The effect of scan diameter on retinal nerve fiber layer thickness measurement using stratus optic coherence tomography.

OBJECTIVE To study how customizing the peripapillary scan diameter on the basis of optic nerve head (ONH) diameter affects retinal nerve fiber layer (RNFL) thickness measurements using Stratus optical coherence tomography (OCT). METHODS Retinal nerve fiber layer was examined using 1 fixed-diameter circular scan (3.4 mm) and 2 customized-diameter scans (at 0.5 mm and 1 mm from the ONH edge) in 81 healthy subjects. RESULTS Using fixed-diameter scans, the mean RNFL thickness increased with larger ONH vertical diameters (r = 0.3425, P =.002), whereas using customized-diameter scans, negative correlations were detected (r = -0.3004 [P =.006] at 0.5 mm and r = -0.2369 [P =.03] at 1 mm from the ONH edge). The mean values obtained by customized-diameter scans showed lower standard deviations in most measurements, meaning a tendency toward lower interindividual variability. CONCLUSIONS When RNFL thickness is measured at a constant distance from the ONH edge, larger discs exhibit a thinner RNFL. Hence, the correlation between large discs and thicker RNFLs observed using the standard fixed-diameter scan probably represents a technical artifact reflecting the shorter distance between the scan and the ONH edge. A new normative database, stratified not only on the basis of age but also on the basis of ONH size, is suggested.