Relationship between electrophysiological, psychophysical, and anatomical measurements in glaucoma

To evaluate the relationship between electrophysiological, psychophysical, and structural measurements in normal and glaucomatous eyes and to test the hypothesis that there is a continuous structure-function relationship between ganglion cell numbers and visual field sensitivity.

Thirty-four normal subjects and 40 patients with glaucoma were examined with the pattern electroretinogram (PERG), perimetry and retinal tomography. Transient and steady state (SS) PERGs were recorded, and peak (P)-to-trough (N) amplitude was measured. The unit of differential light sensitivity (DLS) in perimetry is the decibel. The decibel is 10. log(1/Lambert), where the Lambert is the unit of test spot intensity. PERG amplitudes were correlated with decibel and 1/Lambert DLS for the central 18 degrees of the visual field and with neuroretinal rim area in the temporal part of the optic disc. Age-related changes in the structural and functional measurements were sought. The correlation between variables was investigated by linear and quadratic regression analysis. A quadratic (y = ax + bx(2) + c) fit was taken to be significantly better than a linear fit, if the coefficient (b) for the x(2) term was significant at P < 0.05.

A quadratic fit between decibel DLS and PERG amplitude (transient PERG: R(2) = 0.40, P = 0.0000; SS PERG: R(2) = 0.32, P = 0.0000) was significantly better than a linear fit. There was a linear correlation between 1/Lambert DLS and PERG amplitude (transient PERG: R(2) = 0.44, P = 0.0000; SS PERG: R(2) = 0.35, P = 0.0000). There was a linear correlation between temporal neuroretinal rim area and PERG amplitude (transient PERG: R(2) = 0.17, P = 0.0003; SS PERG: R(2) = 0.20, P = 0.0001). A quadratic fit between decibel DLS and temporal neuroretinal rim area (R(2) = 0.38, P = 0.0000) was significantly better than a linear fit. There was a linear correlation between 1/Lambert DLS and temporal neuroretinal rim area (R(2) = 0.30, P = 0.0000). Both DLS and PERG amplitude declined with age in the normal subjects. The rate of decline was -0.17%, -0.74%, -0.75%, and -0.78% per year for decibel DLS, 1/Lambert DLS, transient PERG, and SS PERG, respectively.

There is a curvilinear relationship between decibel DLS and both PERG amplitude and neuroretinal rim area, and a linear relationship between 1/Lambert DLS and PERG amplitude and neuroretinal rim area. These findings support the hypothesis that there is no ganglion cell functional reserve but a continuous structure-function relationship, and that the impression of a functional reserve results from the logarithmic (decibel) scaling of the visual field.