Irregular Astigmatism, Part 1
This first article in a three-part series examines management with small-diameter corneal lenses.
By Susan Kovacich, OD
A young woman comes into the office, not wearing her glasses or contact lenses. She has not had an eye examination in years. She reports that her glasses “do not work” and that she stopped using them long ago. Her rigid contact lenses became so uncomfortable that she eventually stopped wearing those, too. Entering uncorrected visual acuities are 20/400 in both eyes. The patient denies a history of keratoconus or other eye condition, surgery, or trauma. She is having trouble taking care of her children, she cannot drive, and she is frustrated with her quality of life. But, before she leaves, she will be presented with options that will improve her vision significantly and give her a new outlook on life—all because of contact lens alternatives that can better address her previously undiagnosed irregular astigmatism due to keratoconus.
Regular astigmatism by definition has meridians of curvature that are regular and at right angles from each other (90° apart). Depending on the amount, regular astigmatism is relatively straightforward to correct with spectacles, soft toric lenses, or conventional GP lenses.
Irregular corneal astigmatism, however, is much more difficult to correct. Additionally, if the vision is significantly impacted, patients usually require a GP lens and may require a special design to not only correct vision, but to remain on the eye and to maintain corneal health. GP lenses leverage both the smooth optical surface that a rigid lens provides and the tear lens, which compensates for the corneal irregularity. As an additional complication, many patients are intolerant of corneal GP lenses due to discomfort from the lens interaction with the upper eyelid (Polse et al, 1999).
Irregular astigmatism should be ruled out when vision cannot be corrected to 20/20 with spectacles, the ocular media is clear, and the retina and optic nerve appear healthy. Often, an irregular (“scissors”) reflex is observed during retinoscopy, or a patient has monocular diplopia complaints (Snellen letters appear doubled, or they have shadows or tails.)
Irregular astigmatism should also be suspected if a patient has a history of corneal surgery or trauma. Keratometric mires are often distorted. Corneal topography is more useful in making the diagnosis of irregular astigmatism, as a keratometer measures only a small area of the central cornea, so off-axis corneal irregularities can be missed.
Contact Lens Correction Options Irregular astigmatism can be managed by a variety of different lens designs. This three-part series will cover small-diameter corneal lenses used in keratoconus, large corneal lens designs, scleral lenses, piggyback systems, hybrid lenses, and specialty soft lenses. While it is true that scleral lenses are being increasingly employed to manage irregular corneas, not all patients can be fit with or can afford scleral lenses. Additionally, all of the different modalities have a place in managing irregular astigmatism. Lens selection will be determined by the location and severity of the condition and by the ability of the patient to tolerate and handle the lenses, lens care, and cost.
Causes of Irregular Astigmatism
Irregular astigmatism can result from corneal degenerations, corneal surgery, or trauma.
Degenerations These include keratoconus, pellucid marginal degeneration, and keratoglobus. Keratoconus is one of the more common degenerations and often requires a specialty GP contact lens design. It is usually diagnosed in adolescence to the early 20s (Marcos et al, 2001). Patients will often complain of blurred vision that is not corrected by glasses and will have high amounts of astigmatism or astigmatism that is rapidly changing.
This painless, progressive corneal ectasia (thinning) results in a bowing forward of the cornea, resulting in a cone-shaped deformity. As the disease progresses, iron may deposit at the base of the cone, (Fleischer ring), and vertical tension lines may form at Descemet’s membrane (Vogt’s striae), which are pathognomonic of the condition. Keratoconus can progress rapidly, often resulting in 20/400 vision. The elevation of the cone makes conventional GP lenses difficult to impossible to fit.
Corneal collagen cross-linking (CXL), which is not yet approved by the U.S. Food and Drug Administration (FDA), is expected to change the progression and management of the disease. This procedure utilizes ultraviolet light and riboflavin to stiffen the cornea and prevent further development of the disease.
