An attempt to detect glaucomatous damage to the inner retina with the multifocal ERG

To detect glaucomatous damage to the inner retina using the multifocal electroretinogram (mERG).

The stimulus array consisted of 103 hexagons with a mean luminance of 100 cd/m2 and a contrast of 50%. The mERG was recorded from 13 control subjects, 18 patients with open-angle glaucoma (OAG), 4 glaucoma suspects, and one patient with ischemic optic neuropathy (ION). Individual responses, as well as responses summed within quadrants or across the entire array, were measured in a number of ways. Humphrey visual fields were obtained for all patients, and the mean total deviation (MD) values for the 18 patients with OAG ranged from -2.2 to -18.2 with a mean (SD) of -7.3 (4.5).

The mERG measure that best discriminated between the patients and the control subjects was the ratio of the amplitude at 8 msec after the peak response to the amplitude at the peak. Although the value of this ratio fell below the median of the control group for 16 of the 18 OAG patients, only 6 of these patients had ratios that fell below the normal range. Other measures of first- and second-order kernels did not do as well. Both within and across patients, the correlation between local field loss and the mERG ratio measure was poor. Furthermore, although in some patients the mERG waveform is clearly different from normal, in other patients (including the patient with ION) the waveform approximates the normal even in visual field areas with substantial sensitivity loss.

Because glaucomatous damage is known to affect the ganglion cell axon, these data suggest that damage to ganglion cell axons is not a sufficient condition to produce changes in the mERG as measured here and that in patients with clear changes in mERG waveforms, these changes do not appear to be well localized and local waveforms are poorly correlated with local changes in field sensitivity.