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

Test-retest variability in structural parameters measured with glaucoma imaging devices

In addition to classical stereo-disc photography, various glaucoma imaging devices were developed in the last two decades to quantitatively measure and record glaucoma-related structural parameters of the eye. In determining whether or not the glaucomatous damage progressed from baseline and in estimating the number of test results' optimal frequency needed to confirm disease progression, information relating to the test-retest variability of measurement results provided by each imaging device is indispensable. Such information enables the clinician to apply these devices in practice. The test-retest variability of a system is usually estimated using the Bland-Altman analysis and by calculating the coefficient of variation (CV), intraclass correlation coefficient (ICC), and minimum detectable changes (MDC). The reported CV, ICC, and MDC values for glaucoma-related structural parameter measurement results of stereo-disc photographs, confocal scanning laser ophthalmoscopes, scanning laser polarimeters, time-domain optical coherence tomography (OCT), spectral-domain OCT (SD-OCT), anterior-segment OCT, and ultrasound biomicroscope are systematically reviewed in this manuscript, which will enable the clinician to interpret measurement results provided by each glaucoma imaging devices and thus be useful in practice. Although SD-OCT systems may be currently prevailing because of the volume of information provided and the relatively better test-retest variability, these systems need improvement in their test-retest variability measurement capabilities.

Reproducibility of the optic nerve head topography with a new laser tomographic scanning device

BACKGROUND

Laser scanning tomography has been shown to be an accurate and reliable method for the assessment of the three-dimensional optic disc topography. The authors investigate the reliability of morphometric measurements with the Heidelberg retina tomograph, a new instrument which was designed based on this technology, which simplifies handling and is much smaller than the laser tomographic scanner.

METHODS

Three independent measurements of the optic disc were performed in 39 eyes of 39 patients which were equally divided into the following three groups: glaucoma, glaucoma suspects, and controls.

RESULTS

The mean coefficient of variation for measurement in the glaucoma, glaucoma suspect, and control groups was 2.9%, 5.0%, and 3.4%, respectively, for cup area; 4.9%, 4.6%, and 4.6%, respectively, for cup volume; 5.2%, 3.8% and 3.3%, respectively, for mean cup depth; and 5.2%, 4.1%, and 4.0%, respectively, for maximal cup depth. The mean standard deviation for one pixel of the total image was 30 +/- 6 microns, 28 +/- 7 microns, and 22 +/- 6 microns for the three groups, respectively.

CONCLUSION

The Heidelberg retina tomograph enables fast and reliable measurement of the optic disc topography and therefore may allow exact follow-up of patients.

Reproducibility of topometric data acquisition in normal and glaucomatous optic nerve heads with the laser tomographic scanner

The reproducibility of optic disc cup measurements was analyzed in 24 eyes of 24 patients [8 normals, 8 glaucoma patients, 8 glaucoma suspects] using the Laser Tomographic Scanner. The mean coefficient of variation in triple measurements was 5.0% for the cup area, 5.4% for the rim area, 7.0% for the cup volume, 4.0% for the mean cup depth, and 4.3% for the maximum cup depth. Mean reliability between two of the three measurements performed in each eye was better than 0.988 for the cup area, 0.995 for the cup volume and 0.996 for the mean cup depth readings. These results suggest that laser scanning tomography allows highly reproducible measurements in living eyes and adds an important tool to the ophthalmologist's armamentarium for the diagnosis and follow-up of glaucoma patients.

Laser scanning tomography and stereophotogrammetry in three-dimensional optic disc analysis

Laser scanning tomography (LST) and computed stereophotogrammetry (CSP) are sophisticated diagnostic tools for the three-dimensional analysis of optic nerve head topography. The two methods are based on different physical principles. To compare the information about the shape of the cup of an optic nerve head obtained by LST and CSP, we evaluated the volume profile (VP; i.e., the cross-sectional area of the cup from top to bottom) in 36 discs of 36 patients (20 control group discs C, 16 glaucoma discs G). The Spearman correlation coefficient between the photogrammetric and the laser scanning VP-slope measurements was rs = 0.931; P < 0.001 (rs = 0.935 G, P < 0.001; rs = 0.910 C, P < 0.001). The results suggest that confocal laser scanning provides readings of the shape of the optic disc cup that are similar to the measurements of computed stereophotogrammetry.

Age-related decline of disc rim area in visually normal subjects

Optic disc parameters and axial lengths were measured in 81 visually normal young and elderly white adults (young [n = 41]: 27.2 +/- 5.7 years versus elderly [n = 40]: 68.8 +/- 8.4 years) using Rodenstock Optic Disc Analyzer and A-scan sonography. Axial lengths correlated positively with optic disc parameters (Spearman correlation analysis: rim area: P = 0.042, disc area: P = 0.052, and cup volume: P = 0.010). Older subjects had a shorter axial length (23.09 +/- 1.02 mm) and smaller disc rim area (1.171 +/- 0.338 mm2) than younger subjects (23.60 +/- 1.15 mm and 1.325 +/- 0.314 mm2; Mann-Whitney U test: P = 0.011 and P = 0.013, respectively). While both axial length and disc rim area declined with age (0.011 mm and 0.003 mm2 per year; Spearman correlation analysis: P = 0.032 and P = 0.020, respectively), the cup-to-disc and rim area-to-disc area ratios appeared to remain relatively constant throughout adult life. The age-related decline of disc rim area is consistent with histologic evidence of age-related decline of ganglion cell axons. The rim area-to-disc area ratio seems less affected by age, and is thus a better parameter to isolate age-related change from disease-related change of optic nerve in a longitudinal follow-up of diseases involving the optic nerve head.