Optimal parameters of transcorneal electrical stimulation (TES) to be neuroprotective of axotomized RGCs in adult rats

Optimal transcorneal electrical stimulation parameters for preserving photoreceptors in a mouse model of retinitis pigmentosa

JOURNAL/nrgr/04.03/01300535-202419110-00034/figure1/v/2024-03-08T184507Z/r/image-tiff Retinitis pigmentosa is a hereditary retinal disease that affects rod and cone photoreceptors, leading to progressive photoreceptor loss. Previous research supports the beneficial effect of electrical stimulation on photoreceptor survival. This study aims to identify the most effective electrical stimulation parameters and functional advantages of transcorneal electrical stimulation (tcES) in mice affected by inherited retinal degeneration. Additionally, the study seeked to analyze the electric field that reaches the retina in both eyes in mice and post-mortem humans. In this study, we recorded waveforms and voltages directed to the retina during transcorneal electrical stimulation in C57BL/6J mice using an intraocular needle probe with rectangular, sine, and ramp waveforms. To investigate the functional effects of electrical stimulation on photoreceptors, we used human retinal explant cultures and rhodopsin knockout (Rho-/-) mice, demonstrating progressive photoreceptor degeneration with age. Human retinal explants isolated from the donors' eyes were then subjected to electrical stimulation and cultured for 48 hours to simulate the neurodegenerative environment in vitro. Photoreceptor density was evaluated by rhodopsin immunolabeling. In vivo Rho-/- mice were subjected to two 5-day series of daily transcorneal electrical stimulation using rectangular and ramp waveforms. Retinal function and visual perception of mice were evaluated by electroretinography and optomotor response (OMR), respectively. Immunolabeling was used to assess the morphological and biochemical changes of the photoreceptor and bipolar cells in mouse retinas. Oscilloscope recordings indicated effective delivery of rectangular, sine, and ramp waveforms to the retina by transcorneal electrical stimulation, of which the ramp waveform required the lowest voltage. Evaluation of the total conductive resistance of the post-mortem human compared to the mouse eyes indicated higher cornea-to-retina resistance in human eyes. The temperature recordings during and after electrical stimulation indicated no significant temperature change in vivo and only a subtle temperature increase in vitro (~0.5-1.5°C). Electrical stimulation increased photoreceptor survival in human retinal explant cultures, particularly at the ramp waveform. Transcorneal electrical stimulation (rectangular + ramp) waveforms significantly improved the survival and function of S and M-cones and enhanced visual acuity based on the optomotor response results. Histology and immunolabeling demonstrated increased photoreceptor survival, improved outer nuclear layer thickness, and increased bipolar cell sprouting in Rho-/- mice. These results indicate that transcorneal electrical stimulation effectively delivers the electrical field to the retina, improves photoreceptor survival in both human and mouse retinas, and increases visual function in Rho-/- mice. Combined rectangular and ramp waveform stimulation can promote photoreceptor survival in a minimally invasive fashion.

Application of electrostimulation and magnetic stimulation in patients with optic neuropathy: A mechanistic review

Visual impairment caused by optic neuropathies is irreversible because retinal ganglion cells (RGCs), the specialized neurons of the retina, do not have the capacity for self-renewal and self-repair. Blindness caused by optic nerve neuropathies causes extensive physical, financial, and social consequences in human societies. Recent studies on different animal models and humans have established effective strategies to prevent further RGC degeneration and replace the cells that have deteriorated. In this review, we discuss the application of electrical stimulation (ES) and magnetic field stimulation (MFS) in optic neuropathies, their mechanisms of action, their advantages, and limitations. ES and MFS can be applied effectively in the field of neuroregeneration. Although stem cells are becoming a promising approach for regenerating RGCs, the inhibitory environment of the CNS and the long visual pathway from the optic nerve to the superior colliculus are critical barriers to overcome. Scientific evidence has shown that adjuvant treatments, such as the application of ES and MFS help direct thetransplanted RGCs to extend their axons and form new synapses in the central nervous system (CNS). In addition, these techniques improve CNS neuroplasticity and decrease the inhibitory effects of the CNS. Possible mechanisms mediating the effects of electrical current on biological tissues include the release of anti-inflammatory cytokines, improvement of microcirculation, stimulation of cell metabolism, and modification of stem cell function. ES and MFS have the potential to promote angiogenesis, direct axon growth toward the intended target, and enhance appropriate synaptogenesis in optic nerve regeneration.

Chronic electrical stimulation with a peripheral suprachoroidal retinal implant: a preclinical safety study of neuroprotective stimulation

Purpose

Extraocular electrical stimulation is known to provide neuroprotection for retinal cells in retinal and optic nerve diseases. Currently, the treatment approach requires patients to set up extraocular electrodes and stimulate potentially weekly due to the lack of an implantable stimulation device. Hence, a minimally-invasive implant was developed to provide chronic electrical stimulation to the retina, potentially improving patient compliance for long-term use. The aim of the present study was to determine the surgical and stimulation safety of this novel device designed for neuroprotective stimulation.

Methods

Eight normally sighted adult feline subjects were monocularly implanted in the suprachoroidal space in the peripheral retina for 9-39 weeks. Charge balanced, biphasic, current pulses (100 μA, 500 µs pulse width and 50 pulses/s) were delivered continuously to platinum electrodes for 3-34 weeks. Electrode impedances were measured hourly. Retinal structure and function were assessed at 1-, 2-, 4-, 6- and 8-month using electroretinography, optical coherence tomography and fundus photography. Retina and fibrotic thickness were measured from histological sections. Randomized, blinded histopathological assessments of stimulated and non-stimulated retina were performed.

