Spatial cone activity distribution in diseases of the posterior pole determined by multifocal electroretinography

Comparison of retinal parameters between rhesus and cynomolgus macaques

Nonhuman primates are important research models for basic vision research, preclinical pathogenesis, and treatment studies due to strong similarities in retinal structure and function with humans. We compared retinal parameters between 10 healthy normal rhesus macaques (Macaca mulatta) and 10 cynomolgus macaques (Macaca fascicularis) by optical coherence tomography and electroretinography. The Heidelberg Spectralis® HRA+OCT and Roland multifocal electrophysiometer were used to analyze retinal morphology, multifocal electroretinograms (mfERGs), and full-field electroretinograms (ff-ERGs). Mean retinal thickness was lowest in the central fovea of macaques and did not differ significantly between species, but the retinal thicknesses of the nerve fiber ganglion cell layer and the inner plexiform layer were significantly different. The amplitude density of the N1 wave was lower in rhesus macaques than in cynomolgus macaques in ring and quadrant areas. Dark-adapted 3.0 oscillatory potentials (reflection of amacrine cell activity) and light-adapted 30-hz flicker ERG (a sensitive cone-pathway-driven response) waveforms of the ff-ERG were similar in both species, while the times to peaks in dark-adapted 0.01 ERG (the rod-driven response of bipolar cells) and dark-adapted 3.0 ERG (combined rod and cone system responses) as well as the implicit times of the a- and b-waves in light-adapted 3.0 ERG (the single-flash cone response) were substantially different. This study provides normative retinal parameters for nonhuman primate research on basic and clinical ophthalmology, as well as a reference for researchers in the appropriate selection of rhesus or cynomolgus macaques as models for ophthalmology studies.

Multi-Angular Electroretinography (maERG): Topographic mapping of the retinal function combining real and virtual electrodes

GOAL

The full-field electroretinogram (ffERG) is an objective tool to assess global retinal function, though as it is currently done, it is unable to localize sources of retinal dysfunction or damage. To overcome this, we have developed a new way to record multiple spatial derivations of the ERG using the rotating capability of the eye, thus creating virtual electrodes. We have termed this the multi-angular ERG (or maERG). With only 3 real electrodes and 11 varying gaze positions, we create 33 virtual electrodes.

METHODS

We created a realistic electrophysiological and anatomical eye model (i.e. forward model) to reconstruct the retinal activity (i.e. inverse problem) from the 33 virtual electrodes. We simulated 2 pathological scenarios (central and peripheral scotomas), which were compared to their respective theoretical source configurations using an Area under Receiver Operator Characteristic curve metric.

RESULTS

Our simulations show that the low-resolution brain electromagnetic tomography algorithm (LORETA) is the best method tested to reconstruct retinal sources when compared to the Minimum Norm Estimate algorithm. Furthermore, a signal to noise ratio of 50dB is needed to accurately reconstruct the retinas functional map.

CONCLUSION

Our proposed maERG recording method, combined with our solution to the electromagnetic inverse problem enables us to reconstruct the functional map of the human retina.

SIGNIFICANCE

We believe that this new functional retinal imaging technique will permit earlier detection of retinal malfunction and consequently optimize the clinical monitoring of patients affected with retinopathies.

Diagnostic ability of multifocal electroretinogram in early multiple sclerosis using a new signal analysis method

PURPOSE

To determine if a novel analysis method will increase the diagnostic value of the multifocal electroretinogram (mfERG) in diagnosing early-stage multiple sclerosis (MS).

METHODS

We studied the mfERG signals of OD (Oculus Dexter) eyes of fifteen patients diagnosed with early-stage MS (in all cases < 12 months) and without a history of optic neuritis (ON) (F:M = 11:4), and those of six controls (F:M = 3:3). We obtained values of amplitude and latency of N1 and P1 waves, and a method to assess normalized root-mean-square error (FNRMSE) between model signals and mfERG recordings was used. Responses of each eye were analysed at a global level, and by rings, quadrants and hemispheres. AUC (area under the ROC curve) is used as discriminant factor.

RESULTS

The standard method of analysis obtains further discrimination between controls and MS in ring R3 (AUC = 0.82), analysing N1 waves amplitudes. In all of the retina analysis regions, FNRMSE value shows a greater discriminating power than the standard method. The highest AUC value (AUC = 0.91) was in the superior temporal quadrant.

CONCLUSION

By analysing mfERG recordings and contrasting them with those of healthy controls it is possible to detect early-stage MS in patients without a previous history of ON.

Surrogate electroretinographic markers for assessing therapeutic efficacy in the retina

Visual acuity remains the primary surrogate marker for clinical trials in ophthalmology (and the primary outcome for most US Food and Drug Administration applications) due to its long history, ease of measurement and clear relationship to clinically meaningful characteristics of daily life. However, treatment trials are being planned for diseases that are currently untreatable where visual acuity may not be the most appropriate outcome measure. Specialized electroretinographic tests can be powerful surrogate markers in such trials. The selection of outcome measures and surrogate markers depends in part on whether the goal is to preserve remaining vision in a progressive retinal degeneration or to improve vision in an eye that has already undergone extensive degeneration. Among the electroretinographic tests available are those involving the whole retina (full-field electroretinographic), the posterior pole (pattern electroretinographic) or focal areas within the macula (multifocal electroretinographic). The advantages and disadvantages of each will be discussed along with selected applications of each test to a specific category of disease.

Multifocal electroretinography

Multifocal electroretinography (mfERG) provides functional and objective evidence of retinal dysfunction. We have found mfERG to be especially useful in the management of occult outer retinopathy and Stargardt's disease.

Comparing mfERGs with estimates of cone density from in vivo imaging of the photoreceptor mosaic using a modified Heidelberg retina tomograph

The spatial variation in central retinal function determined from mfERG was compared to co-localised measurements of cone density in two normal subjects. Individual cone cells in the parafoveal region of the retina were identified from 1 degrees x1 degrees images of the photoreceptor mosaic using a modified Heidelberg retina tomograph (HRT). The variation in cone density compared well with previous histology and retinal imaging studies and was strongly linearly correlated (r=0.98, p<0.001) with mfERG amplitude within the central retina. Retinal function determined from mfERG amplitude appears to directly reflect the density of the cone cells in this region.

