The structure-function relationship in glaucoma is only marginally strengthened by improving visual field and retinal nerve-fiber layer thickness measurements

Abstract

 

Purpose

 

The relationship between measured structure and function in glaucoma is weak. This study aimed to see whether the structure-function relationship can be strengthened by measuring structure and function more extensively in a laboratory setting, and connecting the two with an individualised structure-function map.

 

Methods

 

People with glaucoma (n=23) were tested at 2 visits. A Zippy Estimation by Sequential Testing (ZEST) procedure with uniform prior probability was used to measure visual field sensitivities, using the Open Perimetry Interface (Turpin et al., JOV, 12:1-5,2012). Visual fields were measured with a high spatial resolution 3 X 3 degree grid (158 locations). Retinal nerve fiber layer (RNFL) thickness was measured using spectral domain optical coherence tomography (OCT) (4 scans, 2 per visit). Blood vessels were removed manually from the OCT scans. An individualised map was used to relate structure and function (Denniss et al., IOVS, 53:6981-90, 2012) using biometric data. Structure-function correlations were calculated between 24-2 locations (n = 52) of the first tested visual field and RNFL thickness from the first OCT scan, using the sectors of the G-H map (Garway-Heath et al., Ophthalmology, 107:1809-15, 2000). We added additional data (averaged visual field and OCT data, additional 106 visual field locations and OCT data without blood vessels, individually customised map) and calculated the correlations with each in turn. Spearman correlations are reported, with increasing rho being taken as a measure of improvement in the strength of structure-function relationship.

 

Results

 

The correlations for individual sectors are shown in the table. The first column shows the simple case. The second column reports correlations post averaging of visual field and OCT data. The third column shows the effect of adding additional visual field locations and removing blood vessels from the OCT. The final column adds custom structure-function mapping to the previous.

 

Conclusions

 

Increasing the amount of data collected for OCT and visual fields, combined with individual structure-function maps, only marginally improves the strength of the cross-sectional relationship between structural and functional measures.

 

 

Table: Spearman correlation (bootstrap 95% confidence intervals) between structure and function for 6 sectors.

 

Keywords: 758 visual fields • 642 perimetry • 550 imaging/image analysis: clinical