Article Date: 5/1/2003

Fitting the Irregular Cornea
Use contact lenses to visually rehabilitate irregular corneas caused by surgery or disease.
LORETTA B. SZCZOTKA, OD, MS, FAAO

Significant visual loss may be associated with irregular corneas that are surgically induced or relate to corneal dystrophies, cor-neal degenerations or other corneal pathology. Many of these conditions induce focal epithelial and stromal abnormalities which disrupt the regularity of the anterior refracting corneal surface and induce excessive light scatter. In fact, don't tell patients with irregular corneas that they cannot be helped until a rigid contact lens is placed on their eye and a sphero-cylindrical over-refraction is performed. Occasionally, a specialty soft lens may used if the best-corrected spectacle acuity is no worse than 20/40. In the last few years, a variety of specialty rigid lens designs, such as scleral and reverse geometry, have become popular with specialty contact lenses fitters.

This article reviews the indications in which contact lenses may improve vision in patients with irregular corneas, discusses the traditional contact lens options for the most common conditions, provides pearls for fitting each of these subgroups of patients and looks at newer products and fitting techniques that have become increasingly popular in managing patients with irregular corneal surfaces.

Epithelial and Stromal Irregularities

Conditions such as epithelial basement membrane dystrophy, herpes simplex keratitis, healed corneal lacerations and healed ulcers with residual scarring are some of the disorders which present with epithelial and/or stromal areas of corneal irregularity. If the affected area is within the visual axis or if the more peripheral topographic effects extend into the visual axis, then spectacle-corrected visual acuity is compromised.

Figure 1. Epithelial basement membrane dystrophy

Epithelial basement membrane dystrophy Most corneal dystrophies result in an irregular corneal surface and affect vision. Patients with corneal dystrophies that affect the epithelium and/or stroma may achieve improved vision from contact lenses. Epithelial basement membrane dystrophy (a.k.a. Cogan's microcystic epithelial dystrophy or map-dot-fingerprint dystrophy) is the most common dystrophy which presents with focal or diffuse epithelial and/or stromal areas of corneal irregularity (Figure 1). Reis-Buckler's dystrophy is an uncommon bilateral corneal disorder which results in frequent episodes of photophobia, erosions, anterior stromal and subepithelial scarring over time and visual impairment. Corneal stromal dystrophies (granular, macular and lattice) also result in corneal opacity and surface irregularities.

The rigidity of a GP lens and its lacrimal lens usually significantly improves the poor vision induced from the focal irregularities of epithelial and stromal dystrophies. Because epithelial erosions may be exacerbated by the mechanical friction of a GP lens, soft contact lenses are often tried as the first line of contact lens therapy. Firmer or thicker hydrogel or silicone hydrogel lenses are required to mask the irregularity enough to improve vision sufficiently. Choose either a prism ballasted spherical or toric design, such as CooperVision's Hydrasoft Daily Wear or Preference Standard lenses or CIBA Vision's Focus Night & Day lens because these lenses may mask some irregular astigmatism due to their high modulus and/or increased center thickness. Even such lenses often have limited success, and patients with significantly reduced best-corrected spectacle acuity or monocular diplopia from irregular corneal astigmatism rarely improve sufficiently with soft lenses.

GP lenses not only improve visual acuity by neutralizing the corneal irregularity, but wearing such lenses may reduce the severity of the basement membrane changes in epithelial basement membrane dystrophy. When fitting GP lenses, avoid harsh lens bearing over areas of "maps and fingerprints" to prevent mechanical friction and potential corneal erosions. A mild to moderate apical clearance lens-to-cornea fitting relationship is usually recommended if the area of irregularity is central or paracentral, which is typical if the patient is seeking improved vision with GP correction.

Herpes keratitis and scarred corneas Herpes keratitis with residual scarring, healed ulcers and corneal trauma or lacerations with residual scarring usually present with abrupt zones of corneal irregularity which can induce local or diffuse corneal topographic changes and irregularity both at the site of the scar and away from it. The National Society to Prevent Blindness reported that ocular trauma is second only to cataracts in producing visual impairment in the United States. Corneal surface alterations from penetrating trauma and subsequent repair have the greatest effect at the site of the injury and immediately adjacent to it. Suturing a wound may have a profound effect on topography. Good suturing is critical to the rehabilitation process and visual outcome. GP lenses are usually required to correct this type of irregularity.

