Abstract
purpose. To investigate the variability of the standard reference height (SRH) in consecutive Heidelberg Retina Tomograph 3 (HRT3) examinations and its influence on five main stereometric parameters.
methods. HRT3 results of all patients attending our glaucoma center from August to October 2007 were retrospectively reviewed. Only one eye per patient with at least three consecutive HRTs and a quality control label of acceptable or better was selected. An SRH difference ≥10% compared with baseline was considered as excess SRH variability.
results. A review of 641 patients yielded 150 patients (150 eyes) who met the inclusion criteria, representing a total of 556 examinations. The mean total number of HRTs per patient was 3.7 (range, 3–7), and the mean follow-up time was 3.5 years (range, 1.4–6.7). The mean proportion of consecutive HRT3 examinations with intraindividual excess SRH variability was 46% (95% CI, 40–52), whereas the mean intraindividual excess SRH variability was 24% (95% CI, 20–28). The median absolute SRH difference was 8.6% (interquartile range, 3.9%–16.2%). Furthermore, 55.5%, 75.3%, 74.7%, 0.0%, and 19.4% of the variability in rim area, rim volume, retinal nerve fiber layer thickness, cup shape measure, and height variation contour, respectively, could be attributed to SRH variability.
conclusions. There is considerable SRH variability between HRT3 examinations of the same eye, and this could explain more than half the variability of the parameters RA, RV, and RNFL. These findings indicate that changes of HRT3 parameters should be considered with caution when excess SRH variability is present.
Glaucoma is a chronic optic neuropathy characterized by optic nerve damage, visual field defects, and often also elevated intraocular pressure.
1 2 Changes in the optic nerve head morphology are of major importance in glaucoma progression, and these often precede the onset of visual field defects.
3 4 5 Optic disc biometry by means of scanning laser ophthalmoscopy with the Heidelberg Retina Tomograph (HRT; Heidelberg Engineering, Heidelberg, Germany) has been widely used to assess structural changes of the optic nerve head and is expressed in different descriptive parameters.
6 7 8 Other imaging techniques used for this purpose are optical coherence tomography, scanning laser polarimetry, or stereophotographs.
9 10 11 12
Optic disc changes with the HRT3 can be assessed by either of two modalities, stereometric parameters or topographic change analysis (TCA). The first method depends on the standard reference height (SRH) and a manually drawn contour line essential for cross-sectional studies, whereas the second method is less dependent on these.
7 8 During each HRT3 scan, an SRH is automatically determined. The reference height is expressed in micrometers and represents the location of the reference plane relative to the mean height of the peripapillary retinal surface.
8 This reference plane is required to separate the cup and the neuroretinal rim of the disc. Structures located more deeply than the reference plane are considered cup, and structures located above the reference plane are considered rim. Ideally, the reference plane should be located at the inner surface of the sclera and beneath the retinal nerve fiber layer. As an approximation of this, the reference plane is automatically defined such that it is located 50 μm deeper than the average surface height of the papillomacular bundle at the contour line (350°–356°) and is parallel to the peripapillary retinal surface. The papillomacular bundle was chosen because it is assumed to be the last to change in the process of glaucomatous structural damage to the optic nerve head.
Variability of the SRH does occur in consecutive HRT3 measurements.
8 Therefore, we wanted to investigate the frequency and magnitude of SRH variability and its impact on the variability of the stereometric HRT3 parameters.
Given the widespread use of the HRT to detect optic disc damage progression, measurements of the stereometric parameters should be expected to be reliable. However, almost all stereometric parameter outcomes are based on an automatically determined SRH, which can vary.
8 Several studies have investigated the usefulness of different reference planes for the HRT.
16 17 18 19 20 Little has been reported on the frequency, magnitude, and impact of SRH variability on the main HRT parameters. Based on previously published work and as informally suggested by the HRT3 manufacturer, a cutoff value of 10% variability was chosen. Our study clearly indicates that excess SRH variability does occur in a considerable proportion of follow-up HRT3 scans (46% of all HRT3 scans) and was 24% on average. In a large sample of consecutive HRT3 examinations of good quality, more than half the variability in RNFL, RA, and RV could be attributed to variability in the SRH. Strouthidis et al.
21 reported that intertest differences in reference height and image quality had a strong relationship with intertest RA differences and that together these were responsible for 70% of the intertest variability of RA measurements. In addition, Tan et al.
22 found that the most frequent contributor to RA variability was the position of the reference plane with respect to the optic nerve head. A more stable reference plane was recently suggested (Kappou V, et al.
IOVS 2008;49:ARVO E-Abstract 3650). Regarding the reproducibility of HRT parameters, Miglior et al.
23 concluded that image acquisition-induced variability seemed larger than operator-induced variability. Other factors to be taken into account are measurement variability and the noise of HRT measurements.
24
In fact, a more stable SRH or an appropriate correction for SRH variability is necessary. In this study, the within-patient SD of the SRH (0.033), representing the SRH fluctuations within a patient, amounted to one-third of the between-patient SD of the SRH (0.105). Considering the mean SRH (0.29), the within-patient SD of the SRH indicated that intraindividual SRH fluctuations were nonnegligible.
The HVC, as an HRT3 parameter, was only slightly influenced by SRH variability, whereas the CSM appeared to be independent of SRH. The latter finding suggests that CSM could be a preferred HRT3 parameter to determine glaucoma progression. Despite its independence of the SRH, one can graphically observe that considerable variability in CSM still exists. HVC represents the height variation of the retinal surface along the contour line—that is, the height difference between the most elevated and the most depressed points of the contour line.
8 In light of this definition, with HVC also independent of SRH, our results still demonstrate a limited influence of SRH variability on HVC changes over time, probably because of measurement variability. Our findings indicate that TCA, provided appropriate image alignment correction and considering guidelines for clinical versus statistical significant progress, may be preferred to assess glaucoma progression.
In summary, we investigated SRH variability in consecutive HRT3 examinations of good quality and its influence on stereometric parameters. We found that 45.8% of intraindividual follow-up HRT3 examinations showed excess SRH variability. The impact rates of SRH variability on the variability of the stereometric parameters RA, RV, RNFL, CSM, and HVC were 55%, 75%, 74%, 0%, and 19%, respectively. These findings indicate that changes in stereometric HRT3 parameters should be considered with caution when the variability of the SRH exceeds 10% compared with baseline. Therefore, researchers and the HRT manufacturer are challenged to further refine this technique or the software to obtain a more stable reference plane, resulting in more repeatable and reliable stereometric HRT parameters for the detection of glaucoma progression.
Presented at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, April 2008.
Submitted for publication May 24, 2008; revised July 21, 2008; accepted September 10, 2008.
Disclosure:
C. Breusegem, None;
S. Fieuws, None;
I. Stalmans, None;
T. Zeyen, None
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “
advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Corresponding author: Thierry Zeyen, Department of Ophthalmology, Leuven University Hospitals, Campus St. Rafaël, Kapucijnenvoer 33, 3000 Leuven, Belgium;
[email protected].
Table 1. Variability of HRT3 Parameters Explained by Variability in SRH
Table 1. Variability of HRT3 Parameters Explained by Variability in SRH
HRT3 Stereometric Parameter | Percentage of Variability Explained by SRH Variability |
Rim area | 55.5 |
Rim volume | 75.3 |
Retinal nerve fiber layer thickness | 74.7 |
Cup shape measure | 0.0 |
Height variation contour | 19.4 |
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