Pellucid marginal degeneration (PMD) is a pathology that affects older patients and results in a thinning of the inferior margin of the cornea, which also makes standard GP lenses difficult to fit.
Keratoglobus, which is a steepening of the entire cornea, is a rare congenital condition. Any other corneal disease that results in an irregular corneal surface may require a GP lens to correct the vision adequately.
Corneal Surgery This can also result in irregular astigmatism. Penetrating keratoplasty (PK or PKP), in which a donor button or graft of corneal tissue is sewn into the recipient’s corneal tissue, will often be irregular where the button is sutured to the donor tissue. Grafts can be elevated, sunken, or tilted, inducing astigmatism. Corneal transplantation has been the treatment of choice in end-stage keratoconus, cases in which contact lenses could not be successfully fit, or when scarring was significantly impairing vision.
Partial thickness or lamellar corneal surgeries, such as deep anterior lamellar keratoplasty (DALK), disrupt the corneal surface less compared to PK and result in smaller amounts of irregular astigmatism; these are being used more in the management of keratoconus and other corneal conditions. Refractive surgery on myopes flattens the prolate cornea and creates an oblate surface that may be a challenge to fit and requires a reverse curve GP design.
Radial keratotomy (RK), photorefractive keratectomy (PRK), and laser in-situ keratomileusis (LASIK) all disrupt the cornea in different ways. RK has not been performed since the early 1990s, but it can result in a strong hyperopic shift and daily fluctuations in vision that make vision correction a challenge. Myopic refractive surgeries can result in corneal ectasia and irregular corneas. All of these refractive surgeries can also cause dryness from severing corneal innervation, induce aberrations by changing the prolate nature of the cornea (Olivares Jiménez et al, 1997), and cause glare. Decentered procedures and residual central islands are especially problematic to fit.
Corneal Trauma This can permanently disrupt the cornea and can also result in irregular astigmatism. Corneal laceration, which requires sutures, will cause disruptions in the corneal structure. Thinning of the cornea and scarring from infections—such as herpes simplex, herpes zoster, and microbial keratitis—also cause corneal warpage. Corneal scarring in a young child, which causes irregular astigmatism, may result in amblyopia.
Small Rigid Corneal CLs to Correct Irregular Astigmatism in Keratoconus
Knowing where the steepest area of the cornea is located, the degree of steepening, and how large the affected area is will dictate what type of lens or lens system will be used to manage an irregular cornea. Rigid corneal contact lenses (overall diameter: 8.0mm to 12.5mm) (van der Worp et al, 2013) will tend to center on the steepest area of the cornea.
As previously discussed, keratoconus results in a particular type of irregular cornea, which can require special rigid corneal designs. Generally, bitoric rigid contact lenses, which are quite successful for correcting regular astigmatism, are not indicated for managing keratoconus and other irregular corneas. Historically, small-diameter (less than 9.8mm), steep corneal lenses have been used to manage keratoconus, a technique that is most effective for small, steep, centrally located (“nipple”) cones (Figure 1).
Figure 1. A small, central cone.
Keratoconic corneas tend to be highly eccentric, changing rapidly in elevation from the steepest part of the cone to the flatter base of the cone, which causes problems with GP corneal contact lens fitting. One complication can be poor apposition of the contact lens at the inferior portion of the cone, or inferior edge standoff. The lower lid can dislocate the lens at this area of standoff, resulting in a displaced lens on the anterior surface of the eye, or worse, a lens that is ejected from the eye.
To avoid this, the overall diameter (OAD) of early corneal lenses was made smaller so that the area of standoff was eliminated or minimized. As rigid contact lens designs were improved, multiple flat peripheral curves were also employed to help spherical GP lenses position and align better on the eye.
Highly eccentric keratoconic corneas can also result in bubbles that form under the lens between the apex and base of the cone. As contact lens manufacturing progressed, back-surface aspheric lens designs were created to better align to the cornea. The optical zone diameter (OZD) was also reduced in some keratoconic designs to prevent bubble formation at the base of the cone.
