Optic nerve head measurements: the optic nerve head analyzer--its advantages and its limitations

The Optic Nerve Head Analyzer (ONHA) calculates by means of computer-assisted analysis of stereo images different parameters of the optic disc: disc diameter, disc size, cup/disc ratio (CDR), neuroretinal rim area, and excavation volume for the disc quadrants and for the total disc. To obtain first indications of the clinical value of ONHA measurements for diagnosis and follow-up in glaucoma we examined the reproducibility of measurement results for different diseases. Furthermore, we studied the mean values of the different disc parameters in healthy eyes. It was investigated: 1) whether the reproducibility is different in the different disc parameters; 2) whether the reproducibility in different eye diseases is different as compared to healthy eyes; 3) by which criteria the reproducibility is influenced; 4) whether there is a correlation between disc size and size of rim area. The reproducibility was studied in 178 eyes of 178 patients, who were all examined twice with the ONHA. The mean difference between the results of first and second measurement was calculated for the different disc parameters. Differences were found in the reproducibility of the parameters: e.g., the mean difference between first and second measurement was 1.8 percent for the disc size, and 5.6 percent for the neuroretinal rim area. For the disc quadrants, the reproducibility of values was worse than for the total disc. No marked differences of reproducibility of disc parameters were found for different diseases. Reproducibility depends, for instance, upon correct determination of the disc margin. In healthy eyes, a significant correlation was found between rim area and disc size: larger discs have a larger neuroretinal rim area than smaller discs. Thus, the rim areas of different eyes are only comparable for equally sized discs. Relative values, as for instance, cup/disc ratio, or the quotient of rim area and disc size, are therefore better suited for comparison of different eyes than absolute values. The recent developments in automatic disc analysis equipment and the clinical relevance of the results for diagnosis and follow-up in glaucoma are discussed.