The impact of retardance pattern variability on nerve fiber layer measurements over time using GDx with variable and enhanced corneal compensation

Retinal nerve fiber layer progression in glaucoma: a comparison between retinal nerve fiber layer thickness and retardance

OBJECTIVE

To investigate the performance of spectral-domain optical coherence tomography (OCT) and scanning laser polarimetry to detect progressive retinal nerve fiber layer (RNFL) changes and to determine whether reduction of the RNFL retardance occurred before thinning of the RNFL in glaucoma.

DESIGN

Prospective, longitudinal study.

PARTICIPANTS

One hundred eighty-four eyes of 116 glaucoma patients and 43 normal eyes of 23 healthy individuals.

METHODS

Patients were followed up every 4 months for at least 36 months with RNFL retardance (GDx Enhanced Corneal Compensation; Carl Zeiss Meditec) and RNFL thickness (Cirrus HD-OCT; Carl Zeiss Meditec) measured in the same visit. Progressive RNFL retardance and thickness changes were evaluated with event-based analysis (Guided Progression Analysis; Carl Zeiss Meditec) with reference to the RNFL retardance change map and the RNFL thickness change map, respectively. The area and frequency distribution of RNFL changes were examined by overlaying the RNFL retardance change maps and the RNFL thickness change maps in the latest follow-up. The agreement of RNFL retardance and RNFL thickness progression was evaluated with κ statistics.

MAIN OUTCOME MEASURES

Number of eyes with progressive RNFL changes over time.

RESULTS

A total of 2472 OCT thickness maps and 2472 RNFL retardance maps were collected and reviewed with a mean follow-up of 55.1 months. Twenty-seven eyes (14.6%; 26 glaucoma patients) showed progressive RNFL thinning, whereas 8 eyes (4.3%; 8 glaucoma patients) showed progressive reduction of RNFL retardance. Seven eyes (3.8%; 7 glaucoma patients) had progression that was detected by both instruments, all with progressive RNFL thinning detected before progressive reduction of RNFL retardance became evident, and the mean lag time was 13.4 months (range, 4.0-37.6 months). The agreement between RNFL thickness and RNFL retardance progression was fair (κ, 0.357). Progressive loss of RNFL thickness was observed most frequently at the inferotemporal 223° to 260°, whereas the inferotemporal 227° to 263° and superior 56° to 117° were observed most commonly for progressive loss of RNFL retardance. In the normal group, no eyes showed reduction in RNFL thickness or retardance.

CONCLUSIONS

At a comparable level of specificity, progressive RNFL thinning was detected more often than progressive reduction of RNFL retardance. For eyes with progressive loss of RNFL thickness and RNFL retardance, the former preceded the latter.

Retinal nerve fiber layer measurements by scanning laser polarimetry with enhanced corneal compensation in healthy subjects

PURPOSE

To evaluate the (i) effects of biological (age and axial length) and instrument-related [typical scan score (TSS) and corneal birefringence] parameters on the retinal nerve fiber layer (RNFL) measurements and (ii) repeatability of RNFL measurements with the enhanced corneal compensation (ECC) protocol of scanning laser polarimetry (SLP) in healthy subjects.

METHODS

In a cross-sectional study, 140 eyes of 73 healthy subjects underwent RNFL imaging with the ECC protocol of SLP. Linear mixed modeling methods were used to evaluate the effects of age, axial length, TSS, and corneal birefringence on RNFL measurements. One randomly selected eye of 48 subjects from the cohort underwent 3 serial scans during the same session to determine the repeatability.

RESULTS

Age significantly influenced all RNFL measurements. RNFL measurements decreased by 1 µm for every decade increase in age. TSS affected the overall average RNFL measurement (β=-0.62, P=0.003), whereas residual anterior segment retardance affected the superior quadrant measurement (β=1.14, P=0.01). Axial length and corneal birefringence measurements did not influence RNFL measurements. Repeatability, as assessed by the coefficient of variation, ranged between 1.7% for the overall average RNFL measurement and 11.4% for th nerve fiber indicator.

CONCLUSIONS

Age significantly affected all RNFL measurements with the ECC protocol of SLP, whereas TSS and residual anterior segment retardance affected the overall average and the superior average RNFL measurements, respectively. Axial length and corneal birefringence measurements did not influence any RNFL measurements. RNFL measurements had good intrasession repeatability. These results are important while evaluating the change in structural measurements over time in glaucoma patients.