Results

All subjects tolerated the surgical and stimulation procedure with no evidence of discomfort or unexpected adverse outcomes. The device position was stable after a post-surgery settling period. Median electrode impedance remained within a consistent range (5-10 kΩ) over time. There was no change in retinal thickness or function relative to baseline and fellow eyes. Fibrotic capsule thickness was equivalent between stimulated and non-stimulated tissue and helps to hold the device in place. There was no scarring, insertion trauma, necrosis, retinal damage or fibroblastic response in any retinal samples from implanted eyes, whilst 19% had a minimal histiocytic response, 19% had minimal to mild acute inflammation and 28% had minimal to mild chronic inflammation.

Conclusion

Chronic suprathreshold electrical stimulation of the retina using a minimally invasive device evoked a mild tissue response and no adverse clinical findings. Peripheral suprachoroidal electrical stimulation with an implanted device could potentially be an alternative approach to transcorneal electrical stimulation for delivering neuroprotective stimulation.

Glial cells as a promising therapeutic target of glaucoma: beyond the IOP

Glial cells, a type of non-neuronal cell found in the central nervous system (CNS), play a critical role in maintaining homeostasis and regulating CNS functions. Recent advancements in technology have paved the way for new therapeutic strategies in the fight against glaucoma. While intraocular pressure (IOP) is the most well-known modifiable risk factor, a significant number of glaucoma patients have normal IOP levels. Because glaucoma is a complex, multifactorial disease influenced by various factors that contribute to its onset and progression, it is imperative that we consider factors beyond IOP to effectively prevent or slow down the disease's advancement. In the realm of CNS neurodegenerative diseases, glial cells have emerged as key players due to their pivotal roles in initiating and hastening disease progression. The inhibition of dysregulated glial function holds the potential to protect neurons and restore brain function. Consequently, glial cells represent an enticing therapeutic candidate for glaucoma, even though the majority of glaucoma research has historically concentrated solely on retinal ganglion cells (RGCs). In addition to the neuroprotection of RGCs, the proper regulation of glial cell function can also facilitate structural and functional recovery in the retina. In this review, we offer an overview of recent advancements in understanding the non-cell-autonomous mechanisms underlying the pathogenesis of glaucoma. Furthermore, state-of-the-art technologies have opened up possibilities for regenerating the optic nerve, which was previously believed to be incapable of regeneration. We will also delve into the potential roles of glial cells in the regeneration of the optic nerve and the restoration of visual function.

Noninvasive electrical stimulation as a neuroprotective strategy in retinal diseases: a systematic review of preclinical studies

BACKGROUND

Electrical activity has a crucial impact on the development and survival of neurons. Numerous recent studies have shown that noninvasive electrical stimulation (NES) has neuroprotective action in various retinal disorders.

OBJECTIVE

To systematically review the literature on in vivo studies and provide a comprehensive summary of the neuroprotective action and the mechanisms of NES on retinal disorders.

METHODS

Based on the PRISMA guideline, a systematic review was conducted in PubMed, Web of Science, Embase, Scopus and Cochrane Library to collect all relevant in vivo studies on "the role of NES on retinal diseases" published up until September 2023. Possible biases were identified with the adopted SYRCLE's tool.

RESULTS

Of the 791 initially gathered studies, 21 articles met inclusion/exclusion criteria for full-text review. The results revealed the neuroprotective effect of NES (involved whole-eye, transcorneal, transscleral, transpalpebral, transorbital electrical stimulation) on different retinal diseases, including retinitis pigmentosa, retinal degeneration, high-intraocular pressure injury, traumatic optic neuropathy, nonarteritic ischemic optic neuropathy. NES could effectively delay degeneration and apoptosis of retinal neurons, preserve retinal structure and visual function with high security, and its mechanism of action might be related to promoting the secretion of neurotrophins and growth factors, decreasing inflammation, inhibiting apoptosis. The quality scores of included studies ranged from 5 to 8 points (a total of 10 points), according to SYRCLE's risk of bias tool.

CONCLUSION

This systematic review indicated that NES exerts neuroprotective effects on retinal disease models mainly through its neurotrophic, anti-inflammatory, and anti-apoptotic capabilities. To assess the efficacy of NES in a therapeutic setting, however, well-designed clinical trials are required in the future.

Effectiveness and Safety of Non-Invasive Neuromodulation for Vision Restoration: A Systematic Review and Meta-Analysis

