A Keratoconus Fitting Primer
BY BEZALEL SCHENDOWICH, OD, FIACLE
Keratoconus (or KC, as seasoned clinicians call it) allows contact lens practitioners to employ their deepest understanding of corneal structure, spectacle and contact lens application and the psychology of individual sufferers. From the moment of diagnosis until the decision to perform a penetrating keratoplasty, the primary and most efficient treatment for approximately 85 percent of all keratoconics is optical and contact lens care. (Figures show that only 10 percent to 20 percent undergo surgery.)
An optometric triage for KC should run as follows:
- The simplest contact lenses that will restore vision, maybe even soft spheres
- And, when the going gets tough, piggybacking
The point is, you never know which option will best suit a patient until you try. An elderly gentleman who has reduced tactility and can't manage lens application -- let alone removal -- might feel satisfied with gross anisometropia and approximately 7.00D of cylinder in his glasses.
This article will provide several tips that just might help you interpret both contact lens fitting session results and complaints from patients.
Choosing a KC Lens Design
A summary article appeared in this journal a few years ago by Shelley Cutler, OD, FAAO ("Managing Keratoconus with Proprietary Designs," October, 1999). She listed design and fitting characteristics of the many then-available KC lenses. Much of that information is still current; much is not. The principle remains the same: A fitter needs trial lenses to evaluate in situ the complexities of the topography. A map isn't always enough!
How many trial sets does an efficient fitter require? In the best of cases, he probably needs as many as different labs are willing to make available to him. Each KC cornea is different; each cone has properties similar to, but mostly different from, almost every other cone. Some lens designs are "more universally useful" than others. Experience with the different designs will lead to either an understanding of what works best on which cone types or to new and different designs.
Which lens is likely to fit better: the one that's more comfortable or the one that's less comfortable? In the vast majority of cases, when patients complain that one lens is far more irritating than the other, even a cursory observation will likely show that the "bothersome" lens has a nearly perfect textbook fit and the phenomenally comfortable lens is bound to the cornea with aggravated scattered punctate keratopathy and number-two conjunctival injection.
Fitting Relationship Goals
In the early days of fitting KC corneas, we attempted to achieve a forgiving fitting relationship by designing smaller lenses with steeper base curves. No matter what we tried, as the base curve radius (BCR) shortened, the lenses became more likely to bind. Topography of the peripheral cornea has shown us that as the cone steepens, the more peripheral surface of the cornea tends to be relatively flatter than what you'd expect. You can enhance corneal health, overall comfort and longer wearing times by:
- Seeking alignment or minimal central clearance to avoid causing erosions and abrading the apex of the cone.
- Achieving peripheral clearance to ensure that the lens won't bind to the cornea or cause a peripheral circular incision.
The optic zone diameter (OZD) also plays a crucial role in overall lens comfort. If the OZD is too wide, then it'll vault the Fleischer's ring, which is located at the base of the cone where the cone ectates from the cornea proper. The potential space under the lens holds a reservoir of tears. As wearing time increases and the eye dries, the volume of tears in this pool tends to decline, allowing bubble formation and engendering dimple veiling. The pressure of the lens on the veil can cause erosions in this sensitive location.
Geometry of the KC Cornea
Fitting a GP lens on a normal cornea requires fluorescein pattern evaluation. In general, "pooling" means a steep-fitting lens and "touch" a flat-fitting lens.
On a KC cornea, central zone fluorescein evaluation still follows that rule. But secondary zone evaluation requires a reverse interpretation: Outside of the most central zone of the fluorescein pattern, a flat lens creates a pool. The flatter the lens (relative to the cornea), the deeper the pool and vice versa. As the lens gets steeper relative to the cornea (central pooling), the mid-periphery and periphery become tight and may appear to be bearing.
Keeping Eyes Happy
Even if your patient experiences great comfort with his new lenses, he'll almost inevitably begin the rapid descent to dry, red eyes while wearing his contact lenses.
In many cases, you may notice a partial blink with these symptoms. You must remain alert -- the earlier you diagnose scattered punctate keratopathy; three o'clock and nine o'clock staining; and horizontal, engorged conjunctival capillaries ("the red line"), the sooner you can reinstruct your patient about proper blinking techniques or possibly redesign his lens. Redesigning helps reduce the lens/lid sensation, which drives the patient to partial closure. A clean lens and well-lubricated eye will maintain a happy eye.
Several design factors change as KC progresses: Lenses get steeper to keep up with changes in corneal curvature, and in many designs they become smaller as well. As the BCR decreases, the (minus) power of the lens must increase to compensate for the larger plus-powered lacrimal lens it induces. As power increases, so does edge thickness. Eventually, we may try an awkward lens shape that in many cases is understandably uncomfortable and may quite easily become dislodged by tightly squeezed lids.
When no lens design is comfortable or even wearable without frequently abrading corneal tissue, we consider piggybacking lenses. Use a highly permeable soft contact lens, generally of low plus or minus power, which will remain stable on the cornea (without inferior buckling) and place the current GP on top. You can manage small alterations in overall refraction by the correct choice of the soft lens power.
Implementing this technique vastly improved overall comfort for a patient who required an extremely steep lens of 37.00D. By using a 9.00D soft contact lens as a carrier, we significantly reduced his GP's minus power and edge thickness. Our patient then achieved his long-time ambition of participating in a scuba diving class. He wore his piggyback combo inside his diving mask.
The main problem with piggybacking is permeability. These patients need more frequent monitoring, but do quite well.
When to Stop
Lens fitters who specialize in KC corneas must make an honest effort using the lens designs at their disposal to comfortably restore visual acuity without compromising corneal health or integrity.
Our job is rewarding. Our rapport with our patients needs to include our capacity to announce the end of our efforts. Nevertheless, paraphrasing a well known parable, "There's always another lens/fitter." From time to time, the best move that we can make is to refer a patient who has an advanced cone to another fitter who may have access to other lens designs or to a corneal surgeon for a penetrating keratoplasty work up.
In 1936, Applebaum (Archives of Ophthalmology) estimated the prevalence of KC at about one in 2,000 patients. In 1959, Hofstetter (American Journal of Optometry, Academy) raised the ante to around one in 450. Today's diagnostic instrumentation and tomorrow's genetic testing will show more definitively how many of us are keratoconics. Iatrogenic secondary ectasias will also account for a growing number of induced KC cases. It's incumbent upon every eye clinician to become familiar with the diagnosis and efficient contact lens fitting of keratoconus.
Dr. Schendowich is a preceptor in the Optometry Clinic at the Hadassah Academic College in Jerusalem, Israel and is an adjunct assistant clinical professor of Optometry at SUNY - Optometry in the Optometry Clinic at the Sha'are Zedek Medical Center, Jerusalem, Israel.
Contact Lens Spectrum, Issue: March 2005