Modeling the effect of forward scatter and aberrations on visual acuity after endothelial keratoplasty

Purpose. To evaluate the effects on visual acuity of forward scatter and aberrations typical of those after Descemet stripping endothelial keratoplasty (DSEK). Methods. Twenty normal eyes of 20 subjects (ages 22-57 years) were examined with best spectacle correction. Under photopic conditions, high-contrast visual acuities (HCVAs) were measured by using ETDRS charts. Visual acuity was also measured by using aberrated charts that simulated the typical high-order aberrations at 12 months after DSEK. Forward scatter was induced by viewing the eye charts through a 1-mm-thick layer of scattering solution (Amco Clear, at a concentration of 4000 nephelometric turbidity units) and was measured with a straylight meter. Results. Forward scatter increased from 1.19 ± 0.11 log straylight parameter (log[s]; mean ± SD) without induced scatter to 1.57 ± 0.06 log(s) with induced scatter (P < 0.001). Induced scatter reduced HCVA on the nonaberrated chart by 2.7 Snellen letters, from 20/19 (Snellen equivalent) to 20/21 (P < 0.001) and by 2.1 letters on the aberrated chart, from 20/25 to 20/28 (P = 0.005). Addition of aberrations reduced HCVA by more than twice the number of Snellen letters than did induced scatter, by 6.4 letters with low scatter (P < 0.001), and by 5.8 letters with high scatter (P < 0.001). Conclusions. Under typical clinical testing conditions, increased forward scatter has minimal effect on visual acuity. High-order aberrations are a more likely cause of degraded visual acuity than is forward scatter in eyes with clear corneas after DSEK.