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Advances in automated perimetry

ByDonald L. Budenz, MD

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White-on-white perimetry remains the gold standard for glaucoma diagnosis and for monitoring disease progression in patients with glaucoma. Continuing developments in automated perimetry, however, may expand options for ophthalmologists.

Glaucoma progression analysis has been available for several years and was of immediate benefit for ophthalmologists. The Early Manifest Glaucoma Trial (EMGT) provided a scientific rationale for its use.^1,2

EMGT researchers adapted existing glaucoma change probability maps to analyze changes in test points in successive visual fields based on pattern deviation, thus overcoming distorting effects of increasing media opacities.¹

Two other technologies of value to ophthalmologists are frequency doubling technology (FDT) and Swedish interactive thresholding algorithm (SITA) short-wavelength automated perimetry (SWAP). Newer iterations of these technologies appear promising but have not been fully validated in clinical studies.

Glaucoma progression analysis

Glaucoma progression analysis is software for the Humphrey Field Analyzer (HFA II, Carl Zeiss Meditec, Dublin, Calif.). The program adjusts for the reduced hill of vision in patients with cataract and other conditions that result in diffuse depression of the visual field.

Glaucoma progression analysis works well with baseline full threshold or SITA fields, although follow-up fields must be done using the SITA algorithm. The program uses the criteria from EMGT to judge progression at individual points and is similar to the Glaucoma Change Probability program.

The EMGT researchers defined a change as a deterioration of three or more test points at the P < .05 level at the same location on three consecutive fields. Glaucoma progression analysis utilizes that principle.

The software uses open, half-darkened and darkened triangles to help ophthalmologists evaluate possible glaucoma progression (Figure 1). If three or more points have changed at the same location on two consecutive tests, then the software reports “possible progression,” which prompts the ophthalmologist to perform another test. If the findings are confirmed on the third test, then the software reports “likely progression” (Figure 2).

Ophthalmologists must consider several limitations when using glaucoma progression analysis. At least five fields are required to diagnose progression. The system has not been validated in a data set independent of EMGT. The initial selection of the baseline fields is automatic, but ophthalmologists may need to review the selection and change it if necessary.

A possible limitation of the software is a potential for false-positive indications of glaucoma progression caused by branch vein occlusions and other non-glaucomatous visual field changes. Clinical correlation looking for other causes of field loss should be obtained before concluding that the progression is due to glaucoma. Lastly, the program does not exclude artifacts of visual fields such as rim artifact from analysis.

SWAP

SWAP has been a useful tool to detect glaucoma, and a new software upgrade, SITA-SWAP (Carl Zeiss Meditec), has shortened the duration of the test.

SWAP uses a blue, Goldmann size-V stimulus projected on a yellow background that tests cells in the koniocellular pathway. It may be more sensitive than standard automated perimetry (SAP) for early glaucoma and for glaucoma progression.^3,4

The increased sensitivity may be due to the existence of less redundancy in the koniocellular pathway, thus early damage may be more easily identified with this technique.

Current limitations of the SWAP technology include false-positive readings. Another limitation is the presence of cataracts or age-related macular degeneration, which are two common conditions among elderly patients who are at greater risk for glaucomatous progression. There is no way to compensate for the interference from these two conditions, although they give different patterns of visual field loss than glaucoma.

Another shortcoming with the test is that, in its current form, it takes 20 minutes per eye to perform, which may be onerous for patients, especially the elderly. The new SITA-SWAP software, according to the manufacturer, can reduce the test-taking time from 20 minutes to 4 to 6 minutes.

FDT

FDT has mostly been used for community screening for glaucoma. FDT tests ganglion cells in the magnocellular pathway, which also have less redundancy in the visual system. Subsequently, this technology may detect abnormalities earlier than standard white-on-white perimetry, and studies have shown that it predicts future SAP defects.^5,6

A new iteration of FDT called Matrix (Carl Zeiss Meditec) may expand the potential of FDT (Figure 3). The Matrix perimetry has smaller targets than existing FDT software (5° vs. 10°) and examines 69 targets in a 30-2 threshold, thus enabling ophthalmologists to detect glaucomatous defects earlier than with previous FDT software. Potential uses for Matrix-FDT in evaluating glaucomatous progression include patients who have ocular hypertension and normal results from a SITA visual field or patients with abnormal discs and normal SITA fields.

Other potential patients are glaucoma suspects for whom an early functional test may be desired and patients with questionable visual field defects on SITA.

Some drawbacks exist with Matrix-FDT that may limit its use in the clinic and must be considered by ophthalmologists. The Matrix tool has not been validated, and ophthalmologists cannot necessarily extrapolate data from FDT to the Matrix. Evidence that Matrix should replace SAP does not exist yet.

Conclusion

Although the advances in SITA-SWAP and Matrix have not undergone rigorous testing, the technologies appear more promising than earlier versions. As clinical trials using the software systems continue, data may confirm the software’s value. Glaucoma progression analysis, however, has been validated in the EMGT and, despite its limitations, allows ophthalmologists to monitor glaucomatous changes through successive testing.

References

1. Heijl A, Leske MC, Bengtsson B, et al, for the Early Manifest Glaucoma Trial Group: Reduction of intraocular pressure and glaucoma progression: Results from the Early Manifest Glaucoma Trial. Arch Ophthalmol. 2002;120:1268-1279.
2. Leske MC, Heijl A, Hyman L, Bengtsson B, The Early Manifest Glaucoma Trial Group: Early Manifest Glaucoma Trial: Design and baseline data. Ophthalmology. 1999;106:2144-2153.
3. Demirel S, Johnson CA. Short wavelength automated perimetry (SWAP) in ophthalmic practice. J Am Optom Assoc. 1996;67:451-456.
4. Budenz DL, Rhee P, Feuer WJ, et al. Sensitivity and specificity of the Swedish interactive threshold algorithm for glaucomatous visual field defects. Ophthalmology. 2002;109:1052-1058.
5. Johnson CA, Samuel SJ. Screening for glaucomatous visual field loss with frequency-doubling perimetry. Invest Ophthalmol Vis Sci. 1997;38:413-425.
6. Medeiros FA, Sample PA, Weinreb RN. Frequency doubling technology perimetry abnormalities as predictors of glaucomatous visual field loss. Am J Ophthalmol. 2004;137:863-871.