Effects of reference electrode location on monopolar-derived multifocal electroretinograms in cynomolgus monkeys

Clinical usefulness of binocular multifocal electroretinography in patients with monocular macular disease

Purpose

To evaluate the clinical usefulness of binocular multifocal electroretinography (mfERG) by comparing results with conventional monocular mfERG in patients with monocular macular disease.

Methods

mfERG testing was conducted on 32 patients with monocular macular disease and 30 normal subjects. An initial mfERG was simultaneously recorded from both eyes with two recording electrodes under binocular stimulation. A second mfERG was subsequently recorded with conventional monocular stimulation. Amplitudes and implicit times of each ring response of the binocular and monocular recordings were compared. Ring ratios of the binocular and monocular recording were also compared.

Results

In the macular disease group, there were no statistical differences in amplitude or implicit time for each of the five concentric rings between the monocular and binocular recordings. However, with binocular simulation, the ring ratios (ring 1 / ring 4, ring 1 / ring 5) were significantly reduced in the affected eye. In the normal control group, there were no statistical differences in any parameters between the monocular and binocular recordings.

Conclusions

Binocular mfERG could be a good alternative to the conventional monocular test. In addition, given that the test needs stable fixation of the affected eye during the binocular test, the reliability of the test results could be improved, especially for patients with monocular macular disease.

The effect of pentobarbital sodium and propofol anesthesia on multifocal electroretinograms in rhesus macaques

We compared the suitability of pentobarbital sodium (PB) and propofol (PF) anesthetics for multifocal electroretinograms (mfERGs) in rhesus macaques. mfERGs were collected from 4 ocularly normal rhesus macaques. All animals were pre-anesthetized with intramuscular ketamine (10-15 mg/kg). Intravenous PB induction/maintenance levels were 15 mg/kg/2-10 mg/kg and for PF, 2-5 mg/kg/6-24 mg/kg/h. There were 3 testing sessions with PB anesthesia and 5-7 testing sessions with PF anesthesia. All PB sessions were carried out before PF. First-order (K1) and second-order (first slice) kernels (K2.1) response density amplitude (RDA), implicit time (IT), and root mean square signal-to-noise ratios (RMS SNR) of the low-frequency (LFC) and high-frequency (HFC) components were evaluated. The use of PF or PB anesthesia resulted in robust, replicable mfERGs in rhesus macaques; however, RMS SNR of K1 LFC in ring and quadrant analyses was significantly larger for PF than for PB. Additionally, K1 RDA under PF was significantly larger than under PB for N1, P1, and P2 components (ring and quadrant) and for N2 (quadrant). PF IT was significantly prolonged (<1 ms) relative to PB IT for N1, P1 (ring), and N1 (quadrant), while PB IT was significantly prolonged (0.8-4.2 ms) relative to PF IT for N2 and P2 (ring and quadrant). K1 HFC and K2.1 LFC did not differ significantly between PB and PF in the ring or quadrant analyses. The response differences found with PB and PF anesthesia likely arise from variable relative effects of the anesthetics on retinal γ-aminobutyric acid (GABA(A)) receptors, and in part, on glycine and on glutamate receptors. Given the advantages of a stable anesthetic plane with continuous intravenous infusion and a smoother, more rapid recovery, PF is an appealing alternative for mfERG testing in rhesus macaques.

Functional and anatomic consequences of subretinal dosing in the cynomolgus macaque

OBJECTIVE

To characterize functional and anatomic sequelae of a bleb induced by subretinal injection.

METHODS

Subretinal injections (100 μL) of balanced salt solution were placed in the superotemporal macula of 1 eye in 3 cynomolgus macaques. Fellow eyes received intravitreal injections (100 μL) of balanced salt solution. Fundus photography, ocular coherence tomography, and multifocal electroretinography were performed before and immediately after injection and again at intervals up to 3 months postinjection. Histopathologic analyses included transmission electron microscopy and immunohistochemistry for glial fibrillary acidic protein, rhodopsin, M/L-cone opsin, and S-cone opsin.

