By Deepak P. Edward, MD
Drs. Boel Bengtsson and Anders Heijl have developed a new test called the glaucoma progression index (GPI) that appears to more accurately estimate visual function in patients with cataract compared to the standard index for estimating the rate of glaucomatous progression, the Mean Deviation Index (MDI).
Because progressive cataract formation can falsely suggest higher rates of progression, especially on the MDI, Bengtsson and Heijl sought to develop a new age-corrected visual field index for assessment of progression that was largely independent of cataract and reflected more accurately the relative importance of the central and more peripheral visual fields to patient visual function. The GPI is expressed as a percent of a normal visual field, making it possible to calculate a rate of functional loss.
They chose to present results relative to patient age to emphasize the role of life expectancy considerations in determining a therapeutic course.
The GPI was designed for the Humphrey 30-2 and 24-2 test point patterns. Visual field findings were adjusted for age, and were converted into a percentage so that the GPI of a perimetrically normal field was set to 100 percent, and the GPI of a perimetrically blind field was set to 0 percent.
To avoid the effects of cataract on the new GPI, they assessed sensitivity at each point using a percent scoring index and weighting procedures. The authors tested the GPI on 29 patients, all with a long series of visual fields, and found their approach effectively corrected the effects of cataract, except in eyes with very advanced visual field loss.
They also tested the GPI in a pilot study of 135 patients divided into three groups. Group 1 included eyes developing cataract, group 2 were eyes without cataract, and group 3 consisted of group 1 patients after cataract surgery.
- Mean loss of visual field per year in group 1 was 3.6 percent for the MDI and 2.1 percent for the GPI.
- Mean loss of visual field per year in group 2 was 2.7 percent for the MDI and 2.6 percent for the GPI. The difference was not statistically significant.
- In Group 3 the confidence limits for the rate of progression were significantly smaller with the GPI than with the MDI, suggesting that the progression rates as measured by GPI were significantly better than measured with MDI.
Some of the potential strengths of GPI include the ability to calculate rates of progression and also the possible ability to stage the disease. The results of this important study need to be validated in a prospective, longitudinal study. It would also be interesting to know how the new indices would perform or be affected by different grades and morphological types of cataract.