Spherical GP lenses are most commonly prescribed because the irregularity and resulting visual compromise stem from the cornea. Front surface toric lenses are used only when the patient has sufficient interual astigmatism that reduces vision even with a spherical GP lens that corrects the corneal astigmatism. However, back surface and bitoric lenses may be indicated when the astigmatism is "global" rather local. "Global" refers to the predominance of typically regular astigmatism over the corneal surface which may be induced from a peripheral corneal laceration and resultant scar. Astigmatism which covers a relatively small corneal area is "local" astigmatism, and a spherical GP works to vault this localized irregularity.

When fitting corneas with localized scars, the fitting philosophy is based on the characteristics of secondary shape changes induced on the cornea. Scarred herpetic keratitis and healed ulcers often leave residual depressions, which pool with fluorescein under GP lenses. Healed lacerations create a flattened area over the scar, similar to a radial keratotomy (RK) incision, which can act as an elevated pivot point during GP lens fitting. The goals in GP lens fitting is to attain appropriate vision with a lens that has good centration, movement and tear exchange to preserve the corneal integrity. Attempt to vault the injured or diseased area. Fluorescein pooling adjacent to the scar is expected, but try to minimize chronic bubbles within the pool to prevent or minimize dimple veil stain. Because the corneal contours and resultant fluorescein patterns in these patients are atypical and asymmetrical, it is difficult to vault the affected area without excessively impinging on adjacent healthy tissue.

 

TABLE 1 Pearls to Fitting Success in Epithelial and
Stromal Irregularities

 

  • A moderate to high Dk GP material preserves the integrity of the diseased or scarred corneal epithelium and stroma

  • Meticulous lens cleaning, routine protein removal or the use of a strong emulsifying surfactant to solubilize deposits (such as the Claris system by Menicon or Optimum by Lobob) is recommended to remove back surface lens deposits which can increase the friction of higher Dk GP lenses and lead to breakdown of the fragile corneal surface

  • Recommend digital cleaning of the inside surface of the GP lens with the pinky finger or a cotton swab to keep the inside lens surface clean

  • Large overall diameters between 9.5mm and 11.0mm are often the most stable, comfortable and best fitting lenses

  • The GP lens will center itself over the steepest or most elevated hemi-meridian of the cornea, which may be over the scar itself. If the elevated scar is in the far corneal periphery, it may act as an obstruction to centration and the GP lens will slide "downhill." In that case, fit the largest diameter possible to allow for full pupil coverage

As with corneal dystrophy patients, thick soft lenses may provide only some masking of irregular astigmatism in corneas with prominent scars. Soft torics often are frustrating to fit on irregular corneal topography. Corneal surface asymmetry, elevations and depressions induced by some scars lead to unpredictable soft toric lens behavior. Patients with corneal scars should be monitored regularly, especially if corneo-scleral lacerations or iris material is trapped in wound sites. Such patients can develop new blood vessels racing up their corneal scars, and this may leak lipids into the clear cornea and cause further opacity (Table 1).

Corneal Degenerations and Ectasias

Salzmann's Nodular Degeneration and Terrien's Marginal Degeneration are uncommon conditions, and they are managed well with GP lenses. Spectacle-corrected acuity deteriorates in Terrien's Marginal Degeneration secondary to increased irregular and regular astigmatism produced by progressive thinning of the peripheral cornea. Salzmann's Nodular Degeneration is characterized by elevated subepithelial nodules and irregular astigmatism results. Fit the GP lens based on the underlying corneal shape produced. If the corneal astigmatism is "global" and fairly regular, a bitoric lens provides the best stability. In Terrien's Marginal Degeneration, larger diameter lenses work best to provide full pupil coverage should the lens decenter toward the steepest hemi-meridian. In Salzmann's Nodular Degeneration, however, the elevated nodules act as a barrier to lens movement, as the nodules are typically peripheral and often mechanical obstacles to GP lens decentration. Fit the lens small to stay within the clear central zone and to avoid constant lens impingement against the nodules, which leads to tissue breakdown and discomfort.

Contact lens practitioners may encounter patients with corneal ectasias, such as keratoconus (KC) and pellucid marginal degeneration (PMD), that require contact lenses. In the early stages of KC or PMD, soft spherical or toric lenses can be tried, but do not expect the visual acuity to be significantly better than the best-corrected spectacle acuity. New soft lens designs are advocated for some KC patients, such as Innovations In Sight Super Nova Complex Curve Design, which attempt to correct low levels of corneal irregularity with thick centers and stiffer materials. As KC progresses, the symptoms and signs include worsening of best-corrected spectacle acuity, fluctuating visual acuity, increased regular and irregular corneal astigmatism due to a progressively thinned and displaced ectasia of the cornea, deposition of iron in a ring or ring segment in the corneal epithelium surrounding the base of the cone (Fleischer's ring), Vogt's striae in the posterior corneal stroma and central corneal scarring. These patients are best fit with GP lenses to correct the irregularity.

