Neural Conduction Along Postretinal Visual Pathways in Glaucoma

Abstract

Purpose: This study was conducted in order to evaluate retinal ganglion cell (RCG) function and the neural conduction along the postretinal large and small axons and its correlation with retinal nerve fiber layer thickness (RNFL-T) in open-angle glaucoma (OAG) eyes. Methods: Thirty-seven OAG patients (mean age: 51.68 ± 9.83 years) with 24-2 Humphrey mean deviation (MD) between -2.5 and -20 dB and IOP <21 mmHg on pharmacological treatment (OAG group) and 20 age-matched controls (control group) were enrolled. In both groups, simultaneous pattern electroretinograms (PERG) and visual evoked potentials (VEP), in response to checks stimulating macular or extramacular areas (the check edge subtended 15' and 60' of visual arc, respectively), and RNFL-T (measured in superior, inferior, nasal, and temporal quadrants) were assessed. Results: In the OAG group, a significant (ANOVA, p < 0.01) reduction of 60' and 15' PERG P50-N95 and VEP N75-P100 amplitudes and of RNFL-T [overall (average of all quadrants) or temporal] with respect to controls was found; the values of 60' and 15' PERG P50 and VEP P100 implicit times and of retinocortical time (RCT; difference between VEP P100 and PERG P50 implicit times) were significantly (p < 0.01) increased with respect to control ones. The observed increased RCTs were significantly linearly correlated (Pearson's test, p < 0.01) with the reduced PERG amplitude and MD values, whereas no significant linear correlation (p < 0.01) with RNFL-T (overall or temporal) values was detected. Conclusions: In OAG, there is an impaired postretinal neural conduction along both large and small axons (increased 60' and 15' RCTs) that is related to RGC dysfunction, but independent from the RNFL morphology. This implies that, in OAG, the impairment of postretinal neural structures can be electrophysiologically identified and may contribute to the visual field defects, as suggested by the linear correlation between the increase of RCT and MD reduction.

Keywords: OCT imaging; PERG; VEP; glaucoma; visual pathways.

Figure 1

Examples of Humphrey Field Analyzer (HFA) 24-2, simultaneous pattern electroretinogram (PERG) and visual evoked potentials (VEP) recordings and retinal nerve fiber layer thickness (RNFL-T) analysis observed in one representative control eye (#15) and three representative OAG eyes (#16, #24, #35). MD, mean deviation; 60′ and 15′, check edges subtending 60 min (60′) and 15 min (15′) of the visual angle for PERG and VEP visual stimuli; N75 and P100 refer to the first negative and the first positive peak of VEP recordings (the implicit time of P100 and the peak-to-peak N75-P100 amplitude were considered); P50 and N95 refer to the first positive and the second negative peak of PERG recordings (the implicit time of P50 and the peak-to-peak P50-N95 amplitude were considered); the retinocortical time (RCT), indicated by the red arrow, is the difference between VEP P100 and PERG P50 implicit times. msec, milliseconds; μV, microvolt; T, temporal; S, superior; N, nasal; I = inferior refer to the RNFL sectorial thickness expressed in microns. In OAG eyes, with respect to the control eye, it is possible to observe reduced P50-N95 PERG and N75-P100 VEP amplitudes, delayed P50 PERG and P100 VEP implicit times, increased RCT, and reduced RNFL-T.

Figure 2

OAG eyes: linear correlation between retinocortical time (RCT) and pattern electroretinogram (PERG) P50-N95 amplitudes (A,B) and Humphrey Field Analyzer (HFA) 24-2 mean deviation (C,D). RCT, difference VEP 100 minus PERG P50 implicit times; 60′ and 15′, visual stimuli with checks subtending 60 and 15 min of visual arc, respectively; msec, milliseconds; μV, microvolt.

Figure 3

OAG eyes: linear correlation between retinocortical time (RCT) and retinal nerve fiber layer (RNFL) thickness. RCT, difference VEP 100 minus PERG P50 implicit times; 60′ (A) and 15′ (B), visual stimuli with checks subtending 60 and 15 min of visual arc, respectively; msec, milliseconds; μ, microns.