Prescribing for Astigmatism

First Half Game Plan: Corneal GP Keratoconus Prescribing

BY ELAINE CHEN, OD; COLTON HEINRICH, OD; & TIMOTHY B. EDRINGTON, OD, MS, FAAO

Patients who have keratoconus often require GP contact lenses to achieve optimal vision. Fitting keratoconus patients may seem daunting. But, in this two-part article, we outline both corneal and scleral GP lens fitting one step at a time.

Step by Step

Following are our steps for fitting corneal GPs for keratoconus.

Step 1: Measure corneal curvatures manually or with topography. For manual keratometry, you can extend the range of your keratometer by ~8.00D with a +1.25D spectacle trial lens or by ~16.00D using a +2.25D trial lens over the patients’ side of the keratometer.

Step 2: Select a trial lens base curve between average K and steep K value. If the keratoconus is more advanced, lean toward the steep K value.

Step 3: Evaluate the fluorescein pattern. If there is apical touch, try a steeper base curve (BC) to increase the sagittal depth; if there is excessive central clearance, try a flatter BC. The goal is to find the “steepest” trial lens that just clears the apex of the cone. Bracket base curves to achieve optimal fit.

We utilize the first definite apical clearance lens (FDACL) and believe that vaulting the apex of the cone slightly minimizes excessive mechanical rubbing over the steepest area of the cornea.

Some keratoconus fitting philosophies advise fitting flat over the cone. While this may potentially provide patients with sharper vision, it does not impede cone progression and may lead to abrasions and eventual corneal scarring.

Step 4: Evaluate fluorescein pooling beneath the optic zone, and decrease the optic zone diameter (OZD) if there is excessive pooling and bubbles. As disease severity increases, the optimal OZD tends to decrease. A keratoconus patient needing a 45D BC may be prescribed a 7.0mm to 7.5mm OZD; a patient who needs a 50D BC may be prescribed a 6.5mm to 7.0mm OZD; and a patient who needs a 60D BC may be prescribed a 6.0mm to 6.5mm OZD.

It is important to remember the relationship between sagittal depth and OZD (i.e., decreasing OZD may flatten the fit). We suggest beginning with an overall lens diameter (OAD) of approximately 9.0mm.

Step 5: Evaluate the peripheral curve system. Flatten the peripheral curve radius if the periphery is too tight, and steepen the peripheral curve if there is excessive edge lift. Despite steeper central corneal curvature, the midperipheral and peripheral corneas of keratoconus patients are usually similar to a “normal” patient’s corneal anatomy; therefore, a secondary curve around 8.50mm is usually a good starting point.

Step 6: Perform a sphero-cylinder over-refraction using your best fitting trial lens. Don’t forget to vertex the equivalent sphere power if indicated. In general, there is little to no correlation between your manifest refraction and the contact lens power for a keratoconus patient. Due to the tear lens, the final contact lens power tends to be much more minus.

But Wait, There’s More!

We believe that corneal GP lenses provide good vision to the majority of keratoconus patients. If a patient has very advanced keratoconus or inferior steepening, it may be very difficult to achieve an adequate fit with a corneal lens. In these cases, we generally prescribe scleral GP contact lenses. We will discuss our tips for these in our next column. CLS

Dr. Chen and Dr. Heinrich are the current cornea and contact lens residents at the Southern California College of Optometry (SCCO) at Marshall B. Ketchum University (MBKU). Dr. Edrington is the cornea and contact lens residency coordinator at SCCO. He is also a Fellow of the American Academy of Optometry and a Diplomate in its Cornea, Contact Lenses, and Refractive Technologies Section.