The electroretinogram: a useful tool for evaluating age-related macular disease?

With an ageing population, the number of age-related macular disease (ARMD) cases will inevitably rise. This gives greater impetus for the need to identify the disease earlier and assess treatments to slow disease progression. Differing electroretinogram (ERG) modalities have been reviewed in relation to the objective assessment of retinal function in ARMD and for monitoring the effectiveness of clinical interventions. Conflicting results have been found with regard to the efficacy of ERG findings in the investigation of ARMD in previous years. The newer multifocal ERG paradigm provides spatial topographical information about retinal function in ARMD. It has shown promising results in monitoring effectiveness of clinical interventions and studies are continuing in this area. Better knowledge of retinal function in ARMD may lead to enhanced treatments at each phase of the disease.

Multifocal electroretinogram for functional evaluation of retinal injury following ischemia-reperfusion in pigs

BACKGROUND

Multifocal electroretinogram (mfERG) has the power to discriminate between localized functional losses and overall retinal changes when evaluating retinal injury. So far, full-field ERG has been the gold standard for examining retinal ischemia and the effects of different neuroprotectants in experimental conditions. The aim of the present study was to establish mfERG, with simultaneous fundus monitoring, for analyzing the localized functional response in the retina after ischemia-reperfusion in the porcine eye.

METHODS

70 kg pigs underwent pressure-induced retinal ischemia (1 hour) followed by reperfusion. mfERG recordings were obtained before and after ischemia, followed by 1 and 5 hours of reperfusion. Individual components of the summed mfERG responses were correlated to ischemia and the time of reperfusion.

RESULTS

The visual streak area had significantly higher amplitudes than the optic nerve head and the area in between, suggesting that the mfERG monitors localized functional retinal responses. The mfERG recordings were altered following ischemia-reperfusion. In one group of animals, there was a complete flattening of the mfERG waveforms, indicating complete ischemic injury. In the other group of animals, ischemia-reperfusion altered the mfERG such that the implicit time was increased (20.82 +/- 0.18 before ischemia and 21.57 +/- 0.21 after ischemia and 1 hour of reperfusion, in the visual streak area, p < 0.05) and the amplitude was decreased (13.16 +/- 2.3 before ischemia and 11.47 +/- 0.88 after ischemia and 1 hour of reperfusion, in the visual streak area, p < 0.001), suggesting partial ischemic injury.

CONCLUSIONS

In conclusion, the porcine model of pressure-induced retinal ischemia-reperfusion results in mfERG changes, typical for retinal ischemia. mfERG may be a useful tool for evaluating and monitoring localized cone dysfunction after an ischemic injury.

Ocular and systemic safety evaluation of calcium formate as a dietary supplement

PURPOSE

The objective of this study was to perform a preliminary evaluation of the ocular and systemic safety of calcium formate, a dietary calcium supplement for prevention and management of osteoporosis. Although formate is an endogenous product of metabolism, high concentrations are associated with toxicity during methanol overdose.

METHODS

In this prospective clinical trial, 12 healthy women ingested calcium formate (1,300 mg) three times a day for 14 days. Study evaluations included physical and ocular examination, extensive laboratory testing, serum calcium and formate levels, Early Treatment Diabetic Retinopathy Study (ETDRS) visual acuity, color vision, visual fields, visual evoked potential (VEP), and full-field, pattern, and multifocal electroretinograms (MERG).

RESULTS

The mean baseline serum level of formate was 0.572 +/- 0.06 mM. Peak serum levels and final serum formate did not differ significantly from baseline. The final concentration was 0.582 +/- 0.091 mM. Accumulation of serum formate did not occur. There was also no evidence of toxicity with calcium formate ingestion. All examinations and tests remained normal, including optic nerve and retinal function. Three subjects had mild transient symptoms attributable to any calcium formulation.

CONCLUSIONS

Calcium formate is highly bioavailable and well-tolerated. Serum formate remained at basal levels and did not accumulate with repeated dosing. Systemic and ocular safety was demonstrated by objective testing. Given its high oral bioavailability, calcium formate may be a good choice for calcium supplementation in the prevention and management of osteoporosis. Further study will be needed to evaluate its long-term safety in a larger group of subjects representing more varied age, health, dietary, and nutritional status.

Eccentricity-dependent changes in local onset and offset responses in patients with progressive cone dystrophy

Shinoda and colleagues hypothesized that patients with cone dystrophy (CD) might suffer from a selective ON-system deficit, based on the local nature of the disease [Shinoda, K, Ohde, H, Inoue, R, Ishida, S, Mashima, Y, & Oguchi, Y (2002). ON-pathway disturbance in two siblings. Acta Ophthalmologica Scandinavica, 80, 219-223]. The purpose of the current study was to test this hypothesis by examining onset and offset responses as a function of eccentricity in a group of patients with CD using long-duration LED stimuli. Nine patients with CD participated in this study (mean age of 36.1 years and visual acuity 20/200). For this study, the following measures were obtained: Humphrey threshold visual fields, standard multifocal ERGs (mfERGs) as well as mfERGs to long duration stimuli recorded using the Retiscan stimulator (Roland Instruments). This display contained 61 scaled hexagons and the LEDs were on for 100ms (180cd/m(2)) and off for 100ms. In addition, standard full-field photopic and flicker ERGs using Ganzfeld stimulation were obtained. For the control subjects, the onset responses were larger than the offset responses at all eccentricities; whereas for the patients, there was overlap between the amplitudes of the onset and offset responses. For the patients, the amplitude ratios (relative to the control data) indicated that the difference between the onset and offset responses was greatest for the central-most ring and this difference decreased with increasing eccentricity. For the onset responses, Humphrey thresholds and mfERG amplitudes, performance was poorest for the center ring and best for the most peripheral ring; for the offset responses, the opposite pattern of results was obtained. The differences in the pattern of results in the long duration mfERG data are consistent with a selective loss of the onset responses in our patient population.