We carried out a systematic review and meta-analysis to determine the effectiveness and safety of non-invasive electrical stimulation (NES) for vision restoration. We systematically searched for randomised controlled trials (RCTs) comparing NES with sham stimulation, for vision restoration between 2000 and 2022 in CENTRAL, MEDLINE, EMBASE, and LILACS. The main outcomes were as follows: visual acuity (VA); detection accuracy; foveal threshold; mean sensitivity as the parameter for the visual field; reading performance; contrast sensitivity (CS); electroencephalogram; quality of life (QoL), and safety. Two reviewers independently selected studies, extracted data, and evaluated the risk of bias using the Cochrane risk of bias 2.0 tool. The certainty in the evidence was determined using the GRADE framework. Protocol registration: CRD42022329342. Thirteen RCTs involving 441 patients with vision impairment indicate that NES may improve VA in the immediate post-intervention period (mean difference [MD] = -0.02 logMAR, 95% confidence intervals [CI] -0.08 to 0.04; low certainty), and probably increases QoL and detection accuracy (MD = 0.08, 95% CI -0.25 to 0.42 and standardised MD [SMD] = 0.09, 95% CI -0.58 to 0.77, respectively; both moderate certainty). NES likely results in little or no difference in mean sensitivity (SMD = -0.03, 95% CI -0.53 to 0.48). Compared with sham stimulation, NES increases the risk of minor adverse effects (risk ratio = 1.24, 95% CI 0.99 to 1.54; moderate certainty). The effect of NES on CS, reading performance, and electroencephalogram was uncertain. Our study suggests that although NES may slightly improve VA, detection accuracy, and QoL, the clinical relevance of these findings remains uncertain. Future research should focus on improving the available evidence's precision and consistency.

Monocular transcorneal electrical stimulation induces ciliary muscle thickening in contralateral eye

Transcorneal electrical stimulation (TES) is used as therapy for retinal diseases such as retinitis pigmentosa (RP) and was suggested for assessing retinal sensitivity by determining phosphene thresholds, subjective luminance impressions caused by retinal stimulation. Further applications concerned the accommodation process, revealing an improved accommodative amplitude in presbyopic eyes after TES treatment. The respective changes of the ciliary muscle (CM), the structure most important for near vision, during TES are yet unknown. In a pilot study, we aimed to assess whether monocular TES leads to morphological and functional CM changes and whether central accommodation control is affected. Ten healthy, near-emmetropic adults participated in the trial (4 females, age 26.3 ± 3.6 years). Using a wavefront and a stimulus generator, a biphasic square-wave stimulus (2 s positive and 6 s negative amplitude) of 0 μA average current was produced and transferred to the eye by means of a Dawson-Trick & Litzkow electrode. Prior to the stimulation, an individual determination of phosphene thresholds served to define individual TES current amplitudes, which ranged between 60 and 100 μA. Optical coherence tomography (OCT) imaging of the right eye's temporal ciliary muscle was performed before and during ipsi-as well as contralateral monocular TES in randomized order in the morning and afternoon of the same day. During imaging, subjects fixated a target at 4 m distance and refraction was simultaneously recorded via eccentric infrared photorefraction. OCT images were assessed using previously published custom-developed software, allowing the definition of selective CM thickness (CMT) readings, and plotting of continuous CMT profiles along the muscle border. CMT profiles revealed that both stimulations, on the ipsi- and contralateral eye, induced a thickening of the CM compared to the non-stimulated state. The selective CMT readings confirmed a significant increase with ipsi- (31 ± 30 μm; p = 0.010) and contralateral (25 ± 16 μm; p = 0.001) TES. However, refraction during far vision was not significantly affected by either stimulation (ipsilateral [n = 5]: median Δ_w/-w/o = 0 D; contralateral [n = 7]: Δ_w/-w/o = 0.13 D). Pupil size on average increased during TES, but without reaching significance (ipsilateral [n = 5] median Δ_w/-w/o = 0.23 mm, contralateral [n = 7] Δ_w/-w/o = 0.39 mm). Ipsilateral CM thickening could be explained by local changes within the stimulated ciliary muscle, such as increased blood flow or interstitial fluid rise induced by TES. However, the CMT increase in the right eye when TES was performed contralaterally, on the left eye, indicates an involvement of the central control circuit of accommodation. Further possible explanations for this finding are a synchronization of neuronal activities in the visual pathway, the release of vasoactive neuropeptides, or effects on the central blood pressure regulation. Given a neuromodulation effect on the CM function, TES might have implications for children with accommodation insufficiencies and as additional therapy in myopia control management, e.g. in combination with multifocal contact lens treatment. Our study is important for the clinical application of TES, and the outcome might add crucial knowledge to the current understanding of the accommodation process and inform research and treatment of both myopia and presbyopia.

Transcorneal Electrical Stimulation Dose-Dependently Slows the Visual Field Loss in Retinitis Pigmentosa

Purpose

To assess whether transcorneal electrical stimulation (TcES) current-dependently slows progressive loss of visual field area (VFA) in retinitis pigmentosa (RP).

Methods

Data from 51 patients with RP who received monocular TcES treatment once weekly over 1 year in an interventional, randomized study have been analyzed a posteriori. Current amplitudes were 0.1 to 1.0 mA in the TcES-treated group (n = 31) and 0.0 mA in the sham group (n = 20). VFA was assessed in both eyes (semiautomatic kinetic perimetry, Goldmann targets V4e, III4e). Annual decline rate (ADR) of exponential loss and model-independent percentage reduction of VFA at treatment cessation were correlated to current amplitude.

Results

For V4e, mean ADR was -4.1% in TcES-treated eyes, -6.4% in untreated fellow eyes, and -7.2% in placebo-treated eyes; mean VFA reduction in TcES-treated eyes was 64% less than in untreated fellow eyes (P = 0.013) and 72% less than in placebo-treated eyes (P = 0.103). Individual VFA reductions correlated with current amplitude (P = 0.043) and tended toward zero in patients who received 0.8 to 1.0 mA. For III4e, there was a marginally significant current-dependency of interocular difference in reduction (P = 0.11). ADR and VFA reduction did not significantly correlate with baseline VFA.

