Contact Lens Fitting of
IRREGULAR CORNEAS

BY JUDY CHAN, O.D., B. OPTOM, M.S., & DENNIS BURGER, O.D., F.A.A.O.
APR. 1997

This post-refractive surgery patient, who was also a noncompliant extended wear patient, presented us with a dual contact lens fitting challenge.

As refractive surgery becomes more widely available to a broader patient population, contact lens practitioners will likely see more irregular corneas. Irregular corneas present unique challenges to contact lens practitioners. Cases may be as simple as a successful PRK patient entering presbyopia or as complex as the patient we saw recently.

PATIENT HISTORY

A.S., a 54-year-old man, was referred to our clinic for contact lens fitting of his left eye, which had received radial keratotomy (RK) 15 years prior. He reported good vision for 12 years after the surgery until he developed a corneal abscess in one of the 16 incisions. Six months before we saw him, A.S. received photorefractive keratectomy (PRK) to treat his residual refractive error. His vision improved for about three weeks post PRK, after which time corneal neovascularization developed and his vision steadily declined. He had not been wearing a contact lens in the left eye since the RK surgery.

For the last 10 years, A.S. had worn an extended wear soft contact lens in his unoperated right eye up to eight months at a time with replacement upon discomfort. He had been wearing his present lens, which he had purchased from a mail-order outlet, for four months. A.S.'s last full general eye examination had been about 10 years ago. He had no personal or family history of ocular disease, and his general health history was unremarkable. He denied taking any medications.

INITIAL EVALUATION

A.S.'s entrance visual acuity was 20/50+ OD with the current soft contact lens and 20/200- OS uncorrected. Refraction was: OD -15.00 -0.25 x 30, 20/50; OS -4.25 -2.00 x 120, 20/40. The referring doctor's report indicated A.S.'s best corrected vision in the left eye was 20/40 with -3.25D sphere two years previously, as well as 20/40- with -1.25 -3.50 x 180 three months after PRK (three months before visiting our clinic).

Slit lamp examination of the left eye showed the 16-incision RK, an inactive corneal abscess in one of the inferotemporal incisions (Fig. 1), central corneal scarring about one millimeter in diameter, moderate corneal haze over the entire corneal surface, and neovascularization which encroached five millimeters into the inferior cornea (Fig. 2). In the right eye, we observed neovascularized vessel loops extending three millimeters both superiorly and inferiorly (Fig. 3). We noted severe corneal staining upon contact lens removal (Fig. 4).

FIG. 1: CORNEAL ABSCESS, LEFT EYE.	FIG. 2: NEOVASCULARIZATION, LEFT EYE.
FIG. 3: NEOVASCULARIZATION, RIGHT EYE.	FIG. 4: CORNEAL STAINING AFTER LENS REMOVAL, RIGHT EYE.

Corneal topography of the unoperated right eye showed a pattern suggestive of keratoconus with inferior steepening; the left eye had a flat cornea with irregular flattening effect (Fig. 5).

FIG. 5: TOPOGRAPHY MAPS. RIGHT EYE (TOP), LEFT EYE (BOTTOM).

Central keratometry readings in the right eye were 45.50/46.00@90 with grade 1 distortion. For the left eye, we affixed a circular template with four eccentric fixation points (4mm from center) to the keratometer and asked A.S. to eccentrically fixate at those points.

Fundus examination revealed peripapillary atrophy and posterior vitreous detachment in both eyes, OD greater than OS, with 0.1 cup-to-disc ratio in each eye. We also observed nuclear sclerotic changes: grade 1 OD and trace OS.

DIAGNOSIS

Right eye: corneal neovascularization secondary to extended soft contact lens wear and possible keratoconus. Left eye: postoperative RK/PRK with multiple radial incisions, dense scarring centrally and inferotemporally, moderate corneal haze over the entire cornea, and neovascularization encroaching five millimeters onto the inferior cornea. Both eyes: peripapillary atrophy and posterior vitreous detachment.

CONTACT LENS SELECTION

Because we suspected A.S. had keratoconus in the right eye based on the corneal topographic map, we evaluated the fluorescein pattern using a rigid gas permeable contact lens. We chose a trial lens that was a half-diopter flatter than the flattest central K and expected an alignment pattern. The lens centered well with the fluorescein pattern resembling keratoconus, three-point touch with one millimeter central bearing (Fig. 9).

FIG. 9: RGP LENS IN RIGHT EYE -- 7.50MM BASE CURVE, 9.0MM DIAMETER.

