Management of Irregular Corneal Astigmatism
By John Mark Jackson,
Contact lenses offer patients with irregular astigmatism hope for better vision.
Most astigmatism is defined as regular; that is, the minimum and maximum powers in the refraction are perpendicular. Correction with spectacles or conventional contact lenses poses no significant problem for these patients. Irregular astigmatism presents a much greater management challenge for practitioners.
Irregular astigmatism can also encompass irregularities in the surface shape of the cornea, and can result from a variety of causes, including degenerative processes (keratoconus and pellucid marginal degeneration), surgical (corneal transplants or refractive surgery), infectious (bacterial or viral ulcerative keratitis) or trauma. The resulting corneal irregularities can preclude adequate correction with spectacles, but contact lenses offer hope for better vision in many of these patients. The types of treatment options available vary depending on the etiology.
Keratoconus is probably the most common degenerative cause of corneal irregularities. The well-known inferior corneal steepening results in distorted vision and sometimes multiple images. In mild cases of keratoconus, when spectacle acuity is still relatively good, soft spherical or toric lenses can provide adequate visual improvement. As soft lenses drape over the cornea, however, they cannot effectively mask the corneal irregularities in more advanced cases.
Most commonly, keratoconus is managed with rigid gas permeable (RGP) contact lenses. The rigid lens surface allows tears to collect under the lens and fill in the irregularities, effectively improving the patient's vision where glasses alone cannot. Practitioners espouse a number of fitting philosophies for keratoconus, but the most common approach is to achieve a bulls-eye pattern or three-point touch. This method appears to balance the weight of the lens evenly across the cornea and not place too much pressure on the fragile cone apex. Small lens diameters and optic zone diameters are the norm for these lenses. Many contact lens laboratories have engineered specially designed lenses for keratoconus, including the ComfortKone from Metro Optics and the Rose K lens from Lens Dynamics. These lenses may offer simplified fitting procedures for these patients.
Fluorescein pattern of three-point touch keratoconus RGP
Not every keratoconic patient is able to wear RGPs. Due to the irregular nature of the keratoconic cornea, fitters may encounter difficulty in achieving adequate lens centration. Some patients develop painful abrasions from excessively flat fits or from overwear of their lenses. These patients may benefit from piggybacking, in which a soft lens is worn beneath an RGP. This method usually improves both comfort and centration. The drawbacks of this technique include increased complexity of lens care (two systems instead of one) and decreased oxygen delivery to the cornea through the doubled system. Using high Dk RGP materials and either high water content or newer silicone hydrogels for the soft lens can decrease hypoxic stress to the cornea.
Other options for keratoconics include hybrid lenses such as the SoftPerm lens from Wesley Jessen. This lens, with its rigid center and soft "skirt," provides many patients with the optics of an RGP and the comfort of a soft lens. The relatively low Dk of this lens and somewhat limited parameters restrict its usefulness. In addition, the MacroLens, a semi-scleral RGP lens from C & H Contact Lens, may be an option for some patients. Specialty Ultravision has recently received FDA approval to market EpiCon, a large diameter rigid lens made of carbosilfocon material which the company claims to have high oxygen transmissibility and excellent comfort.
|Fluorescein pattern of an RGP lens on a
||Fluorescein pattern of an RGP lens on a post-penetrating keratoplasty
Case 1: Keratoconus
C.B. was referred to our clinic for evaluation after his optometrist diagnosed keratoconus. He had worn soft lenses successfully for several years, but had noticed his vision steadily worsening over the last several months, which he attributed to needing a boost in his prescription. His entrance acuities with his previous highly myopic spectacle prescription were 20/80 (PH 20/30) OD, 20/60 (PH 20/30) OS. No significant change in acuity was found with a new refraction. Corneal topography showed significant steepening inferiorly OD>OS and very distorted ring images. Slit lamp findings included classic signs of keratoconus including vertical striae and Fleischer's ring in both eyes. We confirmed the diagnosis of keratoconus and proceeded with RGP fitting.