Today, manufacturing techniques have improved so much that a rigid lens can be designed with different base curves in one or more quadrants, and/or with different peripheral curves in different quadrants, to align to different areas of the cornea and avoid inferior standoff and fitting problems such as touch or bubbles.
Corneal topography has facilitated the fitting of keratoconic corneas (especially with software that assists in empirical fitting), but it is not required. If a small-diameter corneal lens can be successfully fit, it is between a flat-fitting lens and a fit that is too steep. The lens should not fit too flat (touch/bear on the apex of the cone), which can result in pain and/or corneal abrasions and scarring.
A lens that is too steep with too much apical clearance over the cone, or in which bubbles form under the lens, is also unacceptable. Visual acuity tends to decrease with too much apical clearance. “Feather touch,” in which a light area of bearing is observed and fluorescein can be observed flowing up under the lens with the blink, is considered acceptable.
In general, small-diameter lenses are fit with either a “three-point-touch system” or with minimal apical clearance. With three-point touch, there is light bearing in the periphery nasally and temporally (two points) and at the apex of the cone (third point) (Figure 2). Anterior segment optical coherence tomography (OCT) can be useful to make sure there is sufficient clearance of the cone. The lens should be fit with minimal inferior edge lift or standoff, should move well with the optical zone over the visual axis, and should not be dislocated with eye movement.
Figure 2. Small-diameter lens with three fits: Apical touch or flat fit (A); “Feather Touch” or three-point touch (B); and apical clearance or steep fit (C).
Because of the interaction with the upper eyelid, there is more initial discomfort with corneal GP lenses. With a reasonable adaptation period—usually about one to two weeks—the lens should be comfortable for most of the day. Patients who cannot adapt to this modality will need to be refit with one of the lenses/lens systems that minimizes upper eyelid interaction.
Let’s take another look at the keratoconus patient described at the beginning of the article. She exhibited many of the classic signs and symptoms of keratoconus patients even though she denied being diagnosed with the condition. She had gone through several pairs of glasses that didn’t seem to work, she had been fit with GP lenses that at some point had become too uncomfortable to wear. Although she was a young patient, her vision was reduced and could not be corrected to 20/20 with spectacles, no reflex was seen with retinoscopy, and she refracted with a high amount of myopia and some astigmatism. Topography measurements in the right eye were very steep and consistent with keratoconus, and her left cornea was so irregular that topography measurements couldn’t be obtained. She had small, steep cones that were slightly inferior of the visual axis. She had Fleischer rings and Vogt’s striae in both corneas.
In the end, her vision was corrected by small (8.3mm), steep, specialty corneal GP contact lenses, and her quality of life (and the quality of life of her family) improved significantly after she was successfully fit. She corrected to 20/25- OD and 20/40- OS. The lenses moved and centered on the cones with three-point touch. After a week, the patient could wear the lenses comfortably for 12 to 14 hours a day. She was thrilled with her vision, and she felt that she was a better mother to her children and was “really living life again.” Her affect was significantly improved, and she appeared to be much more excited about life.
There are many contact lens tools available for managing irregular cornea patients. While there has been a well-merited increasing emphasis on scleral lenses, with these patients we must still remember that traditional small-diameter GP lenses are still an option, notably with keratoconus patients exhibiting a nipple-type cone.
The second article in this series, which will appear in the May issue of Contact Lens Spectrum, will focus on managing irregular corneas with large corneal and scleral lenses. CLS
For references, please visit www.clspectrum.com/references and click on document #245.
Dr. Kovacich graduated from Indiana University (IU) School of Optometry and completed a hospital-based residency at the St. Louis VAMC. After practicing for 10 years, she returned to IU and is an associate clinical professor in the Cornea and Contact Lens Service. She was a consultant to Alcon and has received travel funding from Alcon, Bausch + Lomb, Johnson & Johnson Vision Care, Inc., and CooperVision.