Increasing the Role of GDx Scanning Laser Polarimetry
New software helps better monitor progression of glaucoma, as demonstrated by this expert's use of the technology.
By Robert N. Weinreb, M.D.
Staging and following glaucoma is essential for management of the disease. Both require the use of perimetry and examination of the optic disc and the retinal nerve fiber layer (RNFL).
Staging disease allows a physician to appropriately place an individual patient on the glaucoma continuum. Knowing which patients to treat, and also how to treat them, is at the core of glaucoma management. Not all patients will advance through the disease continuum to functional impairment and blindness, and some patients will progress faster than others.
This supplement focuses on the assessment of glaucoma progression. It will discuss Guided Progression Analysis (GPA) software for assessing visual fields and for evaluating the results of scanning laser polarimetry (GDx) and optical coherence tomography (Stratus OCT).
New Software
Carl Zeiss Meditec has developed software, under the direction of Dr. Qienyuan Zhou, to measure longitudinal progression with the GDx. There has never been any question that this technology is needed to improve our ability to evaluate glaucoma progression.
One method for diagnosing glaucoma is to look for characteristic changes in the optic disc, the RNFL and the visual field. A more specific way, perhaps, is to look for a change in one or more of the structural or functional parameters. With a method for longitudinally assessing the RNFL, it is possible that a more specific method for diagnosing glaucoma will be available.
Figure 1. The GDx GPA printout features three separate algorithms to find RNFL change, ranging from focal to diffuse. Results from each are summarized in the box at the upper left.
| Future Goal: Integrate Structural And Functional Testing |
|---|
| It is important to remember that both structural and functional tests are needed when managing glaucoma patients. To diagnose glaucoma and measure progression, the appearance of the optic nerve should be observed and documented, and visual field testing needs to be performed. The new GDx GPA software, currently undergoing validation, should facilitate the estimation of progression. Our goal should be the integration of structure and function in order to better organize and utilize the various measurements. |
Mapping Our View
GDx GPA can be used to address some key clinical needs. It can:
■ Help identify progressive RNFL loss, using terminology that is similar to what is used by GPA on the Humphrey Field Analyzer
■ Determine the rate of progression, in terms of microns per year and the confidence interval
■ Help assess treatment efficacy by comparing progression rates before and after intervention.
Figure 1 shows a sample printout with the key elements.
Interpreting the GDx GPA Printout
At the top of the printout, there is a summary box that indicates if statistically significant change has been flagged, and if so, by which algorithm(s). The following color code is used:
■ Yellow: Possible progression
■ Red: Likely progression
■ Purple: Possible increase.
The three check boxes in the summary box communicate whether any of the three algorithms flagged significant change. These are:
Image progression map: Most sensitive to narrow, focal change (located at center top). The image map contains more than 9,000 pixels. At least 150 adjacent pixels in the areas being compared must show significant change.
TSNIT progression graph: Most sensitive to broader focal change (located at right top). The area between the calculation circles around the optic nerve is divided into 64 segments, and change must be detected in four adjacent segments.
Summary parameter charts: Most sensitive to diffuse change (located in middle of printout). These chart the following three parameters: TSNIT Average, Superior Average and Inferior Average. If Likely Progression is found, they display it both graphically and numerically in terms of microns per year. Two trends can be displayed on the same chart to compare progression before and after intervention.
Example 1: Focal RNFL Loss
Figure 2 shows a patient who was examined in June 2004 (left side) and then again in June 2006 (right side). In both images, there is a superotemporal wedge-shaped RNFL defect, which does not appear to have changed.
However, in just 2 years, a new focal RNFL defect appeared in the inferotemporal region. This correlates with the visual fields shown in Figure 3. In June 2004, shown on the left, the pattern deviation indicates inferior loss corresponding to the superior RNFL defect. In June 2006, there is additional superior loss, corresponding to the new inferotemporal RNFL defect.
