Modulation of the human photopic ERG luminance-response function with the use of chromatic stimuli

Retinal dysfunction in Parkinson's disease-results of the extended protocol for photopic negative response (PHNR) full-field electroretinogram (ERG)

BACKGROUND

We investigated whether the photopic negative response (PhNR) in the electroretinogram (ERG) was affected in Parkinson's disease (PD) patients and whether it was associated with retinal changes on optical coherence tomography (OCT).

METHODS

Thirty-two patients with PD and 31 age and sex-matched healthy controls from a single tertiary centre were included in the study. Hoehn and Yahr scale scores and the presence of REM sleep behaviour were recorded. PhNR, a-wave and b-wave responses in photopic ERG (red on blue background) and retinal layer thicknesses in OCT were obtained.

RESULTS

The mean age was 61 ± 10.4 in the PD group (female/male: 18/14) and 60.9 ± 7 in the control group (female/male: 18/13). The amplitudes of the PhNR, a- and b-waves in the ERG were significantly decreased in the PD group, but the implicit times were not significantly different. BCVA was significantly correlated with Hoehn and Yahr scores (p < 0.001, r = - 0.596). There was a significant correlation between BCVA and a-wave amplitude (p = 0.047, r = - 0.251). On OCT analysis, the thickness of the nasal INL was increased, and the temporal and inferior OPL and temporal peripapillary RNFL were decreased in the PD group compared to healthy controls (p = 0.032, p = 0.002, p = 0.016 and p = 0.012, respectively).

CONCLUSION

This study demonstrated reduced a-wave, b-wave and PhNR-wave amplitudes on ERG measurements in PD patients. These findings suggest that the whole ERG response, not just the PhNR, is attenuated in patient with PD, suggesting a possible involvement of the visual system in the disease.

Comparing the RETeval® portable ERG device with more traditional tabletop ERG systems in normal subjects and selected retinopathies

Purpose

Our study aimed to determine if ISCEV standard-like ERGs recorded with the LKC RETeval® portable ERG unit compared to those obtained using the more traditional tabletop unit.

Methods

ERGs recorded from normal subjects and patients affected with retinal ON and OFF pathway anomalies were compared. Analysis included peak time and amplitude measurements as well as time–frequency domain analysis with the discrete wavelet transform of waveforms obtained with the two systems.

Results

Although both systems were similarly able to record reliable and highly reproducible ERG responses, there were major discrepancies in ERG responses between the portable and tabletop units, pointing toward a weaker stimulation of the retinal OFF pathway with the portable RETeval® unit.

Conclusion

The portable RETeval® unit appears to be able to record highly reproducible and diagnostically useful clinical ERGs, albeit with some significant differences in waveform composition compared to those obtained with more standard tabletop systems. Given the unknown origin of these waveform discrepancies, if left uncorrected, these differences could potentially lead to erroneous interpretation when used in the clinical context and/or compared to ERGs recorded using more traditional table top units. Clearly, more research is warranted before handheld devices, such as the RETeval®, can be homologated as a diagnostically sound ERG devices.

The photopic negative response (PhNR): measurement approaches and utility in glaucoma

Purpose

Visual electrophysiological testing continues to generate interest among glaucoma experts because of its potential help in clarifying disease pathophysiology and promoting early detection of glaucomatous damage. The photopic negative response (PhNR) is a slow negative component of the full-field electroretinogram that has been shown to provide specific information about retinal ganglion cells (RGCs) activity. The purpose of this article is to review the literature to explore the currently available measurement methods and the utility of PhNR in glaucoma diagnostic process.

Methods

We gathered publications related to the origins, types of stimuli used, measurements methods and applications of the PhNR of ERG in animal models and humans through a search of the literature cited in PubMed. Search terms were: “PhNR”, “photopic negative response”, “glaucoma”, “glaucomatous optic neuropathy”, “ERG”, “electroretinogram”.