Due to an irregular corneal surface and vascularization in the left eye, we chose an RGP with base curve based on the average of the four peripheral corneal readings obtained. The initial diagnostic lens was: Fluorocon, 8.2mm base curve, -3.00D power, 10.0mm diameter, 8.2mm optic zone diameter (OZD), 9.1/0.4mm posterior secondary curve radius/width (PSCR/W), 10.8/0.5mm posterior peripheral curve radius/width (PPCR/W).

Overrefraction yielded visual acuity of 20/50. We noted a central bubble, although the peripheral system appeared adequate (Fig. 10).

FIG. 10: RGP LENS IN LEFT EYE -- 8.20MM BASE CURVE, 10.0MM DIAMETER.

We placed a second diagnostic lens with a flatter base curve on the left eye: Polycon II, 8.45mm base curve, -6.00D power, 10.0mm diameter, 8.2mm OZD, 9.3/0.4mm PSCR/W, 11.2/0.5mm PPCR/W. Visual acuity obtained with overrefraction was again 20/50, but there was a better central pattern with minimal apical clearance, good centration and adequate peripheral system (Fig. 11). Because of the neovascularization in both eyes, we chose a 60 Dk fluorosilicone acrylate material to provide better oxygen permeability for both corneas. We did not choose a material higher than this because of our desire to minimize flexure and warpage.

FIG. 11: RGP LENS IN LEFT EYE -- 8.45MM BASE CURVE, 10.0MM DIAMETER.

The final lenses ordered were: (right) Fluorocon, 7.50mm base curve, -14.75D power, 9.2mm diameter, 7.4mm OZD, 8.5/0.5mm PSCR/W, 10.5/0.4mm PPCR/W, 0.16mm center thickness; (left) Fluorocon, 8.45mm base curve, -10.75D power, 10.0mm diameter, 8.0mm OZD, 10.0/0.5mm PSCR/W, 12.0/0.5mm PPCR/W, 0.16mm center thickness; medium blend.

FOLLOW-UP VISITS

At his first progress examination, A.S. reported discomfort in the right eye, but good comfort in the left eye. Visual acuity was 20/30- OD and 20/50 OS. Overrefraction improved the vision to 20/25 OD and 20/50 OS. Slit lamp examination of the left eye showed a high-riding lens with minimal apical clearance and adequate peripheral system. In the right eye, we saw central superficial punctate keratitis with central bearing. We later removed the right lens and repeated keratometry readings and corneal topography. The keratometry reading was 46.00/47.50@90 with grade 1 distortion. The corneal topographic map showed an almost spherical cornea (Fig. 12). We ordered a new lens with a steeper base curve for the right eye: Fluorocon, 7.40mm base curve, -15.00D power, 9.0mm diameter, 7.8mm OZD, 8.4/0.4mm PSCR/W, 10.7/0.2mm PPCR/W, 0.15mm center thickness.

FIG. 12: TOPOGRAPHY MAP OF THE RIGHT EYE.

When A.S. returned for his new right lens, visual acuities were 20/25 OD and 20/40- OS. The right lens centered well with minimal apical clearance, but the left eye showed 2+ conjunctival injection nasally that the patient noticed. Slit lamp examination showed that the lens was rubbing against the nasal pinguecula. We ordered a smaller diameter lens to avoid contact with the pinguecula and still maintain stable lens movement (Fig. 13). Lens parameters were: Fluorocon, 8.45mm base curve, -10.75D power, 9.5mm diameter, 7.7mm OZD, 10.0/0.5mm PSCR/W, 12.0/0.4mm PPCR/W, 0.16mm center thickness.

FIG. 13: RGP LENS ON LEFT EYE -- 8.45MM BASE CURVE, 9.5MM DIAMETER.

When A.S. returned for his new left lens, corrected visual acuity was 20/25 OD and 20/40- OS. Slit lamp examination showed stable lens movement on the left eye and no contact with the nasal pinguecula.

NEOVASCULARIZATION AND CORNEAL EDEMA

Although we observed no signs at the slit lamp examination, the initial corneal mapping and RGP fluorescein pattern of the right eye showed indications of keratoconus. However, the corneal mapping two weeks after the new lens was dispensed appeared different. The initial lens seemed adequate at first, but was unacceptable at the follow-up visit. The most plausible explanation is that corneal changes occurred after the patient discontinued the extended wear soft contact lens and began wearing the daily wear RGP.

The inferior steepening we noted at the initial visit was the result of corneal edema secondary to four months of continuous extended wear of a soft contact lens. Holden et al. (1983) showed that a high minus soft contact lens caused an average of 13.9 percent overnight swelling and 5.5 percent during the day. Corneal hypoxia secondary to wearing a high minus soft contact lens produces greater swelling peripherally than centrally, and can result in neovascularization and corneal edema.

