Multifocal Electroretinogram Photopic Negative Response: A Reliable Paradigm to Detect Localized Retinal Ganglion Cells' Impairment in Retrobulbar Optic Neuritis Due to Multiple Sclerosis as a Model of Retinal Neurodegeneration

Abstract

The measure of the full-field photopic negative response (ff-PhNR) of light-adapted full-field electroretinogram (ff-ERG) allows to evaluate the function of the innermost retinal layers (IRL) containing primarily retinal ganglion cells (RGCs) and other non-neuronal elements of the entire retina. The aim of this study was to acquire functional information of localized IRL by measuring the PhNR in response to multifocal stimuli (mfPhNR). In this case-control observational and retrospective study, we assessed mfPhNR responses from 25 healthy controls and from 20 patients with multiple sclerosis with previous history of optic neuritis (MS-ON), with full recovery of visual acuity, IRL morphological impairment, and absence of morpho-functional involvement of outer retinal layers (ORL). MfPhNR response amplitude densities (RADs) were measured from concentric rings (R) with increasing foveal eccentricity: 0−5° (R1), 5−10° (R2), 10−15° (R3), 15−20° (R4), and 20−25° (R5) from retinal sectors (superior-temporal (ST), superior-nasal (SN), inferior-nasal (IN), and inferior-temporal (IT)); between 5° and 20° and from retinal sectors (superior (S), temporal (T), inferior (I), and nasal (N)); and within 5° to 10° and within 10° and 20° from the fovea. The mfPhNR RAD values observed in all rings or sectors in MS-ON eyes were significantly reduced (p < 0.01) with respect to control ones. Our results suggest that mfPhNR recordings may detect localized IRL dysfunction in the pathologic condition of selective RGCs neurodegeneration.

Keywords: multifocal electroretinogram; multiple sclerosis; neurodegeneration; photopic negative response; retinal ganglion cells.

Figure 1

Multifocal photopic negative response (mfPhNR) stimulus (A) consisting of a circular dartboard of 60 elongated scaled elements and the relative obtained trace array (B) subtending 30° of the visual field from a representative control eye. The average response amplitude density (RAD) of the mfPhNR expressed in nanoVolt/degree² (nV/deg²) (C) was measured as baseline-to-trough with an implicit time between 50 and 90 milliseconds (ms) from the stimulus onset. In the analysis of mfPhNR responses, we considered three different topographies. (D) The ring analysis analyzes mfPhNR RADs from five concentric annular areas (rings, R) with increasing eccentricity from the fovea, depicted in different colors: ring 1 (R1) in red from 0° to 5°; ring 2 (R2) in green 5° to 10°; ring 3 (R3) in purple from 10° to 15°; ring 4 (R4) in blue from 15° to 20°; and ring 5 (R5) in orange from 20° to 25°. (E) The sector analysis, covering an area of 20° of foveal eccentricity, analyzes RAD responses recorded from S1 (corresponding to R1, in red, see above), a circular area of 5° of centered on the fovea, and from the more external quarters of annulus within 5° and 20°, localized in the superior-temporal (ST, in blue), superior-nasal (SN, in green), inferior-nasal (IN, in orange), and inferior-temporal (IT, in purple) areas with respect to the fovea. (F) The ETDRS sector analysis analyzes the mfPhNR responses from nine sectors corresponding to the ETDRS map configuration. The central sector corresponds to the R1 of the ring analysis and to S1 of the sector analysis (in red, see above); the external sectors analyze the superior (S, in blue), temporal (T, in purple), inferior (I, in orange), and nasal (N, in dark green) areas within 5° and 10° from the fovea; and the outermost sectors analyze the S (in light blue), T (in pink), I (in yellow), and N (in light green) sectors within 10° and 20° from the fovea.

Figure 2

Multifocal photopic negative response (mfPhNR) ring (R) analysis. (A) MfPhNR averaged responses from ring 1 to 5 (R1 to R5) are presented from a representative control eye (#7) and from an eye of a multiple sclerosis patient affected by optic neuritis (MS-ON) (#3). The mean mfPhNR response amplitude density (RAD) measure is indicated by an arrow (↕). (B) Mean values of mfPhNR RAD (expressed in nanoVolt/degrees², nV/deg²) plotted as a function of foveal eccentricities (R1 to R5 refer to ring analysis (see Methods)). Vertical bars represent one standard deviation of the mean values. Dashed lines indicate the exponential fitting for mfPhNR RADs (Controls: r² = 0.97; MS-ON eyes: r² = 0.95). The relative functions show a progressive decrease of mfPhNR RADs in both controls and MS-ON eyes with increasing eccentricities (from R1 to R5). The slope of the mean mfPhNR RAD function of both groups presents a greater steepness proceeding from R2 to R3 (the center to periphery transitional retinal areas: from 5° to 10° to 10° to 15° of foveal eccentricity). * indicates the statistically significant (p < 0.01) difference between MS-ON and control groups. The values of the statistical analysis are reported in Table 1.

Figure 3

Multifocal photopic negative response (mfPhNR) sector analysis. (A) MfPhNR averaged are presented from a representative control eye (#7) and from an eye of a multiple sclerosis patient affected by optic neuritis (MS-ON) (#3). The mean mfPhNR response amplitude density (RAD) measure is indicated by an arrow (↕). We analyzed five sectors covering an area of 20° of eccentricity from the fovea. The first sector (S1) corresponding to R1 (0–5°) is reported on Figure 1; the more external sectors (between 5° to 20°) were quarters of annulus, localized in the superior-temporal (ST), superior-nasal (SN), inferior-nasal (IN), and inferior-temporal (IT) areas with respect to the fovea. (B) Mean values of mfPhNR RAD (expressed in nanoVolt/degrees², nV/deg²) are plotted as a function sector analysis. Vertical bars represent one standard deviation of the mean values. Dashed lines indicate the linear fitting for mfPhNR RADs (controls: r² = 0.17, MS-ON: r² = 0.96). The relative functions show almost constant values of mfPhNR RADs in both controls and MS-ON eyes between sectors although the trend is actually linear only in the MS-ON group. * indicates the statistically significant (p < 0.01) difference between MS-ON and control groups. The values of the statistical analysis are reported in Table 3.

Figure 4

Multifocal photopic negative response (mfPhNR) ETDRS sector analysis. MfPhNR averaged are presented from a representative control eye (#7) and from an eye of a multiple sclerosis patient affected by optic neuritis (MS-ON) (#3). The mean mfPhNR response amplitude density (RAD) measure is indicated by an arrow (↕). We analyzed nine sectors covering an area of 20° of eccentricity from the fovea. The first sector (S1) corresponding to R1 (0–5°) is reported on Figure 1; the more external sectors between 5° to 10° (A) and between 10° to 20° (C) were quarters of annulus, localized in the temporal (T), superior (S), nasal (N), and inferior (I) areas with respect to the fovea. Mean values of mfPhNR RAD (expressed in nanoVolt/degrees², nV/deg²) are plotted as a function of sector analysis on 5° to 10° (B) and between 10° to 20° (D). Vertical bars represent one standard deviation of the mean values. Dashed lines indicate the linear fitting for mfPhNR RADs (5°–10°: controls: r² = 0.58, MS-ON: r² = 0.28; 10°–20°: Controls: r² = 0.59, MS-ON: r² = 0.91). The relative functions show almost constant values of mfPhNR RADs in both controls and MS-ON eyes between sectors. * indicates the statistically significant (p < 0.01) difference between MS-ON and control groups. The values of the statistical analysis are reported in Table 5.