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RNFL thickness threshold linked to visual function loss in open-angle glaucoma
Br J Ophthalmol. 2012;96(1):47-52.
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Detectable loss of visual function correlated with a roughly 17% reduction in retinal nerve fiber layer thickness in glaucomatous eyes, according to a study.
"Substantial structural loss ... appears to be necessary for functional loss to be detectable using the current testing methods," the study authors said.
Investigators set out to determine the retinal nerve fiber layer (RNFL) thickness at which visual field damage could be detected and linked to structural loss.
The prospective study included data from 72 healthy subjects and 40 open-angle glaucoma patients culled from the Pittsburgh Imaging Technology Trial, a longitudinal analysis designed to assess changes in ocular structure. Healthy subjects had a mean age of 50 years; glaucoma patients had a mean age of 65.5 years.
All subjects underwent unilateral visual field examination and optical coherence tomography imaging of the optic disc. Investigators used statistical analysis to determine a "tipping point" at which visual field values were associated with diminished RNFL thickness.
"The tipping point allows us to define the RNFL threshold where strong association with [visual field] abnormalities can be expected," the authors said.
Study results showed that the mean RNFL thickness tipping point for visual field loss was 75.3 µm, with RNFL thickness reduced 17.3% from expected age-matched normal values and a 25% reduction in the superior and inferior quadrants.
Above the tipping point, the slope for RNFL thickness and threshold was 0.03 dB/µm; below the tipping point, the slope was 0.28 dB/µm. The difference was statistically significant (P < .001).
"A similar pattern was observed for quadrant and clock-hour analysis," the authors said.