RGP insights

Keratoconus Problem Solving

BY TIMOTHY B. EDRINGTON, OD, MS
MARCH 1998

Keratoconus patients are often the most satisfying cases for the contact lens practitioner. Prescribing RGP contact lenses for keratoconus patients has received much attention recently, but even with the best initial lens designs, patients may present at a follow-up visit with symptoms and/or signs that challenge your professional skills.

Troubleshooting the Problem

Poor tear exchange may cause decreased contact lens wearing time, a common symptom which often triggers referral for penetrating keratoplasty. If the fluorescein pattern indicates minimal peripheral clearance or binding in the midperiphery of the lens, enhance the tear pump by flattening the existing secondary or peripheral curves or by further blending the junctions between the peripheral curves. The peripheral curve system of your lens design should be similar in radius of curvature to a non-keratoconus cosmetic RGP lens design (secondary curve radius 8.50mm to 9.00mm). However, some labs will "correct" your peripheral curves by correlating them with the steeper base curve radius. This may result in a peripheral curve system that's too steep and too tight.

While dimple veiling is rare today due to oxygen permeable materials, you may observe dimple veiling and stipple staining around the base of the cone in keratoconus patients (Fig. 1). This indicates excessive pooling of metabolic debris underneath the optic zone, which you can reduce or resolve by decreasing the optic zone diameter. Blend the new junction between the optic zone and peripheral curve system to enhance tear exchange.

Coalesced, abrasive staining of the apex of the cone might indicate that the lens is too flat or that there are excessive deposits on the posterior lens surface. If the fitting relationship is too flat, select a new base curve radius by evaluating the fluorescein patterns of steeper diagnostic lenses. By achieving minimal apical clearance at the trial lens fitting, you can quantify the fit of the keratoconus patient's habitual lenses. I recommend prescribing a base curve that lightly touches the apex of the cone (about 0.10mm flatter than the base curve of the flattest lens in your trial set that provides an apical clearance fluorescein pattern).

Other Solutions

Because of the steep base curves often prescribed, keratoconus patients may have trouble adequately removing deposits from the posterior lens surface. So rule out excessive deposit coating before you reorder a new lens due to staining of the corneal apex. I sometimes tell patients with large fingers to clean this area using a cotton swab soaked in cleaning solution but to avoid swabs with wooden sticks, which may scratch the lens surface. Increased enzyme treatments may also help.

Reducing the edge thickness by contouring or plussing the anterior edge of an existing lens or by ordering a lenticular design will reduce the incidence of 3 and 9 o'clock staining and spontaneous lens ejection. Rewetting drops may also help, but if an unacceptable amount of staining persists despite your best lens design efforts, consider prescribing a disposable soft contact lens to be worn beneath the rigid lens. This piggyback lens system may be prescribed as a temporary measure to allow the peripheral cornea to rehabilitate.

Dr. Edrington is a professor and chief of contact lens services at the Southern California College of Optometry.