Fundus autofluorescence technology provides a unique image that captures the early stages of age-related macular degeneration.
New imaging technology has arrived that offers a non-invasive way to view subtle changes in the structures of the retina. It’s already having a profound impact on improving the delivery of eyecare by optometrists, particularly for age-related macular degeneration (AMD).
Fundus autofluorescence (FAF) occurs when a specific wavelength of light is absorbed by the lipofuscin in the retinal pigment epithelium (RPE), which results in fluorescence of the lipofuscin. The confocal scanning laser ophthalmoscopy technique uses a laser with an excitation wavelength of 488nm and a barrier filter at 500nm or 520nm, the same wavelengths used for fluorescein angiography. Fluorescence occurs when molecules absorb electromagnetic energy, which excites them to a higher energy state and produces the emission of light at a longer wavelength. The ocular fundus displays a normal background autofluorescence which will vary to a small degree from person to person.
The greater the concentration of lipofuscin in the RPE cells, the brighter the appearance as compared to the normal background autofluorescence of the retina. This indicates that the involved area of fluorescence has RPE cells that are compromised by the inability to biodegrade the rhodopsin liberated by the outer segments following photo transformation. In other words, the cells degenerate due to a particular cause, such as age or hereditary. The greater the autofluorescence, the more dysfunctional the cells become. However, the fact that autofluorescence is detected does not indicate that the length of time abnormal metabolism has been present. An increase in the severity of the underlying condition can only be judged by detecting an increase in fluorescence or greater area of involvement on subsequent imaging.
Less concentration of intracellular lipofuscin will produce the darker gray color to the fundus appearance (the greater the loss of lipofuscin concentration the darker gray the color). The total absence of lipofuscin (such as the loss of the RPE cells) will result in a black appearance of the involved area(s). Also, the pigmented area of the retina in the RPE or anterior to the RPE will block the fundus fluorescence. This is most often caused by congenital hypertrophy of the RPE or RPE hyperplasia. Nevi will not cause darkening of the fundus autofluorescence because the pigmented lesions are below (external) to the RPE.
FAF IMAGING FOR AMD
Early dry AMD is composed of a few to numerous drusen (less than 63mu) and/or RPE degeneration with RPE hyperplasia. On FAF, these changes are seen as small spots of slightly elongated areas of hyper-autofluorescence and often associated with spots of pigment clumping (RPE hyperplasia) causing hypo-fluorescence. These findings may be seen in or just outside the macula (often along the temporal arcades). These age-related FAF findings can also be seen adjacent to the optic disc. Pigmented lesions can be seen on the FAF image as dark areas.
Of great interest is the fact that the hyper-fluorescence areas may be short and linear in appearance. In more advanced cases, there are long linear areas of hyper-fluorescence. The sensory retina, RPE or choriocapillaris, can’t form small thin linear lesions, as seen in the picture below left, but it is possible in the basal lamina, Bruch’s membrane and cuticular basement membrane. These are likely to represent the fractures in the basal lamina as seen on histopathology sections of AMD. I suspect that if the fractures are wide enough, it makes it possible for neovascular vessels from the choriocapillaris to grow though the basal lamina into the subretinal space.
Intermediate dry and geographic atrophy are seen as larger areas of involvement, often with more linear lesions of hyper-fluorescence. Note in the photo, bottom right, that most of the area of geographic atrophy involvement is darker then the background autofluorescence, which indicates decreased viability of the RPE. The small black area indicates a loss of RPE cells. The numerous thin linear lesions radiating away from the main central portion of the geographic area would seem to indicate that the fractures are located at the leading zone of progressing degeneration.
Wet AMD would show many areas of hypo autofluorescence due to loss of RPE functioning, scarring and possibility of hemorrhaging.
FAF is a valuable, non-invasive tool for the primary care clinician to diagnose and manage patients with retinal disease. OO
William L. Jones, OD, FAAO, is founding president of the Optometric Retina Society and serves on the Optometric Office editorial advisory board.
WHERE TO FIND IT:
800.970.7227 | usa.canon.com/eye.care
Carl Zeiss Meditec, Inc.
800.342.9821 | meditec.zeiss.com/usa
Heidelberg Engineering, Inc.
800.931.2230 | heidelbergengineering.com
Kowa American Corporation
800-966-5692 | kowa-usa.com
800.854.3039 | optos.com
Topcon Medical Systems, Inc.
800.223.1130 | topconmedical.com