Prescribing for Astigmatism
Second Half Game Plan: Scleral GP Keratoconus Prescribing
BY COLTON HEINRICH, OD; ELAINE CHEN, OD; & TIMOTHY B. EDRINGTON, OD, MS, FAAO
Welcome back to the second half of the keratoconus gameday plan. In our April column, we discussed fitting corneal GP lenses for keratoconus. This second half will outline the general process of scleral GP fitting for moderate-to-severe keratoconus. If you are new to scleral lens fitting, we recommend familiarizing yourself with one scleral design before branching out to multiple designs.
Step by Step
Following are our steps for fitting scleral GPs for keratoconus.
Step 1: Measure corneal curvatures. ProTip: Evaluate curvature maps and elevation maps. This will aid in determining cone location and disease severity.
Step 2: Select a trial lens. Follow the manufacturers’ guides for initial lens selection. Consider a reverse geometry design if the central cornea is flatter compared to the midperipheral cornea (e.g., pellucid marginal degeneration).
ProTip: Scleral fitting is all about sagittal depth, which can be changed by altering the overall diameter and/or base curve. In general, choose smaller diameters initially for milder cases. Larger diameters may be required in advanced cases.
Step 3: Evaluate the initial corneal clearance. Instill sodium fluorescein along with application solution into the lens bowl and evaluate with a white light optic section. It is important to know the center thickness of the lens to estimate corneal clearance by comparison. The goal is approximately 300 microns of initial apical corneal clearance and 150 to 250 microns after lens settling.
ProTip: Lenses tend to settle approximately 100 microns in the first 20 minutes of wear.
Step 4: Evaluate limbal clearance. To protect the limbal stem cells, scleral lenses should not bear on the limbus. Ideal clearance is approximately 50 to 60 microns.
Pro Tip: A blue light filter, sodium fluorescein in the lens, and a diffuse slit lamp beam will help you evaluate whether there is adequate clearance at the limbus when the vault is very thin.
Step 5: Evaluate the landing zone, and document areas of vessel blanching and compression as well as areas of edge lift. It is important to properly align the peripheral curve system to the scleral contour, which is often toric. A tight fit that causes vessel blanching can create a hypoxic environment. A loose peripheral curve system may cause bubbles and debris formation under the lens. Prescribe toric peripheral curves when indicated.
ProTip: Remove the lens to check for lens suction as well as to monitor for a compression ring or conjunctival staining.
Step 6: Perform a sphero-cylindrical over-refraction (OR) using your best-fitting trial lens. Vertex the OR when indicated.
ProTip: If considering a front-surface toric correction, utilize toric peripheral curves or prism ballast to stabilize lens rotation prior to the addition of cylinder.
Step 7: Call a lab consultant to discuss the design and to order the lens.
We believe that corneal GP lenses provide good vision and corneal physiology for many keratoconus patients. If a patient has advanced keratoconus, inferior corneal steepening, or becomes intolerant to corneal GP lenses, scleral lenses are an alternative treatment option. Different lens designs can provide unique features to aid in the scleral fitting process, so it is important to understand the scleral lens designs to provide your patients with optimal vision, comfort, and eye health. CLS
Dr. Heinrich and Dr. Chen 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.