There are three general "philosophies" to fit GP contact lenses in keratoconus: apical bearing, which may have minimal peripheral stabilization, three-point-touch and the apical clearance. Most contact lens practitioners use the first two techniques. The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study found that 88 percent of the patients were fit this way. However, some fitters believe that apical touch fitting may cause or increase apical scarring. The only prospective randomized trial to attempt to associate the fitting relationship with scarring was performed by Korb et al in 1982. In this small trial of seven KC patients, one eye was randomized to be fit with a central touch and the other eye to a central clearance. After 12 months, 57 percent of the eyes fit with central touch scarred, but none of the eyes fit with central clearance scarred.

In the three-point-touch method, the goal is to achieve mild apical touch and then mid-peripheral touch on the slopes of the cone to distribute lens bearing/pressure across a relatively large area and to reduce the likelihood of excessive pressure in a single area. Adherents of the apical clearance philosophy believe the thinned corneal apex should not be assaulted with the mechanical rubbing of a lens, but rather the lens should vault over the thinned and steep corneal apex and distribute bearing pressure only along the slope of the cone.

In addition to various fitting techniques for keratoconics, no uniformly accepted baseline data points exist for initial lens selection. My approach is to look at the corneal shape via topography and then to apply logical standards to achieve a selected fitting approach. The global shape of the keratoconic cornea is that of an irregular prolate ellipse. To use topography qualitatively, assess the morphological shape of the cornea based on a tangential map display. Usually the cone type can be classified as either a central (nipple) cone or a sagging (oval) cone. In a central cone, the area of steepening is located very near the visual axis, small in diameter and uniformly surrounded by corneal flattening. A sagging cone is characterized by a larger area of steepening located usually infero-temporal to the visual axis, and it may have a thinner zone of flattening peripheral to the cone in the same quadrant as the cone dislocation.

Once you identify the cone shape, adopt a quantitative topographic approach to GP fitting. For this you should use axial maps to choose an initial trial lens. The axial steep simulated keratometry reading (SIM-K) provides an apical clearance lens on a central cone. In the sagging/decentered cone subgroup, the curvature at the peak of the apex on an axial map provides a lens with minimal apical touch. Fitting GPs to the decentered cone in keratoconus is one of the greatest challenges to the contact lens fitter. Because GPs tend to center themselves over the steepest portion of any cornea, the lens drifts inferiorly and creates a sub-optimal fitting relationship, lens awareness and insufficient pupil coverage with resultant glare and monocular diplopia. Attempting to fit with apical clearance often makes things worse, because inferior lens displacement and a steep base curve-to-cornea fitting relationship classically results in GP lens adherence (Table 2).

 

 

TABLE 2 Summarized Fitting Approach

 

CENTRAL CONE:

  • Achieve faint apical touch or vault the apex

  • Aim for well centered lens

  • Use small lens diameters (<8.7mm) and optical zones (<6.7mm)

  • Use steep SimK value from the axial topographic map for your first trial GP base curve

SAGGING CONE:

  • Use larger lens diameters (>9.5mm) and optical zones (>7.0mm)

  • May have to accept inferior lens decentration

  • Use apical reading from axial topographic map for mild apical touch

  • Allow greater apical touch. For this approach, use the topographic reading from the apex off the axial map and flatten by 2.00D to 3.00D for the first trial lens base curve

Pellucid marginal degeneration (PMD) is a less common form of corneal ectasia than KC and it is characterized by a narrow, arcuate band of thinning usually located between 4 o'clock and 8 o'clock in the inferior cornea. The central cornea is of normal thickness, protrudes forward and creates irregular and against-the-rule astigmatism. The classic topographical sign is a butterfly-shaped curvature map with a central against-the-rule bow tie pattern and an inferior steep region (Figure 2).

Figure 2. Pellucid marginal degeneration patient exhibiting butterfly map.

PMD's features are similar to those in KC, such as progressive thinning, irregular astigmatism, hydrops and/or scarring. In contrast to KC, PMD has no Fleischer's ring, Vogt's striae or conical shape. In PMD, specialty soft torics may be fit with regular astigmatism and good spectacle-corrected acuity. If there is concurrent corneal irregularity, GP lenses are more routinely prescribed, and bitoric lenses are often required for the most appropriate physical fitting relationship (Table 3).