Conclusions

Loss of VFA (V4e) in patients with RP was significantly reduced in treated eyes compared to untreated eyes by regular use of TcES in a dose-dependent manner. No dependence of effects on the initial extent of VFA loss was found.

Translational Relevance

TcES provides potential for preservation of visual field in patients with RP.

Advances in transcorneal electrical stimulation: From the eye to the brain

The mammalian brain is reported to contain about 10⁶-10⁹ neurons linked together to form complex networks. Physiologically, the neuronal networks interact in a rhythmic oscillatory pattern to coordinate the brain's functions. Neuromodulation covers a broad range of techniques that can alter neuronal network activity through the targeted delivery of electrical or chemical stimuli. Neuromodulation can be used to potentially treat medical conditions and can serve as a research tool for studying neural functions. Typically, the main method of neuromodulation is to electrically stimulate specific structures in both the central and peripheral nervous systems via surgically implanted electrodes. Therefore, it is imperative to explore novel and safer methods for altering neuronal network activity. Transcorneal electrical stimulation (TES) has rapidly emerged as a non-invasive neuromodulatory technique that can exert beneficial effects on the brain through the eyes. There is substantial evidence to show that TES can change the brain oscillations in rodents. Moreover, the molecular data clearly shows that TES can also activate non-visual brain regions. In this review, we first summarize the use of TES in the retina and then discuss its effects in the brain through the eye-brain connection. We then comprehensively review the substantial evidence from electrophysiological, behavioral, and molecular studies on the role of TES on modulating neurons in the brain. Lastly, we discuss the implications and possible future directions of the research on TES as a non-invasive tool for neuromodulation of the brain via directly stimulating the mammalian eye.

Trans-Sclera Electrical Stimulation Improves Retinal Function in a Mouse Model of Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a photoreceptor-degenerating disease with no effective treatment. Trans-corneal electrical stimulation has neuroprotective effects in degenerating retinas, but repeated applications cause corneal injury. To avoid the risk of corneal damage, here we tested whether repetitive trans-sclera electrical stimulation (TsES) protects degenerating retinas in rd10 mice, a model of RP. At postnatal day 20 (P20), the right eyes of rd10 mice were exposed to 30 min of TsES daily or every other day till P25, at the amplitude of 50 or 100 μA, with zero current as the sham. Immunostaining, multi-electrode-array (MEA) recording, and a black-and-white transition box were applied to examine the morphological and functional changes of the treated retina. Functionally, TsES modified the retinal light responses. It also reduced the high spontaneous firing of retinal ganglion cells. TsES at 100 μA but not 50 μA increased the light sensitivities of ganglion cells as well as their signal-to-noise ratios. TsES at 100 μA increased the survival of photoreceptors without improving the visual behavior of rd10 mice. Our data suggest that repetitive TsES improves the retinal function of rd10 mice at the early degenerating stage, therefore, it might be an effective long-term strategy to delay retinal degeneration in RP patients.

Corneal Neuro-Regenerative Effect of Transcutaneous Electrical Stimulation in Rabbit Lamellar Keratectomy Model

Purpose

This study aimed to evaluate the effect of transcutaneous electrical stimulation (TES) on corneal nerve regeneration in rabbits injured from superficial lamellar keratectomy (SLK).

Methods

New Zealand White rabbits were used in this experimental study. To induce corneal nerve damage, SLK was performed using a 7.0-mm trephine. TES was applied for 28 days after the corneal nerve injury. Corneal sensitivity, Western blotting, real-time polymerase chain reaction (PCR), and immunofluorescence were performed to observe changes in the corneal tissue.

Results

In the 2-Hz and 20-Hz electrical stimulation groups, the degree of corneal wound healing increased by more than 10% compared to the control group, but no significant difference was observed. Conversely, the electrical stimulation (2-Hz or 20-Hz) group showed significantly increased corneal sensitivity compared to the control group. Western blot analysis revealed that small proline-rich protein 1A (SPRR1a), a regeneration-associated protein was significantly increased in the 2-Hz group on days 1 and 7 compared to that in the other groups. Once again, nerve regeneration in the 2-Hz group was supported by the results of PCR, in which a significant increase in the nerve growth factor (NGF) on day 1 was observed compared with the other groups. Moreover, immunofluorescence after 28 days of electrical stimulation showed significant nerve regeneration in the 2-Hz group.

Conclusions

TES promoted corneal nerve regeneration in rabbit SLK model. The application of electrical stimulation of 2-Hz frequency was more effective than the 20-Hz frequency, showing potential clinical applications for corneal diseases.

Translational Relevance

This study shows how application of TES to the eyes that exhibit corneal nerve damage can improve corneal nerve regeneration examined by histologic analysis.

An in-silico analysis of retinal electric field distribution induced by different electrode design of trans-corneal electrical stimulation

OBJECTIVE

Trans-corneal electrical stimulation (TcES) produces therapeutic effects on many ophthalmic diseases non-invasively. Existing clinical TcES devices use largely variable design of electrode distribution and stimulation parameters. Better understanding of how electrode configuration paradigms and stimulation parameters influence the electric field distribution on the retina, will be beneficial to the design of next-generation TcES devices.

APPROACH

In this study, we constructed a realistic finite element human head model with fine eyeball structure. Commonly used DTL-Plus and ERG-Jet electrodes were simulated. We then conducted in silico investigations of retina observation surface (ROS) electric field distributions induced by different return electrode configuration paradigms and different stimulus intensities.