The clinical applications of multifocal electroretinography: a systematic review

Multifocal electroretinography (mfERG) is an investigation that can simultaneously measure multiple electroretinographic responses at different retinal locations by cross-correlation techniques. mfERG therefore allows topographic mapping of retinal function in the central 40-50 degrees of the retina. The strength of mfERG lies in its ability to provide objective assessment of the central retinal function at different retinal areas within a short duration of time. Since the introduction of mfERG in 1992, mfERG has been applied in a large variety of clinical settings. This article reviews the clinical applications of mfERG based on the currently available evidence. mfERG has been found to be useful in the assessment of localized retinal dysfunction caused by various acquired or hereditary retinal disorders. The use of mfERG also enabled clinicians to objectively monitor the treatment outcomes as the changes in visual functions might not be reflected by subjective methods of assessment. By changing the stimulus, recording, and analysis parameters, investigation of specific retinal electrophysiological components can be performed topographically. Further developments and consolidations of these parameters will likely broaden the use of mfERG in the clinical setting.

Nuclear kinesis, neurite sprouting and abnormal axonal projections of cone photoreceptors in the aged and AMD-afflicted human retina

Tissues often respond to damage by recapitulating developmental programs. We have investigated whether anatomical signs of developmental recapitulation are evident in cone photoreceptors of the aged and AMD-afflicted human retina. Radial migration of cell nuclei mediated by microtubules is a characteristic feature of cells in the developing retina. Similarly, neurite outgrowth is a feature of developing neurons. We have examined whether nuclear kinesis and neurite outgrowth from cone photoreceptors is evident. Calbindin-positive cone photoreceptor nuclei are normally positioned as a single layer of somata at the outer border of the outer nuclear layer. In AMD-afflicted retinae, many nuclei are translocated, with some somata abutting the outer plexiform layer (OPL) and others outside the outer limiting membrane whilst many nuclei are present at intermediate levels. The axonal processes of many cones were also aberrant, displaying tortuous pathways as they projected to the OPL, with occasional evidence for bifurcation at points where the axon changed direction. We suggest that tangential extension of collateral neurites and the rapid retraction of the original process may give rise to the tortuous axonal projections observed. Since microtubules are key mediators of both neurite extension and nuclear kinesis we examined expression of microtubule associated protein 2 (MAP2) which is an important regulator of neurite extension. The strong expression of MAP2 observed in those cells with aberrant morphologies supports the notion that abnormal microtubule-mediated remodelling events are present in the AMD retina and to a lesser extent in normal aged retinas, allowing cone photoreceptors to recapitulate two key features of development.

Wide field multifocal and standard full field electroretinographic features of hemi retinal vein occlusion

The purpose of this study was to document the standard full field electroretinographic (ERG) and wide field multifocal electroretinographic (WF-mfERG) findings in eyes with recent onset hemi-retinal vein occlusion (HRVO) and to compare the electro-diagnostic findings in the affected and fellow eyes with reference to normative data. Eight patients with HRVO were assessed using ERG and WF-mfERG. WF-mfERG first order responses from the affected hemi-retinae and the unaffected hemi-retinae in each affected eye were compared. WF-mfERG responses from each affected hemi-retina and from the symmetrical hemi-retina of each fellow eye were compared. ERG responses between affected and unaffected eyes were also compared. All electrodiagnostic tests were compared to normative data (5-95% confidence limits derived from age-related controls). WF-mfERG P1 and N1 implicit times were greater for the affected hemi-retinae than for the unaffected hemi-retinae (p <0.05). WF-mfERG N1 and P1 implicit times were prolonged (p < 0.05) and WF-mfERG P1/N1 amplitude ratios were significantly reduced (p < 0.05) for the affected eyes when compared with the fellow eyes. Maximal b-wave, cone b-wave and flicker implicit times were prolonged (p < 0.05) when comparing affected and fellow eyes. These results indicate that retinal injury due to HRVO culminates in significant delay of both ERG and WF-mfERG implicit times. These results suggest that WF-mfERG in combination with ERG may have a role in the management of HRVO.

Examination of short binary sequences for mfERG recording

The mfERG, when first introduced by Erich Sutter used long sequences with short periods (approximately 15 ms) between steps (flashes). Since then a number of studies have introduced slower or modified sequences to emphasise Oscillatory Potentials (OPs), Optic Nerve Head Components (ONHC) or the s-wave. With this reduction in the rate of presentation many of the investigators have reduced the length of the sequence to allow a shorter recording period. This is also desirable for patient comfort and co-operation in diagnostic investigations. When reducing the length of the sequence there is a risk that it will be too short to ensure orthogonality of the first order component and all significant higher order components, particularly when a large number of areas (hexagons) are stimulated. This paper aims to verify that a short sequence (using the sequence used by the Roland Retiscan stimulating 19 hexagons) is capable of keeping responses of both first and higher orders separate for each stimulating area. The sequence was investigated by placing photodiodes connected to a Diagnosys Espion and then exported to Excel and MATLAB for analysis. It was determined that the sequences used were m-sequences length n = 9. The photodiode only responded to flashes of light so was unable to detect a correcting 0 at the end of sequence. The sequences driving each hexagon were then determined and found to be shifted 26 steps from each other. The correlation coefficients between all sequences was found to be -1/(2n -1). The sequences to decode the second order kernels were determined and the correlation coefficients between each of these sequences, and between these and the original sequences, were also -1/(2n -1). This work provides a mathematical validation of the use of short sequences for slow mfERG, and describes an empirical test method.

Influence of Dopamine Deficiency in Early Parkinson’s Disease on the Slow Stimulation Multifocal-ERG

PURPOSE

In animal studies intravitreal injection of tetrodotoxin (TTX) results in mfERG waveform changes similar to those observed in glaucoma. As TTX blocks amacrine as well as ganglion cells, there is still a question regarding the underlying cell population responsible for these changes in waveform. In an attempt to assess the contribution of the amacrine cells to these changes, a mfERG was obtained from patients with Parkinson's disease as some amacrine cells are mediated by dopamine, a substance lacking in Parkinson's.