Post-surgical Corneas

Surgically-induced irregular cor-neal astigmatism occurs in an estimated 25 percent of eyes after penetrating keratoplasty (PK). The most common complications after PK include both regular and irregular postoperative astigmatism and anisometropia, which limits successful spectacle wear. Contact lenses are the best possible optical correction in many cases. Irregular astigmatism occurs in between 1 to 11 percent of eyes after kerato-refractive procedures.

Soft contact lens use should be limited for full-time visual correction in post-keratoplasty eyes. Although safe soft contact lens wear has been documented post-operatively, soft lenses also have been directly implicated in neovascularization and possible rejection of the donor tissue. The transplanted corneal tissue is avascular and immunologically privileged. Neovascularization immune host modulators enter the tissue and disrupt the immuno-privileged status. Hyper Dk silicone hydrogel lenses are an option if the patient's refractive status is correctable with presently available spherical lens parameters, but this is an unlikely scenario. Soft lenses may be used in the post kerato-refractive eye because rejection is not a concern. Watch for neovascularization along incision lines after radial keratotomy (RK) to preserve the tissue should transplantation be required.

 

 

TABLE 3 Fitting Pearls for Corneal Degenerations and Ectasias

 

  • In Salzmann's Degeneration, although manual or simulated keratometry usually reveals high amounts of astigmatism, bitoric lenses are usually not indicated because the irregularity is diffuse and asymmetric and the cornea has unequal curvature variations along a given meridian

  • In KC with a decentered apex, accept inferior lens standoff if the lens is fit acceptably to the central cornea.

  • Always ensure wide and satisfactory peripheral edge lift to prevent lens adherence if a lens continually decenters.

  • Toric designs are rarely needed in KC, even with substantial cylinder in the refraction and keratometry.

  • When fitting bitoric lenses to PMD, the lens may not orient in accord with the theoretical assumption that the steep base curve will orient along the steep (180 degree) central corneal meridian, and the flat base curve will orient along the flat (90 degree) central corneal meridian. Due to excessive steepening at the 6 o'clock hemi-meridian, the steep base curve may orient vertically

When fitting GP lenses, corneal topography measurement is critical. Topography provides an analysis of the overall corneal shape, which is necessary to choose the best design for a post-operative eye and to provide peripheral corneal data to select posterior curves for traditional and reverse geometry lens designs. The five major post-penetrating keratoplasty corneal shapes that have been described by Waring et al are prolate, oblate, mixed prolate and oblate, asymmetric and steep to flat graft tilt. After complicated kerato-refractive procedures, the predominant shapes are oblate (for a pre-operative myope) or prolate for a (pre-operative hyperope).

Figure 3. Macrolens.

Prolate corneal shapes can be fit with standard (or keratoconic) lens designs since prolates may closely resemble normal corneal asphericity. Oblate corneal shapes often require reverse geometry (RG) lenses, which have "plateau" posterior surfaces with secondary curves 2.00D to 8.00D steeper than the base curve to better align with the oblate corneal contour. Due to the asymmetry of the steep-to-flat pattern as well as the asymmetric pattern, GPs may be very difficult to center. Therefore, large overall and optical zone diameters may be required to aid in centration and pupillary coverage. Aspheric lenses have been suggested to improve centration; however, if an aspheric lens remains decentered, visual acuity may be diminished secondary to eccentric viewing through the aspheric periphery.

Post-operative oblate corneal shapes are difficult to fit with standard GP designs and fitting techniques. Traditional GP nomograms for base curve selection are not appropriate, and the selection of base curves depends on the corneal curvature measurements available. The desired lens-to-cornea fitting relationship is a vaulting of the central flat corneal zone and alignment to the mid-peripheral cornea. When using pre-operative keratometry readings, the initial trial base curve should be selected 0.50D to 1.00D flatter than flat K. When using post-operative Ks, the initial base curve should be at least 1.00D to 1.50D steeper than the post-op flat K. In study of post-LASIK eyes, the base curve averaged about 2.10D steeper than the mean postoperative keratometric power. Corneal topography is the most sophisticated method of assessing corneal curvature, and it is arguably the most appropriate method for GP lens fitting after corneal surgery. When fitting a standard GP design, the initial trial lens base curve should be selected equal to the corneal curvature value 3.5mm to 4.0mm superior to the visual axis on axial topographic map. This provides lens stabilization and alignment along this area of the cornea with vaulting and tear pooling over the flat central zone (Table 4).