RESULTS

Retinas were reattached by 2 days postinjection (seen by ocular coherence tomography). Multifocal electroretinography waveforms were suppressed post-subretinal injection within the subretinal injection bleb and, surprisingly, also in regions far peripheral to this area. Multifocal electroretinography amplitudes were nearly completely recovered by 90 days. The spectral-domain ocular coherence tomography inner segment-outer segment line had decreased reflectivity at 92 days. Glial fibrillary acidic protein and S-cone opsin staining were unaffected. Rhodopsin and M/L-cone opsins were partially displaced into the inner segments. Transmission electron microscopy revealed disorganization of the outer segment rod (but not cone) discs. At all postinjection intervals, eyes with intravitreal injection were similar to baseline.

CONCLUSIONS

Subretinal injection is a promising route for drug delivery to the eye. Three months post-subretinal injection, retinal function was nearly recovered, although reorganization of the outer segment rod disc remained disrupted. Understanding the functional and anatomic effects of subretinal injection is important for interpretation of the effects of compounds delivered to the subretinal space.

CLINICAL RELEVANCE

Subretinal injection is a new potential route for drug delivery to the eye. Separating drug effects from the procedural effects is critical.

Serial multifocal electroretinograms during long-term elevation and reduction of intraocular pressure in non-human primates

The purpose of this study was to evaluate the relationship between elevations of intraocular pressure (IOP) and the multifocal electroretinogram (mfERG) in non-human primates. Experimental glaucoma was induced in 4 rhesus and 4 cynomolgus monkeys by laser trabecular meshwork destruction (LTD) in one eye. To evaluate the contribution of ganglion cells to mfERG changes, one monkey of each species had previously underwent unilateral optic nerve transection (ONT). After >or=44 weeks of elevation, the IOP was reduced by trabeculectomy in 2 non-transected animals. In the intact (non-transected) animals, there was an increase in the amplitude of the early mfERG waveforms (N1 and P1) of the first-order kernel (K1) throughout the period of IOP elevation in all of the rhesus, but not all of the cynomolgus monkeys. A species difference was also present as a decrease of the second-order kernel, first slice (K2.1) in all of the cynomolgus monkeys but only in 1 of the rhesus monkeys (the 1 with the ONT). Similar IOP effects on the mfERG were seen in the ONT animals. Surgical lowering of IOP resulted in a return of the elevated K1 amplitudes to baseline levels. However, the depressed K2.1 RMS in the cynomolgus monkeys did not recover. These results demonstrate species-specific changes in cone-driven retinal function during periods of elevated IOP. These IOP-related effects can occur in the absence of retinal ganglion cells and may be reversible.

Using multifocal ERG ring ratios to detect and follow Plaquenil retinal toxicity: a review : Review of mfERG ring ratios in Plaquenil toxicity

Multifocal ERG ring ratios provide a sensitive and objective method to detect ocular toxicity in patients taking hydroxychloroquine (Plaquenil). In order to measure ring ratios, the average mfERG amplitude was calculated for each of five concentric rings of a 61-hexagon mfERG. The age-corrected amplitude of the central hexagon (R(1)) and the ratios of R(1) to each of the successive rings (R(1)/R(2), R(1)/R(3), etc.) were then computed. Normative values for ring ratios were established from a population of 67 normal controls. In the study population, a ring was considered abnormal if it was above the 99% confidence limits for the normal population. The technique was evaluated on 131 eyes of 62 patients taking Plaquenil for a variety of conditions including rheumatoid arthritis, systemic lupus erythematosus, and Sjögren's syndrome. Patients who had taken Plaquenil for an extended period showed a higher incidence of retinal toxicity, regardless of the condition for which they were taking the drug. Among patients who had taken a cumulative dose of less than 1,250 g, 7 of 67 eyes (10%) showed a characteristic mfERG defect, while in patients with a cumulative dosage of 1,250 g or more, 26 of 64 eyes (41%) showed one of these defects. In at least one patient, the technique was able to detect the early onset of Plaquenil toxicity followed by reversal of the toxic effects after the medication was discontinued. It appears appropriate to recommend that mfERG testing be done on all patients on Plaquenil therapy.

Detection of early hydroxychloroquine retinal toxicity enhanced by ring ratio analysis of multifocal electroretinography

PURPOSE

To assess decreased retinal function associated with high cumulative doses of hydroxychloroquine using multifocal electroretinography (mfERG).

DESIGN

Retrospective cross-sectional study.

