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Monica Chen, Ali Raza, Elaine Su, Jeffrey Odel, C. Gustavo De Moraes, Jeffrey Liebmann, Robert Ritch, Donald Hood; A Comparison of Transition Zones from Relatively Healthy to Severely Affected Retinal Nerve Fiber Layer in Glaucoma and NAION Patients. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2352.
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To study structural changes in the transition zone (TZ) from relatively healthy to severely affected regions in glaucoma and non-arteritic anterior ischemic optic neuropathy (ION) using frequency domain optical coherence tomography (fdOCT).
Peripapillary fdOCT scans (black circle, Fig. 1) were obtained from 17 eyes of 13 glaucoma patients (62.1±15.2 yrs) with central arcuate defects on 10-2 fields, 8 eyes of 7 ION patients (67.1±10.7 yrs), and 54 eyes of 54 controls (53.2±8.1 yrs). Macular vertical line scans (black bar, Fig. 1) were obtained from the glaucoma patients and 16 eyes of 16 controls (55.5±10.7 yrs). Using a hand-corrected algorithm,[1,2] retinal nerve fiber layer (RNFL) thicknesses were obtained across the TZ in the macular (M) and peripapillary (P) region.
For the glaucoma (GL) M (blue) and P (green) and the ION P (red) analyses in Fig. 2A, the patients’ RNFL thickness profiles were averaged after shifting relative to the point (tz0) where thicknesses fell below a 95% confidence limit in controls. Absolute thickness loss was determined by subtracting the mean control thickness (Fig. 2B) and relative thickness loss by dividing the absolute thickness loss by the mean control thickness (Fig. 2C). The rate of thickness change at the edge of TZ (slope) was determined in a 0.2 mm window near tz0 (dotted lines). The slopes were: dissimilar for the shifted data (Fig. 2A): -.002 (GL M), -.051 (GL P), and -.106 (ION P); less discrepant for absolute thickness loss (Fig. 2B): -.042, -.113, and -.075; but relatively similar for relative thickness loss (Fig. 2C): -.116, -.102, -.085 %/μm. Surprisingly, there was a modest correlation (R2=.33) between the slopes of the relative thickness losses for GL P and GL M.
When expressed as relative thickness loss, the slopes of the TZs are similar for macula and peripapillary regions affected by glaucoma and for peripapillary regions affected by glaucoma and ION. This suggests that while different numbers of axons are affected in each TZ, about the same relative number of axons is lost. TZ analyses may constrain models of mechanisms of diseases affecting the RNFL. 1. Raza et al AO 2011; 2. Yang et al Opt Exp 2010.
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