Specialty Lenses

Reverse geometry lenses (RGL) used in orthokeratology have become popular post-surgical lens designs.

Standard or aspheric GP lenses (which progressively flatten in the periphery) work well on:

Reverse geometry lenses work well on:

Combination (hybrid) designs The SoftPerm design (CIBA Vision) uses a low Dk rigid gas permeable center (n-butyl-styrene) and a low Dk hydrogel skirt. It may be ideal for many irregular corneas as it offers the comfort and stability of a soft lens with the optics of a rigid lens. However, due to its very low oxygen transmissibility, acute corneal edema and chronic hypoxia leading to neovascularization limit its use. The lens should be used with special caution due to the potential for inflammation from entrapped tear film debris, poor tear exchange, or to the possibility of a "steep, bound lens syndrome." The fitting technique involves choosing a base curve slightly steeper than the mean keratometric cylinder and analyzing the fluorescein pattern with high molecular weight fluorescein. Steepening a Softperm base curve may contra-intuitively increase movement, as it reduces lens flexure.

Piggyback Fitting

Piggyback lens fitting is most often used for patients with KC or corneal dystrophies to solve comfort problems and manage recurrent abrasions. When fitting patients with KC, areas prone to breakdown include the cone apex, raised scars covered with hypertrophied epithelium and proud nebula. Usually steep (base curve <8.5mm) daily or two-week disposable lenses work best for moderate to advanced KC patients. When fitting patients with corneal dystrophies, the areas prone to breakdown with a GP are raised epithelial ridges seen with negative staining patterns during slit lamp fluorescein evaluations. Fit these patients with thin "membrane" lenses to limit movement and friction induced with the overlying GP.

Scleral Lenses

GP scleral lenses are preferred. Semi-sclerals, preformed scleral lenses and limbus-to-limbus gas permeable lens designs available from a variety of manufacturers. The Macrolens (C&H Labs, Dallas, TX) and the Dyna Intralimbal Lens (Lens Dynamics, Golden, CO) are two such designs that have worked well in my experience. These lenses can be manufactured in almost any GP material including high Dk polymers.

The Macrolens ranges in diameter from 13.9mm to 15.0mm. The comfort claims arise from using diameters similar to soft lenses which minimize lens movement and lid interactions. The bearing pressure is distributed on the central and mid-peripheral cornea along with the sclera. The Dyna Intralimbal Lens is a corneal lens with a starting diameter of 11.2mm, and available in any GP material..

Dr. Szczotka is an associate professor at Case Western Reserve University Dept. of Ophthalmology and Director of the Contact Lens Service at University Hospitals of Cleveland.

To receive references via fax, call (800) 239-4684 and request document #93. (Have a fax number ready.)

 

 

TABLE 4 Pearls to Fitting Post-surgical Eyes

 

  • GPs can be fit as early as three months after PK as long as all sutures are covered by epithelium and all knots are buried. If a suture breaks or is removed to control astigmatism, wait at least three weeks for resultant topographic changes to stabilize prior to GP fitting or re-fitting

  • Symptoms of loose or broken sutures are constant foreign body sensation, redness and tearing. Inform patients that sutures are still present so that possible suture breakage may be reported early

  • After PK, if astigmatism is minimal and localized to the central 5.0mm of the graft, it may be possible to vault the entire astigmatic area with a standard spherical lens design

  • After PK, if astigmatism is regular and expands into the mid-periphery, the cornea is classified as "mixed prolate and oblate," and a bitoric lens design is indicated

  • Elevation maps are very useful in predicting the appearance of what a GP fluorescein pattern may look like on a given cornea without having to interpret curvature maps. On an elevation map, high (red) areas will always displace fluorescein and the low (blue) areas will always pool fluorescein

  • Screen for crossing incisions after radial keratotomy. Because crossing corneal incisions produce excessive corneal flattening and focal irregularity, the GP fitting is complicated by lens impingement at the elevated pivot points created at the intersecting sites

  • Use tangential (instantaneous or true) corneal topography maps to interpret the corneal shape. The tangential algorithm highlights the local curvature better and draws attention to the rapid changes in curvature. Use axial corneal topography maps to choose your posterior GP curves. Axial curvatures (which are "averaged" spherically-biased representations of corneal shape) should provide the most relevant data for posterior GP curvature selection

 


Contact Lens Spectrum, Issue: May 2003