Regional and long-term variability of fundus measurements made with computer-image analysis

We studied the variability of optic disk and peripapillary nerve fiber layer surface contour measurements made by use of computer-image analysis. Six hundred twenty-five measurements of surface contour were made on each eye by use of simultaneous stereoscopic videography. Regional differences in short-term measurement variability were studied in 12 eyes (six normal and six glaucomatous), each imaged nine times over several days. The widths of the 95% confidence interval for the measurements averaged 82 microns for the juxta-papillary surface and 132 microns for the disk surface. Measurements of peripapillary surface contour were significantly less variable than were measurements of the disk surface (P = .000). The greatest variability was detected along large blood vessels and at steep contours. Long-term variability was studied in a separate group of 30 clinically stable patients with glaucoma, each imaged three to six times over a period of more than one year. The widths of the 95% confidence intervals were 132 microns for the peripapillary surface and 217 microns for the disk surface. The long-term variability was significantly greater than the short-term variability (P = .000). The peripapillary nerve fiber layer surface, located away from the margins of large vessels, may provide the most dependable measurements of contour. These estimates of long-term variability of optic disk and peripapillary contour measurements provide clinically relevant confidence intervals with which to detect progressive glaucomatous nerve fiber damage.