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Monica Chen, Ali S. Raza, Donald C. Hood; The Transition Zone from Relatively Healthy to Severely Affected Retina in Glaucoma Patients. Invest. Ophthalmol. Vis. Sci. 2012;53(14):705.
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To better understand the structural changes in the transition zone (TZ) from relatively healthy to severely affected regions in glaucoma patients using frequency domain optical coherence tomography (fdOCT).
High-quality fdOCT macular vertical line scans (3DOCT-2000; Topcon, Inc) were obtained from 12 eyes of 10 glaucoma patients (61.1 ± 7.7 yrs) with central defects and from 15 eyes of 15 healthy controls (55.1 ± 11.0 yrs). Using a previously validated  computer-aided manual segmentation procedure , retinal nerve fiber layer (RNFL) and retinal ganglion cell plus inner plexiform layer (RGC+) thicknesses were obtained and then normalized by subtracting the mean control thicknesses at each retinal point. These normalized thicknesses were plotted relative to the point (tz0) they dropped below controls by 2 SD. A piecewise model was fitted to the plots of RNFL and RGC+ thickness versus retinal location relative to tz0. According to the model, T (normalized thickness) = 0 when x < x1 (healthy region), T = m(x - x1) when x1 ≤ x ≤ x2 (TZ region), and T = r when x > x2 (severely affected region) where x is distance relative to tz0 in mm, m is the rate of thickness change across the TZ in µm/mm, r is the residual thickness (µm) measured in the region severely affected by glaucoma, and w (w = x2-x1) is the width of the TZ.
The model provided a reasonable fit to the average RNFL (R2 = 0.99) and RGC+ (R2 = 0.99) data (model dashed, avg black, ±1 SE gray in Fig). The best fitting values (m; r; w) were -39 μm/mm; -25 μm; 0.64 mm for the RNFL and -53 μm/mm; -32 μm; 0.60 mm for the RGC+. For individual RNFL and RGC+ data, the range of best fitting values for TZ width (w) was 0.38 to 1.04 mm (RNFL) and 0.31 to 1.40 mm (RGC+). An abrupt model (dotted in Fig) did not fit as well [R2 = 0.78 (RNFL), 0.80 (RGC+)].
There is a TZ between healthy and severely affected regions of glaucomatous field defects in the macula. While the TZ is relatively narrow, on average 2.2° (RNFL) and 2.1° (RGC+), it is broader than expected based on an abrupt model. It remains to be determined whether the RGC+ TZ represents "sick cells" or a mixture of healthy and lost cells. In any case, measurement of the TZ may provide a sensitive method for assessing progression. Hood DC et al. OVS. 2011.  Hood DC, Lin CE et al. IOVS. 2009.
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