Author: Dr. Anderson, OD, FAAO
Keratoconus is a bilateral, progressive corneal thinning disorder which manifests as irregular, asymmetric astigmatism. Symptoms include blurred, subacute distorted vision which is usually more pronounced in one eye than the other. Refraction is often difficult and patients may not be adequately corrected with spectacles. Contact lenses are the best form of visual correction in most cases of clinically significant keratoconus. There are many different types of contact lenses available for keratoconus. Depending on the degree of clinical significance, lifestyle and ocular health, the optimal contact lens can be determined for each patient. This article describes the different design options currently available, along with the goals to keep in mind when fitting these patients.
Soft Toric Lenses
Many times, patients in the early stages of keratoconus do well with soft lenses. A satisfactory baseline refraction, which will most likely have a moderate amount of cylinder, is necessary in order to determine the soft lens prescription, which will most likely be toric. Occasionally, a spherical equivalent will produce good visual results. Patients with forme fruste keratoconus may also do well with soft toric lenses since adequate spectacle refraction is often possible.
Rigid Gas Permeable (RGP) Lenses
RGPs are the most commonly prescribed lenses for keratoconus. Traditionally, these lenses were fit with small diameters, three-point touch and variable edge lifts to improve comfort. However, this type of fit often results in a low-riding lens, decentered inferiorly over the pupil which induces aberration. More recent RGP designs for keratoconus have incorporated aspheric or biaspheric optics to decrease aberration, along with larger diameters to improve centration.
Keratoconus patients who are currently fit with RGP lenses oftentimes experience discomfort and decreased wearing time as their condition progresses. The cone steepens and the apex thins to the point of inflammation, abrasion and irritation from RGP bearing. A simple way to halt this cascade of events is to place a soft lens beneath the RGP. Using a low minus power will flatten and protect the corneal surface as well as improve the comfort and wearing time. High Dk soft and RGP materials work well to maintain corneal integrity by decreasing the incidence of neovascularization.
These lenses consist of an RGP center with a soft skirt edge. They combine the benefits of rigid lens optics, including better lens centration and decreased aberrations, along with the comfort of a soft lens. The results are improved vision and increased wearing time. This is especially beneficial in keratoconus patients whose cone apex is very steep, thin and decentered. Traditional small diameter RGP lenses tend to decenter downward over the apex, inducing bothersome aberrations over the pupil. SynergEyes is currently the only manufacturer of hybrid lenses in the U.S. The designs available for keratoconus include the SynergEyes A, KC and ClearKone. Patients with mild to moderate keratoconus may fit into the A lens which has a spherical RGP center. The KC lens has an aspheric RGP center and is reserved for more advanced cones. ClearKone consists of a reverse geometry RGP center which is fit upon elevation or sagittal depth rather than base curve. This allows for clearance, centration and stabilized vision over a decentered cone. Because it achieves clearance by vault, ClearKone offers lower powers and reduced aberrations as compared to lenses fit according to base curve.
Rigid gas permeable lenses with diameters of 13mm or greater fall into this category. The benefits of these large diameter RGPs in keratoconus are a large,well-centered optic zone, minimal movement with blink, stabilized vision and improved comfort. Fitting these lenses requires a great deal of skill, which is acquired from experience. The dynamics are quite different from corneal lenses. The parameters of the larger periphery determine the patient’s ability to wear the lens comfortably and must be fit independently of the central base curve. The ultimate goal in fitting any keratoconus patient with contact lenses is good vision and comfort. This is relatively easy to achieve in early cones and becomes more difficult as corneal thinning and steepening progress. Determining the best lens to fit in order to achieve these goals is as much of an art as it is a science. For example, a patient who has worn small diameter RGPs unsuccessfully may do better in a hybrid or semi-scleral design. Regardless of lens design, finding the best lens begins with careful measurement of corneal curvature and diagnostic lens fitting.