Because A.S. had an irregular cornea and neovascularization, we prescribed a rigid lens to provide better visual acuity and a higher amount of oxygen than a soft lens. There is more tear exchange under a rigid lens than a soft lens (10%-20% vs. 1% per blink), and a rigid lens covers less of the cornea, thus exposing the peripheral cornea to the normal oxygen environment.

This situation emphasizes the importance of educating patients about the danger of wearing a contact lens without removal for extended periods of time and about the importance of follow-up care. If A.S. had not come to us for contact lens fitting of the postsurgical left eye, he would still have been wearing a soft lens on his unoperated right eye on an extended wear basis with substantial neovascularization and corneal edema.

POSTSURGICAL COMPLICATIONS

A.S.'s left eye presented a unique contact lens fitting challenge. He had a history of multiple refractive surgeries as well as a cornea with considerable vascularization and scarring. The initial success of the RK procedure had been compromised by the corneal abscess (intrastromal corneal infection without an open wound) which developed in one of the incisions.

Even when RK is successful, clinical studies such as the Prospective Evaluation of Radial Keratotomy (PERK) and the Casebeer System for Predictable Keratorefractive Surgery indicate that due to residual refractive errors, RK patients may need further vision enhancement either by contact lenses, spectacles or surgery. The PRK enhancement of A.S.'s left eye was complicated by severe neovascularization three weeks after the treatment.

The postoperative refraction had not stabilized after six months, so the effect of the PRK may have regressed because of corneal wound healing, which can continue for 12 months or more. The amount of regression is related to the attempted correction, with more regression occurring after intended corrections of greater than -5.00D. Because of the similar optic disc appearance in both eyes, the refractive error of the left eye preoperatively may have been similar to that of the right eye, which was in the range of -15.00D.

A.S. also had subepithelial haze, which may develop within two weeks of PRK and may last up to six months before decreasing. A.S.'s corneal scarring could have been the result of dense subepithelial haze that didn't resolve secondary to PRK. Scarring has been found in about one to three percent of cases in corrections up to -6.00D, and the incidence increases in corrections of greater than -9.00D. In some cases, a second treatment can remove corneal fibrotic tissue and decrease the haze response. Despite the appearance of the haze, Snellen acuity is usually good because the haze causes light to scatter forward and backward when viewed through a slit lamp, but the patient only sees light scattering forward (light that goes to the retina). Haze is not a problem in RK, since the central cornea is not disturbed.

Fitting A.S.'s left eye with a contact lens was a challenge. Neovascularization developed following PRK, so a soft lens was contraindicated. Determining the base curve for this eye was difficult because of the irregular corneal surface.

Using a circular template with fixation points to obtain keratometry readings is a convenient method to help select a trial lens when corneal topography is unavailable. The technique provides an overview of the corneal flattening effect of RK/PRK using a plastic disc to measure the peripheral cornea at known points. By averaging the readings from the four peripheral positions, you can select a base curve for an initial trial lens that is more stable than a lens selection based on a central keratometric reading. The fluorescein pattern of the lens on the eye guides the selection of the next diagnostic lens.

It's important for visual performance and centration that stability of the RGP lens in the post-RK/PRK eye be achieved utilizing the steeper peripheral cornea. The large diameter permits lid interaction without creating excessive superior bearing or inferior lift, thus providing better stability. For A.S., we decreased the diameter from 10.0mm to 9.5mm because of the constant rubbing of the lens against the pinguecula.

We chose Fluorocon, a high Dk material that provides good oxygen permeability to the cornea without too much lens flexure.

Penetrating keratoplasty should be the last treatment choice for this patient because of the potential for separation at the incision sites. Suturing the radial incisions if they separate may reduce the success of the procedure and compromise visual outcome. However, penetrating keratoplasty may be considered when vision is inadequate.

CONCLUSION

A.S. presented us with two separate, unrelated problems. For his right eye, which had been abused by the use of an extended wear soft contact lens causing corneal neovascularization, we prescribed an RGP lens, which provides higher oxygen permeability, more tear exchange and limbal exposure. On his left eye, which had undergone multiple refractive surgeries with corneal scarring, moderate haze, corneal abscess and extensive neovascularization, we placed a similar Dk RGP lens. Both eyes now have acceptable visual acuities. CLS

The Medical Editing Department, Kaiser Foundation Research Institute, provided editorial assistance.

The authors have no financial interest in any of the companies mentioned.

References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #23. (Be sure to have a fax number ready.)