ULTRASONOGRAPHY OF THE OPTIC DISC CUP IN DISCS OF VARIOUS SIZES

Purpose: The aim of the study was to evaluate the sensitivity of modern, eye-dedicated B-scanners in detecting optic disc cups and to correlate the echographic measurements with an optical assessment. A secondary aim was to correlate the rim-to-rim diameter of the optic disc and the width of the retrobulbar optic nerve on standardised A-scan. Methods: A prospective study was conducted on 25 emmetropic subjects (50 eyes) with discs and cups of various sizes. They were examined optically by one observer using a high-magnification slit lamp and the 90 D Volk lens. Another observer measured the B-scan dimensions of the disc cups and the retrobulbar optic nerve. Data from the two observers were compared at the end of the study. Results: High resolution B-scan detected optic disc cups of 0.5 mm or larger in all cases. A statistically significant correlation was obtained between echographic measurements and both vertical optical cup disc diameters and cup disc (C:D) ratios (p<0.0001). Large C:D ratios (0.6 or larger) but small rim-to-rim disc diameter were reported as 'small' on echography. No correlation was found between the measurement of retrobulbar nerve and the diameter of the vertical optical disc diameter. Conclusion: Modern, eye-dedicated ultrasonographic scanners are capable of detecting optic disc cups of 0.5 mm or larger. As there is a normal variation in optic disc rim-to-rim diameter, and this cannot be measured using ultrasound, we suggest the reporting of a C:D ratio on ultrasonography to be potentially misleading. Large disc cups are, however, easily seen and many are likely to be 'pathological'.