Keratoconus is an ocular disorder characterized by progressive corneal thinning and other corneal sequelae, in which the cornea assumes an irregular conical shape. It can be classified according to degree of conicity and characterized morphologically by the shape of the cone. Although early keratoconus with minimal irregular corneal astigmatism may be corrected by spectacles, they cannot be expected to improve visual acuity to the same extent that rigid contact lenses offer, when irregular astigmatism is present or increases.

DIAGNOSIS

Since the onset of this disorder usually occurs in the second decade of life, these patients are active occupationally, socially and recreationally. Therefore demands for good visual acuity among these patients, is a major concern for both the practitioner and the patient. Keratoconus quite often typically progresses for a period of five to seven years at a variable rate. The end point of this progression may range from slight corneal irregularity to severe distortion of the corneal contour and apical scarring. Some of these cases require surgical keratoplastic intervention if the patient cannot achieve acceptable vision with spectacles or is unable to tolerate contact lenses. The most practitioners can do for these patients is offer the best visual acuity possible until this condition reaches some point of remission or until surgical intervention is required.

Since the introduction of photokeratoscopy and, most recently, corneal molding, much has been learned regarding the corneal topographical changes the cornea undergoes during the early stages of keratoconus.

1. The inferior portion of the cornea steepens.
2. The corneal apex steepens and assumes an oblique to "against-the rule" astigmatic configuration.
3. The superior cornea (above the horizontal midline)

As this condition progresses, the curvature of the corneal apex becomes more elongated and the corneal surface bulges more creating increased irregular astigmatism. For these reasons, acceptable visual acuity with eyeglasses becomes increasingly impossible. Soft lenses, due to their flexible nature and limited ability to correct corneal astigmatism, do not provide the degree of visual acuity these young, active patients require to function in their daily lives. By their physical nature, rigid contact lenses offer the best vehicle for providing very good visual acuity to these patients, until their condition reaches a point of remission or until surgical intervention is indicated. These patients typically become more dependent on their rigid contact lenses for day and night time vision correction. It is therefore imperative to maximize lens-wearing comfort for the patient. Because of the prolonged wearing periods, it is also critical to protect the already compromised integrity of the corneal physiology. The use of gas permeable lens materials offering high oxygen transmissibility is extremely important.

Because of the limited and often inaccurate information provided by keratometry, keratoconus lens fitting dictates that diagnostic lenses are used to achieve the best possible fit.

CLASSIFICATION

Mild cones may be classified as having keratometry ("K") readings in both meridians of <48.00 diopters, moderate cones having "K" readings of 48.00 to 54.00 diopters and severe cones as having "K" readings of >54.00 diopters. Since the superior portion of the cornea is relatively normal, it is an important consideration when selecting a lens base curve to avoid impingement in this area.

IDENTIFICATION OF CONE TYPE

It is also helpful for the fitter to be able to identify the shape and size of the cone they are dealing with, to be able to help assist with diagnostic lens selection, minimize the amount of lens parameter changes and subsequent chair-time involved in fitting these cases.

The contact lens fitter can determine the type and size of the cone in one of several ways. Following dilation of the pupils, the "Red reflex" can be used to retro illuminate the cornea and determine the shape and approximate size of the cone. A more direct approach is to place a flat-fitting lens on the eye and view the outline produced by the flat fitting relationship as viewed using sodium fluorescein.

1. <5mm for "nipple" cones—usually located at or near the center of the corneal apex.

2. 5mm to 6mm for "oval" cones. These cones are typically large in size and ellipsoidal in shape. They are located predominantly inferior-temporally and appear to sag due to their larger horizontal dimension.

3. >6mm for "globus" cones—these cones appear more diffuse and are of larger size. They may extend above the horizontal mid-line of the cornea. As progression of the disease is noted and corneal thinning continues, Fleischers ring hydrops, positive Munsons Sigh striae and central scarring are among the signs that may be seen.

RIGID LENS FITTING

While keratoconus can present numerous lens-fitting difficulties, these can be minimized if a disciplined approach to lens fitting is used. In the early stages of this condition, use of rigid spherical and aspheric lenses employing steep base curves, standard designs and average lens diameters will function very well to provide good patient comfort and very good visual acuity.

As the disease progresses, fitting moderate to severe keratoconus requires more attention to identifying the size and shape of the cor-neal aberration. As stated previously, use of diagnostic lenses is the only sure method to achieve an optimal lens fitting relationship. Basic lens design requirements dictate that lenses provide steep base curves, relatively small posterior optical zones and flat peripheral curves. There are several design systems that meet these criteria.

MCGUIRE CONE LENS DESIGN

The McGuire fitting philosophy indicates a "three-point touch" fitting system. The goal is to achieve superior alignment between the cornea and the lens. The ideal fit appearance will exhibit 2mm to 4mm of "feather" apical bearing, with slight edge lift inferiority.

Attempts to vault the corneal apex to reduce inferior edge lift will result in an excessively steep fitting relationship on the more normal superior portion of the cornea.

The fitting system consists of three separate diagnostic lens sets specifically designed for each type of cone configuration. The design is predicated on the lens optical zone size as it relates to the size of the cone varying form 6mm for the nipple-type cones to 6.5mm for oval cones and 7mm for globus cones. Each incorporates identical peripheral curve systems blended together to resemble an aspheric-like flattening in the posterior lens periphery. Four peripheral curves are utilized:

1. -0.5mm flatter than the base curve
2. -1.5mm flatter than the base curve
3. -3.0mm flatter than the base curve
4. -5.0mm flatter than the base curve

1. Mild — (A, B, C, D) less than 48 diopters in either corneal meridian.
2. Moderate — (E, F, G) 48 to 54 diopters in either corneal meridian.
3. Advanced — (H, I, J) >54 diopters in either corneal meridian.

ASPHERIC RIGID LENS DESIGNS

In keratoconus, aspheric designs have proven to not only fit the varied corneal shapes, but also simplify the fitting process by eliminating some of the variables involved with lens design and fitting. These designs provide improved mid-peripheral alignment as compared to spherical designs. Optical zone sizes are generally small but will usually accommodate the larger globus cones as well as oval or nipple-type ones.

Several of these aspheric designs are also "junctionless." That is, the design has been mathematically calculated to allow the various aspheric curves to be joined in a matter in which no junctions are created. This helps to eliminate narrow bearing areas in the mid-periphery, allows lens mass to be distributed more evenly, creating a more uniform tear layer profile under the lens.

OTHER CONTACT LENS OPTIONS

Other means of fitting the keratoconic patient have also been utilized. A "piggyback" fitting systems has been used in cases where corneal contact lenses will not suitably stabilize on the cornea, where rigid lens wear alone is not tolerated by the patient or metabolic problems exist. Using this technique, the patient is first fitted with a large, thin soft lens in the conventional manner, so that both good lens centration and movement are achieved. Once the patient has adapted to full-time soft lens wear, keratometry readings are taken of this "renewed" corneal surface and rigid lens diagnostic fitting is carried out. Use of high-permeability rigid lens materials, designed as thin as possible is essential, since the patient is wearing a "double layer" of lenses on the cornea and the physiology is already impaired. The disadvantage of this system is that the patient must contend with insertion, handling and caring for both types of lenses separately. Use of separate care systems is also costly and time-consuming.

Another option is the use of a hybrid lens such as the SoftPerm lens. The advantage of this system is that the soft lens and gas permeable lenses are polymerized together into one unit.

Consideration must be given to the physiological impact that these fitting systems may have on the cornea with regard to providing adequate oxygen.