METHODS

Eight patients with early Parkinson's disease underwent ophthalmologic examination, testing of contrast sensitivity and electrophysiological examination according to ISCEV standard at least 12 h following their last medication with Dopamine. A slow stimulation mfERG was obtained with a stimulus base interval of 53.3 ms and with a stimulus base interval of 106.6 ms. During MF-ERG recordings 103 hexagons stimulated the central 50 deg of the retina simultaneously and independently (m-sequence 2(13), L(max): 200 cd/m(2), approximately 100% contrast).

RESULTS

Contrast sensitivity and ISCEV standard electrophysiological testing was unremarkable. When the mfERG was analyzed, only four patients had an adequate signal-to-noise ratio to allow further data analysis - one of whom was diagnosed with a multi system atrophy in retrospect. The first order response component was analyzed at a filter setting of 10-300 Hz and at 100-300 Hz (OPs) and compared to mfERGs of a control group. On average, in patients, the amplitude of N1P1 was slightly lower in the central and nasal response averages. When the three OPs at a latency of 72-89 ms were analyzed in the 53.3 ms base interval recording, the most marked difference in amplitude was observed in the superior nasal response average of the first OP. Here a mean amplitude of 1.3 nV/deg(2) in patients compared to a mean amplitude of 1.9 nV/deg(2) in the control group (P: 0.08).

DISCUSSION

In contrast to our previous findings in NTG, there was a consistent presence of three OPs. Under the stimulus conditions applied, we did not find an influence of dopaminergic amacrine cells on the mfERG in our patients with moderate stages of Parkinsion's. The difficulties in obtaining an adequate signal-to noise ratio due to e.g. muscle artifacts even in Parkinson patients of moderate disease stages render a success of mfERG recording in patients with more advanced stages unlikely. The question of the influence of dopaminergic amacrine cells on the mfERG could possibly be addressed using MPDT in animal research.

High-Resolution Retinal Imaging of Cone–Rod Dystrophy

PURPOSE

This study examines a patient with cone-rod dystrophy using high-resolution adaptive optics retinal imaging. Conventional ophthalmoscopes provide limited resolution due to their inability to overcome the eye's optical aberrations. In contrast, adaptive optics ophthalmoscopes correct these aberrations to provide noninvasive high-resolution views of the living retina. To date, adaptive optics ophthalmoscopy has been used mainly to examine the normal retina. Here we use adaptive optics ophthalmoscopy to image cone-rod dystrophy in vivo and compare these results with standard clinical tests.

DESIGN

Observational case report.

METHODS

High-resolution retinal images of a patient with cone-rod dystrophy were obtained with the University of Rochester adaptive optics flood-illuminated ophthalmoscope and the adaptive optics scanning laser ophthalmoscope located at the University of Houston and compared with standard clinical tests, including fundus photography, Goldmann visual fields, fluorescein angiography, optical coherence tomography, electroretinography, and multifocal electroretinography.

MAIN OUTCOME MEASURES

Direct measurement of cone density and diameter and comparison of adaptive optics images with standard clinical imaging and functional tests.

RESULTS

Adaptive optics images were acquired at multiple retinal locations throughout a clinically detected bull's-eye lesion. Within the atrophic regions, we observed large areas devoid of wave-guiding cones. In contrast, regions that appeared relatively spared by clinical examination contained a completely tiled cone mosaic. However, in these areas the cones were abnormally large, resulting in a 6.6-fold reduction from the normal peak cone density (patient peak density: 30 100 cones/mm2, normal peak density: 199 200 cones/mm2). Multifocal electroretinography confirmed a 5.5-fold reduction in amplitude of the central peak (10.8 nanovolts/degree2 vs. 59.8 nanovolts/degree2).

CONCLUSIONS

Adaptive optics ophthalmoscopy is a noninvasive technique to observe a patient's retinal pathology directly at a cellular level. It can provide a quantitative measurement of photoreceptor loss in retinal disease.

Multifocal electroretinography and fluorescein angiography in retinal vein occlusion

PURPOSE

To interpret the responses of multifocal electroretinography (mfERG) of the central macula in retinal vein occlusion and correlate them with findings of fluorescein angiography, optical coherence tomography (OCT), and visual acuity.

METHODS

In a prospective observational case series, 25 patients with branch or hemiretinal vein occlusion were examined by means of mfERG, fluorescein angiography, OCT, and visual acuity.

RESULTS

In eyes with macular ischemia shown by fluorescein angiography, the implicit time was significantly longer than that in eyes without macular ischemia (P = 0.006; Mann-Whitney test). The amplitudes in the first-order kernel responses in the ischemic eyes were more reduced (P = 0.04; Mann-Whitney test), and the amplitudes in the central macula correlated well with visual acuity (P = 0.006; Pearson correlation). There was no significant correlation between the degree of retinal thickening as measured by OCT and the mfERG responses, although there was a trend toward reduced amplitudes in the maculae with central retinal thickening (P = 0.08; Pearson correlation).

CONCLUSION

Macular ischemia as measured by fluorescein angiography correlates well with the prolonged implicit time on the first-order kernel response in mfERG, but no significant correlation was detected between the responses of mfERG and the OCT findings. Further investigations will demonstrate if this can be used as a prognostic instrument in the selection of patients, who will most likely benefit from future treatments.

Vitamin A Deficiency Modifies the mfERG: A Case Study of Rod Influence on the mfERG

We report an observational case study which showed that the rod pathway may have a significant effect on the multifocal electroretinogram (mfERG). A 68-year-old lady diagnosed with Vitamin A deficiency underwent standard ERG and mfERG. ERG rod and cone function were analysed and the first order mfERG responses were determined. At presentation, there was no demonstrable rod function on ERG testing and normal cone function. mfERG P1 amplitude responses were normal but mfERG P1 latency responses were significantly delayed. Four weeks after initiating treatment with intramuscular Vitamin A, ERG rod function significantly improved. However, cone amplitude was reduced and the 30 Hz flicker latencies decreased. Furthermore, mfERG P1 latency returned to normal and interestingly mfERG P1 amplitude responses showed a significant reduction. These results alter our understanding of the mfERG response by indicating that there is a possible inhibitory influence on mfERG responses from rod pathway systems.

Glare sensitivity and professional drivers' safety: a case of rod-cone dystrophy with negative electroretinogram

BACKGROUND

To obtain a driver's licence certain requirements for visual acuity and visual field have to be fulfilled. Mesopic contrast and glare sensitivity are not regularly tested and are not crucial to passing the driving test.

