What Do OCT Scans and Fundus Autofluorescence Images Show?
Yesterday was visit 6 for the clinical trial. It turned out to be a long day of testing. There are a lot of variables they are assessing. Lots of measurements they want to have about the progress of my disease.
What does an optical coherence tomography (OCT) scan show?
Optical coherence tomography is the scan that shows the depth and width of my lesions. The laser can reach into my eye and take images at different depths like so many slices of cheese, one on top of the others. They then take a cross-section of all those slices. That is the image I can see up on the screen when I go to see the doctor for the actual exam. The OCT scans show the hole where my macula used to be.
What does a fundus autofluorescence (FAF) image show?
That is old damage. However, the purpose of this research is to see what is happening with the damage that is currently happening. If you want to see if a treatment slows the damage process, you need to look “where the action is”. Looking at where the current damage is occurring is the job of a different scanner. That scan is called autofluorescence imaging. You may have seen it abbreviated as FAF imaging. The FAF stands for fundus autofluorescence.
What is a fundus?
That’s nice. What is a fundus? (Not fungus as my spell checker keeps insisting. Jeez!) The fundus is the back of the eye. It is the part you can see if you look through the pupil. If you shine a super bright light onto the fundus, parts of it will auto-fluoresce. Auto means by itself and fluoresce means to shine. So in other words, light up the back of your eye with enough energy and it will shine back at you.
Interesting party trick, but what purpose would that have in age-related macular degeneration? Glad you asked!
What do retinal pigment epithelial cells do?
It turns out, according to the article The Nuts and Bolts of Autofluorescence Imagery, that photoreceptors shed their damaged parts to be cleaned up by the retinal pigment epithelial cells.1 For RPEs cleaning up means ingesting. The RPEs ingest the shred parts as well as the chemicals in these shed parts.
It is possible for RPEs to have as much as a quarter of their body weight made up of the photoreceptor wastes and the byproducts made from these wastes. The RPEs that are ingesting and processing the most of this waste are right there where the damage is occurring. Because they have so much of the photoreceptor waste in their cells, hit with a light of the right frequency, these RPEs will glow. The atrophy lesion does not glow. There is nothing there to glow. Healthy cells are not stuffed with the glow stuff and don’t glow that much. It is the RPEs on the “front lines” and full of the glow stuff from dead and dying photoreceptors that light up like Broadway.
Unfortunately, there is nothing happy or festive about the light these cells throw back. The light is coming from the battleground. Our troops are being overwhelmed and dying. That is what that glow means is happening.
Monitoring macular degeneration progression
Not good news, but by using FAF to monitor where cell death is occurring, we can get some idea of how rapidly the disease process is happening. Compare the size of the area of autofluorescence and how fast it is spreading to measures taken before treatment and you can get a good idea if the treatment is doing anything. If we are losing ground more slowly, FAF imaging can tell us that.
Ocular coherence tomography and fundus autofluorescence imagery. Right now these are two systems documenting our losses. Let us hope they will soon document our victories.
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