One of the most frustrating situations is a “successful” scleral lens fit for a keratoconus patient that results in suboptimal 20/30 or worse visual acuity, despite a relatively clear cornea without central or paracentral scar tissue. The most likely causative factor for reduced best-corrected vision for this patient population is significant residual higher-order aberrations (HOAs).
The most dominant and visually disabling HOA with keratoconus is coma (Pantanelli et al, 2007). Coma originates from both the front and back surface of the keratoconic cornea. The opposite sign of back-surface coma partially negates the negative coma that originates from the front surface (Chen and Yoon, 2008). A scleral lens effectively masks front-surface irregularity and secondary HOAs but does not inherently correct aberrations from the corneal back surface, including positive coma. Additionally, static decentration of a scleral lens can induce HOAs, including positive coma (Sabesan et al, 2013). Figure 1 shows a decentered lens on the right eye of a keratoconus patient. Figure 2 shows a wavefront map with a best-corrected scleral lens. Note the substantial residual coma.
The first step to decreasing residual HOAs is to improve lens centration. Fitting a small-diameter scleral lens or customizing the back surface with a toric- or quadrant-haptic back surface will often improve the centration of a scleral lens.
Custom wavefront-guided correction can be added to a scleral lens’ front surface to correct residual HOAs (Sabesan et al, 2013; Marsack et al, 2014). Marsack et al (2014) were able to successfully reduce HOAs of 10 out of 14 keratoconic eyes with wavefront-guided scleral lenses to the same level of normal eyes. Figure 3 shows a simulated 20/25 Snellen E of the patient from Figures 1 and 2 through a scleral lens. Figure 4 is the same simulated letter with coma corrected.
Studies have shown that despite successfully correcting residual HOAs of keratoconus patients, they do not achieve comparable visual acuity results to matched normals (Sabesan et al, 2013; Marsack et al, 2014). Neural therapy using adaptive optics or longstanding wear of wavefront-guided scleral lenses may eventually help patients improve to normal visual acuity (Sabesan and Yoon, 2013). CLS
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