Correlation between depth and area of retinal nerve fiber layer defect as measured by spectral domain optical coherence tomography

Background: To evaluate the correlation between the depth and area of retinal nerve fiber layer (RNFL) defect, as measured on an RNFL map of spectral-domain optical coherence tomography (SD-OCT). Methods: The RNFL of 472 glaucoma subjects and of 217 healthy subjects was imaged by an SD-OCT. RNFL defect depth and area on the RNFL map were expressed as an RNFL defect depth percentage index (RDPI) and an RNFL defect area index (RDAI), respectively, according to the following two formulas: 100×[1–{summation of thicknesses of RNFL defects/summation of thicknesses of upper 95th percentile range of age-matched healthy subjects in areas corresponding to defects}]; 100×[number of superpixels of RNFL defects/(46 × 46–superpixels inside optic disc or β zone parapapillary atrophy)]. The best-fitting model describing the relationship between the two parameters was derived by fractional polynomial analysis. Results: Logarithmic fit was determined to be the best-fitting model in describing the relationship of the RDPI against the RDAI (y = 53.4 + 3.7 ln(x) and y = 50.9 + 5.9 ln(x) in superior and inferior hemifields, respectively). The expected RDAIs at the point where the RDPI and RDAI rates of change were the same were 3.7 and 5.9 %; the corresponding upper 95 % confidence interval limits of the RDPI 59.0 and 61.8 % in the superior and inferior hemifields, respectively. Conclusions: The correlation between the RNFL defect depth and area, as derived from the RNFL map, was best described by the logarithmic fit. Changes were more marked in depth than in area, especially for mild localized defects.