Prescribing for Astigmatism

A New Comfort Zone

prescribing for astigmatism
A New Comfort Zone

By necessity, toric soft contact lenses are thicker and larger than spherical lenses, so corneal changes due to low transmissivity are more common. In fact, physiological changes, such as neovascularization, myopic creep, microcysts and corneal distortion, usually seen only in extended wear of spherical lenses, are all too common with daily wear of toric soft contact lenses. And experience has shown that, despite our advice to the contrary, toric soft lens wearers also nap or sleep while wearing their lenses just like their spherical lens-wearing counterparts.

Fortunately, we're now seeing the emergence of silicone hydrogel lenses for astigmatism, which should help prevent these problems in new wearers, and allow rejuvenation of the long-term toric lens-wearing cornea. In this article, I'll discuss what to watch for during the refitting process.

Learning From Experience

Restoring normal oxygen levels to a chronically hypoxic cornea is clearly desirable, but it's not without potential surprises during recovery. The lessons we learned from refitting long-term PMMA lens wearers into gas permeable lenses and, more recently, from comparative studies of overnight wear with low- and high-Dk spherical lenses, will help us understand some of the changes we'll see when refitting long-term toric soft lens wearers.

Corneal topography of long-term wearers of low-Dk toric soft lenses often show inferior steepening (Figure 1). This may be due in part to the shape and thickness of the lenses, and it does resolve when patients stop wearing their lenses. In many cases, refitting with a high-Dk toric lens also will resolve this problem, perhaps because of
the change in lens design, but more likely because of improved physiology.

Myopic creep that occurs over a long time may be subtle, and it may take a month or more to recover. With the astigmatic patient this may manifest in the form of altered cylinder power or axis, not just an increase in the spherical component.

Microcyst formation is considered an indicator of chronic hypoxia and is usually subtle in daily wear patients. The phenomenon of microcyst rebound has been well-documented in patients refit from low-Dk into high-Dk extended wear. Sometimes we're surprised to see this seemingly paradoxical increase in microcysts in the first week or two after refitting into the new material. You can expect this effect in some patients being refit into the newer high-Dk toric lenses as well, even if on a daily wear schedule.

The other common marker for chronic hypoxia has been corneal neovascularization. Unfortunately, this has been most common with toric soft lens wear, with the inferior cornea affected most often. Refitting the patient with high-Dk lenses usually promotes draining of the new vessels.

After the Refit

The introduction of new and better materials for toric soft lens patients brings both opportunity and responsibility to the practitioner. Patients deserve to know about these significant improvements, and you can expect many will want to try these lenses. Refitting toric soft lenses on a cornea that's expected to change in the process is a challenge for which you and your patients must be prepared.

In most cases, refitting is no more problematic than in any other circumstance. In some patients, however, significant refractive shifts may occur that require a lens change or two before the fitting is finalized. In these cases, you can reassure your patient that the changes are a positive sign of corneal rejuvenation. The extra visit or extra lens is worth the effort and justifies a higher fitting fee. These are the patients who will benefit most from being refit into the higher Dk lenses.

Figure 1: Corneal topography (top) shows inferior steepening associated with 10 years of toric soft contact lens wear. Corneal topography (bottom) 6 weeks after refit with high-Dk toric soft lenses.

Can you predict which patients will experience refractive shift? Patients are more likely to experience lens-induced corneal distortion if they have a history of long-term wear of low-Dk/t lenses, especially if you've seen evidence of significant refractive shift since fitting them originally. Presence of microcysts or neovascularization provides clues regarding chronic hypoxia or suggests that the patient frequently naps or sleeps while wearing his lenses. Corneal topography that shows inferior steepening, or keratometer mires that show distortion are also evidence of corneal compromise.

No Patient Is the Same

The initial refitting should proceed normally. Apply the lens that best fits the eye and corrects the refractive error. It's unlikely the fitting relationship will change significantly over time. The major concern is with the final refractive correction.

Depending on the level of corneal distortion you suspect, you may not need or want to "go for broke" on the first visit, particularly when the possibility of the need for change is fairly high. These patients should be told from the start that the first lenses they try might not be their final correction. Having patients wear the new lenses for a week or two should provide a good indication of how much change will occur.

If a patient's visual acuity changes after he's worn the new lenses for more than a week, you should over-refract to determine the needed change. Use a conventional cross-cylinder calculator (as opposed to Tori-Track), as there still may be some corneal distortion. Repeat a lens-off refraction and corneal topography to look for a trend toward recovery and stability. If significant change has occurred, you may need to dispense a second trial lens and follow up in another week or two.

As many of us have learned while managing contact lens-
induced distortion before refractive surgery, there are no hard-and-fast rules on how long it takes for recovery. In most cases, little or no change occurs and the initial fit is likely to be satisfactory. However, you must be alert to the occasional patient who will show significant change in the first few weeks of the process.

Look to the Future

Over the next few years, we'll likely refit most of our toric soft lens patients from low-Dk into new high-Dk lenses. Among the many benefits is the potential for making overnight wear a reasonable option. Being alert to the possibility of refractive changes will make the process more acceptable for you and your patients.

Dr. Bergenske is a past chair of the American Academy of Optometry's Section on Cornea and Contact Lenses. He has practiced for more than 20 years in Wisconsin and is on the faculty at Pacific University College of Optometry.
E-mail him at: