New approaches to ophthalmic electrodiagnosis by retinal oscillatory potential, drug-induced responses from retinal pigment epithelium and cone potential

Comparisons of oscillatory potentials and 30 Hz flicker electroretinograms for discriminating eyes with diabetic retinopathy from normal eyes

PURPOSE

To compare the amplitudes and implicit times of the oscillatory (OPs) of the full-field electroretinograms (ERGs) to those of the 30 Hz flicker ERGs in differentiating eyes with diabetic retinopathy (DR) from normal eyes.

STUDY DESIGN

Single-center observational study.

METHODS

Full-field ERGs were recorded in 55 patients with Type 2 diabetes mellitus (DM) and 20 normal control subjects. The amplitudes and implicit times of the OPs and of the 30 Hz flicker ERGs were measured. Optical coherence tomography angiography (OCTA) was used to record 3×3 mm enface images of the retina from which the vascular density (VD) of the superficial capillary plexus (SCP) and deep capillary plexus (DCP) were obtained. The receiver operating characteristic (ROC) curves were used to determine the ability of each ERG parameter to discriminate diseased eyes from normal eyes. The significance of the correlations between each ERG parameter and the VD of the SCP and DCP was determined.

RESULTS

The area under the ROC curves (AUCs) was significantly larger for the implicit times than for the amplitudes of each ERG component (P<0.005). There were no significant differences in the AUCs between the OPs and 30 Hz flicker ERGs in differentiating eyes with DM or DR from normal eyes. The implicit time of the 30 Hz flicker ERG had the highest significant correlation coefficient with the VD of the DCP (r = - 0.31, P <0.001).

CONCLUSIONS

The OPs and 30 Hz flicker ERGs have equal ability in differentiating eyes with DR from normal eyes but with better ability for the implicit times than the amplitudes. The implicit time of the 30 Hz flicker ERG is the most sensitive parameter that is correlated with the reduction of VD among the full-field ERG components.

Effects of fixed cutoff filtering on dark- and light-adapted ERG components and the application of variable cutoff filter

Purpose

Human oscillatory potentials (OPs) are derived from dark-adapted (DA) electroretinograms (ERGs) with fixed frequency cutoff filters while light-adapted (LA) OPs are generally not isolated from ERGs. Our purpose was to analyze the effect of cutoff frequencies on DA and LA ERG components using a series of fixed and variable filters.

Methods

DA and LA ERGs were recorded from 10 healthy eyes of 10 subjects (mean age = 20.5 ± 6.7 years) following ISCEV standards. Each signal was filtered in the Fourier domain to acquire slow (a- and b-waves; below cutoff frequency) and fast (OPs; above cutoff frequency) components. Fixed cutoff frequencies ranged from 60 to 105 Hz and a variable cutoff frequency was calculated. Results were analyzed with statistical tests and specific models.

Results

DA ERG components were slightly influenced by the filter cutoff frequency. In contrast, fixed and variable filters significantly changed LA components: the lower the cutoff frequency the smaller the b-wave and OP3 and the higher the OP2/OP4 amplitudes. Analyzing the filter frequency limits a transition range between 68.9 Hz and 83.9 Hz was observed where amplitudes vary.

Conclusions

The present report shows that DA OPs may be isolated from ERGs using filtering procedures with high-pass cutoff frequency at about 75 Hz as recommended by ISCEV. On the other hand, the spectral distribution of low-frequency and high-frequency LA ERG components may overlap. Accordingly, filtering the signal using different cutoff frequencies is not necessarily separating b-wave and OPs.

Pemafibrate Prevents Retinal Dysfunction in a Mouse Model of Unilateral Common Carotid Artery Occlusion

Cardiovascular diseases lead to retinal ischemia, one of the leading causes of blindness. Retinal ischemia triggers pathological retinal glial responses and functional deficits. Therefore, maintaining retinal neuronal activities and modulating pathological gliosis may prevent loss of vision. Previously, pemafibrate, a selective peroxisome proliferator-activated receptor alpha modulator, was nominated as a promising drug in retinal ischemia. However, a protective role of pemafibrate remains untouched in cardiovascular diseases-mediated retinal ischemia. Therefore, we aimed to unravel systemic and retinal alterations by treating pemafibrate in a new murine model of retinal ischemia caused by cardiovascular diseases. Adult C57BL/6 mice were orally administered pemafibrate (0.5 mg/kg) for 4 days, followed by unilateral common carotid artery occlusion (UCCAO). After UCCAO, pemafibrate was continuously supplied to mice until the end of experiments. Retinal function (a-and b-waves and the oscillatory potentials) was measured using electroretinography on day 5 and 12 after UCCAO. Moreover, the retina, liver, and serum were subjected to qPCR, immunohistochemistry, or ELISA analysis. We found that pemafibrate enhanced liver function, elevated serum levels of fibroblast growth factor 21 (FGF21), one of the neuroprotective molecules in the eye, and protected against UCCAO-induced retinal dysfunction, observed with modulation of retinal gliosis and preservation of oscillatory potentials. Our current data suggest a promising pemafibrate therapy for the suppression of retinal dysfunction in cardiovascular diseases.

Advanced Retinal Imaging and Ocular Parameters of the Rhesus Macaque Eye

Purpose

To determine the range of normal ocular biometry and perform advanced retinal imaging and functional assessment of the rhesus macaque eye.

Methods

We performed ocular phenotyping on rhesus macaques at the California National Primate Research Center. This process consisted of anterior and posterior segment eye examination by ophthalmologists, advanced retinal imaging, and functional retinal electrophysiology.

Results

Full eye examinations were performed on 142 animals, consisting of pupillary light reflex, tonometry, external examination and photography, anterior slit lamp examination, and posterior segment examination by indirect ophthalmoscopy. Ages of the rhesus macaques ranged from 0.7 to 29 years (mean, 16.4 ± 7.5 years). Anterior segment measurements such as intraocular pressure (n = 142), corneal thickness (n = 84), lens thickness (n = 114), and axial length (n = 114) were acquired. Advanced retinal imaging in the form of fundus photography (n = 78), optical coherence tomography (n = 60), and quantitative autofluorescence (n = 44) was obtained. Electroretinography (n = 75) was used to assay retinal function. Quantitative analyses of the macular structure, retinal layer segmentation, and rod and cone photoreceptor electrical responses are reported. Quantitative assessments were made and variations between sexes were analyzed to compare with established sex changes in human eyes.

Conclusions

The rhesus macaque has an ocular structure and function very similar to that of the human eye. In particular macular structure and retinal function is very similar to humans, making this species particularly useful for the study of macular biology and development of therapies for cone photoreceptor disorders.

Translational Relevance

Rhesus macaques are an ideal model for future vision science studies of human eye diseases.

