The relationship between retinal ganglion cell function and retinal nerve fiber thickness in early glaucoma

PURPOSE. To compare relative reduction of retinal ganglion cell (RGC) function and retinal nerve fiber layer (RNFL) thickness in early glaucoma by means of steady-state pattern electroretinogram (PERG) and optical coherence tomography (OCT), respectively. METHODS. Eighty-four persons with suspected glaucoma due to disc abnormalities (GS: mean age 56.6 +/- 13.8 years, standard automated perimetry [SAP] mean deviation [MD] -0.58 +/- 1.34 dB) and 34 patients with early manifest glaucoma (EMG, mean age 65.9 +/- 10.7 years, SAP [MD] -2.7 +/- 4.5 dB) were tested with PERG and OCT. Both GS and EMG patients had small refractive errors, corrected visual acuity >= 20/25, and no systemic or retinal disease other than glaucoma. RESULTS. MDs from age-predicted normal values were larger for PERG amplitude (GS: -1.113 dB; EMG: -2.352 dB) compared with the PERG-matched RNFL thickness (GS: -0.217 dB; EMG: -0.725 dB). Deviations exceeding the lower 95% tolerance intervals of the normal population were more frequent for PERG amplitude (GS: 26%; EMG: 56%) than PERG-matched RNFL thickness (GS: 6%; EMG: 29%). CONCLUSIONS. In early glaucoma, reduction in RGC electrical activity exceeds the proportion expected from lost RGC axons, suggesting that a population of viable RGCs in the central retina is dysfunctional. By combining PERG and OCT it is, in principle, possible to obtain unique information on reduced responsiveness of viable RGCs.