CASE REPORT

We report a case of a 39-year-old professional bus driver whose only complaint was increased glare sensitivity. After he had been involved in four traffic accidents, ophthalmological investigations revealed binocular annular scotomata and night blindness, leading to the diagnosis of rod-cone dystrophy.

DISCUSSION

Enhanced glare sensitivity is a common complaint in elderly people or people with the beginnings of cataract but may also represent an initial symptom of a retinal disorder. It is therefore advisable for traffic safety if drivers with such complaints undergo a complete ophthalmological investigation including visual field testing.

mfERG in normal and lesioned rabbit retina

BACKGROUND

To evaluate and describe the cone function in the normal and lesioned rabbit retina using the multifocal electroretinogram (mfERG).

METHODS

Twelve animals underwent a two-port vitrectomy with subsequent retinectomy in one eye. The area of removed retina was located in the nasal part of the visual streak, and measured approximately 1-2 disc diameters. Both eyes were investigated with mfERG preoperatively and up to 13 weeks postoperatively. A Burian-Allen bipolar contact lens with built-in infrared emitters was used to visualize the retina during the recordings. The averages of the trace array amplitudes in the lower nasal and temporal quadrants were calculated and statistically analyzed at the different time intervals. All eyes were examined histologically with hematoxylin and eosin staining.

RESULTS

The retina could be visualized during the mfERG examinations. Postoperatively, up to 3 weeks, amplitudes were reduced over the entire stimulated area in retinectomized eyes (2.20 microV+/-1.22 SD) as compared with preoperative examinations (3.40 microV+/-1.00 SD). After 7 weeks the amplitudes in the quadrant including the retinectomized area remained low (2.62 microV+/-1.02 SD), whereas they were higher than at earlier postoperative examinations in the lower unlesioned temporal quadrant (3.56 microV+/-0.71 SD) with a statistical difference between the quadrants. At 13 weeks this was even more pronounced. In unoperated eyes, the area corresponding to the visual streak displayed significantly higher amplitudes than the area superior to the myelinated streak, corresponding well to the cone distribution. High amplitudes were also detected in the area of the myelinated nerve fibers and optic nerve head, most likely as a result of scattering light. In histological sections, the retinectomized area had a diameter of 1-3 mm.

CONCLUSIONS

This study shows that the mfERG technique can be used as a tool in experimental retinal research involving the rabbit eye, where retinal lesions down to at least 1 mm can be detected. One difficulty involves scattering light emanating from the relatively large optic disc and the myelinated nerve fibers, which makes the use of a mfERG system, in which the fundus can be visualized during stimulation, mandatory.

Peripheral cone dystrophy*1A variant of cone dystrophy with predominant dysfunction in the peripheral cone system1

PURPOSE

To determine the phenotype of 3 patients from 2 pedigrees with an unusual form of cone dystrophy in which the peripheral cone system is more affected than the central cone system, and whose rod system is relatively normal.

DESIGN

Three observational case reports with electrophysiologic and psychophysical studies.

METHODS

Three patients underwent fundus photography, fluorescein angiography, color vision testing, Goldmann visual field testing, full-field electroretinograms (ERGs), focal macular cone ERGs, and rod-cone perimetry, in addition to routine ophthalmologic examinations. Multifocal ERGs also were recorded from 2 patients.

RESULTS

The fundus examination and fluorescein angiogram results essentially were normal except for a mild temporal pallor of the optic disc in 2 patients. The corrected visual acuity ranged from 20/16 to 20/100. Color vision was normal in 1 patient, but was abnormal in 2 patients. A relative paracentral scotoma was detected in 2 patients. Full-field ERG cone responses were reduced significantly, but rod responses were normal in all patients, as in patients with typical cone dystrophy. However, the focal macular cone ERGs were well preserved in all patients. Psychophysical rod-cone perimetry demonstrated that the peripheral cone system was impaired, whereas the rod sensitivity was completely normal. The results of the multifocal ERG in 2 patients supported the findings made by the full-field and focal macular ERGs.

CONCLUSIONS

These findings demonstrate that there is a subgroup of patients with cone dystrophy where the peripheral cone system is more affected than the central cone system.

Multifocal stimulation techniques in ophthalmology -- Current knowledge and perspectives

The purpose of this paper is to give a brief overview on the application of multifocal stimulation techniques in ophthalmology. The use of m-sequences as a stimulus sequence allows a high-resolution topographic mapping of sensory function. Outer retinal dysfunction can readily be detected with the multifocal ERG (mfERG). When the inner retinal contribution to the mfERG response is enhanced through adapting the stimulation sequence, the sensitivity of the mfERG to detect retinal dysfunction in glaucoma can be increased. Testing of the entire visual pathway with multifocal cortical evoked visual potentials is also possible and recent studies have focused on reducing interindividual variability. The use of m-sequence stimulation in magnetic encephalography offers new ways to study visual processing without the need to apply electrodes.

Assessment of multifocal electroretinogram abnormalities and their relation to morphologic characteristics in patients with large drusen

OBJECTIVES

To determine the extent of functional changes in the first-order kernel multifocal electroretinogram (mfERG) responses in patients with large drusen by means of a localized analysis and to determine correlations between mfERG responses and morphologic changes.

METHODS

Thirty-one eyes from 20 patients ages 58 to 84 years with large drusen (> or =5 drusen > or =63 microm diameter) were studied. The mfERGs were recorded with a stimulus of 103 hexagons and a flash intensity of 2.67 candela (cd).s-1.m-2. Each of the 103 single first-order kernel mfERG responses was analyzed and compared with those of age-matched healthy control subjects. Imaging studies, including color stereo fundus photography, red-free fundus photography, and fluorescein angiography, were performed in all patients, and morphologic changes (drusen in red-free fundus photography, staining or window defect in fluorescein angiography) were determined with a digital measurement tool. The mfERG responses were correlated to areas with and without morphologic changes.