MAIN RESULTS

Our results suggested that the ROS electric field distribution could be modulated by re-designing TcES electrode settings and stimulus parameters. Under far return location (FRL) paradigms, either DTL-Plus or ERG-Jet approach could induce almost identical ROS electric field distribution regardless where the far return was located. However, compared with the ERG-Jet mode, DTL-Plus stimulation induced stronger nasal lateralization. In contrast, ERG-Jet stimulation induced relatively stronger temporal lateralization. The ROS lateralization can be further tweaked by changing the DTL-Plus electrode length.

SIGNIFICANCE

These results may contribute to the understanding of the characteristics of DTL-Plus and ERG-Jet electrodes based electric field distribution on the retina, providing practical implications for the therapeutic application of TcES.

Mechanisms of electrical stimulation in eye diseases: A narrative review

Background

In the last two decades, electrical stimulation (ES) has been tested in patients with various eye diseases and shows great treatment potential in retinitis pigmentosa and optic neuropathy. However, the clinical application of ES in ophthalmology is currently limited. On the one hand, optimization and standardization of ES protocols is still an unmet need. On the other hand, poor understanding of the underlying mechanisms has hindered clinical exploitation.

Main Text

Numerous experimental studies have been conducted to identify the treatment potential of ES in eye diseases and to explore the related cellular and molecular mechanisms. In this review, we summarized the in vitro and in vivo evidence related to cellular and tissue response to ES in eye diseases. We highlighted several pathways that may be utilized by ES to impose its effects on the diseased retina.

Conclusions

Therapeutic effect of ES in retinal degenerative diseases might through preventing neuronal apoptosis, promoting neuronal regeneration, increasing neurotrophic factors production in Müller cells, inhibiting microglial activation, enhancing retinal blood flow, and modulating brain plasticity. Future studies are suggested to analyse changes in specific retinal cells for optimizing the treatment parameters and choosing the best fit ES delivery method in target diseases.

Transcorneal electrical stimulation enhances cognitive functions in aged and 5XFAD mouse models

Dementia is a major burden on global health for which there are no effective treatments. The use of noninvasive visual stimulation to ameliorate cognitive deficits is a novel concept that may be applicable for treating dementia. In this study, we investigated the effects of transcorneal electrical stimulation (TES) on memory enhancement using two mouse models, in aged mice and in the 5XFAD model of Alzheimer's disease. After 3 weeks of TES treatment, mice were subjected to Y-maze and Morris water maze tests to assess hippocampal-dependent learning and memory. Immunostaining of the hippocampus of 5XFAD mice was also performed to examine the effects of TES on amyloid plaque pathology. The results showed that TES improved the performance of both aged and 5XFAD mice in memory tests. TES also reduced hippocampal plaque deposition in male, but not female, 5XFAD mice. Moreover, TES significantly reversed the downregulated level of postsynaptic protein 95 in the hippocampus of male 5XFAD mice, suggesting the effects of TES involve a postsynaptic mechanism. Overall, these findings support further investigation of TES as a potential treatment for cognitive dysfunction and mechanistic studies of TES effects in other dementia models.

Electrical neurostimulation in glaucoma with progressive vision loss

Background

The retrospective study provides real-world evidence for long-term clinical efficacy of electrical optic nerve stimulation (ONS) in glaucoma with progressive vision loss.

Methods

Seventy glaucoma patients (45 to 86 y) with progressive vision loss despite therapeutic reduction of intraocular pressure (IOP) underwent electrical ONS. Closed eyes were separately stimulated by bipolar rectangular pulses with stimulus intensities up to 1.2 mA sufficient to provoke phosphenes. Ten daily stimulation sessions within 2 weeks lasted about 80 min each. Right before ONS at baseline (PRE), vision loss was documented by static threshold perimetry and compared to the same assessment approximately 1 year afterwards (POST). Mean defect (MD) was defined as primary outcome parameter. Perimetries with a reliability factor (RF) of max. 20% were considered.

Results

Perimetry follow-up of 101 eyes in 70 patients fulfilled the criterion of a max. 20% RF. Follow-up was performed on average 362.2 days after ONS. MD significantly decreased from PRE 14.0 dB (median) to POST 13.4 dB (p < 0.01). 64 eyes in 49 patients showed constant or reduced MD as compared to baseline (PRE 13.4 dB vs. POST 11.2 dB). In 37 eyes of 30 patients, MD increased from PRE 14.9 dB to POST 15.6 dB.

Conclusions

Innovative treatments that preserve visual function through mechanisms other than lowering IOP are required for glaucoma with progressive vision loss. The present long-term data document progression halt in more than 63% of affected eyes after ONS and, thus, extend existing evidence from clinical trials.