Results

The most reliable PhNR measurements are obtained using a red stimulus on a blue background, without requiring refractive correction, fixation monitoring, or ocular media transparency. Given its direct correlation with RGCs response, the PhNR measured as baseline-to-trough (BT) represents the most reliable parameter of evaluation. Glaucoma patients with evident perimetric defects show pathologic PhNR values. Even though the PhNR is promising in detecting early RGCs impairment, distinguishing between healthy subjects and suspect patients at risk of developing glaucomatous damage still remains challenging.

Conclusion

The PhNR is a useful additional tool to explore disorders that affect the innermost retina, including glaucoma and other forms of optic neuropathy. In particular, comparing reports of the standard examinations (optic disc assessment, OCT RNFL measurement, standard automated perimetry) with the results of electrophysiological tests may be helpful in solving clinical diagnostic and management dilemmas. On the one hand, the PhNR of the ERG can examine the parvocellular pathways; on the other hand, the steady-state pattern ERG optimized for glaucoma screening (PERGLA) can explore the magnocellular pathways. This could give ophthalmologists a useful feedback to identify early RGCs alterations suggestive of glaucoma, stratify the risk and potentially monitor disease progression.

Electrophysiological measures of dysfunction in early-stage diabetic retinopathy: No correlation between cone phototransduction and oscillatory potential abnormalities

PURPOSE

To define the relationship between abnormalities in the activation phase of cone phototransduction and the oscillatory potentials (OPs) of the light-adapted electroretinogram in diabetics who have mild or no retinopathy.

METHODS

Subjects included 20 non-diabetic controls and 40 type-2 diabetics (20 had no clinically apparent diabetic retinopathy [NDR] and 20 had mild nonproliferative DR). Single flash responses for a series of stimulus retinal illuminances were measured under light-adapted conditions using conventional techniques. The a-waves of the responses were fit with a delayed Gaussian model to derive R_mp3 (maximum amplitude of the massed photoreceptor response) and S (phototransduction sensitivity). OPs were extracted from the responses by conventional band-pass filtering.

RESULTS

Analysis of variance (ANVOA) indicated that both diabetic groups had significant OP amplitude and S reductions compared to the controls, whereas R_mp3 did not differ significantly among the groups. Although log OP amplitude and log R_mp3 were significantly correlated for the control subjects for each flash retinal illuminance (all r > 0.49, p < 0.03), log OP amplitude and log R_mp3 were not correlated for either diabetic group for any flash retinal illuminance (all r ≤ 0.36, p ≥ 0.13). Log OP amplitude and log S were generally not correlated significantly for the control or diabetic groups.

CONCLUSION

OP amplitude losses do not appear to be related to reduced cone sensitivity in early-stage diabetic retinopathy. This suggests that diabetes may separately affect cone function, as evidenced by cone phototransduction sensitivity losses, and inner-retina function, as evidenced by OP amplitude losses.

A Temporal White Noise Analysis for Extracting the Impulse Response Function of the Human Electroretinogram

PURPOSE

We introduce a method for determining the impulse response function (IRF) of the ERG derived from responses to temporal white noise (TWN) stimuli.

METHODS

This white noise ERG (wnERG) was recorded in participants with normal trichromatic vision to full-field (Ganzfeld) and 39.3° diameter focal stimuli at mesopic and photopic mean luminances and at different TWN contrasts. The IRF was obtained by cross-correlating the TWN stimulus with the wnERG.

RESULTS

We show that wnERG recordings are highly repeatable, with good signal-to-noise ratio, and do not lead to blink artifacts. The wnERG resembles a flash ERG waveform with an initial negativity (N1) followed by a positivity (P1), with amplitudes that are linearly related to stimulus contrast. These N1 and N1-P1 components showed commonalties in implicit times with the a- and b-waves of flash ERGs. There was a clear transition from rod- to cone-driven wnERGs at ∼1 photopic cd.m-2. We infer that oscillatory potentials found with the flash ERG, but not the wnERG, may reflect retinal nonlinearities due to the compression of energy into a short time period during a stimulus flash.

CONCLUSION

The wnERG provides a new approach to study the physiology of the retina using a stimulation method with adaptation and contrast conditions similar to natural scenes to allow for independent variation of stimulus strength and mean luminance, which is not possible with the conventional flash ERG.

TRANSLATIONAL RELEVANCE

The white noise ERG methodology will be of benefit for clinical studies and animal models in the evaluation of hypotheses related to cellular redundancy to understand the effects of disease on specific visual pathways.

Assessing the Contribution of the Oscillatory Potentials to the Genesis of the Photopic ERG with the Discrete Wavelet Transform

The electroretinogram (ERG) is composed of slow (i.e., a-, b-waves) and fast (i.e., oscillatory potentials: OPs) components. OPs have been shown to be preferably affected in some diseases (such as diabetic retinopathy), while the a- and b-waves remain relatively intact. The purpose of this study was to determine the contribution of OPs to the building of the ERG and to examine whether a signal mostly composed of OPs could also exist. DWT analyses were performed on photopic ERGs (flash intensities: −2.23 to 2.64 log cd·s·m⁻² in 21 steps) obtained from normal subjects (n = 40) and patients (n = 21) affected with a retinopathy. In controls, the %OP value (i.e., OPs energy/ERG energy) is stimulus- and amplitude-independent (range: 56.6–61.6%; CV = 6.3%). In contrast, the %OPs measured from the ERGs of our patients varied significantly more (range: 35.4%–89.2%; p < 0.05) depending on the pathology, some presenting with ERGs that are almost solely composed of OPs. In conclusion, patients may present with a wide range of %OP values. Findings herein also support the hypothesis that, in certain conditions, the photopic ERG can be mostly composed of high-frequency components.

Advance in ERG analysis: from peak time and amplitude to frequency, power, and energy

Purpose. To compare time domain (TD: peak time and amplitude) analysis of the human photopic electroretinogram (ERG) with measures obtained in the frequency domain (Fourier analysis: FA) and in the time-frequency domain (continuous (CWT) and discrete (DWT) wavelet transforms). Methods. Normal ERGs (n = 40) were analyzed using traditional peak time and amplitude measurements of the a- and b-waves in the TD and descriptors extracted from FA, CWT, and DWT. Selected descriptors were also compared in their ability to monitor the long-term consequences of disease process. Results. Each method extracted relevant information but had distinct limitations (i.e., temporal and frequency resolutions). The DWT offered the best compromise by allowing us to extract more relevant descriptors of the ERG signal at the cost of lesser temporal and frequency resolutions. Follow-ups of disease progression were more prolonged with the DWT (max 29 years compared to 13 with TD). Conclusions. Standardized time domain analysis of retinal function should be complemented with advanced DWT descriptors of the ERG. This method should allow more sensitive/specific quantifications of ERG responses, facilitate follow-up of disease progression, and identify diagnostically significant changes of ERG waveforms that are not resolved when the analysis is only limited to time domain measurements.

Estimating ON and OFF contributions to the photopic hill: normative data and clinical applications

BACKGROUND

With progressively brighter stimuli, the amplitude of the b-wave of the human photopic electroretinogram (ERG) first increases to a maximal value (Vmax) and then decreases to finally reach a plateau, a phenomenon known as the photopic hill (PH). A mathematical model combining a Gaussian (G) and a logistic (L) growth function was previously proposed to fit this unusual luminance-response curve, where the G and L functions were suggested to represent, respectively, the OFF and ON retinal pathway contributions to the building of the PH.

METHOD

The PHs of patients presenting stationary diseases affecting specifically the ON (3 CSNB-1) or OFF (4 CPCPA) retinal pathways as well as patients affected with retinitis pigmentosa (14 RP) of different stages or etiology were analyzed using this mathematical model and compared to the PHs of a group of 28 normal subjects.

RESULTS

The PH of the CSNB-1 patients had a much larger contribution from the G function compared to normal subjects, whereas the opposite was observed for the CPCPA patients. On the other hand, analysis of data from RP patients revealed variable G-L contributions to the building of their PH.

CONCLUSION

In this study, we confirm the previous claim that the luminance-response function of the photopic ERG b-wave can be decomposed into a Gaussian function and a logistic growth function representing, respectively, the OFF and ON retinal pathways. Furthermore, our findings suggest that this mathematical decomposition could be useful to further segregate and potentially follow the progression of retinopathies such as RP.

