Article Date: 4/1/2007

Topographical maps can give you clues about how to best fit challenging contact lens cases.

A Map to Contact Lens Wear Success

BY KENNETH A. LEBOW, O.D., F.A.A.O. Virginia Beach, Va.

The corneal topographer probably represents the single most important diagnostic and fitting tool in the contemporary contact lens practice. While corneal topography was originally developed to identify inappropriate candidates for refractive surgery, its application in the contact-lens practice far exceeds its worth in refractive surgery.

This article reviews some case histories that demonstrate cor-neal topography’s worth in your clinical contact lens practice.

Routine examination

Sometimes, visual acuity doesn’t correlate with the results of your examination. For example, a young, very healthy patient presented with no visual complaints, yet one eye O.D. was only correctable to 20/30. Internal and external examination revealed no retinal pathology or cataracts, but this patient was simply not correctable to 20/20 in spite of a normal correction in the contralateral eye.

A corneal topography map in this circumstance often reveals a subtle deviation in corneal shape, which may account for the patient’s reduced acuity. Although this patient didn’t fit the typical keratoconic profile, the map did indeed reveal an unusual keratoconic-like pattern, which adversely affected his vision. This patient was diagnosed with sub-clinical keratoconus.

Figure 1 (right): Spherical cornea after replacing a high-modulus silicone hydrogel contact lens that was worn inside out.
Figure 2 (left): Difference map demonstrates the rehabilitation effect of a high modulus silicone hydrogel lens in the proper position.

While performing topography at every routine eye examination is often unnecessary, using it in special circumstances like this one, helps explain deviations from the norm and can help you provide clearer educational information to your patients about their individual corneal conditions. Monitoring this change more closely than usual is very appropriate for these cases. Follow-up examinations, including corneal topography, are indicated every six months to evaluate potential changes.

Figure 3 (right): Contact lens-induced corneal warpage with a reversal of customary radial symmetry as a result of inappropriate GP-lens wear.
Figure 4 (left): Difference map demonstrating corneal rehabilitation after fitting a geometrically centered contact lens.

Resolving corneal warpage

Fitting hydrogel and silicone hydrogel contact lenses is relatively straightforward. Most practitioners don’t rely on cor-neal topography fitting software to select the appropriate contact lens base curve for these patients. Certainly topography maps obtained prior to lens application are extremely important to rule out conditions, such as displaced corneal apex, keratoconus, pellucid marginal degeneration and distorted corneas. Their value in the routine cosmetic contact lens fitting (GP or soft lenses), however, is to establish a baseline corneal map from which you can evaluate changes in curvature as contact lens wear progresses. Specifically, look for contact lens-induced corneal distortion, which has been shown to occur in both soft and rigid contact lens wear.

An asymptomatic contact lens wearer presented with a ring compression caused by a silicone hydrogel contact lens. Her visual acuity with her contact lenses hadn’t changed, her wearing time was normal, and she didn’t report any spectacle blur after removing her contact lenses and wearing glasses.

While the PathFinder analysis (proprietary software on the Zeiss Humphrey Atlas topographer) demonstrates ‘normal’ topography, there was central steepening associated with a mid-peripheral area of flattening and a peripheral ring steepening. Since nothing other than the patient’s corneal topography appeared unusual, we applied a new silicone hydrogel lens and asked the patient to return in one week to repeat topography.

Figure 1 represents the new topography map upon follow-up, which demonstrates a more spherical and typical pattern. The difference map (see figure 2) clearly demonstrates the mid-peripheral steepening that occurred during the rehabilitation process. Actually, this patient had worn her high-modulus silicone hydrogel lens inside out and developed an orthokeratologic effect, which was reversed once the lens was placed properly on the eye.

In many cases of corneal distortion, the normal prolate corneal curvature (steeper central curvature) becomes oblate (steep-er peripheral curvature), resulting in a loss of customary radial symmetry. The cornea shown in figure 3 demonstrates marked central flattening and inferior steepening, a moderately irregular surface and an extremely low prolate shape factor. While this may be confused with keratoco-nus, which also shows marked inferior steepening along with superior flattening, the absence of a highly prolate shape factor (SF) confirms the problem is contact lens-induced.

Since the contact lens was displaced inferiorly, the goal of resolving this problem was to center the lens with a steeper base-curve-to-corneal-fitting relationship. We selected a contact lens that had a base curve 1.00D steeper than the patient’s flat keratometry (K) reading and found it would geometrically center over the corneal apex with 2mm of blink-induced lens movement.

After wearing this lens for approximately one month, the patient returned and reported improved contact-lens comfort and longer wearing times. The corneal topography map obtained upon removal of the patient’s lenses revealed a much different picture than the first visit. This time, normal corneal symmetry was present with steeper central and flatter peripheral curvatures. The central cornea had literally ‘popped’ out and returned to normal once the compression from the old contact lens was removed.

