Article Date: 9/1/2003

astigmatism
Upgrade Your Toric Know-How
With the diversity and abundance of toric contact lenses on the market, you have new options for fitting patients who have astigmatism.
BY DEEPAK GUPTA, O.D., Stamford, Conn.

Experts estimate that more than one third of spectacle-corrected patients have cylindrical corrections of -0.75D or greater. However, many of these patients are wearing eyeglasses instead of contact lenses. What's worse is that many patients who have astigmatism aren't even aware that they're candidates for contact lens wear. Or, many of them have been told by their eye doctors that they have to wear rigid gas permeable (GP) lenses or conventional (yearly) toric contact lenses.

With the diversity and abundance of toric contact lenses on the market, the notion of having to wear GP lenses is ridiculous, whether your patient has 0.75D of astigmatism or 5.75D. In short, there's no legitimate reason why you can't fit the vast majority of your patients into soft, planned replacement contact lenses. Furthermore, with the quality of lenses on the market, there's no reason why we shouldn't aim for 20/15 vision, as we often do with our spherical patients. Let's discuss some of the finer points of soft toric contact lens fitting for your astigmatic patients.

Maintaining alignment

Regardless of modality, soft toric contact lenses require some means of keeping their cylinder axis properly aligned with that of the patient's refractive error. Most employ one or a combination of the following three methods:

1. Prism ballasting. This common method of stabilization entails adding more lens material mass to the inferior portion of the lens, making that part of the lens thicker. Doing this helps prevent the lens from rotating. This method offers a good compromise between patient comfort and good stability.

2. Lens truncation. This design involves cutting off the bottom portion of the lens, leaving it flat or truncated. The flat edge of the lens usually aligns with the lower lid, and the bottom "corners" help hold the lens in place. Lens truncation is an effective stabilization system, but it tends to make the lens less comfortable for the patient to wear.

3. Thin zones. This design is also called "dynamic stabilization" and "double slab-off." It involves removing lens material at both the apex (superior portion) and base (inferior portion) of the contact lens, creating thin zones at the top and bottom. These thin zones allow the eyelids to exert pressure on the lens and hinder its rotation. This toric contact lens design offers the greatest comfort of the three stabilization methods, but it also provides the least resistance to rotation.

Schedules to fit every need

Toric lenses are available for a daily, two-week, one-month and three-month disposal schedule (see "Some of the Common Planned Replacement Soft Toric Lenses" in the sidebar below). Many manufacturers offer ready-made toric lenses for astigmatism of up to 2.25D around the clock.

For higher powers or for patients who have unusual topography, you can prescribe custom-made toric lenses for monthly or quarterly replacement. What's really nice about these toric lenses is that they're often available in one-degree increments around the clock so you can obtain a precise fit.

Assessing rotation

All toric contact lenses have their own unique marking that allows the practitioner to assess lens rotation. Most lenses have markings at 6 o'clock, while others have them at 3 o'clock and 9 o'clock. When the markings are ideally aligned, the cylinder axis is located as marked on the trial lens. If the markings are rotated, then so is the cylinder axis.

We can take advantage of this by visualizing the rotation as clock hours where each clock hour is equivalent to 30 degrees. When a particular lens doesn't align properly, we can compensate for rotation via the Left Add Right Subtract (LARS) rule.

Clockwise rotation means the bottom portion of the lens has rotated to the left. Therefore, we add 10 degrees to the lens axis when ordering the diagnostic contact lens. Counterclockwise rotation means that the bottom portion of the contact lens rotated to the right, so you subtract this value when ordering the diagnostic contact lens.

For example, for a refraction of -3.25 -1.25 x 90 and a diagnostic lens rotation 10 degree clockwise, we'd order a lens with power -3.00 -1.25 x 100. This assumes that when we place the new lens on the eye, the lens will again rotate that same 10 degrees clockwise, making the effective axis at 90 degrees.

Fitting guidelines

In years past, toric fitting involved taking keratometry readings and refractions and estimating at a toric lens, which we had to order. This meant a long and involved fitting process for both practitioner and patient. Fortunately with the emergence of planned replacement lenses, this practice is obsolete.

