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Analyses exploring relationships between structural and functional losses in glaucoma show that early structural damage as measured by spectral domain OCT can occur in the absence of visual field changes.
However, VF changes occur once about 20% of retinal nerve fiber layer (RFNL) thickness is lost, reported Gadi Wollstein, MD, at the annual meeting of the American Glaucoma Society.
The analyses were based on data collected from 72 healthy participants and 40 patients with glaucoma representing the spectrum from early to advanced disease, said Dr. Wollstein, associate professor of ophthalmology and director, Ophthalmic Imaging Research Laboratories, University of Pittsburgh School of Medicine.
Data on RNFL thickness were obtained from 200 × 200 cube scans using spectral domain OCT (Cirrus HD-OCT, Carl Zeiss Meditec). VF testing was performed with automated perimetry (Humphrey Field Analyzer, Carl Zeiss Meditec).
A "broken-stick" method of analysis was used to find the inflection, or "tipping point," where the slope changed in the relationship between VF and RNFL thickness.
Using data for global RNFL thickness, the analysis showed visual field threshold values remained stable at high RNFL thickness values but then began to decrease at an average RNFL thickness of about 75 microns, which was about 20% below the average RNFL thickness for healthy controls at the same age.
The slopes for the relationships between VF threshold values and RNFL thickness were statistically significantly different for thicknesses higher and lower than the inflection point.