Here’s what you should know about the options for diagnosing and treating keratoconus.
Keratoconus affects a small segment of the U.S. population—an estimated 1 in 2,000, according to the National Eye Institute—but those individuals with the disorder experience blurring and distortion and, in some cases, may eventually require corneal transplant. When the OD asks you to assist in the diagnosis of this disorder, you need to know why and what to expect.
Thinning and Bulging
In keratoconus, the cornea progressively thins and starts to bulge slightly in the center, eventually taking on a cone shape.
Keratoconus is bilateral, but it is usually asymmetrical, with one eye more involved that the other. It may be so asymmetrical that only one eye needs to be treated.
In its earliest stages, keratoconus causes slight blurring and distortion of vision and increased sensitivity to light. These symptoms usually appear when individuals are in their late teens and early 20s. Keratoconus may progress for 10 to 20 years and then slow or stabilize, usually when individuals are in their early 30s.
A symptom of keratoconus is a painless decrease of vision over time. It may manifest as intolerance to contact lenses or inability to wear soft lenses.
The cause of keratoconus is unknown. Patients may have a family history, yet the disease is not inherited. Also, keratoconus is not associated with other serious ocular or systemic conditions.
Although the cause of keratoconus is unknown, it is easy diagnose and treat. A corneal topographer provides different maps to show changes indicative of keratoconus.
Keratometry, or K, readings are another important part in diagnosing keratoconus. If the mean K is less than 50.00D, the cone can be considered early stage. A mean K-reading of 50.00D to 55.00D is advanced, a reading greater than 55.00D is severe.
In the past, optometrists treated keratoconus in the following order: eyeglasses, gas permeable contact lenses and corneal transplant. Today, there are more treatments available.
Eyeglasses or soft contact lenses may be used to correct the mild nearsightedness and astigmatism that occur during the early stages for keratoconus. As the disorder progresses and cornea continues to thin and change shape, gas permeable contact lenses become the preferred treatment. The gas permeability enables these lenses to vault over the cornea, replacing its irregular shape with a smooth, uniform refracting surface to improve vision.
Fitting contact lenses on a keratoconic cornea is challenging and time-consuming. Patients often must make follow-up visits to fine-tune the fit, especially if the keratoconus continues to progress. However, there have been many developments in contact lenses for individuals who have keratoconus.
Custom Soft Lenses
Custom soft contact lenses for correcting mild-to-moderate keratoconus are made-to-order based on detailed measurements of the individual’s keratoconic eye(s). For some individuals, these may be more comfortable than gas permeable or hybrid contact lenses.
Custom soft contacts for the correction of keratoconus include KeraSoft IC from Art Optical Contact Lens Inc. and NovaKone Lenses from Alden Optical, a division of Bausch + Lomb Specialty Vision Products. KeraSoft IC is a high-water-content silicone hydrogel lens with a power range of –20.00D to +20.00D and cylinder powers of –0.25D to –12.00D. NovaKone lenses are medium-water hydrogel lenses and come in powers of –30.00D to +30.00D and cylinder powers up to –10.00D. Both lenses have an extensive range of fitting parameters for a customized fit and are typically larger in diameter than conventional soft lenses for greater rotational stability on a keratoconic eye.
Custom soft toric lenses are an option for part-time wear for individuals with keratoconus who cannot tolerate wearing gas permeable lenses full-time. For example, an individual might wear GP lenses during work and while driving and the soft lenses during leisure activities or as a short break from the GP lenses.
Hybrid contact lenses
Hybrid contact lenses are designed to provide the crisp optics of a gas permeable contact lens and the wearing comfort of soft lenses. The UltraHealth, SynergEyes KC and ClearKone hybrid contact lenses, manufactured by SynergEyes, combine a highly oxygen-permeable rigid center with a soft peripheral skirt.
UltraHealth uses silicone hydrogel material for the soft lens skirt, providing greater oxygen transmissibility than SynergEyes KC’s and ClearKone’s hydrogel skirt. UltraHealth, SynergEyes KC and ClearKone are available in a wide range of parameters to fit the irregular shape of a keratoconic eye.
These lenses were designed specifically for keratoconus, with the central GP zone of the lens vaulting over the cone-shaped cornea for increased comfort.
Scleral and Semi-scleral Lenses
These are large-diameter gas permeable lenses in which the periphery and edge of the lens rest on the sclera of the eye. Scleral lenses cover a larger portion of the sclera, whereas semi-scleral lenses cover a smaller area.
Because the center of scleral and semi-scleral lenses vaults over the irregularly shaped cornea, these lenses don’t apply pressure to the eye’s cone-shaped surface, allowing for a more comfortable fit. And, these larger lenses are more stable than conventional gas permeable contact lenses, which move with each blink because they cover only a portion of the cornea.
In the past few years there have been new treatments for keratoconus in the hope of delaying and possibly preventing the need for corneal transplant.
In 2004, the FDA approved Intacs, which Addition Technology, Inc. acquired from KeraVision, for treating keratoconus and for which OASIS Medical, Inc., is the distributor for the U.S. The surgeon inserts these polymethylmethacrylate, or PMMA, segments in the periphery of the cornea to help reshape the cornea for clearer vision. Intacs may be needed when patients no longer can obtain functional vision with contact lenses or eyeglasses.
Studies have shown that Intacs can improve best spectacle-corrected visual acuity of a keratoconic eye by an average of two lines. Also, the implants can be removed and exchanged. Intacs might delay but can’t prevent a corneal transplant if keratoconus continues to progress.
More recently, Avedro, Inc., received FDA approval in 2016 for Photrexa Viscous and Photrexa riboflavin solutions and the KXL System for corneal cross-linking, or CXL, in patients with keratoconus and with corneal ectasia following refractive surgery. In CXL, the doctor instills the drug formulations onto the ocular surface, and then applies the ultraviolet light to strengthen corneal tissue. This, in turn, halts the bulging of the eye’s surface.
There are two different of corneal cross-linking: FDA-approved epithelium-off cross-linking, in which the outer layer of the cornea is removed to allow penetration of the drug; and epithelium-on (also called transepithelial crosslinking), in which the epithelium is left intact. The epithelium-on method requires different drug formulations and often, more time for the drug to penetrate the cornea, but potential advantages include less risk of infection, less discomfort and faster visual recovery. Epithelium-on CXL has not been approved by the FDA.
The most important thing to remember about keratoconus is that it is treatable and most patients enjoy good vision.
Roberta Beers, CPOT, is an optometric assistant in Erie, PA.
WHERE TO FIND IT:
Addition Technology, Inc. 847.297.8419 | AdditionTechnology.com
Alden Optical, Inc. 800.253.3669 | AldenOptical.com
Art Optical Contact Lens, Inc. 800.253.9364 | ArtOptical.com
Avedro, Inc. 844.528.3376 | Avedro.com
SynergEyes, Inc. 877.733.2012 | SynergEyes.com