Interestingly during this process, a fairly marked amount of central corneal astigmatism manifested. The difference map clearly shows how the new contact lens centered over the cor-neal apex, allowing the shape of the eye to return to normal (see figure 4). The previously steepened inferior cornea flattened, while the previously flattened central cornea steepened. This corneal rehabilitation also resulted in the removal of spectacle blur and enabled the patient to start interchanging an eyeglass prescription in the evening, which allowed the eye to further stabilize.

Keratoconus

One of the single most important uses of corneal topography is the identification of keratoconus. Visually, this is usually described as superior corneal flattening associated with inferior corneal curvature steepening. However, not all corneal topography maps that manifest superior flattening and inferior steepening are indicative of keratoconus. Conflicting maps with similar visual appearance can be displaced cor-neal apex, contact-lens distortion and pellucid marginal degeneration (PMD). An analysis of several, specific corneal indices (shape factor [a variant of corneal eccentricity], corneal irregularity and toric mean keratometry value) aid in differentiating between these similar conditions.

For example, keratoconic corneas typically show moderately high prolate corneal shape factors (SF >0.6), significant corneal irregularity (Corneal Irregularity Measurement [CIM] >2.00), relatively steep flat keratometer curvatures (>45.00D) and elevated mean apical astigmatism (Mean toric keratometry [TKM] >50.00D). Corneas with PMD show very low to oblate corneal shape factors (SF <-0.06), flatter flat keratometer curvatures (flat K = 42.00D) and less mean apical astigmatism (TKM >45.00D). Yet both of these clinical conditions present with a similar visual appearance on a topography map.

You can differentiate cases of contact lens-induced corneal warpage from keratoconus because the former typically have low prolate to oblate shape factor values. Another way these differ from PMD: They feature a prominent oblique, against-the-rule astigmatism. The most difficult differentiation is between true keratoconus and sub-clinical or forme fruste keratoconus. These two conditions have very similar values and may simply represent a progression in the development of the same clinical condition.

Fitting keratoconic corneas is actually a relatively straightforward procedure when using a corneal topographer that’s capable of generating an elevation map. Elevation maps are typically based on a reference sphere. This represents a mathematically generated, best-fit, spherical curvature calculated from the axial map. For non-keratoconic eyes, this value converted into millimeters of curvature is an excellent starting point for GP lens base-curve selection. Since keratoconic eyes usually have steeper curvatures and higher eccentricities, you must adjust the reference sphere using the SF as a new starting point. Usually, the great-er the shape factor, the steeper the base curve adjustment. Figure 5 shows a central or nipple cone in axial view. Figure 6 is an elevation map of the same eye. This map shows the central area of elevation corresponding to the apex of the cone, a reference sphere value of 45.50D and a shape factor of 0.69. We selected a contact lens with a base curve 0.3mm steeper than the convert-ed reference sphere curvature as the initial diagnostic lens for this patient.

Figure 5 (right): Axial map showing central nipple form of keratoconus.
Figure 6 (left): Elevation map of patient shown in figure 5 showing reference sphere and apex of keratoconus.

Penetrating keratoplasty

Figure 7: GP lens for the above patient.

Patients who have undergone corneal transplantation are often the most challenging to fit with contact lenses. Their corneal geometry has been significantly altered by the surgery, and, often, irregular astigmatism precludes their ability to function without GP contact lenses. But selecting an appropriate contact-lens design can often be time consuming and difficult without corneal topography mapping. Topography patterns vary widely. At best, viewing a topography map gives you an approximate starting point from which to design a contact lens. If there isn’t a great deal of astigmatism and the cor-nea shows a normal prolate shape, a spherical or aspheric lens calculated from the reference sphere often provides excellent results. When oblate corneal shapes are present, reverse geometry lenses are typically necessary to achieve lens alignment and centration.

Figure 8: Axial and elevation maps with fluorescein angiography show inferior edge standoff.

A corneal transplant patient was left with an oblate corneal shape (SF = -1.20) with significant corneal astigmatism. A relatively steep, large reverse geometry lens achieved a reasonably aligned base-curve-to-cor-nea fitting relationship, in spite of the excessive corneal astigmatism (see figure 7). When fitting transplanted corneas, your goal should always be to achieve as close to an alignment pattern as possible, realizing that this may not always be realistic, especially in cases of severe astigmatism. Figure 8 demonstrates the axial and elevation maps of a transplanted cornea with just over 20.00D of tor-icity. For cases like this one, unusual fluorescein patterns are quite typical, and problems with edge standoff are the rule rather than the exception.

Corneal topography is vital to a clinical contact lens practice. From monitoring simple hydrogel or silicone hydrogel contact lens fittings for corneal distortion to evaluating keratoconic and transplanted corneas for appropriate lens design, corneal topography simplifies our understanding of corneal geometry and facilitates our ability to visually rehabilitate the associated corneal distortions.

 DR. Lebow operates a private practice in Virginia Beach, Va. specializing in contact lenses. He is a Fellow of the American Academy of Optometry and Past President of the Virginia Optome-tric Association.



Optometric Management, Issue: April 2007