Glaucoma diagnostics

This thesis addresses several aspects of glaucoma diagnostics from both a clinical and a screening perspective. New instruments for diagnosing glaucoma have been developed over the past years, but little information is available regarding their performance as screening methods and their usefulness in ordinary clinical practice. PURPOSE OF THE RESEARCH UNDERLYING THIS THESIS:  The objectives of this research were as follows: to compare the accuracy of results of analysis of the optic nerve head (ONH) achieved by computerized imaging using the Heidelberg Retina Tomograph (HRT) and by subjective assessment performed by physicians with different degrees of experience of glaucoma (paper III); to evaluate the effect of a continuous medical education (CME) lecture on subjective assessment of the ONH for diagnosis of glaucoma (paper II); to investigate subjective assessment of perimetric test results by physicians with varying knowledge of glaucoma with a trained artificial neural network (ANN) and to compare the certainty of the classifications (paper IV); and to compare the diagnostic performance of time-domain Stratus optical coherence tomography (OCT) with that of spectral-domain Cirrus OCT (paper I), frequency doubling technology (FDT) screening perimetry and scanning laser polarimetry with the GDx variable corneal compensator (VCC) in a random population-based sample and in patients with glaucoma of varying disease severity.

METHODS AND RESULTS

  In evaluation of the ONH, use of the HRT statistical tools, Moorfields regression analysis (MRA) and the Glaucoma Probability Score (GPS) was compared with subjective assessment performed by 45 physicians. Optic nerve head images and photographs from 138 healthy and 97 glaucoma subjects were included. The sensitivity of MRA was higher (87-94%) than that of the average physician (62-82%), considerably greater than that of ophthalmologists with subspecialties other than glaucoma (53-77%) and non-significantly better than that of glaucoma experts (72-88%). Sensitivity achieved by GPS (79-93%) was also greater than that of the average physician. MRA correctly classified all eyes with advanced glaucomatous visual field defects, a result that was not achieved by GPS or even by the glaucoma experts. In eyes with small discs, MRA sensitivity (88%) was comparable with that of glaucoma experts (85%) and much better than that of GPS (50%). Also, the group comprising all physicians provided specificity (75-92%) similar to that of both MRA (69 - 86%) and GPS (72-94%) (Andersson et al. 2011a). A 1-hr CME lecture on ONH assessment led to a significant improvement in sensitivity (from 70% to 80%) and a significant decrease in uncertain assessments (from 22% to 13%), whereas specificity remained unchanged (68%) (Andersson et al. 2011b). A rise in sensitivity was seen in all subgroups of physicians, including glaucoma experts. Thirty physicians assessing standard automated perimetry (SAP) test results as Humphrey Field Analyzer single-field analysis printouts with full StatPac information from 99 patients with glaucoma and 66 healthy subjects were compared with a trained ANN regarding diagnostic performance. ANN reached significantly higher sensitivity (93%) than the average physician (83%), whereas specificity was similar for these two groups (91% and 90%, respectively). Diagnostic accuracy was similar among the different groups of physicians and seemingly rather independent of experience. Sensitivity ranged from 82% in the subgroup of other subspecialists to 87% in the glaucoma expert group, and specificity ranged from 88% among general ophthalmologists to 91% for glaucoma experts. The ANN attained certainty of classification that was in parity with that provided by the glaucoma experts and did not make any completely incorrect classifications of the visual fields (i.e. erroneous classifications were in the borderline zone) (Andersson et al. 2012). From a population-based randomly selected sample (n=308) of older subjects (aged ≥ 50 years) living in southern Sweden, 170 subjects underwent a comprehensive examination that included Stratus OCT, Cirrus OCT, an FDT screening programme and the GDx VCC. The same test protocol was applied to 138 randomized clinical patients with different stages of glaucoma. In the population-based sample, both Stratus and Cirrus OCT showed high diagnostic accuracy with area under the receiver-operating curve (aROC) values close to 1.0 (Bengtsson et al. 2012). Both OCT instruments correctly classified all of the clinical glaucoma patients with advanced disease. FDT screening showed high sensitivity (91%) but erroneously gave normal test results for some eyes with advanced disease. GDx VCC had lower sensitivity (73-92%) and also led to a large proportion of examinations with an atypical retardation pattern that is known to affect the diagnostic efficiency of this instrument.

