Different investigators have used different methodologies to evaluate the structure–function relationship in glaucoma. Garway-Heath et al.
10 found that the visual sensitivity expressed in linear scale defined the structure–function relationship better than visual sensitivities expressed as a dB scale. Bowd et al.
11 showed that a linear fit between structure and function with visual sensitivity expressed as a dB scale was comparable to a logarithmic fit in describing the structure–function relationship. Hood and Kardon
22 showed that a simple linear model can describe the structure–function relationship well. Previous studies have used this model to evaluate the structure–function relationship using RNFL,
18,20 inner retinal,
20 and also GCIPL thickness
19,33 measurements at the macula. This model uses visual sensitivity loss as determined on the total deviation numeric plot as the functional measure, and not the visual threshold as has been used in most of the other studies. The total deviation numeric plot adjusts the visual sensitivity loss according to the age of the subject. In this way the age-related variability in the functional measurement is minimized. For the GCIPL measurements, though there are no age-corrected values, the change with age has been reported to be small.
36 We also used lowess curves to estimate the structure–function relationship. The advantage of the lowess curve is that it does not need the specification of a function to model the relationship in a given sample. The shape of the lowess curves and the predicted curve from the Hood and Kardon
22 model shows the lag between structural and functional components in glaucoma. In early stages of glaucoma, the decline in GCIPL thickness is rapid and there is a lag in the visual sensitivity loss. But as the glaucoma damage becomes severe, GCIPL thickness reaches a base level beyond which only the visual sensitivity declines. The base level noted in our study ranged from 49 μm (63% of the thickness in control subjects) in the inferotemporal GCIPL sector to 58 μm (76% of the thickness in control subjects) in the superotemporal sector. A possible limitation in the estimation of
b from our data is that the number of eyes with a mean visual sensitivity loss of >15 dB in different GCIPL sectors ranged from as low as 4 (in superotemporal sector) to as high as 24 (in inferior sector). Raza et al.
19 have also reported GCIPL base level values of 50% to 80% of the control values at different eccentricities in their small sample of glaucomatous eyes. The simple linear model fitted our data reasonably well. However, compared with the lowess curves, the predicted curves seemed to overestimate structural damage in eyes with moderate VF loss. Raza et al.
19 also noticed a systematic bias with the linear model but unlike what was seen in our study, they found that the model underestimated the structural damage in eyes with early VF sensitivity loss. Our results may be partly due to an overestimation of the visual sensitivity loss in our data. On inspection of the VFs of our study, we found that the total deviation probability plots of most of the VFs were worse than the pattern deviation plots in spite of acceptable reliability indices and media clarity. It is important to note that the simple linear model was initially developed for evaluating the structure–function relationship with the RNFL thickness. Therefore more work may be needed to refine the model for structure–function evaluation at the macula and to evaluate the reasons for discordance between the actual data and the model in certain situations.