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New methods of interpreting RNFL thickness, visual field detects glaucomatous change earlier
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KOLOA, Hawaii – Longitudinal data collected recently supports the structure-function relationship in glaucoma progression in which the retinal nerve fiber layer thins to a point where it and visual field begin to worsen simultaneously, a presenter said here.
“Structure generally changes without changes in visual fields early in glaucoma,” Joel S. Schuman, MD, said during the glaucoma section of the Hawaiian Eye meeting.
In other words, during this early part of the curve, abnormality is measurable primarily by structure, he said.
Joel S. Schuman
“We’ve defined a point of RNFL thickness where we began to see visual field loss, so the idea here is that you can be a certain thickness of RNFL and then you get to a point where the RNFL thickness is abnormal enough such that you can start to detect visual field abnormality,” Schuman said.
“For example, if you have somebody with glaucoma who has an RNFL thickness of 95 and they drop down to 80, you may not have any visual field defect,” Schuman said. “You may not be able to measure any visual loss, but in fact, they will have lost tissue.”
As this tissue loss progresses, there will be a point where visual function will begin to change with the RNFL, he said.
“This is what’s called the tipping point,” Schuman told attendees.
At this point, the RNFL and visual field will worsen until RNFL loss becomes difficult to measure, at which point the measurements will bottom out.
“You will lose the ability to measure change by RNFL due to a floor effect and it will look like only the visual field is changing when actually both are getting worse,” he said. “Structure and function seem to change simultaneously when you’re past the tipping point, but before you hit the floor effect for RNFL loss.”
“It’s a change in our paradigm,” Schuman said. “In the past we’ve required visual field abnormality to define the disease. Now, structural change can define the disease, and do it earlier. And these structural changes can be measured by all of us in our offices.” –by Daniel Morgan
Disclosure: Schuman receives royalties from Carl Zeiss Meditec and is an intellectual property/patent holder from Massachusetts Eye and Ear.