We chose our first base curve for trial fitting based on the steep simulated K reading from topography; in this case, we used a 5.6mm (60.25 diopters) radius OD and a 6.4mm (52.75 diopters) radius OS. Both lenses were too steep centrally, vaulted the cone and had no central touch. We tried flatter lenses in 0.1mm increments until minimal touch was achieved. The final base curve selection was 5.9mm (57.25 diopters) OD and 6.9mm (49.00 diopters) OS. We stayed with the small 8.6mm diameter of our trial lenses. We used a 6.0mm optic zone OD and a 6.5mm optic zone OS. A good rule-of-thumb is to make the OZ diameter similar to the base curve radius; this small optic zone allows for a wide peripheral curve system to align with the more normally shaped cornea outside of the steep cone area. We chose a three peripheral curve system with heavy blending to maximize comfort and peripheral alignment. Lens powers based on over-refraction were 16.25D OD and 10.25D OS. High minus powers are typical for moderate to advanced keratoconus.
When the patient returned for dispensing, acuities with the new lenses were 20/30 OD and 20/20 OS. The lenses centered well, provided a light three-point touch pattern and allowed for good tear exchange. On follow-up visits, he was doing well with the lenses and was wearing them for up to 12 hours a day with good comfort.
Pellucid Marginal Degeneration
Pellucid Marginal Degeneration (PMD) is a less common form of corneal degeneration than keratoconus but still produces corneal thinning and irregular astigmatism. The corneal thinning occurs in a narrow, arcuate band in the inferior portion of the cornea, usually more peripherally than keratoconus. The central cornea usually retains its normal thickness but is elevated, causing irregular against-the-rule (ATR) astigmatism. PMD is often misdiagnosed as keratoconus, especially in the early stages, and it is possible to have a combination of the two disorders in the same eye, further complicating diagnosis.
Fluorescein pattern of an RGP lens on a post-penetrating keratoplasty
As with keratoconus, there is no accepted standard for fitting contact lenses in PMD. Previous reports have advocated using aspheric RGPs to fit these highly irregular corneas. You may also achieve success with bitoric RGPs. While bitorics are usually not indicated in irregular astigmatism, they can work in PMD. Although the topography is irregular, the central pattern is often a somewhat regular "butterfly" or bow-tie pattern, similar to a non-diseased eye with high ATR astigmatism. Bitoric lenses can often provide an improved fitting relationship.
Post Penetrating Keratoplasty
Patients who undergo penetrating keratoplasty, or corneal transplantation, are often left with irregular astigmatism due to the irregular corneal topography induced by the procedure. Significant improvements in surgical techniques over the years have helped to minimize these effects, but it remains that these patients are often in need of contact lens correction. While soft sphere or toric lenses can achieve good visual results in mild cases, the risk of complications such as corneal vascularization and graft rejection increases due to the relatively low oxygen transmissibility of the lenses. RGP lenses are usually the best option for these patients.
Assess corneal topography prior to fitting. Numerous corneal shapes can result from the graft procedure, including a normal shape with steeper center and flatter periphery, a drum-head shape with a flatter center and steeper periphery and tilted shapes with no regularity to the pattern. A large diameter is usually used (in the 9.5mm-12.0mm range), partly due to the irregular nature of the pattern and also to cover the entire graft area of the cornea. The normal shape is the most straightforward to fit because of its resemblance to standard corneal topography; a spherical lens often will provide a satisfactory fit. You can fit the drum-head shape with a spherical lens, which would produce a large degree of apical clearance with alignment in the midperiphery. If peripheral seal-off occurs with a spherical lens, or central vaulting appears excessive, a reverse geometry lens may improve the lens-to-cornea relationship. The tilted or otherwise irregular contour provides an additional challenge. Choose an even larger diameter to obtain centration. A rule-of-thumb previously proposed: achieve a balance of touch and clearance in the fluorescein pattern, using one-third touch and two-thirds pooling. This will spread the weight distribution of the lens across the cornea and provide the best fit possible.
