Abstract
Purpose: :
Corneal haze, or backscatter, is frequently present before and after lamellar corneal surgery and is associated with forward light scatter that degrades the retinal image. In this study, we examined how induced forward scatter in the absence of an external glare source affects visual acuity and contrast sensitivity. We confirmed the induction of forward scatter by using a straylight meter, and also assessed whether increased straylight could be detected by an optical metric based on the point-spread-function.
Methods: :
Thirteen normal eyes of 13 subjects (ages 22 to 57 years) were examined with best spectacle correction in the presence and absence of induced forward scatter. A flat glass chamber, 1 mm thick, was placed in the optical path and was filled either with Amco Clear (GFS Chemicals) at a concentration of 4000 NTU to induce forward scatter or with water (no induced forward scatter). Under photopic conditions, high- and low-contrast visual acuity (HCVA, LCVA) were measured by using 100% and 10% contrast ETDRS charts respectively. Contrast sensitivity was measured at 5 spatial frequencies under mesopic conditions (CST 1800 Digital, Vision Sciences Research Corporation). Intraocular straylight was assessed by using a straylight meter (C-Quant, Oculus) and the effect of the induced forward scatter on the retinal point-spread function was assessed by measuring the objective scatter index (OSI) based on the double-pass technique (OQAS, Visiometrics). Differences in variables between induced forward scatter and no induced forward scatter were examined by using paired tests.
Results: :
Forward scatter increased from 1.34 ± 0.15 log straylight parameter (log(s), mean ± SD) without induced scatter to 1.53 ± 0.08 log(s) with induced scatter (p=0.001). Induced forward scatter did not affect HCVA (-0.11 ± 0.09 to -0.09 ± 0.09 logMAR, p=0.36) or LCVA (0.13 ± 0.10 to 0.18 ± 0.13 logMAR, p=0.06), but decreased contrast sensitivity at 12 and 18 cycles/degree (p<0.02). The OSI with induced scatter (1.08 ± 0.47) was greater than that without induced scatter (0.75 ± 0.44, p=0.03). With induced scatter, the coefficient of variation (standard deviation as a percent of the mean) of the straylight parameter (s) and the OSI were 19% and 44% respectively.
Conclusions: :
In the absence of a glare source, increased forward scatter has minimal, if any, effect on HCVA and LCVA, although it slightly degrades contrast sensitivity at high spatial frequencies. Optical changes other than forward scatter associated with corneal haze, such as increased aberrations, should be considered to explain the diminished HCVA after lamellar surgery.
Keywords: optical properties • visual acuity • contrast sensitivity