By: Clark Chang, OD, MS, FAAO
Keratoconus is a non-inflammatory, progressive corneal condition associated with corneal thinning, weakening, and steepening, resulting in corneal optical irregularities (cone shape) and poor vision. An incidence rate of 1:2000 has been reported with no known sexual or ethnic predilection. In addition, familial inheritance has been reported in 6-24% of cases and co-morbidities such as atopic disease and connective tissue diseases have also been reported in the literature. Thus, genetic predisposition, acute eye allergies, and eye rubbing (often as allergic response) have all been proposed as potential underlying causes of keratoconus development. However, no definite causative factor(s) have been proven.
As keratoconus progresses in severity, the increasingly irregular shape of the eye causes optical side effects known as higher order aberrations (think of static noise for TV signals). These aberrations are primarily responsible for the reduction in one’s visual functions. Corneal transplant has traditionally been utilized as the choice of medical treatment for the misshapen cornea. However, the reconstructed tissue interface does not always restore the cornea to its normal (spherical) shape, which means contact lenses are still needed to treat keratoconus after corneal transplantation1. Therefore, with the recent advancements in contact lens technology, non-surgical management methods have been recognized as the leading treatment choice in visual rehabilitation for keratoconus patients. Surgical options are currently being preserved for individuals who either cannot tolerate contact lenses or cannot achieve satisfactory vision with contact lenses2.
Keratoconus Contact Lens Treatment Options
Rigid Gas Permeable lenses (RGP) provide good visual outcomes, which make it the most widely utilized method in rehabilitating an irregular corneal surface. The firm structure of an RGP lens allows a layer of tear fluid to form beneath the lens; the tear layer fills in the irregularities between the cornea and the lens, and the combination of the smooth outer lens surface and the tear layer neutralizes the visual distortions. However, the advantages of good visual outcome and ease of lens handling may not always outweigh the amount of time it takes to adapt to RGP lenses and the issues of discomfort and the potential for RGP lenses to “pop out” of the eye. Therefore, alternative non-surgical treatment options have been developed to overcome such issues.
Because most soft lens materials are flexible they drape over the cornea. If the cornea is irregularly shaped, a soft lens will take the shape of the irregularities hence, this treatment option is only used for patients with very mild keratoconus. Nonetheless, recent custom soft keratoconus lens designs employ enhanced lens thickness in an attempt to mimic the optical benefits offered by RGP lenses. While oxygen permeability may be a concern until new material becomes available, the soft keratoconus lens provides a viable alternative for patients with mild to moderate keratoconus who cannot tolerate an RGP lens.
RGP intolerance mainly stems from irritation caused by the lens touching the eye. A carefully selected bandage soft lens can be placed under the RGP lens in a piggyback system, which decreases irritation and helps stabilize the RGP lens on the eye. However, handling the two-lens system can be complex and inconvenient as there is a potential for reduced oxygen supply to the eye3.
Hybrid lens technology has enabled the bonding of an RGP lens with a soft lens so a single lens system offers both improved visual quality and increased wearing comfort. The recent advances in the 4th generation hybrid lens, ClearKone®, utilize a uniquely designed RGP lens shape to expand on its previous fitting parameters to include patients through all stages of keratoconus. Fitting success rates up to 86.9% have been reported utilizing hybrid lens platforms on keratoconus patients.
Recently improved oxygen permeability in RGP lens materials led to the clinical resurgence of scleral lenses. Scleral lenses comprise the largest diameter lenses within the family of non-surgical treatment options. A scleral lens design can often be more comfortable than its smaller RGP counterpart because a scleral lens design allows its lens edge to rest on sclera (white part of the eye), which has much lower sensitivity than one’s cornea. In addition, similar to the 4th generation hybrid lenses, sclera lenses are designed so the lens does not bear on the cornea, which minimizes irritation. Up to a 93% fitting success rate has been described with modern scleral lens designs1.
Because of the larger lens diameters in both the hybrid and scleral lenses, in comparison to the conventional RGP lenses, the insertion and removal process is different and requires practice during the initial adaptation period. This can hold true even for patients that have worn contact lenses before.
A well-fitted contact lens not only defers the need for more invasive surgical procedures but also significantly improves the quality of one’s vision and life. However, this process requires the combined ingredients of a physician’s clinical expertise and a patient’s determination. Further, it is essential for both the physician and the patient to recognize that no single lens design currently encompasses the complexity of all fitting situations. An open and honest discussion regarding one’s visual expectations and daily functional tasks performed in real life environment can be meaningful in the selection of a tailored management choice from the many non-surgical options that exist today.
1. Rosenthal P. Evolution of an Ocular Surface Prosthesis. Contact Lens Spectrum. 2009 Dec: 24(12):32-38
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4. Garcia-Lledo M, Feinbaum C, Alio JL. Contact lens fitting in keratoconus. Compr Ophthalmol Update. 2006 Mar-Apr; 7(2):47-52.
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