To determine the association between the local VF mean sensitivity and the relative GCA thickness, we based this study on previous work that related the SAP sensitivity to peripapillary RNFL thickness
15–17 or RGC + IPL thickness (GCA) in the macula.
8 Briefly, the assumptions for this model were as follows: the measured GCA thickness
R consists of two components, the thickness
S, which is due to portions of the IPL and RGC layer (GCA) that is affected by glaucoma, and the residual
B, which includes portions of the IPL and RGC layer (GCA) not affected by glaucoma. These areas may include glial cells, blood vessels, bipolar cell axons, and amacrine cell projections. Thus, overall
R =
S +
B. As the MAIA field sensitivity changes, the value of
S will decrease, while
B will remain constant. As the VF sensitivity decreases, the signal portion
S of the local GCA thickness
R will decrease linearly when the VF sensitivity is expressed in linear units. Thus,
R = (
S 0 −
B)
T +
B for
T less than or equal to 1.0, where
S 0 is the median of the control GCA thickness at a particular eccentricity,
T is the relative sensitivity (defined as 10
0.1D), and D is the VF sensitivity minus the mean value of healthy subjects.
T equals 1.0 when there is no loss (0 dB difference from normal) and approaches 0 when there are large losses in the sensitivity. The variable
B was calculated for each zone as the median of the GCA data when the local VF sensitivities were less than 20 dB.