Figure 2. Optic disc photos from June 2004 (left side) and June 2006. Note both images show a superior wedge defect, but a new, inferior defect appears in the second photo after just 2 years.
Figure 3. In June 2004, there is inferior functional loss corresponding to the superior RNFL defect. In June 2006, there is additional superior loss corresponding to the new inferotemporal RNFL defect.
The GDx GPA printout (Figure 4) presents a series of examinations taken in June 2004 and June 2006. The summary box shows that "Likely Progression" was found by the Image Progression Map, which identifies statistically significant progression corresponding very closely to the inferotemporal RNFL defect in Figure 2. Note that the other algorithms did not flag statistically significant loss because they are less sensitive to this type of change.
Figure 4. "Likely Progression" found by the Image Progression Map.
| Why Measure Progression? |
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| As learned from the Ocular Hypertension Treatment Study, certain risk factors facilitate estimation of the potential rate of a patient's progression to glaucoma. For example, older patients with higher IOPs, thin central corneas, large cup-to-disc ratios and visual field losses, as characterized by the pattern standard deviation, are at higher risk to develop glaucoma when they have ocular hypertension. From studies such as the Early Manifest Glaucoma Trial, it has been learned that risk factors help estimate which patients with established glaucoma will progress through the continuum. Other risk factors emerge, including exfoliation and bilateral disease. In clinical practice, it should not be sufficient to estimate and predict which patients will progress. Progression needs to be measured. By measuring progression, more effective management decisions can be made. One can determine which patients are more likely to progress to functional impairment and which patients will need advancing therapy and more intensive therapy to prevent blindness. |
Figure 5. Eye with marked inferior visual field loss corresponding to dense superior RNFL loss shown on the GDx. There is possible inferior RNFL thinning as well.
Figure 6. Functional loss had begun developing superiorly, with corresponding inferior RNFL loss visible on the GDx.
Figure 7. "Possible Progression" found by the TSNIT Progression Graph in the inferior area.
Figure 8. All three algorithms in the GDx GPA summary box indicate "Likely Progression" in the inferior RNFL.
Figure 9. Optic disc photos corresponding to the analysis in Figure 8 in which RNFL loss may have been difficult to identify.
Broader RNFL Loss
Figure 5 shows another patient who was first examined in May 2002. The patient had marked inferior visual field loss. It was difficult to detect corresponding structural damage from the photograph, but the GDx picked up dense RNFL loss superiorly and possibly some thinning inferiorly.
In May 2003, the visual field indicated that functional loss had begun developing superiorly, as well. This also was seen by the GDx as additional inferior RNFL thinning, as shown in Figure 6.
Figure 7 reveals an analysis of this patient using the new GDx GPA. In this example, "Possible Progression" was found by the TSNIT Progression Graph in the inferior area. Change was not flagged by the other algorithms because it was somewhat subtle and in between focal and diffuse. The progression was only flagged in yellow as "Possible" because at the time, there had not yet been a confirming examination.
Inferior Loss Difficult to See Clinically
In another case, a patient received several GDx exams in July 2005 and again in June 2006 (Figure 8). All three algorithms in the GDx GPA summary box indicate "Likely Progression" in the inferior RNFL.
These changes would be difficult to detect by examining the RNFL clinically, viewing photos (Figure 9) or by just viewing the GDx RNFL images. In fact, these images, in many respects, are atypical. Impressively, the software is still able to identify significant change.
Summarizing the Benefits
GDx GPA software allows confirmation or exclusion of a glaucoma diagnosis in patients suspected of having the disease. In addition, it provides a new way to look for a change in the RNFL. This should facilitate more confident treatment decisions, as well as more effective patient education.
Dr. Weinreb is Director, Hamilton Glaucoma Center, Chief, Glaucoma Division and Vice-Chair, Department of Ophthalmology, Shiley Eye Center, San Diego, Calif.
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