Adaptive changes of inner retina function in response to sustained pattern stimulation

Using Noninvasive Electrophysiology to Determine Time Windows of Neuroprotection in Optic Neuropathies

The goal of neuroprotection in optic neuropathies is to prevent loss of retinal ganglion cells (RGCs) and spare their function. The ideal time window for initiating neuroprotective treatments should be the preclinical period at which RGCs start losing their functional integrity before dying. Noninvasive electrophysiological tests such as the Pattern Electroretinogram (PERG) can assess the ability of RGCs to generate electrical signals under a protracted degenerative process in both clinical conditions and experimental models, which may have both diagnostic and prognostic values and provide the rationale for early treatment. The PERG can be used to longitudinally monitor the acute and chronic effects of neuroprotective treatments. User-friendly versions of the PERG technology are now commercially available for both clinical and experimental use.

Compartmental Differences in Macular Retinal Ganglion Cell Function

Purpose

The purpose of this study was to investigate local differences of macular retinal ganglion cell (RGC) function by means of the steady-state pattern electroretinogram (SS-PERG).

Methods

SS-PERGs were recorded in healthy subjects (n = 43) in response to gratings (1.6 c/deg, 15.63 reversals/s, and 98% contrast) presented on an LED display (800 cd/m², 12.5 degrees eccentricity at 30 cm viewing distance) partitioned in triangular sectors (inferior [I]; nasal [N]; superior [S]; and temporal [T]) or concentric regions (central [C] and annulus [A]). For each partition, response amplitude (nV), amplitude adaptation (% change over recording time), phase/latency (deg/ms), and oscillatory potentials (OPs) amplitude (root mean square [RMS] nV) were measured. Data were analyzed with Generalized Estimating Equation (GEE) statistics.

Results

Amplitude differed (P < 0.001) between sectors (I: 254 nV; N: 328 nV; S: 275 nV; T: 264 nV; and N>T, I) as well as concentrically (C: 684 nV; A: 323 nV; and C>A). Latency did not differ between sectors (range = 53–54 ms, P = 0.45) or concentrically (range = 51–51 ms, P = 0.7). Adaptation did not differ (P = 0.66) concentrically (C: −19% and A: −22%) but differed (P = 0.004) between sectors (I: +25% and S: −29%). The OP amplitude did not differ (P = 0.5) between sectors (range = 63–73 nV) as well as concentrically (range = 82–90 nV, P = 0.3).

Conclusions

Amplitude profiles paralleled RGC densities from histological studies. Adaptation profile suggested greater autoregulatory challenge in the inferior retina. Latency profile may reflect axonal conduction time to the optic nerve head assuming a direct relationship between axon length and its size/velocity. Location-independent OPs may reflect preganglionic activity.

Translational Relevance

Normal macular RGC function displays local differences that may be related to local vulnerability in optic nerve disorders.

Shortened Pattern Electroretinogram Latency and Impaired Autoregulatory Dynamics to Steady-State Stimuli in Patients With Multiple Sclerosis

BACKGROUND

The steady-state pattern electroretinogram (PERG) is a sensitive measure of retinal ganglion cell (RGC) function that includes within-test progressive changes-adaptation-reflecting RGC autoregulatory dynamics. Comprehensive PERG assessment in patients with multiple sclerosis (MS) (with or without optic neuritis [ON]) may provide unique information about RGC dysfunction and its progression, as well as a comparison between functional loss and structural loss as measured by optical coherence tomography (OCT). The goal of this project was to measure steady-state PERG components and their associations with intraretinal layer thicknesses in MS.

METHODS

One hundred forty eyes of 70 patients with relapsing-remitting MS and 126 eyes of 63 age- and sex-matched healthy control subjects (HC) were investigated using a new-generation PERG method and ultrahigh-resolution OCT. Of MS eyes, there were 30 eyes with ON (MSON), 22 non-ON fellow eyes (MSFE), and 88 non-ON MS eyes (MSNON). PERG amplitude, phase (latency), and adaptation of amplitude and phase were measured and correlated with OCT-determined thicknesses of intraretinal layers.

RESULTS

The average PERG amplitude in MSON eyes was significantly lower than MSFE (P = 0.007), MSNON (P = 0.002), and HC (P < 0.001). The PERG amplitude in MSFE eyes was also significantly lower than HC (P = 0.039). The PERG latency in MSON eyes was significantly shorter than in MSFE (P = 0.001), MSNON (P = 0.002), and HC (P < 0.001). The PERG latency in MSFE (P = 0.007) and MSNON (P = 0.002) was significantly shorter than in HC. However, no significant differences were found between MSFE and MSNON (P > 0.05). PERG adaptation of amplitude in MSON was significantly lower than that in MSNON (P = 0.039) and HC (P = 0.037). Both the amplitude and latency in the MS eyes were significantly correlated with the thicknesses of the macular retinal nerve fiber layer (mRNFL) and ganglion cell-inner plexiform layer (GCIPL).

CONCLUSIONS

Shortened PERG latency and impaired autoregulatory dynamics occurred in MS, suggesting preferential dysfunction of small, slower RGC axons and decreased ability of RGC to autoregulate their gain in response to PERG stimulus. The established relations of PERG measurements with intraretinal thickness measurements suggested that PERG losses were primarily associated with GCIPL and mRNFL thinning.

Decoding PERG: A neuro-ophthalmic retinal ganglion cell function review

Purpose of review

Currently, the clinical evaluation of neuro-ophthalmologic diseases are mainly focused on identifying stages where structural or functional damage occur. Recognition of retinal ganglion cell (RGC) functional patterns as well as monitoring RGC dysfunction can be performed using steady-state pattern electroretinogram (PERG). The analysis of the amplitude and latency shift aid on providing information on early damage or monitoring of the RGC, allowing for prompt clinical intervention and management modification, potentially changing the natural history of the disease. The purpose of this article is to review the latest findings in PERG, in early manifest glaucoma, non arteritic ischemic optic neuropathy, multiple sclerosis with unilateral recovered optic neuritis and its fellow eyes.

Recent Findings

The steady-state PERG responses provide new and early specific information in neuro-ophthalmic diseases affecting the inner retina.

Summary

Steady state PERG presents specific amplitude and latency outcomes based on the neuro-ophthalmic disease affecting the inner retina, allowing early recognition of changes at the level of RGC and the degree of RGC dysfunction. In addition, PERG alterations may be induced in healthy subjects as well as susceptible eyes using different stress tests such as head down tilting or water drinking tests.

Steady-state PERG adaptation: a conspicuous component of response variability with clinical significance

PURPOSE

To investigate within-test variability of the steady-state PERG (SS-PERG).

METHODS

SS-PERGs were recorded in response to black-white horizontal gratings (1.6 cycles/deg, 98% contrast, 15.63 reversals/s, LED display, 25 deg square field, 800 cd/sqm mean luminance) using skin electrodes. PERG and noise (± reference) signals were averaged over 1024 epochs (~ 2.2 min) and Fourier analyzed to retrieve SS-PERG amplitude and phase. SS-PERGs were split into 16 partial averages (samples) of 64 epochs each, and corresponding amplitudes and phases combined in polar coordinates to assess their dispersion (within-test variability). To assess time-dependent variability, samples were clustered in four successive time segments of ~ 33 s each. Amplitude adaptation was defined as amplitude difference between initial and final clusters, and PERG phase adaptation as the corresponding phase difference. To determine the dynamic range of SS-PERG adaptation, recording was performed in normal controls of different age (n = 32) and patients with different severity of optic nerve dysfunction (early manifest glaucoma, EMG, n = 7; non-arteritic ischemic optic neuropathy, NAION, n = 5).

RESULTS

Amplitude adaptation was largest in younger controls (amplitude adaptation ÷ noise, SNR = 9.5, 95% CI 13.1, 5.9) and progressively decreased with increasing age (older subjects, SNR = 5.5, 95% CI 9.2, 1.8) and presence of disease (EMG: SNR = 2.4, 95% CI 3.5, 1.4; NAION: SNR = 1.9, 95% CI 6.5,-2.2). In 11 young subjects, amplitude adaptation was repeatable (test-retest in two sessions a week apart; intraclass correlation coefficient = 0.59). Phase adaptation was not significantly different from zero in all groups.

CONCLUSIONS

SS-PERG adaptation accounts for a sizeable portion of the within-test variability. As it has robust SNR, sufficient test-retest variability, and is altered in disease, it may have physiological and clinical significance. This study suggests that SS-PERG protocols should include adaptation in addition to SS-PERG amplitude and phase/latency.

Next Generation PERG Method: Expanding the Response Dynamic Range and Capturing Response Adaptation

Purpose

To compare a new method for steady-state pattern electroretinogram (PERGx) with a validated method (PERGLA) in normal controls and in patients with optic neuropathy.