Asymmetrical growth of the photopic hill during the light adaptation effect

In response to progressively stronger flashes delivered against a rod saturating background light, the amplitude of the photopic ERG b-wave first increases, reaches a maximal value (V (max)) and then decreases gradually to a plateau where the amplitude of the b-wave equals that of the a-wave, a phenomenon known as the photopic hill (PH). The purpose of this study was to investigate how the PH grew during the course of the light adaptation (LA) process that follows a period of dark adaptation (DA): the so-called light adaptation effect (LAE). Photopic ERG (time-integrated) luminance-response (LR) functions were obtained prior to (control-fully light adapted) and at 0, 5 and 10 min of LA following a 30-min period of DA. A mathematical model combining a Gaussian and a logistic growth function, suggested to reflect the OFF and ON retinal contribution to the PH respectively, was fitted to the LR functions thus obtained. Our results indicate that the magnitude of the cone ERG LAE is modulated by the stimulus luminance, with b-wave enhancements being maximal for luminance levels that result in the descent of the PH. The Gaussian function grew significantly with LA while the logistic growth function remained basically unchanged. Our findings would therefore suggest that the LAE reflects primarily an increase in the retinal OFF response during LA.

Physiologic significance of steady-state pattern electroretinogram losses in glaucoma: clues from simulation of abnormalities in normal subjects

PURPOSE

To better understand pathophysiologic mechanisms underlying pattern electroretinogram (PERG) losses in glaucoma by simulating either retinal ganglion cell (RGC) dysfunction or RGC loss in normal subjects.

MATERIALS AND METHODS

The steady-state PERG has been recorded in 10 normal subjects (mean age: 31+/-8 y) according to the PERGLA paradigm by means of skin electrodes in response to horizontal gratings (1.7 cycles/degree, 99% contrast, 40 cd/m mean luminance, circular field size 25 degree diameter) alternating 16.28 times/seconds. Simulated RGC dysfunction has been obtained by reducing either contrast and mean luminance or blurring the visual stimulus. Simulated RGC loss has been obtained by reducing stimulus area. Outcome measures were PERG amplitude and phase obtained by discrete Fourier transform of PERG waveforms.

RESULTS

Progressive PERG amplitude reductions spanning the entire dynamic range of PERG response could be obtained by progressively reducing stimulus contrast and luminance, blurring the stimulus, and reducing stimulus area. The same variations in stimulus conditions caused phase changes of disparate sign and magnitude. Phase advanced (latency shortened) by reducing stimulus contrast or blurring the stimulus; phase lagged (latency increased) by reducing stimulus luminance; phase remained constant by reducing stimulus area.

CONCLUSIONS

PERG amplitude and phase are essentially uncoupled, implying that these measures reflect distinct aspects of RGC activity. On the basis of our results and known PERG physiology, we propose a model in which both RGC dendrites and RGC axons contribute to the PERG signal. PERG delays may represent an indication of synaptic dysfunction that is potentially reversible.

The effect of broadband and monochromatic stimuli on the photopic negative response of the electroretinogram in normal subjects and in open-angle glaucoma patients

The aim was to investigate the effects of monochromatic and broadband stimuli on the amplitude of the photopic negative response (PhNR) and to compare the sensitivities of these stimuli for the detection of ganglion cell damage in glaucoma patients. Forty-one healthy subjects were studied, along with 16 patients with open-angle glaucoma. Photopic electroretinograms (ERGs) were elicited with monochromatic red, amber, green, and broadband white stimuli of progressively brighter intensities in a blue background. Pattern ERGs were also recorded using a 0.8 degrees checkerboard pattern on a 21.6 degrees x 27.8 degrees screen. In the photopic ERGs of the control subjects, the PhNR amplitude was significantly higher (P < 0.01) to red than to monochromatic amber, green, and broadband white stimuli of the same intensity. In glaucoma patients, the percentage of amplitude reduction was greater for the PhNR to red (68%, P < 0.001) than to the broadband stimulus (38%, P = 0.001). The PhNR to red monochromatic stimulus appeared to be a more sensitive parameter, with a larger area enclosed by the receiver-operating characteristic curve (0.97) than for the PhNR to broadband stimulus (0.76). Also, the PhNR to red stimulus showed a more significant correlation with the pattern ERG and the visual field defects (P < 0.05) than the PhNR elicited with broadband stimulus. These findings suggest that ganglion cell activity can be more efficiently evaluated with the PhNR elicited with a red than with a broadband stimulus. The PhNR thus appears to be a promising test for the diagnostics of the ganglion cell dysfunction.

