contact lens primer
Fitting Keratoconus
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & JOSEPH T. BARR, OD, MS, FAAO

Corneal surface irregularities present eyecare practitioners with challenging contact lens fits. The goal is to minimize areas of harsh corneal bearing and excessive areas of lens clearance. One achieve the desired fit for post-trauma and post-surgical corneas, it is often necessary to prescribe reverse geometry, aspheric or large-diameter RGPs to best contour these corneas and obtain adequate lens centration. One of the most commonly encountered corneal irregularities that benefits from contact lens prescribing is keratoconus.

One of the most commonly encountered corneal irregularities that benefits from contact lens prescribing is keratoconus.

Figure 1. Mild scarring, Vogt's striae and Fleischer's ring in keratoconus.

Symptoms and Signs

The more severe the disease, generally the more obvious the signs and symptoms. Symptoms may include increased glare, distorted and reduced vision and diplopia. The patient may reveal a family history of keratoconus, allergies and atopic conditions, as well as frequent spectacle prescription changes due to dissatisfaction with vision. Irregular retinoscopy and ophthalmoscopy reflexes are usually present. Keratometry reveals irregular mires with generally steeper findings. Topography shows a steeper inferior cornea when compared to the superior cornea. Slit lamp signs may include Vogt's striae, Fleischer's ring (an iron deposition at the base of the cone) and corneal scarring (Figure 1). Keratoconus is typically a progressive, bilateral and asymmetric condition.

Management

Spectacles, soft sphere and soft toric contact lenses are appropriate if the patient's vision is acceptable and stable. As kerato conus advances and vision becomes unacceptable with spectacles and soft lenses, rigid gas permeable lenses become the choice for most patients. Most fitting philosophies may be categorized as apical touch, including divided support, or apical clearance (Figure 2). Try other options such as the rigid-soft hybrid SoftPerm and piggyback designs if adequate comfort is not achieved with an RGP alone. Large overall diameter RPGs are recommended when traditional designs decenter too inferiorly.

	Figure 2. Slight apical clearance fluorescein pattern.

A Game Plan

Use the steep keratometry or steep simulated K value to select the initial base curve. "Bracket" fluorescein patterns by determining the diagnostic lens that minimally clears the apex of the cone (apical clearance). Verify that a lens approximately 0.2mm flatter exhibits an apical touch fluorescein pattern. Prescribe a base curve that will lightly touch the apex of the cone (for example, the steepest apical touch diagnostic lens base curve). Over-refract and add the vertexed equivalent sphere to the diagnostic lens power to determine the contact lens power.

We recommend overall and optic zone diameters of about 8.6mm and 6.5mm, respectively. The peripheral curve (>11.5mm) system needs to be flat enough to provide adequate edge lift to enhance tear exchange. Secondary curve radii in the range of 8.50mm to 9.00mm are appropriate. A medium or heavy blend of the junction between the OZ and the start of the peripheral curve is indicated. Prescribe a lenticular design with the edges contoured toward the cornea. This minimizes lid-to-lens edge interaction, thereby enhancing comfort and decreasing lens ejection.

Dr. Edrington is a professor and in the contact lens service at the Southern California College of Optometry. E-mail him at tedrington@scco.edu.

Dr. Barr is editor of Contact Lens Spectrum and assistant dean for clinical affairs at The Ohio State University.