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New Glaucoma Progression Analysis helps clinicians recognize change
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Simplistically, glaucoma can be divided into two areas: diagnosis and progression. Both the initial diagnosis and the recognition of progression are difficult tasks. Regarding establishing the diagnosis, signs associated with glaucomatous optic neuropathy are sometimes confusing because there is overlap between the “normal” and “abnormal” optic nerve. The closer the patient is to the imaginary line separating the two, the more difficult the decision becomes.
For optic nerve and nerve fiber analysis, several imaging devices have been developed and are commonly used to aid the clinician in deciding which patient may have glaucoma or is getting worse. The instrument’s role is to confirm the clinician’s suspicion. Visual fields are also a crucial test in managing glaucoma and are used to establish the diagnosis as well as to detect progression. When a person’s field changes from being clean to having several points flagged that are adjacent to each other (cluster defect), that is considered an indicator of developing functional visual loss.
Standard white-on-white perimetry was not designed to detect the earliest damage to the visual system, so subtle damage may be occurring even when a full visual field is present. Pre-perimetric glaucoma describes this situation and is the reason behind the development of newer perimetric tests, such as Short Wavelength Automated Perimetry (SWAP) and Frequency Doubling Technology (FDT). These commercially available tests are designed to recognize early change.
Detecting perimetric change
What about the detection of perimetric change? Years ago, during the period when full threshold was the algorithm of choice, the Zeiss Meditec HFA perimeter (Dublin, Calif.) offered a software analysis tool called Glaucoma Change Probability (GCP) Analysis. This test provided an analysis when at least three fields were available using total deviation maps along with a database (STATPAC II) that was based upon glaucomatous individuals. It provided clues as to whether a patient was stable or getting worse. The SITA algorithm replaced full-threshold approximately 7 years ago and has now become the most commonly used test strategy, which has prevented clinicians from using the once popular GCP tool.
With the loss of the original Glaucoma Change Probability Analysis tool, clinicians were left to compare single fields performed over time or to evaluate overview printouts visually, observing for change at different test locations. This method is crude, leaving the clinician to guess whether observed change is real. It is difficult for a clinician to accurately recognize change by inspecting fields due to the characteristics of the glaucomatous visual field. The biggest culprit that hinders recognition of true change is variability, which describes differences in threshold measurements at the same location when tests are repeated, either immediately (short term) or at a later date (long-term).
The findings at a test location are rarely the same from test to test, as all points have a small amount of variability, even in a normal field. Variability increases as tested points are located further from fixation (eccentricity) and are greater for the superior than inferior field. Variability also increases based upon a patient’s inexperience with perimetry, as well as with advancing damage. A point that appears to worsen may be manifesting variability associated with glaucoma. For this reason, a clinician needs several fields to confirm whether change has occurred, and, even then, it is often a guesstimate.
New Glaucoma Progression Analysis
A new tool has been released for the Carl Zeiss Meditec HFA II perimeter, called Glaucoma Progression Analysis (GPA). This software analysis program works with SITA fields to establish if perimetric findings are stable. A minimum of three tests is needed, and older full-threshold fields can be used only for baseline. GPA uses the basics from the older glaucoma change probability program with some important modifications. (To see an example Glaucoma Progression Analysis, view the PDF file here.)
First, pattern rather than total deviation maps are used. Total deviation refers to change that affects the entire field, which can be due to cataracts, glaucoma, the wrong trial lens, a corneal scar, small pupils or learning. Pattern deviation loss is associated with localized or focal loss, which is associated with glaucoma.
Pattern deviation maps are designed to detect glaucomatous change and ignore diffuse loss that is associated with a worsening cataract. In 1997, several sites evaluated individuals with glaucomatous field loss, with four tests performed in a 1-month period. In this short period, progression should not occur, which allowed the amount of variability associated with glaucomatous field loss to be measured. With variability quantified, this information was factored into the GPA package.
With this program, the first two fields are averaged and become the baseline. Fields with an abnormally high number of false positives are discarded. Each succeeding field is compared to the averaged baseline, with a point-by-point analysis performed. Each point in which measured change exceeds expected variability is flagged with a small open triangle. Points deeply damaged at onset cannot be used to detect further damage, and a small “x” at their location appears on the printouts.
Compares last two fields
Another new feature of the GPA is that it not only compares one test to the baseline, but it also compares it to the last two fields performed. Change is confirmed at each test, and when points are flagged as getting worse a second time, a new symbol is seen: a triangle filled in half way. The concept of confirming change is crucial to understand if true progression is occurring vs. random variability. When points are noted as getting worse in three or more consecutive fields, a dark triangle appears.
To aid the clinician in recognizing change, when the same three points are flagged as getting worse two fields in succession, a reading of “Possible Progression” is marked on the field printout. Three points flagged on three consecutive tests leads to the statement “Likely Progression” printed on the field. The algorithms and criteria for identifying progression incorporated into the GPA were used in the Early Manifest Glaucoma Trial (EMGT).
Also found on the front page of the GPA is a graph performing a regression analysis of mean deviation (MD). This provides information on whether the field is stable. When five fields are available, the rate of change is provided in dB per year. While the overview style printout is commonly used, the GPA symbols are also printed on the single field printout if enough fields are available for analysis. For the single-field printout, points getting worse will be illustrated on a table found on the right side.
The GPA is an excellent tool that allows the clinician to understand if glaucoma is progressing. It is a test that clinicians desperately need. It is easy to understand and will allow detection of change with the subsequent modification of the therapeutic regimen, while also alerting clinicians to change that may not be clinically significant.
For your information:
- The HFA II perimeter is available from Carl Zeiss Meditec, (925) 557-4100; www.humphrey.com.
- Click here to view "Using the Glaucoma Progression Analysis."
These images are stored in an Adobe Acrobat [.PDF] file that requires the Adobe Acrobat Reader software application for viewing. The application is available for download at no charge, and is available in various formats (stand alone, plug-in) for most of the major operating systems.
