Long-term fluctuations of the normalised rim/disc area ratio quotient in normal eyes

The assessment of the cup of the optic disc in follow-up of glaucomatous optic nerve heads depends on the variability of the cup area over time. We examined the variability of topographic measurements depending on scan focus settings and evaluated the long-term fluctuations of the normalised rim/disc area ratio quotient of normal subjects for 1 year.

Part 1. Evaluation of the influence of scan focus and corneal curvature on the number of pixels per millimetre in one emmetropic volunteer. The scan focus was varied using contact lenses of different refraction. Part 2. HRT examinations of the optic nerve head of five eyes of five volunteers were repeatedly recorded during a 12-month period. The contour line from the first mean topographic image was transferred into the following seven mean topographic images of each subject.

Part 1. The size of pixels depends on the adjusted scan focus for mean corneal curvature radius between 6 and 9 mm. However, after image to image scaling the size of pixels is nearly independent to the scan focus. Part 2. We found the highest variation of normalised rim/disc area ratio for the location of the temporal horizontal sectors, followed by the temporal inferior and the temporal superior sectors. Variation was smallest in the nasal sectors. Mean fluctuation of the normalised rim/disc area ratio quotient of the 95th percentile was 0.09 following image to image scaling compared to 0.16 when using individual image scaling.

Scan focus variation during HRT follow-up examinations can influence optic disc parameters, especially when image to image scaling has not been performed. A confidence area of an individual optic disc could now be calculated. These confidence bands could help to distinguish more easily between progress of glaucomatous optic nerve head damage and physiological intertopographic variation of optic cup measurements. A decrease of more than 43% in the fluctuations of the normalised rim/disc area ratio could be achieved using image to image scaling techniques. For that reason follow-up of glaucomatous optic nerve heads should only be performed using this software application.