Abstract
Purpose: :
The area and shape of zones of geographic atrophy (GA) appear to be different on short wavelength and near-infrared autofluorescence (AF) images. To quantify this observation, we measured the area of zones of GA in images obtained with both AF imaging modes.
Methods: :
AF images using both short wavelength (488 nm) and near infrared (787 nm) excitation were recorded in 18 patients with dry AMD (mean age: 79±7.3 years), using an HRA (Heidelberg Retina Angiograph) camera. The area of 34 zones of distinct GA in these patients, ranging in mean diameter from 0.4 to 4.4 mm, were measured in both AF-488 and AF-787 images using an image analysis method that defines the junction as the half grey level point at 75 to 400 points along the perimeter. The zones of GA were also traced manually from the color images of each patient. The area of each GA was expressed as the radius of an equal-area circle. Differences in magnification of the different images were accounted for.
Results: :
The mean area of zones of GA from AF-787 images was 14±17% (range: -9 to 62%) larger than that measured from AF-488 images (p<0.0001). The area derived from color images was not significantly different than that from AF-787 images (p=0.8) and larger than that from AF-488 images (p=0.02). The mean equivalent radius of GA’s from AF-787 images was 36±32 microns larger than that from AF-488 images (range: -17 to 90 microns). This increase in radius was not correlated to the area of the GA (p=0.5), nor with iris color (p=0.3). We also observed that other smaller foci of atrophy or depigmentation seen in AF-787 images were often not or barely visible in the AF-488 images.
Conclusions: :
AF-787 is attributed to components closely associated with melanin (RPE and choroid) whereas AF-488 reflects the distribution of lipofuscin and melanolipofuscin in the RPE cells. The larger areas of GA measured in AF-787 may be the result from loss of melanin in degenerating RPE cells that are still loaded with lipofuscin and melanolipofuscin and/or from chemical changes that would reduce the efficiency of melanin AF. Note that membraneous debris in the subretinal space would have the opposite effect as it would attenuate 488 nm light more efficiently than 787 nm light. Longitudinal studies of the relative progression of the GA measured by the two imaging modes may provide information on the biology of the degenerating RPE.
Keywords: age-related macular degeneration • imaging/image analysis: clinical • ipofuscin