Retinal dysfunction induced in a mouse model of unilateral common carotid artery occlusion

Background

Retinal ischemic stresses are associated with the pathogenesis of various retinal vascular diseases. To investigate pathological mechanisms of retinal ischemia, reproducible, robust and clinically significant experimental rodent models are highly needed. Previously, we established a stable murine model of chronic hypoperfusion retinal injuries by permanent unilateral common carotid artery occlusion (UCCAO) and demonstrated chronic pathological processes in the ischemic retina after the occlusion; however, retinal functional deficits and other acute retinal ischemic injuries by UCCAO still remain obscure. In this study, we attempted to examine retinal functional changes as well as acute retinal ischemic alterations such as retinal thinning, gliosis and cell death after UCCAO.

Methods

Adult mice (male C57BL/6, 6–8 weeks old) were subjected to UCCAO in the right side, and retinal function was primarily measured using electroretinography for 14 days after the surgery. Furthermore, retinal thinning, gliosis and cell death were investigated using optical coherence tomography, immunohistochemistry and TUNEL assay, respectively.

Results

Functional deficits in the unilateral right retina started to be seen 7 days after the occlusion. Specifically, the amplitude of b-wave dramatically decreased while that of a-wave was slightly affected. 14 days after the occlusion, the amplitudes of both waves and oscillatory potentials were significantly detected decreased in the unilateral right retina. Even though a change in retinal thickness was not dramatically observed among all the eyes, retinal gliosis and cell death in the unilateral right retina were substantially observed after UCCAO.

Conclusions

Along with previous retinal ischemic results in this model, UCCAO can stimulate retinal ischemia leading to functional, morphological and molecular changes in the retina. This model can be useful for the investigation of pathological mechanisms for human ischemic retinopathies and furthermore can be utilized to test new drugs for various ischemic ocular diseases.

Pemafibrate Protects Against Retinal Dysfunction in a Murine Model of Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the leading causes of blindness globally. Retinal neuronal abnormalities occur in the early stage in DR. Therefore, maintaining retinal neuronal activity in DR may prevent vision loss. Previously, pemafibrate, a novel selective peroxisome proliferator-activated receptor alpha modulator, was suggested as a promising drug in hypertriglyceridemia. However, the role of pemafibrate remains obscure in DR. Therefore, we aimed to unravel systemic and retinal changes by pemafibrate in diabetes. Adult mice were intraperitoneally injected with streptozotocin (STZ) to induce diabetes. After STZ injection, diet supplemented with pemafibrate was given to STZ-induced diabetic mice for 12 weeks. During the experiment period, body weight and blood glucose levels were examined. Electroretinography was performed to check the retinal neural function. After sacrifice, the retina, liver, and blood samples were subjected to molecular analyses. We found pemafibrate mildly improved blood glucose level as well as lipid metabolism, boosted liver function, increased serum fibroblast growth factor21 level, restored retinal functional deficits, and increased retinal synaptophysin protein expression in STZ-induced diabetic mice. Our present data suggest a promising pemafibrate therapy for the prevention of early DR by improving systemic metabolism and protecting retinal function.

No flow through the vitreous humor: How strong is the evidence?

When analyzing vitreal drug delivery, or the pharmacological effects of drugs on intraocular pressure, or when interpreting outflow facility measurements, it is generally accepted that the fluid in the vitreous humor is stagnant. It is accepted that for all practical purposes, the aqueous fluid exits the eye via anterior pathways only, and so there is negligible if any posteriorly directed flow of aqueous through the vitreous humor. This assumption is largely based on the interpretation of experimental data from key sources including Maurice (1957), Moseley (1984), Gaul and Brubaker (1986), Maurice (1987) and Araie et al. (1991). However, there is strong independent evidence suggesting there is a substantial fluid flow across the retinal pigment epithelium from key sources including Cantrill and Pederson (1984), Chihara and Nao-i, Tsuboi (1985), Dahrouj et al. (2014), Smith and Gardiner (2017) and Smith et al. (2019). The conflicting evidence creates a conundrum-how can both interpretations be true? This leads us to re-evaluate the evidence. We demonstrate that the data believed to be supporting no aqueous flow through the vitreous are in fact compatible with a significant normal aqueous flow. We identify strong and independent lines of evidence supporting fluid flow across the RPE, including our new outflow model for the eye. On balance it appears the current evidence favors the view that there is normally a significant aqueous flow across the RPE in vivo. This finding suggests that past and future analyses of outflow facility, interpretations of some drug distributions and the interpretation of some drug effects on eye tissues, may need to be revised.

Retinal Neurodegeneration as an Early Manifestation of Diabetic Eye Disease and Potential Neuroprotective Therapies

PURPOSE OF REVIEW

Diabetic retinopathy (DR) is a major cause of visual impairment and blindness throughout the world. Microvascular changes have long been regarded central to disease pathogenesis. In recent years, however, retinal neurodegeneration is increasingly being hypothesized to occur prior to the vascular changes classically associated with DR and contribute to disease pathogenesis.

RECENT FINDINGS

There is growing structural and functional evidence from human and animal studies that suggests retinal neurodegeneration to be an early component of DR. Identification of new therapeutic targets is an ongoing area of research with several different molecules undergoing testing in animal models for their neuroprotective properties and for possible use in humans. Retinal neurodegeneration may play a central role in DR pathogenesis. As new therapies are developed, it will be important to develop criteria for clinically defining retinal neurodegeneration. A standardization of the methods for monitoring neurodegeneration along with more sensitive means of detecting preclinical damage is also needed.

Bioelectrical function and structural assessment of the retina in patients with early stages of Parkinson's disease (PD)

Purpose

To determine bioelectrical function and structural changes of the retina in patients with early stages of Parkinson’s disease (PD).

Materials and methods

Thirty-eight eyes of 20 patients with early idiopathic PD and 38 eyes of 20 healthy age- and sex-matched controls were ophthalmologically examined, including assessment of distance best-corrected visual acuity (DBCVA), slit lamp examination of the anterior and posterior segment of the eye, evaluation of the eye structures: paramacular retinal thickness (RT) and retinal nerve fiber layer (RNFL) thickness with the aid of OCT, and the bioelectrical function by full-field electroretinogram (ERG). Additionally, PD patients were interviewed as to the presence of dopamine-dependent visual functions abnormalities.

Results

In patients with early PD, statistically significant changes in comparison with the control group were observed in ERG. They contained a reduction in mean amplitudes of the scotopic a-wave (rod–cone response), the scotopic oscillatory potentials (OPs)—OP2 and OP3, the photopic b-wave, and a reduction in the overall index (OP1 + OP2 + OP3) and a prolongation of mean peak times of the scotopic OP1, OP2, OP3, OP4 (p < 0.05). A questionnaire concerning abnormalities of dopamine-dependent visual functions revealed that PD patients with abnormal peak times of OP1, OP2, and OP3 reported non-specific visual disturbances more frequently in comparison with PD patients with normal peak times of OPs. Other analyzed parameters of ERG, DBCVA, RT, and RNFL did not significantly differ between patients with PD and the control group.

Conclusion

In patients with early PD, bioelectrical dysfunction of the retina was observed in the ERG test, probably as a result of dopamine deficiency in the retina. The results of our study indicate that ERG may also be a useful tool for understanding the reason for non-specific visual disturbances occurring in PD patients.

Dark-adapted oscillatory potentials in preterm infants with and without retinopathy of prematurity

PURPOSE

To describe the appearance and maturation of dark-adapted oscillatory potentials (OPs) in electroretinograms (ERGs) recorded from preterm infants, and to determine any effect of retinopathy of prematurity (ROP).

METHODS

Dark-adapted ERGs were recorded in conjunction with screening for ROP and at outpatient follow-up, using a flash luminance of 11.3 scot cd s m(-2) (4.06 phot cd s m(-2)). Eligible infants were born before 31 weeks' gestation and/or weighed ≤1,250 grams at birth.

RESULTS

Presence or absence of OPs was established for 68 ERG recordings from 38 infants at maturities ranging from 30 weeks' postmenstrual age (PMA) to 28 weeks' post-term corrected age. 20 infants did not develop ROP, eight developed stage 1, one stage 2 and one stage 3 disease which regressed spontaneously. Eight infants received treatment for threshold ROP. OPs were present in 50 % of infants at 36 weeks' PMA and in all by 50 weeks' PMA. The earliest appearance of OPs was at 30+5 weeks' PMA. Individual OP amplitudes increased and peak time of individual OPs decreased with increasing maturity. For infants with threshold ROP summed OP amplitudes tended to be smaller prior to treatment (6.5 vs 9.9μV, P = 0.09) and were significantly smaller by 50 weeks' PMA (14 vs 30μV, P = 0.007). OP1 was less likely to be present in infants who developed stage 3 or worse ROP (P = 0.000).

CONCLUSIONS

Dark-adapted OPs are recordable in some preterm infants from 30 weeks' PMA. Relative suppression of early OPs is a potential marker for developing ROP.

Neurodegenerative differences in the retinas of male and female patients with type 2 diabetes

PURPOSE

The purpose of our study is to determine whether neuroretinal function, measured by the multifocal electroretinogram, differs between males and females with type 2 diabetes and no retinopathy.

METHODS

This study included 70 eyes from 70 adult subjects (14 control males, 22 control females, 16 males with type 2 diabetes, and 18 females with type 2 diabetes). A template-scaling technique was used to obtain first-order P1 implicit times and N1-P1 amplitudes from photopic multifocal electroretinograms within the central 45 degrees.

RESULTS

The males with type 2 diabetes were significantly more abnormal than their female counterparts in two separate analyses of local neuroretinal function. First, the total number of retinal locations with an abnormally delayed implicit time (z score ≥ 2) was higher (P < 0.001) in the diabetic males (482 locations = 29.2%) compared to the diabetic females (298 locations = 16.1%). Second, in the response topographies that consisted of 103 means of local implicit times for each group, the diabetic males were significantly delayed (P < 0.025) at 23 corresponding positions (22.3%) compared to the diabetic females. At the same time, no corresponding stimulus locations were significantly delayed in the diabetic females compared to the diabetic males.

CONCLUSIONS

Neuroretinal function is more abnormal in males than in females for adults with type 2 diabetes and no retinopathy. These results suggest that, relative to males, females may have some protection from, or resistance to, neurodegenerative changes that precede the development of background retinopathy in type 2 diabetes.

Neurodegenerative influence of oxidative stress in the retina of a murine model of diabetes

AIMS/HYPOTHESIS

Diabetic retinopathy is a progressive neurodegenerative disease, but the underlying mechanism is still obscure. Here, we focused on oxidative stress in the retina, and analysed its influence on retinal neurodegeneration, using an antioxidant, lutein.

METHODS

C57BL/6 mice with streptozotocin-induced diabetes were constantly fed either a lutein-supplemented diet or a control diet from the onset of diabetes, and their metabolic data were recorded. In 1-month-diabetic mice, reactive oxygen species (ROS) in the retina were measured using dihydroethidium and visual function was evaluated by electroretinograms. Levels of activated extracellular signal-regulated kinase (ERK), synaptophysin and brain-derived neurotrophic factor (BDNF) were also measured by immunoblotting in the retina of 1-month-diabetic mice. In the retinal sections of 4-month-diabetic mice, histological changes, cleaved caspase-3 and TUNEL staining were analysed.

RESULTS

Lutein did not affect the metabolic status of the diabetic mice, but it prevented ROS generation in the retina and the visual impairment induced by diabetes. ERK activation, the subsequent synaptophysin reduction, and the BDNF depletion in the diabetic retina were all prevented by lutein. Later, in 4-month-diabetic mice, a decrease in the thickness of the inner plexiform and nuclear layers, and ganglion cell number, together with increase in cleaved caspase-3- and TUNEL-positive cells, were avoided in the retina of lutein-fed mice.

CONCLUSIONS/INTERPRETATION

The results indicated that local oxidative stress that has a neurodegenerative influence in the diabetic retina is prevented by constant intake of a lutein-supplemented diet. The antioxidant, lutein may be a potential therapeutic approach to protect visual function in diabetes.

Effect of estrogen on electroretinographic responses in streptozotocin-induced diabetic female rats

The aim of this study is to investigate the effects of estrogen on functional changes in the retinas of streptozotocin (STZ)-induced diabetic rats by using an electroretinography. Female rats were randomly divided into four treatment groups: (1) Control (sham operation and vehicle administration); (2) STZ (sham operation and STZ administration); (3) OVX (ovariectomy and vehicle administration); and (4) OVX + STZ (ovariectomy and STZ administration). Full-field electroretinograms (ERGs) were recorded before OVX and STZ administration and 4 and 12 weeks after STZ administration. At 4 weeks after STZ administration, although there were no differences in the STZ and OVX groups compared with the Control group, the amplitude of the cone-response was significantly lower in the OVX + STZ group than in the Control group (P = 0.013). At 12 weeks after STZ administration, this response showed a similar tendency in the STZ and the OVX + STZ groups. At 12 weeks after STZ administration, the implicit times of OP3 and OP4 and of the cone-response were significantly delayed in the STZ and OVX + STZ groups (OP3: P = 0.030 and 0.050, OP4: P = 0.0060 and 0.0053, cone-response: P = 0.014 and 0.039), compared with in the Control group. Thus, the retinal functions in STZ-induced diabetic female rats were aggravated by OVX. OVX-induced estrogen deficiency resulted in earlier changes in the amplitudes of cone-response, especially in the diabetes, although this is a transient effect and it is difficult to explain. Recognizing the early neurosensory change would enable a better understanding of the effect of estrogen in the retina.