Effect of Electrical Stimulation on Ocular Cells: A Means for Improving Ocular Tissue Engineering and Treatments of Eye Diseases

Tissue engineering is biomedical engineering that uses suitable biochemical and physicochemical factors to assemble functional constructs that restore or improve damaged tissues. Recently, cell therapies as a subset of tissue engineering have been very promising in the treatment of ocular diseases. One of the most important biophysical factors to make this happen is noninvasive electrical stimulation (ES) to target ocular cells that may preserve vision in multiple retinal and optic nerve diseases. The science of cellular and biophysical interactions is very exciting in regenerative medicine now. Although the exact effect of ES on cells is unknown, multiple mechanisms are considered to underlie the effects of ES, including increased production of neurotrophic agents, improved cell migration, and inhibition of proinflammatory cytokines and cellular apoptosis. In this review, we highlighted the effects of ES on ocular cells, especially on the corneal, retinal, and optic nerve cells. Initially, we summarized the current literature on the in vitro and in vivo effects of ES on ocular cells and then we provided the clinical studies describing the effect of ES on ocular complications. For each area, we used some of the most impactful articles to show the important concepts and results that advanced the state of these interactions. We conclude with reflections on emerging new areas and perspectives for future development in this field.

Protocol to test the efficacy and safety of frequent applications of skin electrical stimulation for Leber hereditary optic neuropathy: a single-arm, open-label, non-randomised prospective study

INTRODUCTION

Leber hereditary optic neuropathy (LHON) is an acute or subacute inherited optic neuropathy caused by mitochondrial mutations. More than 90% of patients with LHON have one of three point mutations (ie, G3460A, G11778A and T14484C). We previously reported that a 12-week session of skin electrical stimulation (SES) with a 2-week interval significantly improved visual acuity and field tests 1 week after the last stimulation and without adverse effects in 10 cases of LHON carrying the mt DNA G11778A mutation. In the present study, we will examine the magnitude and persistence of the efficacy and presence or absence of adverse events using SES with a more frequent stimulation protocol.

METHODS AND ANALYSIS

This study will be a single-arm, open-labelled, non-randomised clinical study that analyses 15 cases of LHON with G11778A mutation. All participants will take a portable SES device home and perform SES by themselves every other day for 12 weeks. The logarithm for the minimum angle of resolution (logMAR) best-corrected visual acuity (BCVA) at 1 week after the last SES will be measured as the primary outcome. LogMAR BCVA will be measured at four and 8 weeks after the last SES treatment. The Humphrey visual field sensitivity test using size V stimulation and critical fusion frequency at 1, 4 and 8 weeks after the last SES session will be secondary outcome measurements. Slit-lamp examination, optical coherence tomography and specular microscopy will also be performed to verify the safety of SES.

ETHICS AND DISSEMINATION

The protocol was approved by the Institutional Review Board at Kobe University, Japan (Approval No.C190030). This study is in progress and deserves Pre-result. All documents communicating with the ethics committee will be reposited by the researcher. Modifications to the protocol will be reviewed by the ethics committee and implemented after approval. Data monitoring will be performed by a researcher who is not involved in the study every 6 months after approval. The research summary results will be registered in the Japan Registry of Clinical Trials (jRCTs) and made available to participants in accordance with the terms described in the documents. In addition, the results of this study will be presented at domestic and international meetings and published in peer-reviewed journals within a year after data is fixed.

TRIAL REGISTRATION NUMBER

jRCTs052200033.

Neuroprotective Effects and Therapeutic Potential of Transcorneal Electrical Stimulation for Depression

Transcorneal electrical stimulation (TES) has emerged as a non-invasive neuromodulation approach that exerts neuroprotection via diverse mechanisms, including neurotrophic, neuroplastic, anti-inflammatory, anti-apoptotic, anti-glutamatergic, and vasodilation mechanisms. Although current studies of TES have mainly focused on its applications in ophthalmology, several lines of evidence point towards its putative use in treating depression. Apart from stimulating visual-related structures and promoting visual restoration, TES has also been shown to activate brain regions that are involved in mood alterations and can induce antidepressant-like behaviour in animals. The beneficial effects of TES in depression were further supported by its shared mechanisms with FDA-approved antidepressant treatments, including its neuroprotective properties against apoptosis and inflammation, and its ability to enhance the neurotrophic expression. This article critically reviews the current findings on the neuroprotective effects of TES and provides evidence to support our hypothesis that TES possesses antidepressant effects.

Effectiveness of Microcurrent Stimulation in Preserving Retinal Function of Blind Leading Retinal Degeneration and Optic Neuropathy: A Systematic Review

OBJECTIVES

To systematically identify and summarize the effectiveness and the parameters of electrical stimulation (ES) for the preservation of visual function in major retinal degeneration and optic neuropathy.

MATERIALS AND METHODS

A systematic review of clinical studies, using ES therapy in patients with blind leading retinal degenerations, including retinitis pigmentosa (RP), age-related macular degeneration (AMD), glaucoma, retinal vein occlusion (RVO), retinal artery occlusion (RAO), and optic neuropathy was conducted. PubMed, EMBASE, Cochrane Library, and Web of Science were searched for relevant interventional studies including randomized controlled trials (RCTs) and observational studies.

RESULTS

A total of 10 RCTs and 15 observational studies were included. Transcorneal ES (TcES), transpalpebral ES (TpES), transdermal ES (TdES), and repetitive transorbital alternating current stimulation (rtACS) were used for the treatment of the patients. ES using 20 Hz biphasic pulses with current strength at 150%-200% of individual electrical phosphene threshold (EPT) for RP patients showed improved retinal function detected by visual acuity (VA), visual field (VF), or electrical retinal graphs (ERG). rtACS on patients with optic neuropathy showed significant preservation of VA and VF. Clinical studies on AMD, RAO, and glaucoma indicated promising protective effects of ES on the visual function, though the amount of evidence is limited.

