Abstract
Purpose:
With aging, the human retina undergoes cell death and additional structural changes that can increase scattered light. We quantified the effect of normative aging on multiply scattered light returning from the human fundus.
Methods:
A confocal scanning laser polarimeter (GDx, Carl Zeiss Meditec, Dublin, CA) using linearly polarized 780nm light scanned 15x15deg visual angle of the retina in a raster pattern. This device was modified by removing the corneal compensator lens. The input polarization angle was varied over a series of images, collected sequentially. Light from the eye enters a polarizing beam splitter and is collected on two detectors: one channel detects light parallel to the input polarization and the other detects light polarized perpendicular to the input polarization. Each detector collects 20 images in <1 sec, which are saved as 256x256 pixels in 8 bit grayscale.<br /> <br /> One foveal centered scan was analyzed for each subject. We tested 120 subjects with normal eye exams, with 20 subjects/decade, from 20 to 80 yrs, with equal numbers of males and females in each group. Custom software in Matlab (Mathworks, Natick, MA) quantified 18 image types based on polarization content. Four measurements of 5x5 pixels were taken using Photoshop (Adobe, San Jose, CA) 1 deg from the fovea in 4 of the 18 computed images. The average pixel grayscale value was computed. The 4 image types were: 1) minimum of the crossed detector, 2) overall average of the two detectors, 3) scattered light ratio and 4) ratio of the crossed to the parallel detector. The scattered light ratio is the ratio of 1 and 2, which provides an estimate of retinal scattered light that does not depend on overall retinal signal. Single factor analysis of variance was done using Excel (Microsoft, Redmond, WA).
Results:
There was a significant difference between age groups (p < 0.05) for each image type (p = 6.33*10^-15 and 1.48*10^-11 for images 1 and 3). These 2 images exhibited a systematic increase in multiply scattered light with age. Image1 y = 0.4122*age+12.639; Image3 y = 0.2968*age+14.298. Image1 ratio old/young = 1.95.
Conclusions:
The increase of multiply scattered light associated with aging is consistent with the histological changes that occur in the fundus of individuals before developing age-related macular degeneration. The increase in scattered light with aging cannot be attributed to changes in retinal reflectivity or pupil diameter.