A comparison of cup-to-disc ratio measurement in normal subjects using optical coherence tomography image analysis of the optic nerve head and stereo fundus biomicroscopy.

Purpose: To determine the relationships between morphometric optic nerve head (ONH) characteristics defined using optical coherence tomography (OCT) and clinical judgments of cup-to-disc (CD) ratios.

Methods: CD ratios were estimated in 20 subjects by two experienced clinicians using stereo fundus biomicroscopy. Sagittal OCT images were obtained in positions defined by clinicians corresponding to the location of their horizontal and vertical CD ratio estimates. To estimate CD ratios using the OCT images, disc width was defined as the separation of the terminations of the retinal pigment epithelium on each side of the image. OCT cup widths were determined using two criteria: the first measured the separation of cup walls halfway down each side and the second estimated the separation 1/3 down each side. Cups that were less than 150 microm deep were defined as flat for the OCT images. Data were analysed using mean and standard deviation of the differences between clinical estimates and imaging techniques as well as using intraclass correlations (ICC).

Results: Between techniques, the range of ICCs for the 1/2 depth criterion was 0.74-0.90 and for the 1/3 criterion was 0.59-0.81. The limits of agreement for the 1/2 criterion were also better than the 1/3 criterion.

Conclusion: Accurate measures of CD ratio may be derived from OCT ONH images. Estimates of clinical CD ratios in normal subjects appear to be made using depth judgments closer to 1/2 of the way down the cup than 1/3.