CONCLUSIONS

ES treatment has promising therapeutic effects on RP and optic neuropathy. More large-scale RCT studies should be conducted to elucidate the potential of ES, especially on AMD, RAO, and glaucoma. A comparison of the effects by different ES methods in the same disease populations is still lacking. Parameters of the electric current and sensitive detection method should be optimized for the evaluation of ES treatment effects in future studies.

Effect of Transcorneal Electrical Stimulation on Patients with Retinitis Pigmentosa

Purpose: In this study, the aim was to evaluate the safety of transcorneal electrical stimulation (TES) treatment in retinitis pigmentosa (RP) patients and to investigate the effect of TES to the visual acuity (VA), visual field (VF), and multifocal electroretinogram (mfERG) findings. Methods: Two hundred two eyes of 101 RP patients with different stages were studied. TES was applied for 30 min once a week for 8 consecutive weeks. Two hundred eyes of 100 RP patients were enrolled as control. After the 2-month TES therapy sessions, patients were followed for 4 months without treatment. Examinations were done at the baseline before TES treatment and 1 and 6 months after the treatment. Best-corrected VA (BCVA), color fundus photography, VF test, optical coherence tomography, and mfERG tests were done at each visit. Results: The mean BCVA and VF tests improved 1 month after the beginning of TES treatment and the improvements were statistically significant (P < 0.05). There was an improvement in p1 wave amplitude in rings 1, 2, and 3 at the first month. The latency of the p1 wave showed a statistically significant shortening in rings 1 and 2. These improvements partially disappeared at 6-month follow-up. There were no serious ocular side effects related to the therapy. Mild dry eye symptoms were observed, which were revealed by artificial tears. Conclusions: TES is a safe therapy without any serious advers effects. Although it can improve VA and VF of RP patients, the beneficial effects could be transient and repeated sessions can be necessary for maintaining the efficiency.

A Single-Arm, Prospective, Exploratory Study to Preliminarily Test Effectiveness and Safety of Skin Electrical Stimulation for Leber Hereditary Optic Neuropathy

Leber hereditary optic neuropathy (LHON) is an intractable disease associated with mitochondrial DNA (mtDNA) mutations. In this preliminary, single-arm, prospective, open-label exploratory trial, we investigated the effectiveness and safety of skin electrical stimulation (SES) for cases of LHON harboring the mtDNA 11,778 mutation. Of the 11 enrolled patients, 10 completed six sessions of SES once every two weeks over a 10-week period. The primary outcome measure was the change in logarithm of the minimum angle of resolution (logMAR)-converted best-corrected visual acuity (BCVA) at one week after the last session of SES. The main secondary outcome measures were the logMAR BCVA at four and eight weeks and Humphrey visual field test sensitivities at one, four, and eight weeks. At all follow-up points, the logMAR BCVA had improved significantly from baseline, [1.80 (1.70–1.80) at baseline, 1.75 (1.52–1.80) at one week, 1.75 (1.50–1.80) at four weeks, and 1.75 (1.52–1.80) at eight weeks; p < 0.05]. At eight weeks of follow-up, five patients showed >2-fold increase in the summed sensitivity at 52 measurement points from baseline. No adverse effects were observed. In conclusion, SES could be a viable treatment option for patients with LHON in the chronic phase harboring the mtDNA 11,778 mutation.

Enayati K, Xie J, Zhao E, Turunen T, Sehic A, Lu L, Utheim TP, Chen DF. Electrical Stimulation Induces Retinal Müller Cell Proliferation and Their Progenitor Cell Potential

Non-invasive electrical stimulation (ES) is increasingly applied to improve vision in untreatable eye conditions, such as retinitis pigmentosa and age-related macular degeneration. Our previous study suggested that ES promoted retinal function and the proliferation of progenitor-like glial cells in mice with inherited photoreceptor degeneration; however, the underlying mechanism remains obscure. Müller cells (MCs) are thought to be dormant residential progenitor cells that possess a high potential for retinal neuron repair and functional plasticity. Here, we showed that ES with a ramp waveform of 20 Hz and 300 µA of current was effective at inducing mouse MC proliferation and enhancing their expression of progenitor cell markers, such as Crx (cone-rod homeobox) and Wnt7, as well as their production of trophic factors, including ciliary neurotrophic factor. RNA sequencing revealed that calcium signaling pathway activation was a key event, with a false discovery rate of 5.33 × 10-8 (p = 1.78 × 10-10) in ES-mediated gene profiling changes. Moreover, the calcium channel blocker, nifedipine, abolished the observed effects of ES on MC proliferation and progenitor cell gene induction, supporting a central role of ES-induced Ca2+ signaling in the MC changes. Our results suggest that low-current ES may present a convenient tool for manipulating MC behavior toward neuroregeneration and repair.

Retinal neurovascular responses to transcorneal electrical stimulation measured with optical coherence tomography

Noninvasive transcorneal electrical stimulation (TES) has emerged as a potential strategy to facilitate visual restoration and promote retinal cell survival for certain retinal and optic nerve diseases owing to its neuroprotective effects. However, the neurovascular responses of retinal neurons evoked by TES have not been completely determined. To investigate this issue, we utilized a custom-designed spectral-domain optical coherence tomography (SD-OCT) to record the retinal neural and vascular responses under TES in vivo simultaneously. Significant increases of both positive and negative intrinsic optical signal (IOS) changes were recorded in all three segmented retinal layers, which mainly related to neural activities. However, the changes of TES-induced retinal vascular responses, including blood velocity, cross-sectional area of vessel, and blood flow, were not significant. It suggests that TES mainly elicited neural responses in retina, while no significant vascular responses were evoked. Our results provide experimental evidence to the mechanism of retinal neurovascular coupling under TES. Additionally, the present study also suggests that SD-OCT could be utilized as a promoting method to explore neurovascular responses under retinal stimulation in clinical treatment and technology.

