Scanning laser tomography is a well established technique that provides reproducible ONH measurements.14,15
The topographic measures produced by the HRT and its predecessor, the laser tomographic scanner (LTS, Heidelberg Engineering, Heidelberg, Germany), have been demonstrated to be repeatable,16,17
and to have less variation compared with other techniques such as computer assisted planimetry.18
Little has been published about the reproducibility of the HRT-II.19,20
As the HRT-II is intended as a “clinical” instrument, its reproducibility under clinical conditions needs to be established. The profile of the subjects in this study was heterogeneous in terms of demographics, disease stage, refractive error, media opacity, and image quality, and therefore simulates the patient profile encountered in clinic. Image quality has previously been shown to be associated with pupil size and the degree of lens opacity (both objective scoring and LOCS III grading). Image quality was seen to improve with pupillary dilation but the improvements were often small.21
Pupil size was therefore not taken into consideration in this study. None of the subjects was taking miotic medications at the time of the study, although this was not a recruitment criterion.
Since the publication of the original reproducibility studies of the HRT,17,22
the Windows based Explorer platform has been introduced. From this perspective, this study is the first to examine the repeatability of HRT defined morphometric parameters using the newer software.
RA and mean cup depth were the most repeatable parameters for both devices. Some caution is required when interpreting coefficients of variability as some parameters, such as cup area and cup shape measure, have mean values of low magnitude (approaching zero). Another difficulty is the interpretation of differences between ICC values of a similar magnitude. It is therefore unlikely that there is any real difference in measurement reliability between mean cup depth, cup area, cup volume, and RA. It should also be noted that the ICC values depend on the variability of the sample population. As our sample was enriched with eyes with lenticular opacity, the ICC values may not be applicable to other populations with less cataract.
Overall, RA and mean cup depth were consistently the most repeatable and reliable of the parameters measured. This concurs with previous findings.23
The findings from another study showed that mean cup depth and cup area were the least variable parameters measured with the HRT-II.19
There is, however, no advantage in measuring cup area as it is merely the difference between disc area (kept constant in Explorer) and RA. As it contains the retinal ganglion cell axons, RA is a meaningful parameter for physicians. It has also been shown to discriminate between normal, glaucoma, and OHT subjects,24–26
and is therefore an appropriate candidate for the assessment of progression.
In this study, the repeatability of RA measurements was similar with both devices (RC
0.2 – 0.3 mm2
), irrespective of observer or test interval. Similar repeatability between imaging performed at the same visit or at different visits is consistent with a previous study, where no difference was identified in the short term and long term variability of topographic measurements.27
The HRT-II performs at least as well as the HRT. The similar level of RA repeatability between HRT-II and HRT Explorer analyses indicate that the two methods could theoretically be used interchangeably in a longitudinal setting.
The sources of variability for the HRT have been documented and include patient/scanner misalignment,28
and interobserver differences in optic disc contour line drawing.18,29,30
The present study identifies inter-test reference height difference to be the most consistent factor related to test-retest variability. It has previously been reported that the use of a 320 μm reference plane reduced RA variability, compared with the Standard reference plane.31
Image quality, as recorded by MPHSD, is another factor consistently found to influence variability. MPHSD is a gauge of the variability of pixel height measurements across the three topographic images used to construct the mean image.14
This study shows that image quality was, in turn, influenced by lens opacity, age, and degree of astigmatism. Sihota et al
also found a significant correlation between the test-retest variability of the HRT-II and age and degree of astigmatism.19
Our results suggest that MPHSD may be an appropriate summary measure for the effect of these factors. It is therefore possible to predict repeatability coefficients for various levels of image quality without having to measure the patient’s age, degree of media opacity, or astigmatism.
In conclusion, this study indicates that RA may be an appropriate measure when monitoring glaucoma progression, as its measurements consistently showed excellent repeatability and reliability. Repeatability was similar with both the HRT and HRT-II, irrespective of observer or test interval. Reference height difference and image quality were found to be the factors that influenced RA variability most. The findings of this study will be used as the basis for suggesting strategies for improving test-retest repeatability. Once this is achieved, strategies for monitoring stereometric parameter progression can be devised and tested.