Volumetric Analysis of Perimetry Tests to Guide Central Testing: The Functional Vulnerability Zone.

Purpose

To use volumetric analysis applied to total deviation (TD) perimetric data to characterize the presence of residual sensitivity measurable using the 10-2 in lieu of the 24-2 test grid in glaucoma (the functional vulnerability zone [FVZ]) and develop a prototype end-user application for clinical guidance.

Design

Cross-sectional study.

Subjects

Six hundred twenty-six pairs of Humphrey Field Analyzer 10-2 Swedish Interactive Thresholding Algorithm (SITA)-Fast and 24-2 SITA-Faster test results of 160 subjects.

Methods

Total deviation values located within 10° from fixation (10-2, 68 locations; 24-2, 12 locations) were extracted. Volumetric analysis was performed to calculate the difference in TD "volume" (10-2 - 24-2) for each pair of results. Then, 24-2 test locations were interpolated across the central 10° to obtain "equivalent" 10-2 test locations used to calculate number of test locations gained (10-2 - 24-2). A positive TD volume difference and positive gain represented the presence of an FVZ. Linear regression and principal components analysis were used to identify 24-2 test features that may predict the presence of an FVZ.

Main outcome measures

Volumetric differences in visual field TD as a surrogate for a FVZ.

Results

Visual field global indices and the number of central 24-2 defects predicted TD volume difference ( P < 0.0001), with 24-2 mean deviation (MD) showing the highest R 2 (0.32). All parameters had low R 2 values when predicting gain. A simplified model comprising 24-2 MD and number of central 24-2 TD probability (TDP) defects ( P < 0.01) provided a method for clinically identifying the likelihood of an FVZ. Principal components analysis revealed 2 principal components (24-2 global indices and pointwise probability scores) that accounted for 79.7% of the variance, with principal component 1 accounting for 69.3% and comprising 24-2 MD and number of central 24-2 TDP scores at P < 2%.

Conclusions

The FVZ offers a data-driven approach to identifying residual dynamic range using the 10-2 test grid. A prototype model for predicting the likelihood of 10-2 utility in progression analysis is proposed.

FINANCIAL DISCLOSURES: The authors have no proprietary or commercial interest in any materials discussed in this article.