Angiotensin II type 1 receptor signaling contributes to synaptophysin degradation and neuronal dysfunction in the diabetic retina

OBJECTIVE

Pathogenic mechanisms underlying diabetes-induced retinal dysfunction are not fully understood. The aim of the present study was to show the relationship of the renin-angiotensin system (RAS) with the synaptic vesicle protein synaptophysin and neuronal activity in the diabetic retina.

RESEARCH DESIGN AND METHODS

C57BL/6 mice with streptozotocin-induced diabetes were treated with the angiotensin II type 1 receptor (AT1R) blocker telimsartan or valsartan, and retinal function was analyzed by electroretinography. Retinal production of the RAS components and phosphorylation of ERK (extracellular-signal regulated kinase) were examined by immunoblotting. Retinal mRNA and protein levels of synaptophysin were measured by quantitative RT-PCR and immunoblot analyses, respectively. In vitro, synaptophysin levels were also evaluated using angiotensin II-stimulated PC12D neuronal cells cultured with or without the inhibition of ERK signaling or the ubiquitin-proteasome system (UPS).

RESULTS

Induction of diabetes led to a significant increase in retinal production of angiotensin II and AT1R together with ERK activation in the downstream of AT1R. AT1R blockade significantly reversed diabetes-induced electroretinography changes and reduction of synaptophysin protein, but not mRNA, levels in the diabetic retina. In agreement with the AT1R-mediated posttranscriptional downregulation of synaptophysin in vivo, in vitro application of angiotensin II to PC12D neuronal cells caused the UPS-mediated degradation of synaptophysin protein via AT1R, which proved to be induced by ERK activation.

CONCLUSIONS

These data indicate the first molecular evidence of the RAS-induced synaptophysin degradation and neuronal dysfunction in the diabetic retina, suggesting the possibility of the AT1R blockade as a novel neuroprotective treatment for diabetic retinopathy.

Evaluation of Intravitreal Kenalog Toxicity in Humans

OBJECTIVE

To evaluate possible functional toxicity of intravitreal Kenalog (commercial triamcinolone acetonide) in patients' retinas.

DESIGN

Observational case series.

PARTICIPANTS

Thirty-two phakic eyes of 16 patients who had nonproliferative diabetic retinopathy and bilateral macular edema refractory to laser therapy, which had no other eye disorder and no previous ophthalmic operation.

INTERVENTION

Kenalog (4 mg/0.1 ml) was injected intravitreally to one eye, whereas the second eye served as the control. The experimental eye was chosen as the eye with worse visual acuity (VA).

MAIN OUTCOME MEASURES

Deterioration of electroretinogram parameters of the study eye measured at 3 months of follow-up when compared with the electroretinogram responses of the fellow, control eye and when compared with electroretinogram responses obtained before injection. Visual acuity, intraocular pressure (IOP), and eventual complications were assessed. No improvement or deterioration of VA or any increase in IOP was regarded as a secondary outcome.

RESULTS

Average maximal response amplitude ratios of the dark-adapted b-wave (treated/control eyes) of the electroretinogram were 0.93 before (P = 0.221) and 0.94 (P = 0.387) 3 months after Kenalog injection. Average ratios of the light-adapted b-wave amplitude (treated/control eyes) of the electroretinogram were 1.04 (P = 0.702) before and 0.86 (P = 0.138) 3 months after Kenalog injection. No significant differences (P>0.05) were found between the electroretinogram parameters obtained from all eyes before and 3 months after Kenalog injection. Average VAs in the treated eyes were 1.08, 0.8, and 1.0 logarithm of the minimum angle of resolution units before and 2 and 4 months after injection, respectively. Temporary elevation of IOP was found in 4 treated eyes of 4 patients (25%).

CONCLUSIONS

No electroretinographic evidence of a retinotoxic effect of intravitreal Kenalog was found in our patients.

Basic difference between brain and computer: integration of asynchronous processes implemented as hardware model of the retina

There exists a common view that the brain acts like a Turing machine: The machine reads information from an infinite tape (sensory data) and, on the basis of the machine's state and information from the tape, an action (decision) is made. The main problem with this model lies in how to synchronize a large number of tapes in an adaptive way so that the machine is able to accomplish tasks such as object classification. We propose that such mechanisms exist already in the eye. A popular view is that the retina, typically associated with high gain and adaptation for light processing, is actually performing local preprocessing by means of its center-surround receptive field. We would like to show another property of the retina: The ability to integrate many independent processes. We believe that this integration is implemented by synchronization of neuronal oscillations. In this paper, we present a model of the retina consisting of a series of coupled oscillators which can synchronize on several scales. Synchronization is an analog process which is converted into a digital spike train in the output of the retina. We have developed a hardware implementation of this model, which enables us to carry out rapid simulation of multineuron oscillatory dynamics. We show that the properties of the spike trains in our model are similar to those found in vivo in the cat retina.

Latency of the mfVEP to diagnose glaucoma?

The multifocal approach is a truly innovative technique

Alcohol- and light-induced electro-oculographic responses in age-related macular degeneration & central serous chorioretinopathy. alcohol- and light-induced EOG responses in ARMD & CSC

The non-photic electro-oculographic (EOG) response induced by alcohol has been proposed as an indicator of retinal pigment epithelial (RPE) integrity, and reported to be abnormal in age-related macular degeneration (ARMD). To evaluate this proposal, we have measured the alcohol-EOG as well as the ISCEV-standard EOG in patients with ARMD (n=11 patients, 4 eyes with drusen, 8 eyes with 'dry' and 7 eyes with 'wet' lesions) and central serous chorioretinopathy (CSC, n=11 patients, 7 eyes with active and 6 eyes with inactive lesions), compared with 29 normal controls. We recorded the alcohol-induced EOG response after a single oral administration of ethanol at 160 mg/kg, followed by an ISCEV-standard EOG. Blood alcohol levels were monitored with a breath analyzer. We found that neither the alcohol-EOG nor the light-induced EOG response showed any difference between either ARMD or CSC patients and normal controls. Nor was there difference among eyes of different ARMD or CSC subgroups. In addition, blood alcohol concentrations near the time of the alcohol-EOG peak showed no obvious relationship with peak/baseline ratios. These data suggest that neither the alcohol- nor the light-induced EOG is a sensitive indicator of these diseases.

Alcohol- and light-induced electro-oculographic responses: variability and clinical utility

The alcohol-induced electro-oculographic (EOG) response has been proposed by Arden as an indicator of retinal pigment epithelial (RPE) integrity. We have evaluated the consistency of the alcohol-EOG with respect to clinical applicability and compared this response to the ISCEV-standard EOG. We recorded, in a group of normal subjects (n=29, 14 men with mean age 42+/-11 years and 15 women with mean age 36+/-13 years), the alcohol response to a single oral dose of ethanol at 160 mg/kg (as 40 proof vodka, drunk in 15 s after 12 h of fasting), followed by an ISCEV-standard EOG 90 min after alcohol administration. Blood alcohol levels were monitored at regular intervals with a breath analyzer. We found a wide range of amplitudes in both light and alcohol responses among participants, from minimal to large values. Subjects had a wide range of blood alcohol concentrations from 0.02 to 0.10%; near the time of the response peak, but there was no relationship between alcohol levels and peak/baseline ratios. In addition, there was no relationship between alcohol peak/baseline ratio and the Arden ratio. Neither the alcohol nor the light response parameters showed any relationship with age or gender. Some of the inter-individual variability in the EOG response to alcohol may reflect variable absorption of oral alcohol. The alcohol-induced EOG has too broad a range of responses to be useful clinically for the one-time evaluation of individual patients. We have similar concerns regarding clinical applications of the standard light-induced EOG.