RESULTS

Reduced responses were found in 10.0% (scalar products) and 4.0% (response densities) and delayed implicit times in 13.8% (N1), 18.9% (P1), and 23.8% (N2) of all mfERGs. Abnormal mfERG responses extended up to 25 degrees in radius. Significant morphologic-functional relations were detected in only a few patients. Abnormal mfERG variables were present in areas without morphologic changes.

CONCLUSIONS

Patients with large drusen exhibit functional changes in the cone-driven pathways evaluated by the mfERG, indexed particularly by implicit times. Morphologically visible changes do not predict retinal function. Large drusen are associated with a more general retinal dysfunction.

MfERG waveform characteristics in the RS1h mouse model featuring a 'negative' ERG

Several retinal disorders lead to a relatively greater attenuation of the b-wave compared to the a-wave of the electroretinogram (ERG), a constellation called 'negative' ERG. To determine the waveform characteristics of multifocal ERGs (mfERGs) and their dependence on recording parameters in such a case, we studied the Rs1h(-/Y) mouse, the model for x-linked juvenile retinoschisis. mfERGs were recorded with a VERIS 4 system connected to a piggyback stimulator prototype that added the stimulus to the optical pathway of a HRA scanning-laser ophthalmoscope (SLO) by means of a wavelength-sensitive mirror. Real-time fundus visualization was achieved with the infrared laser of the SLO (835 nm). High-pass filter settings and the time interval used by the 'artefact removal' feature were varied to study their influence on the waveform. The mfERG in the Rs1h(-/Y) mouse had a 'negative' shape. However, the high-pass filter setting had to be lowered from the usual 10 Hz down to about 2 Hz in order to obtain that result, otherwise the negative shape was lost and mainly a positive peak remained. Similarly, a short time interval used by the 'artefact removal' feature also removed the negative shape. The Rs1h(-/Y) mouse was found to be a valuable model of diseases with a 'negative' waveform shape also in mfERG. Our results underline the importance of a lower high-pass filter cutoff frequency when recording mfERGs in such disorders. In addition, if the 'artefact removal' feature is used, it should be verified that it doesn't distort the waveform shape.

ERGs, cone-isolating VEPs and analytical techniques in children with cone dysfunction syndromes

Photoreceptor and post-receptoral function in children with congenital and acquired cone disorders was measured by full-field electroretinogram (ERG) and transient visual evoked potentials (VEPs). Subjects were five rod monochromats (RM), five with cone dystrophy (CD), and 30 controls. Patients were diagnosed by clinical findings, ERGs, and standard color vision tests. VEP stimuli were check reversals and color grating onsets that stimulated each photoreceptor type (L-, M-, or S-cones) or post-receptoral pathways (L-M, white/black). VEP signal-to-noise ratios (S/N) were calculated by Fourier analysis of VEP epochs. All RM patients showed extinguished cone ERGs. A near normal S-cone VEP was recorded from a blue-cone rod monochromat without any signal from the L- or M-cone stimuli. Two other RM patients were classified as incomplete RM based on a low-level VEP signal from either L- or M-cone stimuli. CD patients had mildly to severely reduced ERGs and VEPs were abnormal to all cone-isolating stimuli. The VEP S/N ratio was not significantly correlated with the amount of rod contrast in the color stimuli. Color VEPs provide an objective assessment of macular cone function in children with cone dysfunction syndromes that is more sensitive to residual central cone function than standard full-field ERGs. VEP techniques may be useful in the early detection of cone loss in children, especially in children who do not tolerate ERG testing.

L- and M-cone driven large-field and multifocal electroretinograms in sector retinitis pigmentosa

The purpose was to study long- (L-) and middle-wavelength-sensitive (M-) cone-driven ERGs and multifocal ERGs (mfERGs) in sector retinitis pigmentosa (sector RP). Two eyes of two patients with sector RP were measured. ERG responses were measured to stimuli which modulated exclusively the L- or the M-cones or the two simultaneously (both in-phase and in counter-phase) with predefined cone contrast leaving the S-cones unmodulated. For comparison, mfERGs were recorded with the visual evoked response imaging system, using a resolution of 61 hexagonal elements within a 30-degree visual field. The two sector RP patients exhibited a general reduction of the L-/M-cone driven ERG sensitivity. Patient 1 exhibited a slight delay of the M-cone driven ERG. In patient 2, L-cone driven ERG was moderately delayed. In both patients, the phases of the L- and the M-cone driven ERGs were positively correlated with cone contrast. The data of the L/M-cone driven ERGs, the mfERGs and the standard photopic ERGs matched each other qualitatively. We conclude that the sector RP patients were clearly different from normal for both the L- and M-cone driven large-field and the multifocal ERGs. Previously, we investigated L- and M-cone driven ERGs in patients with generalized RP and found several features that differ from the sector RP patients. Our data are in agreement with our previous proposition that amplitudes and phases of the L- and M-cone driven ERGs can be differently affected by retinal disorders.

Preoperative and postoperative assessment by multifocal electroretinography in the management of optic disc pits with serous macular detachment

PURPOSE

To evaluate the macular function by means of multifocal electroretinogram (MF ERG) in eyes with congenital optic disc pit and serous macular detachment. The evaluation was performed before and after the successful surgical intervention.

DESIGN

Nonrandomized, comparative (self-controlled), interventional trial.

PARTICIPANTS

Ten patients (10 eyes) with optic disc pit with serous macular detachment were treated with the macular buckling procedure and followed up from March 1999 through May 2001.

METHODS

In all patients included in the study, MF ERG was recorded before and after treatment. For recording, the VERIS III system (Visual Evoked Response Imaging System; Tomey, Nagoya, Japan) was used.

MAIN OUTCOME MEASURES

The retinal response densities were studied before and after treatment in areas 1 and 2 and were compared with visual acuity.

RESULTS

The MF ERG before treatment showed decreased retinal response densities in all 10 cases. Twelve months after the surgical intervention, the electrical response densities in areas 1 (foveal area) and 2 (parafoveal area) improved. Further improvement was noted in 3 of the 10 patients who completed a follow-up of 18 months after treatment. In 8 of the 10 patients, postoperative visual acuity also increased. In the remaining two patients, visual acuity did not change after treatment, despite improvement of the retinal response densities.