Methods

PERGx and PERGLA were recorded in a mixed population (n = 33, 66 eyes) of younger controls (C1; n = 10, age 38 ± 8.3 years), older controls (C2; n = 11, 57.9 ± 8.09 years), patients with early manifest glaucoma (G; n = 7, 65.7 ±11.6 years), and patients with nonarteritic ischemic optic neuropathy (N; n = 5, mean age 59.4 ± 8.6 years). The PERGx stimulus was a black-white horizontal grating generated on a 14 × 14 cm LED display (1.6 cycles/deg, 15.63 reversals/s, 98% contrast, 800 cd/m² mean luminance, 25° field). PERGx signal and noise were averaged over 1024 epochs (∼2 minutes) and Fourier analyzed to retrieve amplitude and phase. Partial averages (16 successive samples of 64 epochs each) were also analyzed to quantify progressive changes over recording time (adaptation).

Results

PERGLA and PERGx amplitudes and latencies were correlated (Amplitude R² = 0.59, Latency R² = 0.39, both P < 0.0001) and were similarly altered in disease. Compared to PERGLA, however, PERGx had shorter (16 ms) latency, higher (1.39×) amplitude, lower (0.37×) noise, and higher (4.2×) signal-to-noise ratio. PERGx displayed marked amplitude adaptation in C1 and C2 groups and no significant adaptation in G and N groups.

Conclusions

The PERGx high signal-to-noise ratio may allow meaningful recording in advanced stages of optic nerve disorders. In addition, it quantifies response adaptation, which may be selectively altered in glaucoma and optic neuropathy.

Translational Relevance

A new PERG method with increased dynamic range allows recording of retinal ganglion cell function in advanced stages of optic nerve disorders. It also quantifies the response decline during the test, an autoregulatory adaptation to metabolic challenge that decreases with age and presence of disease.

RE-PERG, a new procedure for electrophysiologic diagnosis of glaucoma that may improve PERG specificity

PURPOSE

A significant variability of the second harmonic (2ndH) phase of steady-state pattern electroretinogram (SS-PERG) in intrasession retest has been recently described in glaucoma patients (GP), which has not been found in healthy subjects. To evaluate the reliability of phase variability in retest (a procedure called RE-PERG or REPERG) in the presence of cataract, which is known to affect standard PERG, we tested this procedure in GP, normal controls (NC), and cataract patients (CP).

METHODS

The procedure was performed on 50 GP, 35 NC, and 27 CP. All subjects were examined with RE-PERG and SS-PERG and also with spectral domain optical coherence tomography and standard automated perimetry. Standard deviation of phase and amplitude value of 2ndH were correlated by means of one-way analysis of variance and Pearson correlation, with the mean deviation and pattern standard deviation assessed by standard automated perimetry and retinal nerve fiber layer and the ganglion cell complex thickness assessed by spectral domain optical coherence tomography. Receiver operating characteristics were calculated in cohort populations with and without cataract.

RESULTS

Standard deviation of phase of 2ndH was significantly higher in GP with respect to NC (P<0.001) and CP (P<0.001), and it correlated with retinal nerve fiber layer (r=-0.5, P<0.001) and ganglion cell complex (r=-0.6, P<0.001) defects in GP. Receiver operating characteristic evaluation showed higher specificity of RE-PERG (86.4%; area under the curve 0.93) with respect to SS-PERG (54.5%; area under the curve 0.68) in CP.

CONCLUSION

RE-PERG may improve the specificity of SS-PERG in clinical practice in the discrimination of GP.

Can Variability of Pattern ERG Signal Help to Detect Retinal Ganglion Cells Dysfunction in Glaucomatous Eyes?