ON- and OFF-response of the photopic electroretinogram in relation to stimulus characteristics

The aim of this study was to explore the effect of stimulus duration, stimulus intensity, stimulus wavelength and background luminance on the amplitude and waveform of the ON- (b-wave) and OFF- (d-wave) response of the photopic ERG. The following parameters were used in this study: stimulus duration from 5 to 200 ms, the flash intensities from 0.4 to 2.1 log cd s/m2, the background luminance from 20 to 50 cd/m2 and stimulus wavelengths 460, 508 and 667 nm. Prolonging the stimulus duration from 75 to 200 ms did not influence the amplitudes of a-, b- and d-waves significantly. Higher stimulus intensities (1.9 log cd s/m2) broadened the b-wave and increased variability of its implicit time. With a brighter background (50 cd/m2) the b-wave did not broaden and its optimal amplitude was preserved. Stimuli of longer wavelengths (667 nm) significantly reduced the d-wave amplitude. Our results suggest that high intensities and longer wavelengths (red) may not be suitable stimulus parameters in the routine clinical ON- and OFF-response recording.

A Comparison of the Fast Stimulation Multifocal-ERG in Patients with an IOL and Control Groups of Different Age

PURPOSE

It has been shown that a cataract significantly reduces mfERG responses in the central 4-14 degrees . Removing the cataract, leads to a significant increase in the response of the central 4 degrees . In this study we compare the mfERG of Woerdehoff et al.'s patients' [Doc Ophthalmol 2004; 108(1): 67-75] following cataract surgery to a healthy control group in order to assess whether, in the elderly, further influences of age need to be considered in addition to optical effects.

METHODS

Eighteen patients with an IOL following cataract surgery and 29 healthy volunteers (without clouding of the media or retinal changes) underwent testing of the mfERG (103 hexagons stimulating the central 50 degrees , M-sequence 2(15), Lmax: 200 cd/m2, Lmin<1 cd/m2). For the first order response component we compared the latencies of N1,P1 and N2 as well as the natural logarithm (ln) of the amplitudes N1P1 and P1N2 for four group averages: I. the central 4 degrees, II. 4-7 degrees, III. 7-10 degrees and IV. 10-15 degrees.

RESULTS

Mean age was 67 years (SD 10.1) for the IOL patients, 28.5 years (SD 5.6) for a young group of controls (n=15) and 60.2 years (SD 9.2) for the older control group (n=14). Patients with an IOL did not differ in latency from either control group (ANOVA, Tukey). Interestingly, at 10-15 degrees eccentricity, the latency of N2 differed significantly between the younger (41.4 ms, SD 1.4) and the older (43.0 ms, SD 1.9) control group. In the central 4 degrees LnN1P1 amplitudes were significantly lower in the IOL group (mean: 3.7, SD 0.2) than either the younger (mean: 3.9, SD 3.3) or the older (mean: 4.0, SD 0.3) control group. In all other amplitude measures, the older control group had slightly larger mean amplitudes than the younger control group and significantly larger amplitudes than the patients with an IOL, whose amplitudes were lowest.

DISCUSSION

Both, primarily optical but also neural phenomena have been described to affect the mfERG changes observed with age. Our results, are in support of this, as the improvement of the mfERG response following cataract surgery does not seem to reach the level of a healthy control group of equal age. Thus, our results suggest, that a control group with an IOL should be used when retinal function is tested in subjects with an IOL.