CONCLUSIONS

In eyes with congenital optic disc pit with serous macular detachment, the decrease in retinal electrophysiologic response before treatment was not limited to the fovea, but also involved the perifoveal area. In all 10 eyes 12 months after treatment, the electrical activities in areas 1 and 2 improved. This improvement was not always followed by an increase in visual acuity. In 2 of the 10 patients, visual acuity remained unchanged. The values of retinal response densities before treatment cannot be used alone as a prognostic factor for the postoperative functional retinal results. It should also be stressed that the results of this study cannot be compared with the results of studies where spontaneous resolution of the macular detachment occurred.

Influence of photodynamic therapy in choroidal neovascularization on focal retinal function assessed with the multifocal electroretinogram and perimetry

PURPOSE

To study the influence of photodynamic therapy (PDT) on retinal function in patients with a predominantly classic choroidal neovascular membrane.

DESIGN

Comparative observational study.

PARTICIPANTS

Sixteen eyes with a choroidal neovascular membrane eligible for PDT were included.

METHODS

Visual acuity, multifocal electroretinogram (MF-ERG, Electrodiagnostic Imaging, San Mateo, CA) recordings and visual fields (Octopus d32) were obtained before PDT with Visudyne, Novartis AG, Basel, Switzerland, and at follow-up 2 weeks to 3 months later.

MAIN OUTCOME MEASURES

Visual acuity as well as peak-to-peak amplitudes, peak latencies, and focal differences in the scalar product of the two MF-ERG recordings were analyzed. Mean deviation, loss variance, and the absolute difference in sensitivity in the fields obtained were also analyzed.

RESULTS

After PDT, mean visual acuity stabilized or improved in 13 eyes. Except for two eyes with a predominantly parafoveolar choroidal neovascular membrane, the MF-ERG contribution from the central 4 degrees was reduced below the normal level. Although it improved slightly after PDT, it remained below the normal range. In the central 7.5 degrees, retinal sensitivity increased by up to 80 dB and within the paracentral 7.5 degrees to 15 degrees, it increased by up to 183 dB in 13 eyes.

CONCLUSIONS

Consistent with the results of the treatment of age-related macular degeneration with photodynamic therapy (TAP) study, PDT can lead to stabilization of visual acuity. Improvement in parafoveal function can be demonstrated in the central visual field, as well as in MF-ERG recordings, even in patients whose visual acuity remains stable. Therefore, the MF-ERG and central perimetry may aid the assessment of retinal function in treatment trials of patients with age-related macular degeneration.

The multifocal electroretinogram in central serous chorioretinopathy

PURPOSE

To compare multifocal electroretinograms (ERGs) in control subjects and patients with central serous chorioretinopathy (CSC).

METHOD

Eighteen eyes of control subjects and 15 eyes affected by CSC were tested with a VERIS Sciences 4.0 system. The average response densities and latencies of six annular retinal regions in control subjects were compared with those with CSC.

RESULTS

The N1, P1 average response densities of the first and second annuli were decreased and the N1, P1 latencies of first to third rings were delayed in CSC.

CONCLUSION

The multifocal ERG can be used to evaluate retinal dysfunction in CSC.

Mapping of retinal function in Best macular dystrophy using multifocal electroretinography

In order to evaluate the function of the retina in Best macular dystrophy (BMD) 18 patients were examined by means of the multifocal electroretinogram (mfERG). The mfERG peak amplitudes of the central and pericentral responses were significantly reduced in the BMD patients (p<0.001). The ERG amplitude decrease of the central response was significantly correlated with visual acuity loss and with the funduscopic staging. The implicit times in more eccentric groups were slightly but significantly increased. The markedly reduced mfERG amplitudes with only slightly increased implicit times may indicate cone photoreceptor cell loss or damage to the cone outer segments.

A method for comparing psychophysical and multifocal electroretinographic increment thresholds

The multifocal electroretinogram (mfERG) has been commonly used as a method for obtaining objective visual fields. Although qualitative comparisons have been good, quantitative comparisons between the results from mfERG and the results from Humphrey Visual Field Analyser (HVFA) have found variable degrees of agreement depending upon the mfERG response parameter examined and/or the disease studied. Lack of agreement may be due to differences in methodology, differences in the sites of response generation, and/or differences derived from comparing suprathreshold versus threshold responses. In addition, the two procedures are performed at different levels of adaptation. We developed an approach for matching stimulus parameters and compared mfERG and psychophysical thresholds to assess the effects of technique and level of adaptation on the two responses. Psychophysical and mfERG thresholds were obtained as a function of the adaptation level (1.5-4.0 log td) and retinal location. The derived increment threshold-versus-intensity functions for both measures were fitted using the equation logT=logT(0)+log((A+A(0))/A(0))(n). We found that the values of A(0) for the mfERG data were one log unit higher than those for the psychophysical data. In addition, the value of the slope (n) for the mfERG data was shallower (0.8) than that of the psychophysical data (1.0). Predictions were made about comparisons of HVFA threshold and mfERG amplitude data in patients with retinal disease based upon a two-site model of adaptation. The data for some groups of patients could be best-fitted with a model of a disease acting at a site distal to all gain changes, whereas data from other patients were best fitted with a model of a disease acting at a site proximal to all retinal gain. The relationship between the Humphrey visual field threshold losses and mfERG amplitude reductions depends upon the site and mechanism of a particular disease process and the model of retinal gain assumed. In no case is a one-to-one relationship between the losses in the two measures predicted.

Diagnosis and classification of macular degenerations: an approach based on retinal function testing

The results from literature concerning some aspects of retinal function in macular degenerations (MDs) were reviewed in order to evaluate whether (a) specific patterns of retinal dysfunction may be linked to different clinical phenotypes, and (b) distinct functional profiles may help in orienting molecular diagnosis of diseases. Examined clinical phenotypes included: Stargardt disease/fundus flavimaculatus (St/FF), age-related maculopathy (ARM) and macular degeneration (AMD), pattern dystrophies (PD), Best vitelliform dystrophy (BVD), Sorsby's fundus dystrophy (SFD), autosomal cone-rod dystrophies (CRD). The following functional tests were evaluated: (1) electroretinogram (ERG) (scotopic and photopic according to ISCEV standards, rod and cone photoresponses, rod and cone b-wave intensity-response function, focal ERGs); (2) dark adaptometry (pre-bleach sensitivity and post-bleach recovery kinetics); (3) fundus reflectometry (pigment density and regeneration kinetics). Specific patterns of retinal dysfunction were identified for St/FF, ARM/AMD, SFD and BVD, whereas partially overlapping profiles were found for PD and CRD. Specific functional patterns were associated with different peripherin/RDS gene mutations, as well as with CRX mutations. Combined analysis of different retinal function tests may help to identify different phenotypes of MD, and to orient molecular diagnosis for selected genotypes.