Early retinal damage in experimental diabetes: electroretinographical and morphological observations

A growing body of evidence indicates that impairment of retinal function precedes the earliest signs of vascular complications. The aim of this study was to follow the development of retinopathy both functionally and morphologically in a rat model of diabetes mellitus. Diabetes was induced in rats by intravenous injection of streptozotocin (STZ). Age-matched rats raised under similar conditions served as control. The electroretinogram (ERG) was recorded in order to assess retinal function. The expression of glial fibrillary acidic protein (GFAP) in Müller cells was used as a cellular marker for retinal damage. The ERG responses of the diabetic rats were reduced in amplitude compared to the responses recorded from the control rats as early as 2 weeks after onset of diabetes. The b-wave was more affected than the a-wave. GFAP expression in the diabetic retina did not differ from that in the control retina during the first 5 weeks of diabetes. GFAP was demonstrated only in astrocytes in the vitreo-retinal border. After 6-7 weeks of diabetes, GFAP expression in the retinas of the diabetic rats was also detected in the endfeet of the Müller cells. With the progression of diabetes, GFAP expression spreads throughout the entire length of the Müller cells. In the retinas from control rats, GFAP expression was limited to astrocytes and was not detected in Müller cells even at 40 weeks of follow-up. The observations indicate that the functional integrity of retinal cells is compromised already at short time intervals after onset of experimental diabetes in rats.

Age-related changes in the flash electroretinogram and oscillatory potentials in individuals age 75 and older

OBJECTIVES

To evaluate whether the inner plexiform layer of the retina is altered during senescence by examining the oscillatory potentials (OPs) of the flash electroretinogram (fERG) in individuals age 75 and older.

DESIGN

Cross-sectional, observational study.

SETTING

A university-based center.

PARTICIPANTS

Fifty-six healthy volunteers (age 20-88 years).

MEASUREMENTS

fERGs and OPs were first evaluated in scotopic conditions, following pupillary dilation and dark adaptation, in young (20-32 years; n = 30) and older (75-88 years; n = 26) individuals. Electrical signals were recorded with a Dawson-Trick-Litzkow type (DTL) fiber electrode in response to blue and white flashes. Red flashes were subsequently delivered to the test eye for photopic fERG and OP recordings following a period of light adaptation.

RESULTS

The amplitude of the a- and b-waves in response to blue and white flashes was significantly decreased in older people and their implicit time was increased. The latency of the a- and b-waves under photopic conditions was also prolonged with senescence, but only the amplitude of the b-wave was reduced. The amplitude of most OPs recorded under both scotopic and photopic conditions decreased with age, whereas their implicit times were prolonged.

CONCLUSIONS

Our results confirm previous findings regarding the age dependency of the fERG a- and b-waves. Furthermore, we provide novel information concerning the detrimental effects of age on the OPs, indicating that the neuronal elements within the inner plexiform layer of the retina are compromised with senescence.

Inhibition of membrane-bound carbonic anhydrase decreases subretinal pH and volume

PURPOSE

The lipophilic carbonic anhydrase (CA) inhibitor acetazolamide has been shown to enhance subretinal fluid resorption, reduce subretinal pH, and can improve cystoid macular edema, but its clinical use is limited by systemic side effects. While these are most likely a result of inhibiting intracellular CA isoenzymes, retinal pigment epithelial (RPE) transport is thought to be modulated via membrane-bound CA. This study investigates whether benzolamide, a hydrophilic CA inhibitor that does not readily penetrate cell membranes, is sufficient to modulate subretinal volume and pH.

METHODS

Volume and pH were assessed in the subretinal space (SRS) of the perfused chick retina-RPE-choroid preparation by calculating these variables from data obtained with two different double-barreled, ion-selective electrodes (H+ for pH and the extracellular space marker tetramethylammonium (TMA+) for SRS volume). Light induced variations and changes in baseline measurements were recorded before and after addition of 10(-4) M acetazolamide or benzolamide to the basal perfusion.

RESULTS

Basal perfusion with either drug induced both an acidification of the SRS by 0.02-0.04 pH units, which occurred within 60 s, as well as an increase in the amplitude of the light-induced alkalinisation of the SRS. TMA+ concentration in the SRS increased steadily over a period of several minutes after basal perfusion with either of the CA inhibitors, and the calculated SRS volume was reduced by 40% within 8-10 min.

CONCLUSION

The observation that benzolamide had effects equal to acetazolamide suggests that inhibition of membrane-bound CA at the basolateral membrane of the RPE is sufficient to decrease subretinal pH and volume. This may represent a clinically important mechanism for the resorption of sub- and intraretinal fluid.

Electrophysiology in the investigation of acquired retinal disorders

Electrophysiological research on acquired retinal disorders, both common and rare, is reviewed. Age is a major factor influencing electroretinogram (ERG) and electro-oculogram (EOG) findings. Bipolar or Müller cell death in the aging retina could account for much of the amplitude decline that is observed with age. In diabetic retinopathy, the oscillatory potentials can monitor the progression of the disease and indicate neuronal alterations rather than diabetic angiopathy of the retina. Human ERG studies on glaucoma concentrated on ERG measures that are dominated by inner retinal contributions. It has been shown that the pattern ERG can serve as a predictor of ocular hypertension's progression to glaucoma. In retinal disorders caused by endogenous intoxication, such as hepatic retinopathy, or exogenous intoxication from chronic lead exposure, ERG changes give an objective measure of the damage and allow to study the pathophysiological mechanisms that are involved. Inflammations of the choroid and the retina affect the standard ERG when they are diffuse. In central serous chorioretinopathy, functional disturbances can be revealed not only in the photoreceptors but also in the middle and inner retinal layers with the use of focal stimuli. Choroidal melanoma leads to large reductions of the EOG light peak-to-dark trough ratio through its influence on the transepithelial potential of the retinal pigment epithelium (RPE). In cancer-associated retinopathy, both the rod and cone ERGs are reduced. However, selective cone dysfunction has been described. In melanoma-associated retinopathy, the long flash ERG may reveal a specific pathophysiological mechanism, namely the affection of the ON-pathway with preservation of the OFF-pathway. ERG measurements can reveal vitamin A deficiency and are altered in cases with a mutation in the gene for the retinol binding protein in which other organs are not affected. Photochemical damage to the retina from light emission by the operating microscope can be assessed by electrophysiological methods.

