Management of Keratoconus with Contact Lenses
Over the years there have been many different techniques advocated to fit keratoconic patients in contact lenses. Several different methods will work on some keratoconic patients. No single method or philosophy is best for all patients.
For most keratoconic patients the keratometer readings or corneal topography are used only as a rough starting point in determining the rigid contact lens base curve to use. The base curve is a measure of how steep the central portion of the contact lens is. The final determination of which lens is best must be made by evaluating the fluorescein pattern with a diagnostic contact lens on the cornea. Fluorescein (pronounced "floor-uh-seen") is a yellow vegetable dye that glows green under a black light. The dye settles under the contact lens and the eye-care practitioner can evaluate how well the lens fits.
Flat fitting lenses with harsh central bearing areas used to be the mainstay of keratoconic fits. This means that the lens was much flatter than the cone, and therefore pressed down on the center of it. Such a lens, with a large diameter, was often comfortable for the patient. Pictured on the right is such a fit. Note the black area in the center; this is where the lens is bearing down on the cone, and not allowing any dye to get underneath it. The bright ring of green is the dye pooling underneath the edges of the lens. Some practitioners have felt that such fits held the cone back and actually slowed the progression of the disease.
The disadvantages of such fits is that corneal swelling, fluorescein staining on the apex of the cone (which indicates damage to cells), and central scratching of the cornea can occur. On the left is a picture of a central scratch. It is seen in the area of green dye as a small dark spot. There is some evidence that such fits may increase the amount and/or degree of corneal scarring (Korb). Lens intolerance may also be the result of such flat fits.
To counteract this theory, a corneal clearance lens was fit. This lens was made very steep so that it vaulted over the cone to keep from pressing down on it. One problem with this idea is that the lens had to be made so steep to fit over the cone that air bubbles form between the lens and the cornea. These steep lenses can lead to corneal swelling and lens intolerance.
A Compromise Fit
Often the compromise fit for a keratoconic is what is called a "three-point touch" fit. This means that the lens lightly touches the peak of the cone (not the harsh bearing-down of the old style), then a very low vault over the edges of the cone, and lastly a thin band of touching near the edge of the lens. The name "three-point touch" refers to the edge-peak-edge pattern of the lens touching the cornea. The lens is kept as small as is optically possible. Since the lens will center itself over the peak of the cone, an off-center cone needs a bigger lens than a centered cone. Due to the unusual design of the keratoconic lens, a very high minus-powered lens is usually required, as for a person who is very near-sighted. The most accurate way to fit keratoconic patients is to put a diagnostic lens on the eye, check the fit, and modify the fit from there. Since each individual cone is so different, it is a trial and error process, and may require trying on several different lenses.In this photo, the three-point touch fit is illustrated nicely using fluorescein dye. There is a dark band near the edge of the lens; inside that an area of green pooling, and at the very center, a black spot of light touching on the center of the cone. The touch pattern as you go from side to side is edge-peak-edge.
Types of Contact Lenses
Traditional PMMA and Rigid Gas Permeable (RGP) Lenses
PMMA and RGP are abbreviations which describe the material a contact lens is made of. PMMA is rarely used anymore because it allows no oxygen to pass through it. The cornea has high oxygen requirements because it has no blood supply to bring oxygen to it.
Aspheric lenses have been used in order to get a better physical fit to the highly aspheric keratoconic cornea. The aspheric lens has a back surface which is steepest at the center and gradually flattens toward the edges, called the periphery. Various parameters of aspheric lenses can be adjusted to achieve the best fit. The more advanced and steeper the cone the greater the rate of peripheral flattening required for the lens to approximate the corneal shape. While this can give a better fit to the cornea, the disadvantage can be a decrease in visual acuity. The aspheric curve causes a change in power across the lens surface resulting in a gradual increase in plus power and cylinder from the center to the edge of the optical zone. If the lens is not centered over the pupil, as is the case with decentered cones, this becomes a major problem preventing excellent vision with aspheric lenses.
Some keratoconic patients become intolerant to rigid lens wear. For a few of these patients the use of a rigid lens fitted over a hydrogel lens increases comfort resulting in adequate wearing time with good vision (Westerhout). There are obviously some problems with such a system including the handling and care of two different types of lenses, providing sufficient oxygen to the cornea, and difficulty in obtaining centration of the rigid lens. In this photo, the rigid lens can be seen easily just to the right of the pupil. The soft lens, more difficult to see, has an arrow showing its location at the left of the picture. Since the combination of the rigid and soft lenses results in a relatively thick total "lens" on the cornea, swelling due to lack of oxygen is a common problem.
One way to overcome the problems with piggy-back lenses, yet have the optics of a rigid lens with the comfort of a hydrogel, is to fuse a soft rim onto a hard central portion. The SoftPerm is such a lens. The lens can give good comfort with the vision of a rigid lens. It is fitted like hydrogel lens.
There are a number of potential problems with this lens. It often fits too tight with insufficient movement and may be difficult to remove. The central portion can flex resulting in induced astigmatism. This flexing may be worsened by the draping action of the peripheral skirt (Blehl>). The hydrogel portion may split just peripheral to the bond between the two segments with handling although the bond is usually quite adequate and does not separate. The lens is expensive which can be a problem for a patient that splits many lenses.
In spite of the problems, this lens can be useful for some patients with keratoconus.
Scleral (Haptic) Lenses
Scleral lenses were historically the first type of contact lenses fitted and were commonly used for keratoconus. A scleral lens is one that covers the entire visible white part of the eye (the sclera), as opposed to a corneal lens, which only covers the clear front part (the cornea). An advantage of this type of lens is that it rests on the sclera and can bridge the cornea. A rather advanced cone can be cleared with a scleral lens, so that the lens doesn't touch the cone, but rather vaults over it. With a decentered cone corneal rigid lenses will center over the cone and present visual problems. This isn't a problem with scleral lenses, since they rest on the white portion of the eye and can be centered over the clear cornea. Scleral lenses result in excellent optics since they are rigid, allowing the tears to neutralize the distorted cornea, just as with any rigid corneal lenses. Scleral lenses are rather easy to handle and are surprisingly comfortable in comparison to rigid corneal lenses because the edge is under the lids.
The major disadvantage of scleral lenses is the time and skill required in fitting. Scleral lenses have generally been made of PMMA material, which does not allow oxygen transmission, thus corneal swelling has been a problem. PMMA is a good material in that it can be molded to match a mold of the eye to obtain a good fit on keratoconics and other distorted eyes. Scleral lenses made from oxygen permeable materials offer the advantage of minimizing corneal edema (Ezekiel and Ruben and Benjamin 1985). However, the present gas permeable materials are thermosetting plastics which can not be heated and molded to an impression of the eye, which greatly limits the fitting of the lenses to keratoconic corneas . The lenses must be lathe cut and fitted using diagnostic sets. Methods of polymerizing the RGP lens in a mold (Pullum 1987, Bleshoy and Pullum 1988) or using a central button of RGP material fused into a molded PMMA haptic have been used (Lyons et al 1989).