Many practitioners, including myself, advocate that you should have a comprehensive fitting set in your office for your most-used brand of toric lenses. By doing this, you'll save yourself and your patient much time and aggravation. Plus, there's no greater practice builder than immediately fitting your astigmatic patient when he inquires about contact lenses. That patient may return for a contact lens evaluation for a toric lens that you'd have to order, but nothing compares to immediate gratification.

So with the vast selection of lenses available, which one do you fit? As with anything else, there is no right answer. Personally, I've found that there's no one lens that works for every patient. Even the most successful of toric lenses offer a 80% to 90% success rate, which means that you must have a back up lens or two for these patients.

Sometimes, when compensating for rotation or adjusting the base curve fails to yield optimal results, a different brand may work. Also, you should be aware of the subtle differences between the various brands. If a patient has dry eyes, or precise visual needs or long wearing time, you may have to offer a brand other than your "favorite." For more tips, see "Toric Pearls for the Progressive Practitioner".

Act like a specialist

With the wide array of excellent, quality planned replacement soft toric lenses on the market, every practitioner should see a high rate of fitting success. Regardless of what we're legally allowed to call ourselves, as optometrists we all are and should behave as specialists. By keeping up with the latest in contact lens technologies and offering them to patients, that's exactly what our patients will see us as -- specialists.

References available on request

 

Toric Pearls for the Progressive Practitioner

Consider the significance of the patient's cylindrical component. If he has a moderate spherical refractive error with only a small portion of astigmatism, then he's less likely to notice that the cylinder isn't corrected optimally. Even if you're off a little, it doesn't make much of a difference. In contrast, a patient who has a spherical component near plano will be more sensitive to the astigmatic correction and will be more difficult to fit.

Charge fees that are appropriate for a more difficult fitting process. Although many of your toric fits will go well, there will be occasional patients who require multiple visits and multiple trial lenses. Not only should your patient be financially prepared to deal with this up front, but he should also be willing to return for the visits if they become necessary.

Check the patient's sensitivity to astigmatic correction. One of the easiest ways to do this is to apply The Becherer Twist Test. With the refraction in the phoropter, rotate the cylinder axis until the patient first notices blurring of the acuity chart. The more the patient allows you to rotate the axis, the less sensitive the patient is and the more likely it is that you'll have a successful fitting experience.

Quantitatively, the Becherer Twist Test breaks down to the following:

Rotation Success Rate
+ 20 degrees 90% with one lens
+ 15 degrees 90% with two lenses
+ 10 degrees  70% with three lenses
+ 5 degrees Patient must accept variable vision

I don't rely on this method quantitatively in terms of lenses used, but rather qualitatively. The more sensitive a patient is to axis rotation, the less likely you are to have a successful fitting experience.

Increase your options. For your dry eye patients, consider the Encore Toric (CooperVision). It offers a nice, comfortable, stable fit for all-day wear. For higher cylindrical powers, I recommend the Preference Toric (CooperVision), but limit the wear time with this lens because of the low Dk. Another option to open up your lens choices is to perform nasolacrimal punctal occlusion. New drugs on the market such as Systane (Alcon) and Restasis (Allergan) may also increase your contact lens options.

For patients who plan to wear these lenses for long hours or even to sleep in them, I avoid lenses with low DK values. An Acuvue Toric lens (Vistakon) or a Soflens 66 Toric lens (B&L) is sometimes a better alternative because of their higher DK values.

For patients who have high astigmatism (up to 5.75D), the Frequency 55 Toric XR (Cooper Vision) is an excellent choice. This is a made-to-order monthly replacement lens.

For patients demanding precise vision, I use the Biomedics Toric (Ocular Sciences). I have found the first lens successful fitting percentage to be around 90% and an overall success rate of 95%.

 

Dr. Gupta has no financial interests in any of the companies or products mentioned in the article. You can reach him at deegup4919@hotmail.com.

 

 



Optometric Management, Issue: September 2003