Helicoidal peripapillary chorioretinal degeneration: electrophysiology and psychophysics in 17 patients

AIMS

To characterise retinal function using electrophysiological and psychophysical tests in 17 patients with helicoidal peripapillary chorioretinal degeneration.

METHODS

The electroretinogram (ERG) was recorded using gold foil corneal electrodes. The electro-oculogram (EOG) was recorded using a standard protocol. Dark adaptometry was recorded with an SST-1 dark adaptometer and colour vision assessed with Ishihara plates and Farnsworth D-15.

RESULTS

All subjects had a recordable ERG. The amplitudes and implicit times of the a- and b-waves were within normal limits at all luminances in five subjects (age 21-70 years, mean 40 years). The ERG of six (age 26-55 years, mean 40.7 years) had subnormal amplitudes at all luminances, but normal implicit times, and six (age 38-81 years, mean 60.7 years) had abnormal ERGs with marked reduction of a- and b-waves, and delayed implicit times of the b-wave. The implicit times of the a-wave were normal in all subjects. A reduction in the b/a wave ratios was not found, nor was there selective loss of scotopic, mixed rod/cone, or cone responses. The light/dark ratio of the EOG was subnormal (150-185%) or abnormal (below 150%) in all but three subjects. Two patients with normal EOG showed normal ERGs in both eyes, but one had subnormal ERGs in both eyes. The scotopic sensitivity was normal in all subjects and dark adaptation showed a normal time course. Colour vision was normal in all patients.

CONCLUSION

The results suggest that in most cases the function of the retinal pigment epithelium is affected by this disease before any changes in the function of the sensory retina are detectable by our methods, and that retinal dysfunction is focal rather than diffuse.

Electrical responses from diabetic retina

Diabetic retinopathy has long been considered to be a retinal manifestation of systemic diabetic angiopathy. Indeed, it is therapeutically true. However, the prolongation of OP peak latency in diabetic eyes without any angiographic evidence of angiopathy leads us to presume that certain neuronal disorders occur early in diabetic eyes. Even though we cannot neglect the possibility that the prolongation of the OP peak latency may derive from undetectable retinal hypoperfusion, it is still far from conventional diabetic angiopathy. Rather, the status should be properly termed "intraretinal diabetic neuropathy" in that the neurones are the disturbed cells to cause visual dysfunction. Thereafter, the OP amplitude diminishes as retinopathy advances, probably depending on the degree of retinal circulatory disturbance. Marked diminution of the OP amplitude predicts rapid progression and poor prognosis of retinopathy. Diabetic retinal pigment epitheliopathy as manifested by one of our non-photic EOG responses is another kind of early ocular involvement of diabetes. Because its mechanisms are not yet known, so far we have not succeeded in correlating it to any kind of subjective visual index. Routine fundus inspection or fluorescent fundus angiography is incapable of detecting the compromised neural retina and/or retinal pigment epithelial integrity and thus the electrophysiology of vision has the edge in ophthalmology.

Shift of equiluminance in congenital color vision deficiencies: pattern-ERG, VEP and psychophysical findings

We compared electrophysiological responses [pattern-ERG (PERG) and VEP] and psychophysical measures to color stimuli to separate different forms of anomalous color vision. PERG and VEP were recorded from seven normals and 14 subjects with congenital color vision deficiencies. Stimuli were color checkerboards with 0.5 deg check size, phase reversing at 34 rev/sec. The luminances of the red and green parts were varied in opposite direction from 0 to 30 cd/m2, while the hue of individual squares and space-averaged luminance were held constant. This allowed for one equiluminance condition where flicker appeared fused. In the seven normals, the subjective equiluminance was reached at a luminance ratio red/(red + green) = 0.50-0.53. At that point, the PERG amplitude was moderately, and the VEP amplitude sharply reduced. In 14 color anomalous subjects both the PERG and VEP were sharply reduced at equiluminance. These dips were shifted compared to normals and the dip position corresponded to the predicted luminance ratios obtained by calculations from L- and M-cone activation using the Smith-Pokorny transformation. As we found a close correlation of the VEP-dip position and the anomalous quotient, these electrophysiological measures may allow objective assessment of color vision deficiencies.

Selective reduction of the S cone electroretinogram in diabetes

AIMS

To determine whether the short wavelength sensitive (S) cone electroretinogram (ERG) is selectively altered in diabetic patients with and without retinopathy.

METHODS

Ganzfeld spectral flashes in the presence of bright white background illumination were used to elicit S cone ERGs is 15 non-retinopathic diabetics, 16 background retinopathic diabetics, and 16 age matched normal controls.

RESULTS

The amplitude of the S cone ERG b-wave was significantly reduced in both non-retinopathic and retinopathic diabetics. An action spectrum based on equal response criteria revealed a selective loss of S cone sensitivity in diabetics. However, no significant difference was observed in the long and middle wavelength sensitive cone ERG.

CONCLUSIONS

Diabetic patients showed selective reduction of the S cone ERG, which is thought to reflect changes in the outer retina.

The scotopic electroretinogram to blue flashes and pattern reversal visual evoked potentials in insulin dependent diabetes

Pattern reversal visual evoked responses (PR-VEPs), the electroretinogram (ERG) to blue flashes of light in dark adaptation, the steady state ERG to 40 Hz flicker, and alterations in pupil diameter following dark adaptation were studied in 56 juvenile onset diabetics, 34 of whom had no ophthalmoscopic or photographic evidence of diabetic retinopathy (DR-group). The remaining 22 had mild background retinopathy (DR+group). Normal data was obtained from 24 subjects matched for age and sex with diabetics. Skin electrodes were used for all recordings. The scotopic 'b' wave of the ERG was significantly lower in amplitude in the DR- diabetics and it was even more reduced in the DR+group. The PR-VEP was significantly delayed in diabetics but there was no difference between the two groups. The steady state ERG was not significantly different between normals and diabetics. These findings indicate that retinal and more central abnormalities develop early in diabetics. Detection of objective electrophysiological abnormalities may be used to identify persons at risk of developing retinopathy and to monitor the effects of treatment.

Electrophysiology of the retinal pigment epithelium in central serous chorioretinopathy

The pathophysiology of central serous chorioretinopathy is incompletely understood but appears to involve the retinal pigment epithelium. We recorded consecutively the fast oscillation, hyperosmolarity response, acetazolamide response, and light peak from four patients with active central serous chorioretinopathy and three normal subjects to determine whether the affected eyes showed any electrophysical abnormalities. We found essentially no differences in any of the four responses between the active and the inactive eyes of the patients or between patients and normal subjects. Whatever retinal pigment epithelial dysfunction exists in central serous chorioretinopathy is unassociated with clinically evident changes in these retinal pigment epithelial electrophysiologic responses.

