Correct calculation circle location of optical coherence tomography in measuring retinal nerve fiber layer thickness in eyes with myopic tilted discs.

PURPOSE. Retinal nerve fiber layer (RNFL) thickness measured by optical coherence tomography (OCT) can be unreliable in the presence of a myopic tilted disc. The purpose of this study was to investigate the influence of the calculation circle location of spectral domain (SD)-OCT when measuring RNFL thickness in healthy subjects with myopic tilted disc. METHODS. Sixty-nine eyes with myopic tilted disc and no other ocular abnormalities were analyzed. In all eyes, RNFL thickness was measured twice: first, along a calculation circle determined by a built-in algorithm (referred to as circle 1) and then along a manually positioned calculation circle based on the contours of the neural canal opening (NCO) (referred to as circle 2). The mean number of clock hours below normal at the 5% level and the proportion of eyes with abnormally thin clock hours at the 5% level were compared between the two methods: RESULTS. The mean number of clock hours below normal at the 5% level was significantly lower in circle 2 (0.84 ± 1.31) than in circle 1 (1.42 ± 1.43; P < 0.05). The proportion of eyes with abnormally thin clock hours at the 5% level was significantly lower (P < 0.001) in circle 2 (42%) than in circle 1 (69.9%). CONCLUSIONS. In subjects with myopic tilted disc, RNFL thickness measurements along the calculation circle based on the contours of the NCO seem to be more comparable to the normative database of the SD-OCT than does the automatically determined scan position.