METHODS

Sixty-two patients referred for evaluation of hydroxychloroquine retinal toxicity. Controls were 67 normal eyes of 67 patients referred for a variety of conditions in the other eye. Visual symptoms, duration of treatment, daily hydroxychloroquine dose (milligrams and milligrams per kilogram), cumulative dose, condition for which the drug was taken, visual acuity, retinal examination, visual fields, and mfERG amplitude. The average mfERG amplitude was calculated for five concentric rings. The age-corrected amplitude of the central hexagon (R(1)) and the ratios of R(1) to each of the other rings (e.g., R(1)/R(2), R(1)/R(3)) were compared with limits derived from control eyes.

RESULTS

The incidence of characteristic mfERG abnormalities in patients referred for evaluation with cumulative hydroxychloroquine doses of more than 1250 g was nearly 50%. It was 2.8 times that found in patients with cumulative doses less than 1250 g. Significant abnormalities were seen with cumulative doses as low as 400 g. The mfERG abnormality most commonly detected was an increased R(1)/R(2) ratio. Cumulative dose was more predictive of mfERG abnormalities than daily dose (either in milligrams or milligrams per kilogram) or duration of treatment.

CONCLUSIONS

Functional testing of the retina with mfERG shows locally decreased retinal function in a large fraction of patients referred for evaluation who have taken high cumulative doses of hydroxychloroquine. A prudent mfERG testing strategy is proposed.

The multifocal electroretinogram (mfERG) in the pig

PURPOSE

To establish a method allowing multifocal electroretinography (mfERG) recording with simultaneous fundus monitoring on anaesthetized pigs. In addition we characterize the peaks of the porcine mfERG trace, and compare the visual streak area with the optic nerve head, a known non-response area. Finally we illustrate the feasibility of the method by performing mfERG after an induced laser burn in the visual streak.

METHODS

Fifteen pigs underwent mfERG recordings at baseline, and after 1 and 6 weeks of observation. One pig was evaluated before and after retinal diode laser treatment in the visual streak.

RESULTS

The porcine mfERG trace appears similar to the human mfERG trace, and can be described by three peaks named N1, P1 and N2. Significantly faster implicit time was found in the visual streak regarding N1 (P < 0.001) than in areas outside the visual streak. Amplitudes of all three peaks were increased in the visual streak (P < 0.005). The laser-treated area was characterized by a response similar to what is found at the location of the optic nerve head.

CONCLUSION

Porcine mfERG is similar in appearance to the human response and can be described by the same three peaks. Significantly higher amplitudes of all three peaks are found in the visual streak when compared to the optic nerve head and inferior retina. We have detected the functional deficit caused by a laser burn at the size of 3 x 3 mm.

Multifocal visual evoked potentials in the anesthetized non-human primate

PURPOSE

To evaluate monkey multifocal visual evoked cortical potentials (mfVEPs) recorded from central and peripheral fields for reliability and isolation from electroretinographic (ERG) activity.

METHODS

The mfVEP stimulus consisted of a 7-element hexagonal array that subtended 80 degrees of the central visual field. Recordings were made under intravenous pentobarbital sodium (15 mg/kg) anesthesia. Two monkeys with absent optic nerve and ganglion cell function after combined unilateral optic nerve transection and experimental ocular hypertension (ONT/OHT) were followed longitudinally. In a second study, 16 ophthalmologically normal monkeys were tested once.

RESULTS

Testing of the non-transected eye in two transected animals revealed robust first- and second-order kernel, first slice (K1 and K2.1) mfVEPs. Stimulation of the transected eye revealed no contamination of the mfVEP from the concurrently recorded multifocal ERGs. There was complete separation of the root-mean-square (RMS) mfVEP amplitudes from the transected and the fellow eyes tested repeatedly across a 4- to 17- month period. The largest amplitude mfVEP was generated by the central element; however, mfVEPs were recorded from outside the central 20 degrees element. The 16 normal animals showed waveforms similar to the normal eyes of the ONT/OHT animals both in shape and distribution throughout the visual field. A scalar-product measure showed both K1 and K2.1 mfVEPs from central and some peripheral elements were statistically distinct from noise.

CONCLUSIONS

mfVEPs can be reliably recorded from non-human primates anesthetized with pentobarbital. Under the recording conditions described, mfVEPs are not contaminated by ERG activity. mfVEPs may be useful in animal models of diseases that differentially affect macular and peripheral visual field responsiveness.