Clinical applications of multifocal electroretinography

The multifocal ERG using the m-sequence stimulation technique allows the derivation of 61 - 241 local ERG signals in a central visual field of about 60 degree diameter in a short time between 4 and 16 min. A recording in a light adapted state offers local information comparable to cone responses in the full-field ERG. Retinal functional losses due to regional disorders in outer retinal layers can be described in detail with this technique. In maculopathies decreased or absent central ERGs are found surrounded by normal ERG. The extent of the central lesion can be estimated. In diseases of the outer retina the pattern of distribution of multifocal ERG activity is similar to the the pattern of the visual field defect. In addition to decreased ERG amplitudes a delay of implicit time may be an important sign of pathology, i.e. the pronounced delay of implicit times in the periphery in retinitis pigmentosa and the implicit time delays in regions associated with retinal edema like CRVO and cystoid macula edema in intermediate uveitis. No simple correlation of the first order kernel multifocal ERG and field defects could be found in disorders of the ganglion cell layer. The multifocal ERG is therefore useful in the differential diagnosis of retinal and optic nerve diseases.

Multifocal ERG and VEP responses and visual fields: comparing disease-related changes

Static visual perimetry and the multifocal technique both measure the local effects of diseases of the retina and optic tract. The purpose here is to relate the measures obtained from each technique and to describe this relationship in some diseases. It is important to measure both the implicit time and amplitude of the multifocal ERG (mERG) or multifocal VEP (mVEP) responses. Some diseases affect one measure of the responses but not the other. The comparison of either measure to local sensitivity changes measured with static perimetry (e.g. the Humphrey 24-2 and 30-2) presents a problem. Different stimulus displays are employed. Further, the multifocal responses are displayed with arbitrary spacing between the responses. One approach is to measure the amplitude and implicit time of the multifocal responses and display these values on the same coordinates as in the visual field plots. This allows a qualitative comparison of fields and multifocal responses on the same scale. A second approach involves modifying the Humphrey perimeter software so that the test spots are placed in the centers of the multifocal stimuli (e.g. the center of each hexagon of the mERG display). A third approach involves estimating the thresholds for the regions of the multifocal display by interpolating from values at the standard Humphrey locations. The second and third approaches produce a one-to-one mapping of the multifocal and field measures and allow a quantitative comparison between the two. The relationship between visual fields and multifocal responses, determined through one or more of these approaches, is different depending upon whether the disease primarily affects the outer retina (retinitis pigmentosa), ganglion cell (glaucoma), or optic nerve (ischemic optic neuropathy and optic neuritis).

Assessing retinal function with the multifocal technique

With the multifocal technique, as developed by Erich Sutter and colleagues, scores of focal electroretinogram (ERG) responses can be obtained in a matter of minutes. Although this technique is relatively new, it has already provided insights into the mechanisms of retinal disease. However, because it is new, there also remain questions about how it works and what it measures. This chapter considers some of these insights and some of these questions. The first part (Section 2) describes how the multifocal ERG (mERG) is recorded and considers its relationship to the full-field ERG. The mERG responses are shown to be from relatively local regions of the retina and are comprised of the same components as the full-field ERG. The diagnostic advantage of the mERG as compared to the full-field ERG is also illustrated. In Section 3, the effects of damage to different cell layers of the retina are shown to affect the mERG differently, and these changes are summarized within a conceptual framework. It is argued, for example, that when diseases of the receptor outer segment, like retinitis pigmentosa, result in small, depressed mERG responses, then the damage is, as expected, at the outer segment. However, when these diseases result in mERG responses that are reasonably large but very delayed, then the damage is beyond the outer segment, probably in the outer plexiform layer. The implicit time of the mERG, not amplitude, is the more sensitive measure of damage in degenerative diseases of the receptors. On the other hand, diseases, like glaucoma, which act on the ganglion axon, do not result in easily identified changes to the mERG unless inner retinal damage is involved as well. Inner retinal damage changes the waveform of the mERG and decreases the naso-temporal variation normally observed. Finally, diseases, like diabetes, that act on more than one layer of the retina can have a range of effects. In Section 4, recent work with the monkey mERG is reviewed, with emphasis on the relevance to human diseases. For example, blocking the sodium-based action potentials produced by ganglion and amacrine cells eliminates the naso-temporal variation in the monkey mERG and these altered mERG responses resemble those from some patients with diabetes or glaucoma. Finally, in Section 5 the second-order kernel is described. The presence of a second-order kernel has important implications for understanding the shape of the mERG response (first-order kernel). Full-field simulations of the mERG paradigm illustrate that the first-order kernel is comprised of responses with different waveforms. Further, it is argued that the nonlinear, adaptive mechanisms that produce the second-order kernel are involved in shaping the time course of the response. Patients with large, but abnormally delayed mERG responses (first-order kernel), do not have a detectable second-order kernel. It is speculated that a markedly diminished second-order kernel is diagnostic of outer plexiform layer damage, not inner plexiform layer damage as is commonly assumed.

The diagnostic challenge of occult large vessel ischemia of the retina and choroid

Vascular occlusions of the retina and choroid can cause severe visual loss. These occlusions can occur as a result of systemic disease or after surgery. In most cases, the retinal appearance provides evidence of ischemia as the cause of visual loss. On occasion, however, clinical examination shows no objective signs of vascular occlusion, and this can lead the clinician to suspect optic nerve pathology as the cause of visual loss. This paper outlines some of the diagnostic criteria, clinical findings, and ancillary studies that can be used to differentiate between occult occlusion of the retina or choroid and optic nerve disease.