Objective. To evaluate variability of steady-state pattern electroretinogram (SS-PERG) signal in normal, suspected, and glaucomatous eyes. Methods. Twenty-one subjects with suspected glaucoma due to disc abnormalities (GS), 37 patients with early glaucoma (EG), and 24 normal control (NC) were tested with spectral-domain optical coherence tomography (SD-OCT), standard automated perimetry (SAP), and SS-PERG. Mean deviation (MD), pattern standard deviation (PSD), retinal nerve fiber layer (RNFL), and ganglionar complex cells (GCC) were evaluated. The SS-PERG was recorded five consecutive times and the amplitude and phase of second harmonic were measured. PERG amplitude and coefficient of variation of phase (CVphase) were recorded, and correlation with structural and functional parameters of disease, by means of one-way ANOVA and Pearson's correlation, was analysed. Results. PERG amplitude was reduced, as expression of retinal ganglion cells (RGCs) dysfunction, in EG patients and GS subjects compared to NC patients (P < 0.0001). CVphase was significantly increased in EG patients and GS subjects, compared to healthy (P < 0.0001), and it was also correlated with PSD (P = 0.0009), GCC (P = 0.028), and RNFL (P = 0.0078) only in EG patients. Conclusions. Increased intrasession variability of phase in suspected glaucomatous eyes may be a sign of RGCs dysfunction.

Adaptation of the steady-state PERG in early glaucoma

PURPOSE

Previous studies have shown that the onset of high-contrast, fast reversing patterned stimuli induces rapid blood flow increase in retinal vessels in association with slow changes of the steady-state pattern electroretinogram (PERG) signal. We tested the hypothesis that adaptive PERG changes of normal controls differed from those of glaucoma suspects and patients with early manifest glaucoma.

METHODS

Subjects were 42 glaucoma suspects (Standard Automated Perimetry-MD -0.89±1.8 dB), 22 early manifest glaucoma (MD -2.12±2.4 dB) with visual acuity of ≥20/20 and 16 age-matched normal controls from a previous study. The PERG signal was sampled every ~15 seconds over 4 minutes in response to gratings (1.6 cyc/degree, 100% contrast) reversing 16.28 times/s. Amplitude/phase values of successive PERG samples were fitted with a nonparametric locally weighted polynomial regression smoothing function to retrieve the initial and final values and calculate their difference (δ) and the residual SD around the fitted function. The magnitude of PERG adaptive change compared to random variability was calculated as log10 of percentage coefficient of variation (CoV)=100×residual SDr÷δ. Grand-average PERGs were also obtained by averaging all samples of the same series.

RESULTS

The grand-average PERG amplitude [analysis of variance (ANOVA), P=0.02], but not phase (ANOVA, P=0.63), decreased with increasing severity of disease. Adaptive changes [log10 (CoV)] of PERG amplitude were not significantly associated with disease severity (ANOVA, P=0.27) but adaptive changes [log10 (CoV)] of PERG phase were (ANOVA, P=0.037; linear trend, P=0.011).

CONCLUSIONS

The steady-state PERG signal displayed slow adaptive changes over time that could be isolated from random variability. PERG adaptive changes differed from those of grand-average PERGs (corresponding the standard steady-state PERG), thus representing a new source of biological information about retinal ganglion cell function that may have potential in the study of glaucoma and optic nerve diseases.

Visual electrodiagnosis in glaucoma screening: a clinical study

BACKGROUND

The aim of the present study was to investigate the value of pattern visual-evoked potentials (pVEP) and pattern electroretinograms (pERG) in early glaucoma diagnosis.

MATERIALS AND METHODS

Thirty-eight eyes of 38 patients were included. Patients were classified into normal control (NC) and glaucoma patient (GP) groups. Patients underwent a detailed clinical ophthalmic examination and an electrodiagnostic examination using steady-state pVEP and pERG. Differences between groups in the amplitudes of the second harmonic of the pVEP and pERG responses to 480' (A480) and 48' (A48) check sizes and the ratio of the above amplitudes (A48/A480) were examined.

RESULTS

Differences in the 48' and 480' pVEP between groups were not statistically significant. The pVEP A48/A480 ratio was significantly higher in NC than in GP. Differences in pERG between groups were statistically not significant for both 48' and 480' check sizes. In contrast, respective differences in pERG A48/A480 ratio were statistically significant.

CONCLUSIONS

Steady-state pVEP and pERG A48/A480 ratio may be of value in glaucoma diagnosis.

Reduced habituation of the retinal ganglion cell response to sustained pattern stimulation in multiple sclerosis patients

OBJECTIVE

Sustained pattern stimulation in normal subjects induces adaptive changes in pattern electroretinogram (PERG), an effect that has been interpreted as a response of glial cells and retinal ganglion cells (RGC). The aim of this study was to compare the effect in normal subjects and in multiple sclerosis patients without previous optic neuritis.