Colour vision and side-effects during treatment with methazolamide

The retina contains Na+K(+)-ATPase and carbonic anhydrase (CA), enzymes that regulate ion fluxes across cell membranes of photoreceptors. Since inhibition of retinal Na+K(+)-ATPase by digitalis impairs colour vision, we wanted to find out whether this also occurs after inhibition of CA. In a double-masked cross-over study with placebo, 14 male volunteers were given 50 mg q.i.d. of the CA inhibitor methazolamide for 2 weeks. A disturbance of colour discrimination was observed in 8 of the 14 subjects, in the classification phase of Lanthony New Color Test. The presence of the disturbance was not significantly correlated to the degree of acidosis or to other side-effects. Its mechanism could be interpreted as a specific effect of CA inhibition in the retina (or the visual cortex) calculated to more than 99.9%.

The effects of acetazolamide in albino rabbits, pigmented rabbits, and humans

In three separate experiments albino rabbits, pigmented rabbits, and humans were tested following administration of acetazolamide and without acetazolamide. In all three experiments, we recorded electroretinograms (ERGs) under dark adapted and light adapted conditions and measured the b-wave amplitudes. Dark adapted ERG b-wave amplitudes were increased following administration of acetazolamide as compared to control conditions, in albino rabbits, pigmented rabbits and humans. Light adapted b-wave amplitudes showed no statistically significant changes as a function of acetazolamide administration although in all three experiments there was a trend toward light adapted b-wave amplitude reduction following administration of acetazolamide. In the human experiments, ERG a-wave amplitudes were also measured. Light adapted a-wave amplitudes were reduced following administration of acetazolamide. In the human experiments, several behavioral tests were performed, including L'Anthony desaturated D-15, Farnsworth-Munsell 100 hue, Cogan-Gunkel chromatograph, Nagel anomaloscope, Goldmann-Weekers dark adaptometry. There were no consistent changes in the human dark adaptation thresholds or color discrimination, although several measures approached significance.

Sequential recording of photic and nonphotic electro-oculogram responses in patients with extensive extramacular drusen

At present, no clinical electrophysiologic test defines dysfunction of the retinal pigment epithelium. We studied four electrophysiologic responses of the retinal pigment epithelium to compare results from three normal subjects with those from three patients with a diffuse retinal pigment epithelial disorder, extramacular drusen. We recorded the fast oscillation, hyperosmolarity response, acetazolamide response, and light peak by means of a clinical protocol in which these could be elicited consecutively. We found no significant differences between the normal subjects and patients with drusen for any of the four responses. These results suggest that retinal pigment epithelial electrophysiologic function is well maintained despite the widespread physical abnormalities of the retinal pigment epithelium in extramacular drusen. This combined test was well tolerated and may prove useful in characterizing other diseases involving the retinal pigment epithelium.

Comparison of the second and third oscillatory potentials with oscillatory potential power in early diabetic retinopathy

We used an oscillatory potential power method (a measure of the summed oscillatory potential activity) based on fast-Fourier transform analysis to study the oscillatory potentials in early diabetic retinopathy. The method was used in 29 diabetic patients with no ophthalmoscopically visible diabetic retinopathy, 29 diabetic patients with early signs only and 27 control subjects. The reduction in oscillatory potential power was compared with the reduction in the second and third oscillatory potential amplitudes and increase in implicit time in the diabetic patients. The amplitude of the second oscillatory potential was slightly more resistant to diabetic retinopathy than was the amplitude of the third oscillatory potential. Because the oscillatory potentials were detected by means of a high-resolution technique, their implicit times seem to be as discriminating as the oscillatory potential power in the detection of early diabetic retinopathy.

Oscillatory potentials, retinopathy, and long-term glucose control in insulin-dependent diabetes

The main objective of the study was to assess effects of long-term lowering of glucosylated hemoglobin (HbA1%) on neurosensory function in insulin-dependent diabetes. Individual (OP-1, OP-2, OP-3) and summed (OP-sum) amplitudes of oscillatory potentials (OPs) of electroretinography were recorded at study start and 7-years later in 45 patients (the Oslo study). As an overall 7-year change, amplitudes of OP-2, OP-3 and OP-sum were reduced (p < 0.0001-0.01), retinopathy worsened (p = 0.005), intraocular pressure decreased (p < 0.001), systolic blood pressure increased (p < 0.0002), and glycemic control improved from HbA1 of 11.2 +/- 2.2% at study start to a 7-year cumulative mean of 9.5 +/- 1.5% (p < 0.0001). Multiple regression analysis did not identify any independent relations between change in OP-1, OP-2, OP-3, OP-sum and change in glycemic control or background variables, including change in age and duration of diabetes. However, cross-sectional observations at 7 years showed negative correlations between all OPs and age (p < 0.0001-0.003), and between OP-3 and duration (p = 0.003) and counts of microaneurysms (p = 0.02). The data suggest that various clinical background variables may influence individual and summed amplitudes of OPs differently. Reduced neurosensory retinal function (OPs) seemed to appear after 7-years, independently of vascular defects of retinopathy and long-term improvement in glucose control.

Effects of intravenous acetazolamide on retinal pH in the cat

Double-barreled H(+)-selective microelectrodes were used to study the effect on intravenous acetazolamide on intraretinal pH in the cat. Acetazolamide (11.4-27.8 mg kg-1 intravenously) caused a rapid acidification of the subretinal space. This change in pH originated in the most distal portion of the subretinal space and could not be attributed to a change in pH or PCO2 of the arterial blood. Slow light-evoked alkalinizations in distal retina, attributable to a decrease in rod photoreceptor energy metabolism, were relatively unaltered by acetazolamide. This result indicated that acetazolamide had not crossed the blood-retinal barrier in sufficient amounts to change this response. In time, following intravenous perfusion of acetazolamide, continuous depth profiles of intraretinal pH showed an acidification of the entire retina and the vitreous also became more acidic. These results indicate that the rapid or primary effect of acetazolamide is an acidification of the distal portion of the subretinal space, which is thought to originate in a change in the transport of H+ or HCO3- by the retinal pigment epithelium. This is followed by an acidification of the entire retina and vitreous, presumably due to diffusion of acid from the distal retina, although there could be additional causes.

Spectral characteristics of rapid off-response in congenital deuteranomaly in one of monozygotic female twins

The spectral sensitivity of the rapid off-response in the electroretinogram was studied in monozygotic female twins. One case was diagnosed as congenital deuteranomaly, and the other was normal. The log ratio of the sensitivity at 480 nm to the sensitivity at 620 nm (log S480/S620) was within the deutan range in the first case and within the normal range in the second. The two case were determined to be different at the retinal receptor level by study of the rapid off-response. This result of the rapid off-response was consistent with the results of the psychophysical examinations.

