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- To Bi- or Not To Bifocal for Keratoconus
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Corneal thinning, resulting in irregular astigmatism, visual distortion, and deteriorating visual acuity that cannot be redressed with conventional glasses or lenses, is the hallmark of the pathologic condition known as keratoconus. Stromal thinning results in corneal steepening, most commonly centrally (keratoconus) and in some cases peripherally (pellucid marginal degeneration). The progressive ectasia associated with this thinning usually advances from around puberty to presbyopic age, with active and quiet periods of progression and incremental deterioration all along. Hormonal, hereditary, and environmental factors have been implicated in the condition’s etiology.
In eyes with keratoconus, restoration of optical surface quality has been the principal means to restore acceptable to good corneal optics, resulting in restoration of functional visual acuity. In recent years, corneal collagen cross-linking has been used to remedy corneal weakness by strengthening the bonds in the corneal collagen matrix. Cross-linking is widely used in Europe and one company’s system has just recently been approved for us in the United States.
IMPROVING THE SURFACE
The first observation of vision improvement as the effect of bypassing corneal surface irregularity is attributed to Young in the early 19th century. Late in that century, Herschel envisioned using a glass scleral contact lens to mold the cornea. In the mid-20th century, the field of contact lenses exploded with the availability of rigid plastic material. Designs began with scleral lenses, then transitioned to corneal lenses thanks to the innovation of Tuohy in the 1940s, and now the pendulum is swinging back toward scleral lenses. Scleral lenses became practical again, physiologically speaking, with the advent of rigid gas permeable (RGP) material with high oxygen transmissibility.
Soft lenses have not been found to be suitable for use in keratoconus because they cannot establish a regular front corneal surface to improve optical quality. Thicker soft lenses, introduced to overcome this problem, have met with limited success.
The use of hybrid lenses has been suggested for people with heightened corneal sensitivity or poor lens centering. Hybrid lenses are made with an RGP lens centrally surrounded by a soft skirt lens to provide centering and comfort, all in a seemingly seamless pattern based on a patented technology.
The concept of hybrid lenses arose from the so-called “piggyback” mode of fitting. In piggyback fitting, as proposed by Baldone in the 1970s, an RGP lens is placed on top of a soft lens to achieve better centering, stability, and comfort. In some cases, the soft lens can instead be draped over the RGP lens in a reverse-piggyback fashion. Patients sometimes come up with this idea independently through happenstance or improvisation. Another variation is a soft lens with a cutout centrally to accept an RGP lens.
The presbyopic population worldwide in 2005 was estimated to be 1.04 billion.1 The 2010 US Census estimated the number of Americans aged 40 to 64 years to be 102,380,409, with another 12,435,263 people in the age bracket 65 to 69 years. Clearly, the population of presbyopic patients is not small by any means.
Along with the rest of the population, the population of keratoconus patients is growing older, and many are becoming presbyopic. Estimates of the prevalence of keratoconus vary from 1 in 500 to 1 in 2,000. Many of these patients would welcome a bifocal contact lens suitable for keratoconic eyes.
The design of lenses for keratoconus has to take into account an unusually steep, thinned, pathologic area of the cornea surrounded by a flat regular cornea. All popular and successful designs must conform to this irregular corneal surface map, fitting steep and flat curves where mandated. This limits the options available, optically, for designing a successful bifocal keratoconus lens.
A bifocal design with some chance of success in keratoconus would be the widely accepted design used today, with a concentric pattern of foci: distance vision in the center surrounded by a near-vision annulus applied only to the front surface of the lens.
A CASE EXAMPLE
A 44-year-old woman presented to my practice in August 2014 with a history of RGP contact lens wear for more than 30 years. She had red, irritated eyes and reported that she was struggling visually in a profession with high visual demands. She was using lubricant eye drops (Refresh Tears; Allergan) regularly, a few times daily.
During history-taking, she reported a complex recent history of contact lens wear. She had been seen at another center in December 2013. At that time, she was wearing RGP lenses with the following parameters: 9.0 mm diameter; 7.5-mm optical zone in each eye; prescription OD was -5.75 D and OS was -4.25 D.
Her refraction at that time was -5.25 -3.25 x 35 OD, yielding visual acuity of 20/40-2, and -3.00 -3.75 x 125 OS, yielding visual acuity of 20/50+2. She was diagnosed with keratoconus and was brought back to this other center in early January 2014 for refitting. Figure 1 shows the Nidek topography measurements that we obtained from the previous practitioner, dated January 13, 2014. Synergeyes hybrid contact lenses were tried at that time but not ordered.
That practitioner prescribed RGP lenses with the following parameters: base curve 6.30, diameter 8.00 mm, prescription -8.00 D OD; base curve 6.50, diameter 8.20, prescription -7.00 D OS. These lenses were fitted piggyback over Oasys (Acuvue; Johnson & Johnson Vision Care) 8.4 mm, -1.00 D soft lenses in April 2014. Figure 2 shows her topography dated March 24, 2014, the latest Nidek topography we obtained from her previous practitioner.
