Heterochromatic Flicker Photometry vs. Scanning Laser Reflectometry for Determination of Macular Pigment Density

Abstract

Abstract: : Purpose: To compare macular pigment density (MPD) determinations by Heterochromatic Flicker Photometry (HFP) and Scanning Laser Reflectometry (SLR). Methods: This study further investigated preliminary results by Ponce et al and included 26 normal eyes from 18 patients. The MPD was first assessed at 10 minutes, 30 minutes, 1 degree and 2 degrees eccentricity using the HFP technique as described by Wooten et al (1999). MPD was then determined using the SLR technique. Subject's eyes were dilated with 1% Tropicamide, dark adapted for 15 minutes, and photoreceptor photopigment bleached for 2 minutes using the Argon green (514 nm) laser in the Scanning Laser Ophthalmoscope (RodenstockR SLO 101 model 6801 462). 40 degree reflectance images of the posterior pole were acquired with the SLO using Argon blue and green wavelengths (488 and 514 nm respectively). Digital images were averaged using Scion Image b4.0.2 to reduce noise, aligned manually using Adobe Photoshop v5.5R, and converted to tables representing pixel intensity on an 8-bit grayscale using Scion Image b4.0.2. Mean Macular pigment density was then determined as described by Elsner et al (1990) for 10 minutes, 30 minutes, 1° and 2° eccentricities. For comparison to the HFP values, mean SLR MPD at 10 minutes, 30 minutes, 1° and 2° was determined by calculating the average MPD in the two-pixel-wide (0.11°) edge of circles with radii of 10 minutes, 30 minutes, 1° and 2°. All statistical analysis and computation was performed using Microsoft Excel 2000 v9.0.2720R. Results: Mean MPD as assessed by HFP and SLR were as follows: 0.52 (SD 0.13) and 0.44 (SD 0.12) at 10 minutes, 0.40 (SD 0.10) and 0.38 (SD 0.10) at 30 minutes, 0.28 (SD 0.08) and 0.25 (SD 0.06) at 1 degree, and 0.09 (SD 0.07) and 0.07 (SD 0.05) at 2 degrees. Differences in mean MPD are 0.08 at 10 minutes (p<0.04), 0.02 at 30 minutes (p<0.40), 0.03 at 1 degree (p<0.18), and 0.02 at 2 degrees (p<0.30). Comparing the two techniques at all eccentricities yields a correlation coefficient R = 0.78, and a linear regression model relates the two as follows: SLR = 0.70(HFP) + 0.061 Conclusions: The SLR technique of determining MPD yielded consistently lower values compared to HFP. However, the values show a high positive correlation, suggesting that the SLR technique is a viable alternative to HFP assessment of MPD.

Keywords: age-related macular degeneration • macular pigment • laser