Fitting Strategies for Different Forms of Ectasia
BY GREGORY W. DENAEYER, OD, FAAO
Corneal ectasia is a progressive thinning and steepening of the cornea that results in irregularity. There is a variety of classifications of ectasia including keratoconus, pellucid marginal degeneration, and post-surgical ectasia. GP lenses are often used to manage these patients because they are able to correct higher-order aberrations (HOAs) by masking irregularity. Fitting success often depends on choosing a lens design for a particular type of ectasia that will provide stable and comfortable lens wear.
Nipple keratoconus is characterized by central steep symmetrical topography. Corneal GP lenses with diameters between 9mm to 9.5mm are a good choice for successfully fitting these patients. Corneal GP lenses will center on the steep corneal center, making an acceptable fit a relatively straightforward process.
Oval keratoconus is characterized by a broad, inferiorly decentered area of corneal steepness. For mild cases, use large-diameter corneal GP lenses with diameters between 9.5mm and 11.5mm. Larger diameters help to compensate for a lens’ natural tendency to decenter inferiorly toward the corneal steepness. Consider bi-aspheric lens designs where the back-surface asphericity can improve the lens-to-cornea fitting relationship and the front-surface asphericity can compensate for residual HOAs. Patients who have moderate-to-severe oval keratoconus may require mini-scleral or full scleral lenses if adequate centration and stability can’t be achieved with a corneal GP.
In keratoglobus, the entire cornea becomes steep. This subtype of keratoconus is notoriously difficult to fit with GP lenses because it results in extreme amounts of corneal sagittal depth. For keratoglobus, full scleral lenses are often the best option because corneal GPs will be extremely unstable and mini-scleral lenses don’t have enough vaulting capability to achieve clearance.
Creating enough sagittal depth to fit keratoglobus often requires diameters of at least 20mm with optic zones of 10mm or larger. A midperipheral reverse curve can help increase overall clearance.
Pellucid Marginal Degeneration
Pellucid marginal degeneration is characterized by inferior paralimbal steepening. The location of steepening on these corneas makes them challenging to fit, especially with corneal GP lenses because these lenses will want to decenter inferiorly. A mini-scleral or full scleral lens is a better option for achieving centration and stability. A reverse curve is recommended to ensure adequate midperipheral vault.
Managing patients who have corneal ectasia can be a challenging and rewarding experience. Increase your fitting efficiency by choosing lens designs for particular subtypes of ectasia that can offer in best possible outcome. CLS
Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio, and a consultant to Visionary Optics, B+L, and Aciont. You can contact him at firstname.lastname@example.org.
Contact Lens Spectrum, Volume: 28 , Issue: August 2013, page(s): 18