Upon presentation in September 2014 at our clinic, she had 2+ conjunctival injection in each eye; visual acuity was 20/30 in each eye at distance and near. Engorged vessels were evident perilimbally but not invading the cornea. For an unknown reason, the patient was wearing a double piggyback lens OD: a soft lens on the cornea and another soft lens over the keratoconic RGP lens. She was wearing a single piggyback OS: a soft lens under an RGP lens. She was using artificial tears on a regular basis, many times per day.
When these lenses were removed, corneal topography showed a cone that was mostly centrally situated. It was decided to fit the patient with simple keratoconic design lenses but with a bifocal design on the front of the lens. Earlier single-focus small lenses were modified to feature a larger diameter than the conventional small keratoconic design. The reason for this was to allow better centering, which is important in a bifocal mode.
The parameters of these lenses, designed by Lens Dynamics using Boston XO material, were as follows:
OD: base curve 6.94, prescription -4.87 D, diameter 9.6 mm with standard edge, reading add +1.50
OS: base curve 6.73, prescription -7.25 D, diameter 9.6 mm with standard edge, reading add +1.50
With these lenses in place, the patient’s distance and reading visual acuities were 20/25 in each eye. Most important, the patient was comfortable. The injection resolved for obvious reasons; namely, because of a simplified monolens fit versus the cumbersome piggyback or double-piggyback lenses she had been wearing previously.
KERATOCONUS LENS DESIGNS FOR OPTIMAL FIT
Keratoconic designs in the 1970s, ’80s, and ’90s mostly revolved around RGP designs of small 8.0 mm to 9.6 mm diameters. Soper and McGuire designs were in mainstream use, and these were followed by Rose K lenses using modern technologies. These worked well with a so-called nipple cone, centrally located around the visual axis.
In the mid-1990s, newer, more complex cone designs appeared, and these were widely accepted. RGP lenses of larger 10.4 mm to 11.6 mm diameters increased the fitter’s ability to better treat patients with pellucid marginal degeneration or after penetrating keratoplasty or refractive surgery. These designs offered less movement on the eye, increased comfort, better centration, and the ability to mask more of the corneal irregularity of keratoconic eyes. The larger diameters provided better comfort as there was less lid exertion on the lens.
Edge contour is crucial to a comfortable fit. An interesting lens design feature is “quadrant-specific Sym technology.” With the proper lathing technology and software, quadrant-specific edge profiles can be achieved. Many times, irregular corneas have different profiles at different points on the peripheral cornea. By increasing or decreasing the edge lift at either 3, 12, 9, and/or 6 o’clock, the best edge contour can be achieved where required by the irregular corneal contour.
Figure 4 shows a standoff at 6 o’clock, and how that can be corrected with a decreased edge lift in that quadrant, allowing the lens to stabilize on the eye. Such unique designs allow adequate fitting where it would not have been possible before. The lathing technology and software to make quadrant-specific edge profiles and quadrant-specific base curves was developed by Lens Dynamics. It was introduced in 2000, and Lens Dynamics was the only lab making these types of lenses at that time. With the advent of high oxygen transmissible gas permeable materials for better physiological health of the ocular tissue, scleral lenses also reappeared, providing another keratoconus fitting alternative.
Many keratoconic patients who require presbyopic correction cannot be fitted with standard bifocal or multifocal contact lenses. By adding multifocal optics to the front of these special keratoconic designs, we can provide near and far vision for many, centration of the lens being critical. Many of these designs have a small front spherical optical zone to provide the best acuity at distance, and then asphericity is added as we move to the periphery to provide near vision additional power.
Concentric multifocal designs are the most widely used with the highest degree of success. The most frequently used design for these soft bifocal lenses features a center near vision zone surrounded by a peripheral distance vision zone. However, center distance designs can be ordered from certain manufacturers.
As in any multifocal design, centration is crucial for good visual results and a reasonably successful fit; however, pupil size plays an important role as well.
Contact lens fitting in keratoconus is becoming easier with various options to offer the patient. Even hard-to-fit patients are regaining hope, especially if they are able to qualify to undergo a cross-linking procedure.
And when old age sets in, bifocal lens options are beginning to be become available in scleral lenses as well as hybrid lens designs. Specialty contact lenses can be ordered for irregular corneas whether due to keratoconus or trauma.
The author acknowledges expert and friendly help from Al Vaske, president of Lens Dynamics.
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Holden BA, Fricke TR, Ho SM, et al. Global vision impairment due to uncorrected presbyopia. Arch Ophthalmol. 2008;126(12):1731-1739.
Kading DL. Piggybacking: a new approach for scleral lens fogging. Contact Lens Spectrum. December 1, 2015.
Marsack J. Keratoconus: many perspectives, a common goal. Contact Lens Spectrum. February 1, 2016.
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Joseph Hallak, PhD, OD, FAAO
• Private practice in Mineola, New York
• Adjunct clinical assistant professor at the State University of New York College of Optometry in New York.
• Financial interest: none acknowledged