METHODS

PERGs were elicited by a 7.5 Hz pattern stimulus, presented continuously over 152 s. Response cycles were averaged in 20 packets of 60 events each and amplitude and phase of the 2nd harmonic response was measured. Adaptive changes are expressed as amplitude reduction over the full examination time.

RESULTS

In normal subjects PERG amplitude declined progressively to a plateau (dA=-0.46 μV, SE=0.09 μV); in patients the effect size was severely reduced (dA=-0.20 μV, SE=0.04 μV). No significant difference was found in mean amplitude.

CONCLUSIONS

The results show reduced RGC habituation in patients, suggesting an abnormal gain and sensitivity control in the inner retina, even in absence of clinical optic neuritis. Recent findings in astrocyte biology and indications drawn from a mathematical model point to a key role of glial cells in this process.

SIGNIFICANCE

The proposed methodology may have implications in the assessment of MS patients and in understanding the pathophysiology of neurological and retinal disorders.

Characterization of structure and function of the mouse retina using pattern electroretinography, pupil light reflex, and optical coherence tomography

OBJECTIVE

To perform in vivo analysis of retinal functional and structural parameters in healthy mouse eyes.

ANIMAL STUDIED

Adult C57BL/6 male mice (n = 37).

PROCEDURES

Retinal function was evaluated using pattern electroretinography (pERG) and the chromatic pupil light reflex (cPLR). Structural properties of the retina and nerve fiber layer (NFL) were evaluated using spectral-domain optical coherence tomography (SD-OCT).

RESULTS

The average pERG amplitudes were found to be 11.2 ± 0.7 μV (P50-N95, mean ± SEM), with an implicit time for P50-N95 interval of 90.4 ± 5.4 ms. Total retinal thickness was 229.5 ± 1.7 μm (mean ± SEM) in the area centralis region. The thickness of the retinal nerve fiber layer (mean ± SEM) using a circular peripapillary retinal scan centered on the optic nerve was 46.7 ± 0.9 μm (temporal), 46.1 ± 0.9 μm (superior), 45.8 ± 0.9 μm (nasal), and 48.4 ± 1 μm (inferior). The baseline pupil diameter was 2.1 ± 0.05 mm in darkness, and 1.1 ± 0.05 and 0.56 ± 0.03 mm after stimulation with red (630 nm, luminance 200 kcd/m(2)) or blue (480 nm, luminance 200 kcd/m(2)) light illumination, respectively.

CONCLUSIONS

Pattern electroretinography, cPLR and SD-OCT analysis are reproducible techniques, which can provide important information about retinal and optic nerve function and structure in mice.

Efficiency and safety of AAV-mediated gene delivery of the human ND4 complex I subunit in the mouse visual system

PURPOSE

To evaluate the efficiency and safety of AAV-mediated gene delivery of a normal human ND4 complex I subunit in the mouse visual system.

METHODS

A nuclear encoded human ND4 subunit fused to the ATPc mitochondrial targeting sequence and FLAG epitope were packaged in AAV2 capsids that were injected into the right eyes of mice. AAV-GFP was injected into the left eyes. One month later, pattern electroretinography (PERG), rate of ATP synthesis, gene expression, and incorporation of the human ND4 subunit into the murine complex I were evaluated. Quantitative analysis of ND4FLAG-injected eyes was assessed compared with green fluorescent protein (GFP)-injected eyes.

RESULTS

Rates of ATP synthesis and PERG amplitudes were similar in ND4FLAG- and GFP-inoculated eyes. PERG latency was shorter in eyes that received ND4FLAG. Immunoprecipitated murine complex I gave the expected 52-kDa band of processed human ND4FLAG. Confocal microscopy revealed perinuclear expression of FLAG colocalized with mitochondria-specific fluorescent dye. Transmission electron microscopy revealed FLAG immunogold within mitochondria. Compared with Thy1.2-positive retinal ganglion cells (RGCs), quantification was 38% for FLAG-positive RGCs and 65% for GFP-positive RGCs. Thy1.2 positive-RGC counts in AAV-ND4FLAG were similar to counts in control eyes injected with AAV-GFP.

CONCLUSIONS

Human ND4 was properly processed and imported into the mitochondria of RGCs and axons of mouse optic nerve after intravitreal injection. Although it had approximately two-thirds the efficiency of GFP, the expression of normal human ND4 in murine mitochondria did not induce the loss of RGCs, ATP synthesis, or PERG amplitude, suggesting that allotopic ND4 may be safe for the treatment of patients with Leber hereditary optic neuropathy.