneal topography pattern in the central area also indicates poor draping (Fig. 3). In either case, flattening the base curve or using a thinner design allows the lens to drape the cornea more smoothly and usually solves the problem.

Preparation and follow-up -- As important as your lens fitting skills are, mentally preparing the patient is just as critical in achieving success with soft torics. Proper expectations must be set so that if the patient does not attain 20/20 acuity, he will still be a satisfied contact lens wearer. He must also be prepared for a lens reorder or two so that he is not frustrated by extra follow-up visits.

It's also important to interview your patients so that you can better understand their visual tasks at work and home. Visual demands should be moderate, especially for patients with high cylinder requirements. A patient's lifestyle should be such that contact lens wear is of great benefit when compared to spectacle wear. For example, patients who are active, play sports, or are in image-conscious occupations are likely to be motivated to get out of their spectacles. For these patients, it's important to emphasize the advantages of contact lenses by matching their needs to the benefits offered by the lenses. For example, a patient who plays tennis regularly will be bothered by spectacle wear because they will slide down his nose and limit peripheral vision. Contact lenses will eliminate both of those problems to allow the patient to concentrate on hitting the ball instead of keeping his glasses on his face.

Irregular Astigmatism

Not all astigmatism is regular. Some patients have major meridians that are not perpendicular or corneal topographies that are irregular and distorted. These are all given the label "irregular." Irregular astigmatism can be congenital, degenerative, surgically-induced or traumatic in nature. In most cases, spectacle correction is inadequate in providing the patient with good visual acuity. Soft toric lenses also fall short because they provide the same type of astigmatic correction that spectacles do and they drape the cornea such that very little of the surface irregularity is masked. Therefore, RGP lenses are generally the primary lens choice in these cases, but the examples below demonstrate how soft lenses may also be useful in certain cases.

Keratoconus -- RGP lenses are primarily used to provide clear vision in patients with this degenerative thinning of the central cornea. The lacrimal lens is used to fill in corneal irregularities, and the front surface of the RGP lens serves as the new anterior refracting surface of the eye. In mild to moderate cases with little to no corneal scarring, RGP lenses often provide normal visual acuity.

Toric RGP lenses are generally not needed, even if keratometry and corneal topography show that the patient has large amounts of corneal "astigmatism." This is usually not true astigmatism, but is caused by the distorted surface. The only way these instruments can express the irregularity is by yielding astigmatic numbers (Fig. 4). If spherical RGP lenses seem to be unstable because of peripheral corneal toricity, back toric designs can be used. It's possible to visualize peripheral toricity by looking for a differential amount of mid-peripheral and peripheral clearance in the vertical and horizontal meridians (Fig. 5). If it is determined that the patient needs a back toric to improve the fit characteristics of the RGP lens, then an SPE bitoric can be utilized.

Use soft toric contact lenses in mild cases where spectacle-corrected acuity is still good. They are especially useful in patients who are having problems adapting to RGP lenses. Be sure not to fit the lenses too steeply. Design the lens to fit the mid-peripheral and peripheral cornea, and because these curvatures tend to be within the normal range, especially in these mild cases, medium base curves should be ordered initially. On follow-up, the base curve can be adjusted if necessary.

Hybrid lenses, piggyback systems and scleral lenses are specialty designs that can be used when conventional options fail. The advantage of the hybrid lens (SoftPerm) is that it provides rigid lens vision and soft lens comfort. The disadvantages include low-Dk, tight fits and limited base curve choices. For early to moderate cases where RGPs are necessary for vision but the patient cannot tolerate them, the SoftPerm should be utilized.

Piggyback systems can be created by placing a soft spherical contact lens under an RGP lens on the cornea. The soft lens can be as simple as a disposable lens or as complex as a countersunk carrier lens. The RGP is generally fit to the surface of the soft lens. This system is indicated in cases of RGP intolerance, RGP instability or RGP-induced mechanical complications. It's important to watch for hypoxia-related complications in these cases.

Scleral contact lenses are used as a last resort to maintain the patient in contact lenses prior to considering corneal surgery. These are large diameter lenses that are made to fit the sclera and vault the cornea. High-Dk polymers are used to provide a fair amount of oxygen to the cornea despite little tear exchange.

Post penetrating keratoplasty -- Most patients who have had a corneal transplant (Fig. 6) are left with a moderate to large amount of corneal astigmatism. Some also have distorted corneal topographies that can be classified as irregular astigmatism. Spectacle correction in these cases is rarely beneficial, especially in unilateral cases. Rigid lenses are generally the rule when fitting post penetrating keratoplasty corneas. Prior to fitting, an assessment of the corneal topography is essential. Various shapes can result, such as: normal shape with steep center and flatter periphery; drum-head shape with flat center and steeper periphery; and tilted shapes where the graft was inserted at an angle. Normal topographies are the easiest to fit, but because the corneas are somewhat irregular, a larger diameter is generally needed. Choose high-Dk materials to minimize hypoxia effects. Select a base curve that allows reasonable centration and tear exchange and avoid large areas of harsh bearing or excessive clearance. Corneas with lots of toricity can be fit with bitorics, but trial fit with spheres first to confirm that the toricity exists.

The drum-head shape can often be fit with a spherical design RGP. The result would be an apical clearance fit with alignment in the mid-periphery. In cases where a spherical lens results in poor tear exchange, reverse geometry designs become very helpful. Many RGP labs have reverse geometry designs that can be utilized, and most will lend you a fitting set to help you fit the patient.

Tilted grafts are the most difficult to fit. They generally require very large diameters to attain any amount of centration. A good rule to go by is to try and achieve a 1/3 touch and 2/3 clearance fluorescein pattern (Fig. 7). This, coupled with reasonable position and stability, will provide the best lens fit possible.

Throughout your efforts to fit the post transplant cornea, it's important to minimize physical and physiological trauma to the cornea. The last thing you want to do is to induce a graft rejection. By using high-Dk materials, large diameters to vault the graft and well-positioning and moving lenses with good tear exchange, the patient will have the best chance for successful contact lens wear.

Post refractive surgery -- Refractive surgery is successful for the vast majority of patients who undergo it. However, there will always be a few who are unhappy with their post-surgical results. Many of these cases are left with irregular astigmatism. Radial keratotomy (RK) is the most common cause of post-refractive surgery contact lens fitting. Although very little is done today, and that which is done uses improved techniques, some unsatisfied refractive surgery patients will still come through your door. RGP contact lenses are the only choice for these patients. Spherical designs can often be used (Fig. 8), but reverse geometry designs may be necessary if the spherical lens cannot achieve good stability and centration (Fig. 9). Soft contact lenses tend to induce neovascularization into the RK scars and should be avoided.

Photorefractive keratectomy (PRK) and laser in-situ keratomileusis (LASIK) tend to have fewer post-surgical refractive problems. LASIK in particular has been quite successful in providing acceptable vision. Soft lenses can be used in either of these patients without the concern of neovascularization or other soft lens complications. If the patient can achieve good acuity with a subjective refraction, a flat-fitting soft lens works well. If irregular astigmatism is the result (Fig. 10), a spherical or reverse geometry RGP may be necessary. The goal is to maintain reasonable centration and movement while providing better visual acuity.

Conclusion

Approach astigmatism with the confidence that regardless of the type or etiology, you will be able to fit the patient with lenses that will provide good vision and comfort. New soft toric options make contact lenses more attractive and accessible for the majority of regular astigmats.

Dr. Watanabe is an associate professor at the New England College of Optometry and the director of contact lens services at the New England Eye Institute. He has lectured internationally and is in part-time private practice in Boston, Mass