Case 2: Post Penetrating Keratoplasty
G.C. underwent a PKP on his right eye in 1982, secondary to advanced keratoconus. He has mild keratoconus in the left eye as well. As can be seen from his topography map, the graft is irregularly shaped, with no distinct pattern of central steepness or flatness. He had been wearing RGPs in both eyes for many years but was not satisfied with the fit of his right lens. It moved excessively and was becoming more and more uncomfortable for him. The left lens was comfortable and provided adequate vision, and he did not want to change it.
Acuities with his previous RGPs were 20/20 OD and 20/25 OS. His previous right lens had a base curve of 7.4mm (45.62 diopters) and a 9.2mm diameter. The lens decentered nasally and fit very flat with heavy bearing centrally. We steepened the lens and increased the diameter to improve centration and to spread the bearing of the lens over a larger area of the cornea. We chose a spherical tricurve trial lens with a base curve of 7.0mm (48.25 diopters) and diameter of 10.0mm. This lens achieved our goals; it centered much better and provided a good balance between touch and pooling. In fact, his elevation map gives a good representation of his lens fit. The red areas, corresponding to higher areas of elevation, showed touch, and the blue areas, corresponding to relatively lower areas, showed pooling. Elevation maps can be useful tools for envisioning how the fluorescein pattern may look over an irregular cornea.
We ordered an RGP with similar parameters to the trial lens in a high Dk material with his prescription. The new lens centered well, provided the expected even distribution of bearing on the cornea and provided visual acuity of 20/20 in his right eye. Comfort was greatly improved for him as well.
Post Refractive Surgery
While the majority of refractive surgery patients now achieve a successful outcome (however that may be defined), there are many patients for whom spectacles cannot fully correct their residual refractive error. This can be the result of corneal irregularities from a decentered treatment zone and a myriad of other factors. Contact lens correction may provide enhanced visual performance in these patients. This can be a difficult management challenge, as these patients possibly sought refractive surgery to avoid wearing contact lenses and glasses in the first place.
Soft lenses can succeed with some patients, particularly PRK or LASIK patients. Standard soft spheres and torics can be useful, as well as the newer aspheric soft lenses that reduce spherical aberrations, such as the Frequency 55 Aspheric from CooperVision or the Specialty Choice AB from Specialty Ultravision. Soft lenses should be avoided in RK patients, as they can induce vascularization into the RK scars.
If inadequate vision results from a soft lens, RGPs are indicated. The corneal topography of PRK and LASIK patients is similar to the drum-head shape described with PKP patients. The laser-ablated central cornea is flatter than the periphery. Use a large spherical design, or try a reverse geometry design if excess vaulting and seal-off result from a spherical lens fit or if the patient does not attain good centration. The MacroLens may be useful in cases where RGPs were not successful due to comfort issues.
Various topographical maps of Patient G.C.'s post-penetrating keratoplasty cornea.
Infection and Trauma
Irregular astigmatism can also result from infectious keratitis, such as herpetic keratitis and corneal ulceration, and corneal trauma. As the cornea heals in these cases, scarring can occur, resulting in a distorted corneal topography. The scarring can produce irregularities near the actual site of the lesion and can also induce distortion far across the cornea.
As always, the goal is to provide improved visual performance while maintaining good ocular health. Due to the etiology in these cases, corneal health has already been compromised to some degree, and contact lens fitting should be approached carefully to avoid further insult. With RGPs, attempt to vault the damaged area without bearing too harshly on the surrounding normal tissue. This may be difficult owing to the distorted contour of the cornea. Use high Dk materials in these cases to maintain corneal health as much as possible.
Fitting patients with irregular astigmatism can be an intimidating process if you do not regularly fit these patients. Your local CLMA laboratory and the RGP Lens Institute are great resources for practitioners. Video fitting guides are available, and most labs are willing to loan a trial set or two for those difficult fits. With patience and persistence, and some trial and error, you can improve your confidence with these fittings, as well as significantly improve the quality of life of your patients with irregular astigmatism.
Beth Ann Benetz, MA, CRA, and Steven Gushue, CRA, CLEK study photographers, contributed images.
Dr. Jackson is a second-year cornea and contact lens graduate fellow at The Ohio State University College of Optometry. He is currently studying the effects of overnight
Contact Lens Spectrum, Issue: December 2000