Relationships between the electroretinogram a-wave, b-wave and oscillatory potentials and their application to clinical diagnosis

The electroretinogram is the electrical response of the retina to a light stimulus. The amplitude and temporal pattern of its components, the a-wave, the b-wave and the oscillatory potentials, depend on the functional integrity of the retina, on the intensity of test flash reaching the retina and on the ambient illumination. The latter contributions to the normal variability in the electroretinogram can be circumvented by constructing the relationships between the different electroretinogram waves. The electroretinogram responses were recorded from 18 dark-adapted subjects with normal vision. The slope of the a-wave and the amplitude of the b-waves were measured in the time domain. The oscillatory potentials were isolated by a digital filter and were transformed to the frequency domain for quantitative measurement. The relationship between each pair of variables could be fitted by linear segments. Our findings suggest that this mode of electroretinogram analysis can be useful in localizing the site of action of retinal disorders and that the relationship between the a-wave slope and the power density of the oscillatory potentials is a useful index for identifying disorders of the inner retina.

Combined photic and nonphotic electro-oculographic responses in the clinical evaluation of the retinal pigment epithelium

In an attempt to simplify the recording technique in electrophysiologic evaluation of the retinal pigment epithelium, we combined the electro-oculographic light rise, hyperosmolarity and acetazolamide responses in a single recording session. Recordings were performed in six normal subjects and in seven patients with diabetic retinopathy or retinitis pigmentosa. In the patients with background diabetic retinopathy, the hyperosmolarity responses were slightly reduced, while the acetazolamide response and the light rise was normal. In the patients with proliferative diabetic retinopathy, the hyperosmolarity response and light rise were remarkably reduced, while the acetazolamide response was normal. In the patients with retinitis pigmentosa, the hyperosmolarity response and light rise were decreased, while the acetazolamide response was normal. Despite a small study population, we concluded that the clinical results from our combined recording protocol were essentially the same as those reported for each response separately. Because this recording technique simplifies electrophysiologic evaluation of the retinal pigment epithelium, it may help clarify the mechanisms or localization of retinochoroidal and pigment epithelial diseases.

Clinical electrophysiology of the retinal pigment epithelium

There is no ideal electrophysiological test for retinal pigment epithelial (RPE) function. The light-induced responses (EOG, c-wave, fast oscillation) that require photoreception are not pure RPE signals, and even the widely-used EOG has not been associated with any specific physiological disturbance of the RPE or retina. The discovery of non-photic RPE responses (hyperosmolarity, acetazolamide and bicarbonate) has enhanced the possibility of finding tissue-specific RPE tests, but these responses have yet to be correlated with specific RPE functional activity or pathology. We may face a dilemma in our search for RPE tests, insofar as electrophysiology measures membrane changes, but RPE membrane activity is related only indirectly to many functions of the RPE cell. These concerns notwithstanding, RPE electrophysiology can be a valuable clinical tool if one accounts for the physiological limitations and assets of the procedures.

The influence of adaptation on the oscillatory potentials of the human electroretinogram

The influence of adaptation on the oscillatory potentials of the human electroretinogram was studied in the domains of frequency and time. The amplitude of OP1 to OP4, the summed amplitude of OP1 to OP4, as well as the area, decreased from dark adaptation to light adaptation. With increasing intensities of background illumination, they increased slightly and appeared to decrease with the strongest background illumination. The implicit time of oscillatory potentials 3 and 4 increased with stronger background illumination and decreased with the strongest background illumination. The results of the dominant frequency and the total power of the OPs correspond to the results in the time domain. The dominant power decreased from dark adaptation into light adaptation and did not show any systematic changes with increasing intensity of background illumination.

Measurement of the oscillatory potential of the electroretinogram in the domains of frequency and time

The dark-adapted and light-adapted electroretinograms of 13 subjects with 23 normal eyes were analyzed by means of Fourier spectrum. The oscillatory potentials in the time domain were filtered out from the electroretinogram after a corresponding bandpass was given in the frequency domain. The coefficient of variation of total power, dominant power and dominant frequency of the isolated oscillatory potentials in the frequency domain, summed amplitudes and area of the isolated oscillatory potentials, each amplitude and implicit time of the first four major oscillatory potential wavelets in the time domain were compared. The implicit time showed the smallest coefficient of variation; summed amplitudes of OP1 to OP4 showed smaller coefficients of variation than those of the area, the amplitude of each oscillatory potential wavelet, dominant frequency and dominant and total power. The coefficient of variation of these measurement parameters in light-adapted electroretinograms was smaller than those in dark-adapted electroretinograms.

Effects of 2-amino-4-phosphonobutyric acid (APB) and glycine on the oscillatory potentials of the rat electroretinogram

Oscillatory potentials of the electroretinogram were monitored in dark-adapted rats following intravitreal injection of 2-amino-4-phosphonobutyric acid (APB), a glutamate analog that preferentially blocks the light response of depolarizing bipolar cells, or glycine, a known endogenous inhibitory neurotransmitter that suppresses the light response of cells in the inner retina postsynaptic to glycinergic neurons. Oscillatory potentials were abolished in conjunction with the b-wave with APB and selectively reduced or eliminated by glycine: neither agent attenuated the a-wave. The results are compatible with the idea that light-induced depolarizing bipolar cell, hyperpolarizing bipolar cell, and glycinergic amacrine cell responses are all necessary for the generation of oscillatory potentials in the rat. The results also suggest that hyperpolarizing bipolar cells do not contribute to b-wave generation.

The scotopic threshold response in diabetic retinopathy

The scotopic threshold response (STR) is a recently discovered component of the electroretinogram. It is a corneal negative deflection elicited in the fully dark adapted eye to dim stimuli, and appears to originate in the inner retina. The STR was recorded in a group of 50 insulin dependent diabetics with various degrees of diabetic retinopathy, who had not undergone laser photocoagulation. In addition, the scotopic b-wave, oscillatory potentials (OPs) and a pattern electroretinogram (PERG) were recorded. Retinopathy was assessed with stereo colour photographs of the seven standard fields as defined in the Diabetic Retinopathy Study. Retinopathy level was assigned to each eye using a modification of the Airlie House Classification System. Fluorescein angiograms were taken using a 60 degree fundus camera and graded for the presence of leakage and capillary non-perfusion. There was a significant correlation between the severity of retinopathy and the amplitude and latency of the STR. There was a similar correlation with the amplitude and latency of the OPs, a weaker correlation with the amplitude of the PERG, but no significant correlation with the latency of the PERG. These results support an inner retinal origin for the STR and suggest a role for STR in the electroretinographic assessment of diabetic retinopathy.

Oscillatory potentials in early diabetic retinopathy

To study the oscillatory potentials in early diabetic retinopathy the authors developed a new power measurement based on the fast Fourier transform. Three groups totalling 46 patients were examined, varying from nonvisible to preproliferative diabetic retinopathy. The oscillatory potentials expressed in microwatts were measured under scotopic and photopic conditions. The data of the three groups are compared with those of a group of 22 normal individuals. The oscillatory potential power measurement appears to be a reliable method in detecting diabetic retinopathy at an early stage.