RK and Progressive Hyperopia
BY ANN LAURENZI, OD, FAAO
Patients who had radial keratotomy (RK) in the 1980s to rid themselves of spectacles and contact lenses are back in our offices. Only this time, they're usually seeking spectacles or contact lenses to help them with post-surgical refractive error.
Radial incisions made in the paracentral and peripheral cornea sometimes penetrating 90-to-95 percent of corneal thickness weakened the mid- and peripheral cornea. IOP within the eye allows this region to project forward and result in central corneal flattening, which reduced myopia. An average of eight to 16 incisions were usually made, leaving a 4mm central optic zone.
Many of these patients experienced complications of glare from incisions, scarring and topographic irregularity in the optic zone and usually returned to rigid lens wear relatively soon post-operatively. However, a growing number of patients are returning to our clinics some 20 years after surgery because of progressive hyperopia.
The Prospective Evaluation of Radial Keratotomy (PERK) study reported that 40 percent of eyes demonstrated a significant hyperopic shift 10 years after surgery. The study also showed that the hyperopic shift in RK patients continued at an average rate of +0.21D per year for the first six months to two years post surgery and +0.06D per year between two and 10 years post surgery.
Some patients experience severe diurnal visual fluctuation due to corneal steepening from a.m. to p.m. Tissue hydration and/or IOP changes steepen the incised, weak cornea, creating large changes in vision. I have one patient who came to me with five different pair of spectacles she uses as her vision changes 6.00D throughout the day.
Correcting RK Patients with GPs
GP lenses can correct RK patients' residual refractive error, mask corneal irregularity and help control diurnal corneal curvature changes. The topography of post-surgical RK corneas is oblate. Therefore, reverse geometry lenses with large overall diameters (OADs), flatter base curves and steeper peripheral curves are usually required to optimize the fitting contour of the lens to the cornea.
Many reverse geometry designs are commercially available. However, diagnostic fitting is key to achieving a successful fit. A starting point for the central base curve radius is 1.00D steeper than flat K, then titrated to achieve a central alignment fit. You may choose the secondary/reverse curve radius based on a topographical comparison of central and midperipheral corneal curvature. This is where the cornea steepens, usually about 4mm temporal to center — typically 1.00D to 5.00D steeper than the central base curve. Large OADs (9.5mm to 11.5mm) are usually needed to maintain centration.
Reverse geometry designs are different from working with conventional GP lenses, and familiarity with reverse geometry designs will help you make appropriate changes to the fit. For example, a smaller difference between the base curve radius and secondary curve radius will reduce peripheral bearing, allow more tear flow and lens movement and center a low-riding lens. A larger difference, creating a tighter periphery, will cause the opposite to happen.
One last recommendation is to use quadrant-specific designs. RK incisions can create peripheral asymmetry. For example, you may need exceptionally steep peripheral curves to align with the inferior cornea and flatter peripheral curves in the superior quadrant to contour post-surgically induced corneal irregularities.
If you have not seen this patient type in your office yet — you will. Becoming familiar with fitting reverse geometry lenses enables you to help these struggling patients. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #158.
Dr. Laurenzi practices at the Cole Eye Institute in Cleveland, Ohio where she specializes in refractive surgery co-management, contact lenses and clinical research.