CONCLUSIONS

  The HRT MRA performed better than most physicians and was consistent with the glaucoma experts. These results suggest that MRA can be a valuable tool for diagnosing glaucoma in ordinary practice, particularly when only a few glaucoma experts are available. Even though MRA provided 100% sensitivity in eyes with advanced glaucoma, it probably does not offer sufficient specificity to make it suitable as a screening method. Continuing medical education on ONH analysis had a small, but positive effect on diagnostic accuracy for glaucoma. An ANN trained to classify visual fields seemed to perform at least as well as most of the participating physicians, whose performances were remarkably similar regardless of their level of experience. This indicates that available tools for interpreting SAP findings are helpful in assessments of visual field test results. However, SAP is associated with learning effects (Heijl et al. 1989) that may entail low specificity for untrained subjects, and hence, it is not an ideal screening method for glaucoma. By comparison, the screening test of FDT is rapid and easy, but it is probably less suitable for screening purpose, because some eyes with advanced glaucoma were missed in this investigation. GDx VCC images for a relatively large number of eyes could not be analysed and is thus not appropriate for screening. The OCT instruments offer both high sensitivity and high specificity, and all eyes with advanced disease were correctly classified as glaucomatous in this evaluation. However, these instruments are still expensive and require special operator skills. Additional development to obtain OCT instrument that is more compact, easier to use and less expensive might render such tomography suitable as a screening tool for glaucoma.

Comparison of long-term variability of retinal nerve fiber layer measurements made with the RTVue OCT and scanning laser polarimetry

Purpose. To compare long-term variability of retinal nerve fiber layer thickness (RNFLT) measurements made with the RTVue-100 Fourier-domain optical coherence tomograph (RTVue-OCT) and scanning laser polarimetry with variable (GDx-VCC) and enhanced (GDx-ECC) corneal compensation. Methods. One eye each of 110 Caucasian patients (17 healthy, 20 ocular hypertensive, 22 preperimetric, and 51 perimetric glaucoma eyes, of which 10 showed visual field progression) were imaged prospectively at 6-month intervals for 1.5 to 3 years. Results. Median change of Octopus visual field mean defect was -0.300 dB/y for the controls, -0.120 dB/y for perimetric glaucoma group, and 1.231 dB/y for the 10 functionally progressing perimetric glaucoma eyes. Relative (%) variance of all RNFLT parameters measured with RTVue-OCT was significantly (p<<0.001) smaller than that with both of the GDx methods. Relative RNFLT progression slopes (% change/y) did not differ significantly between the methods. Relative variance of the RNFLT parameters did not differ between the control and perimetric glaucoma group with RTVue-OCT, but for most parameters it was consistently higher for perimetric glaucoma with both GDx methods (p=0.01). Relative RNFLT progression slopes, however, did not separate the control and perimetric glaucoma eyes with either method. Conclusions. Long-term RNFLT measurements are less variable with the RTVue-OCT than with GDx-VCC and GDx-ECC. Long-term measurement variability is higher in perimetric glaucoma than in healthy eyes with both GDx methods, but no similar between-group difference is seen with RTvue-OCT. None of the tested methods separated the rate of progression between healthy and well-controlled perimetric glaucoma eyes in 1.5 to 3 years follow-up.

Glaucoma Diagnosis and Monitoring Using Advanced Imaging Technologies

Advanced ocular imaging technologies facilitate objective and reproducible quantification of change in glaucoma but at the same time, impose new challenges on scientists and clinicians for separating true structural change from imaging noise. This review examines time-domain and spectral-domain optical coherence tomography, confocal scanning laser ophthalmoscopy and scanning laser polarimetry technologies and discusses the diagnostic accuracy and the ability of each technique for evaluation of glaucomatous progression. A broad review of the current literature reveals that objective assessment of retinal nerve fiber layer, ganglion cell complex and optic nerve head topography may improve glaucoma monitoring when used as a complementary tool in conjunction with the clinical judgment of an expert.

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.

'Structure-function relationship' in glaucoma: past thinking and current concepts

An understanding of the relationship between functional and structural measures in primary open-angle glaucoma is necessary for both grading the severity of disease and for understanding the natural history of the condition. This article outlines the current evidence for the nature of this relationship and highlights the current mathematical models linking structure and function. Large clinical trials demonstrate that both structural and functional change are apparent in advanced stages of disease, and at an individual level, detectable structural abnormality may precede functional abnormality in some patients, whereas the converse is true in other patients. Although the exact nature of the 'structure-function' relationship in primary open-angle glaucoma is still the topic of scientific debate and the subject of continuing research, this article aims to provide the clinician with an understanding of the past concepts and contemporary thinking in relation to the structure-function relationship in primary open-angle glaucoma.