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Analyses exploring relationships between structural and functional losses in glaucoma show that early structural damage as measured by spectral-domain optical coherence tomography can occur in the absence of visual field changes.
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), and 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 there was a change in slope in the relationship between VF and RNFL thickness.
"Our findings are consistent with previous data based on histological analysis that shows substantial tissue loss occurs before glaucomatous VF changes are detectable," he said.
The message is clinicians can expect to find a substantial zone where patients can experience glaucomatous structural damage without functional change. However, they need to beware that VF loss can be expected to occur once the patient reaches the tipping point so that they can plan for closer follow-up and consider the need for alternative treatment options, he said.
The relationship between structure and function in glaucoma has been the subject for extensive research, and previous studies have shown there is not a simple linear relationship. Regardless of the technology used to measure structure, the results showed a curvilinear relationship where VFs remained relatively unchanged with early structural loss followed by a steep decline demonstrating accelerated functional loss.
"The statistical methods used in previous studies had a 'one-size-fits-all' approach, but the relationship between structure and function in glaucoma may not be the same across the entire spectrum of the disease," Dr. Wollstein said. "The broken-stick method of analysis is a piecewise linear regression that addresses this possibility."