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Lens selection, fitting tricks ensure more successful, efficient astigmatic correction
Contact Lenses and Eyewear
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Industry developments have made soft toric lenses a possibility for nearly every astigmatic patient. The wide selection in toric lenses and manufacturers gives contact lens fitters many options. Aspherical lenses require a defter touch and a more precise fit but, in the hands of an experienced fitter, they can be a valuable addition to any contact lens practice.
One area where the industry has improved is the development of reliable, low cylinder soft lenses. Frank D. Fontana, OD, FAAO, in private practice here, suggests that patients with 0.75 D or more of astigmatism consider toric lenses as part of their correction.
"We have compromised for many years, not worrying about cylinder up to about 1 D. When you get to 1 D, it really visually disturbs the patient in several ways, such as headaches and loss of sharpness of vision," Dr. Fontana said.
Spherical rigid gas-permeable (RGP) lenses can be used to mask low levels of corneal astigmatism. In patients who can tolerate RGPs, Dr. Fontana recommends using spherical rigid lenses for up to 2 D of corneal cylinder, then back toric or bitoric for higher amounts.
RGPs vs. soft torics
In patients with higher levels of astigmatism, Kenneth L. White, OD, in private practice in Mt. Vernon, Wash., recommends rigid toric lenses.
"In a high astigmat - 3 D or more - who is not too sensitive around the eye, I would prefer to fit that patient in an RGP lens because generally I will achieve more stable and crisp vision. Once adapted to the RGP lens, patients will usually have less fluctuation and variable visual acuity," Dr. White said.
In patients who are too sensitive, Dr. White still prefers RGPs, but will compromise by using soft lenses, explaining to the patient that vision will probably not be as crisp, though the comfort will be greater.
Mark P. Andre, FCLSA, director of contact lens services at the Casey Eye Institute in Portland, Ore., is a proponent of soft toric lenses for most corrections. Refractive surgery or penetrating keratoplasty patients and, even in rare instances, keratoconus patients have been successfully fitted with soft lenses, Mr. Andre said.
"There are very few patients out there who cannot be fit into a toric soft lens. One of the biggest mistakes I see is patients who have fairly high degrees of astigmatism being told that they can't wear soft lenses when we have patients with as much as 5 or 6 D of astigmatism wearing a custom toric soft lens," Mr. Andre said.
"Unless they have an irregular astigmatism such as keratoconus or some other irregularity of the cornea, don't rule out any correction as a possible soft lens option," he added.
With-the-rule vs. against-the-rule
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Practitioners should determine the nature of the toricity, whether it is with- or against-the-rule or oblique and whether the astigmatism is corneal or residual, before beginning the fitting process.
For patients with against-the-rule astigmatism, Mr. Andre recommends soft toric lenses because the steeper meridian is along the horizon, and RGP lenses tend to displace.
"Just for the dynamics of the lens fit, it's very difficult to center a rigid lens on an against-the-rule astigmat," Mr. Andre said.
With-the-rule astigmatism allows for easier centration of the lens along the flatter horizontal meridian, allowing for success with either RGPs or soft torics. Mr. Andre makes both options available to patients, and more than half have opted for soft lenses because of the options available in planned replacements and monthly replacements.
Residual astigmatism
In patients with low levels of residual, or internal, astigmatism, spherical RGPs will not mask the cylinder because there is no corneal toricity, Dr. Fontana said. Rigid lenses would require prism ballast and ground in cylinder to correct the astigmatism, a difficult task.
Dr. White said he relies on soft toric lenses in about 90% of his patients with residual astigmatism.
"Generally, small degrees of residual cylinder can be corrected nicely, especially with some of the new designs we have in toric lenses," Dr. White said.
Higher levels of residual astigmatism can be corrected with RGP lenses, but doctors should determine how much of the patient's astigmatism is corneal and how much is residual before fitting the lens, Dr. White said. If the lens is designed to correct only corneal toricity, the residual astigmatism will compromise the patient's vision.
