Purpose To evaluate the effect of the entry position of the scan beam on measurements of optic nerve parameters by using spectral-domain optical coherence tomography (OCT).
Methods 16 eyes from 16 normal subjects were scanned by the Optic disc cube 200x200 protocol of a spectral-domain Cirrus HD-OCT (Carl Zeiss Meditec, Inc., Dublin, CA) under dilated pupils (pupil diameter no less than 7mm). For each eye, the entry position of the scan beam through the dilated pupil was varied horizontally. Parapapillary RNFL thickness, disc area and rim area from 3 different scan beam locations (temporal, central and nasal, respectively) were recorded and compared.
Results Significant differences in   RNFL thicknesses were found among different scan locations (e.g average RNFL thickness for temporal, central and nasal scanning were 92.9 ± 7.4um, 97.0 ± 8.0um, and 101.0 ± 6.8um, respectively, all with p<0.001).No significant difference was detected for disc area (temporal, 1.76 ± 0.35mm2; central, 1.75 ± 0.35 mm2; nasal, 1.75 ± 0.36 mm2; all with p>0.05) and rim area (temporal, 1.26 ± 0.15 mm2; central, 1.24 ± 0.17 mm2; nasal, 1.27 ± 0.14 mm2; all with p>0.05) from different scan locations.
Conclusions Measurement of RNFL thickness but not optic disc parameters was significantly affected by the entry position of the scan beam of Cirrus HD-OCT. Caution should be exercised in interpreting the results of RNFL measurement from different scan beam entry position during longitudinal assessment. Obtaining RNFL measurement at similar scan beam entry position is recommended.