Fitting the lens
When fitting soft toric lenses, Mr. Andre's first step is to determine the lens diameter by measuring the horizontal visible iris diameter and adding approximately 3 mm.
"We're very big on looking at corneal diameter, because corneal diameter is going to affect the sagittal height of the cornea as much or more than the radius of curvature," Mr. Andre said.
The base curve and diameter of the contact lens are determined using a combination of the keratometry readings and the corneal diameter. The spectacle lens prescription is vertexed to the corneal plane, and the patient is fitted with a lens from inventory, or the lens is fitted empirically. After a few weeks, the patient returns for a follow-up visit; if there are no problems, quarterly or monthly replacements are ordered.
Dr. Fontana also prefers fitting lenses empirically and having the patient return for a follow-up visit after wearing the lenses for 1 or 2 weeks.
"That's my trial lens, but it's the trial lens to which the patient must adapt. When I over-refract and check all the other fitting parameters, I know what I've got on the eye, and I know what I need to change, if anything needs changing," Dr. Fontana said.
"Putting a trial lens on in the office has its attributes, but you're only getting a half-hour to an hour of experience with it, and that just doesn't tell the whole tale," he added.
While a loose fit can still be successful for spherical patients, precision is critical in fitting astigmatic patients.
"If the fit isn't perfect with a toric soft lens patient, you're probably not going to have a successful outcome," Mr. Andre said. "It's important to have a perfect fit, and that becomes more important as the astigmatism becomes greater."
A rotation of 10° to 15° is going to severely curtail vision, and the practitioner may have to reorder the lenses with a different axis to compensate for the tilt, Dr. Fontana said.
Confidence, experience help
Some problems many doctors have stem from a lack of confidence in the toric lenses, Dr. Fontana said.
"Contact lenses are not an automatic mechanical thing where you design an object, put it on another object and it works or it doesn't work. A lot has to do with patient control. If a practitioner has very little faith in a lens he or she is fitting, he or she transmits that to the patient," he said.
The policy of soft toric lens manufacturers guaranteeing fits increased his confidence in the lenses and helped the manufacturers secure a niche in the contact lens market, Dr. Fontana said. "That is what has really made the torics a workable reality because they guarantee that if the lens doesn't fit you have no liability. That gives you a lot of confidence to try soft torics. You could be very inexperienced, and this is where you are going to get your experience," he said.
Another area where some practitioners should devote more time is in reading literature about different types of toric lenses, Dr. Fontana said. "We don't know the different types that are available," he said.
A lack of familiarity with available lenses leads some doctors to rely too heavily on a "favorite" lens, and they are reluctant to try anything else if the initial fit is unsuccessful, he said. While he tends to fit the majority of his patients in similar types of lenses, Dr. Fontana said he is open to other lenses and other manufacturers of the same type of lens when a fit is not successful.
One of the most common mistakes Dr. White sees is astigmatic patients being fit in spherical lenses.
"I find a lot of practitioners don't fit patients into astigmatic lenses, and they just use spherical lenses and say, 'You're going to see well enough.' I find that hard to accept because we wouldn't do that in a pair of glasses," Dr. White said.
For Your Information:
- Mark P. Andre, FCLSA, is the director of contact lens services at the Casey Eye Institute, 3375 S.W. Terwilliger Blvd., Portland, OR 97201; (503) 494-5536; fax: (503) 494-4286.
- Frank D. Fontana, OD, FAAO, can be reached at #16 Hampton Village Plaza, Suite 249, St. Louis, MO 63109; (314) 353-6171; fax: (314) 353-0031.
- Kenneth L. White, OD, can be reached at 102 North 15th St., Mt. Vernon, WA 98273; (360) 424-4181; fax: (360) 424-6414.
- None of the clinicians have a direct financial interest in the products mentioned in this article, nor are they paid consultants for any company mentioned.