Impact statement

Noninvasive transcorneal electrical stimulation (TES) has emerged as an effective treatment for certain retinal and optic nerve diseases owing to its neuroprotective effects. However, the retinal neurovascular responses evoked by TES have not been completely determined. To investigate this issue, we utilized a custom-designed spectral-domain optical coherence tomography (SD-OCT) to record the retinal neural and vascular responses evoked by TES in vivo simultaneously. The present study suggested that TES mainly elicited neural responses in retina, while no significant vascular responses were evoked. Our results provide experimental evidence to the mechanism of retinal neurovascular coupling evoked by TES. Additionally, the present study also suggests that SD-OCT could be utilized as a promoting method to explore neurovascular responses under retinal electrical stimulation.

Pulsed Electrical Stimulation of the Human Eye Enhances Retinal Vessel Reaction to Flickering Light

Recent studies indicate therapeutic benefits of electrical stimulation in cases of specific ophthalmic diseases that are associated with dysfunctional ocular microcirculation. This suggests effects of electrical stimulation on vascular functions. In the present study, we investigated the effects of electrical stimulation on retinal vessel reactions using dynamic vessel analysis (DVA). Eighty healthy subjects were randomly assigned to one of three groups receiving electrical stimulation with different current intensities: 400 μA (n = 26); 800 μA (n = 27); 1200 μA (n = 27). The electrode montage for electrical stimulation consisted of a ring-shaped active electrode surrounding one eye and a square return electrode at the occiput. Rectangular, monophasic, positive current pulses were applied at 10 Hz for a duration of 60 s per stimulation period. DVA was used to observe the stimulation-induced reactions of retinal vessel diameters in response to different provocations. In three DVA measurements, three stimulus conditions were investigated: flicker light stimulation (FLS); electrical stimulation (ES); simultaneous electrical and flicker light stimulation (ES+FLS). Retinal vasodilation caused by these stimuli was compared using paired t-test. The subjects receiving electrical stimulation with 800 μA showed significantly increased retinal vasodilation for ES+FLS compared to FLS (p < 0.05). No significant differences in retinal vessel reactions were found between ES+FLS and FLS in the 400 and 1200 μA groups. No retinal vasodilation was observed for ES for all investigated current intensities. The results indicate that positive pulsed electrical stimulation of an adequate intensity enhances the flicker light-induced retinal vasodilation.

Study of Electrical Stimulation with Different Electric-Field Intensities in the Regulation of the Differentiation of PC12 Cells

The strategy of using electrical stimulation (ES) to promote the neural differentiation and regeneration of injured nerves is proven feasible. Study of the possible molecular mechanisms in relation to this ES promotion effect should be helpful for understanding the phenomenon. In this study, it was identified that the neuronal differentiation of PC12 cells was enhanced when the electric field intensity was in the range of 30-80 mV/mm, and a lower or higher electric-field intensity displayed inferior effects. Under ES, however, levels of intracellular reactive oxygen species (ROS), intracellular Ca²⁺ dynamics, and expression of TREK-1 were measured as being gradually increasing alongside higher electric-field intensity. In trying to understand the relationship between the ES enhancement on differentiation and these variations in cell activities, parallel experiments were conducted by introducing exogeneous H₂O₂ into culture systems at different concentrations. Similarly, the effects of H₂O₂ concentration on the neuronal differentiation of PC12 cells, intracellular ROS and Ca²⁺ levels, and TREK-1 expression were systematically characterized. In comparative studies, it was found in two cases that ES of 50 mV/mm for 2 h/day and H₂O₂ of 5 μM in culture medium shared comparable results for intracellular ROS and Ca²⁺ levels and TREK-1 expression. Higher H₂O₂ concentrations (e.g., 10 and 20 μM) demonstrated adverse effects on cell differentiation and caused DNA damage. A stronger ES (e.g., 100 mV/mm), being associated with a higher intracellular ROS level, also resulted in weaker enhancement of the neuronal differentiation of PC12 cells. These facts suggested that the intracellular ROS generated under ES might be an intermediate signal transducer involved in cascade reactions relative to cell differentiation.

Trans-ocular Electric Current In Vivo Enhances AAV-Mediated Retinal Gene Transduction after Intravitreal Vector Administration

Adeno-associated virus (AAV) vector-mediated gene delivery is a promising approach for therapy, but implementation in the eye currently is hampered by the need for delivering the vector underneath the retina, using surgical application into the subretinal space. This limits the extent of the retina that is treated and may cause surgical injury. Vector delivery into the vitreous cavity would be preferable because it is surgically less invasive and would reach more of the retina. Unfortunately, most conventional, non-modified AAV vector serotypes penetrate the retina poorly from the vitreous; this limits efficient transduction and expression by target cells (retinal pigment epithelium and photoreceptors). We developed a method of applying a small and safe electric current across the intact eye in vivo for a brief period following intravitreal vector administration. This significantly improved AAV-mediated transduction of retinal cells in wild-type mice following intravitreal delivery, with gene expression in retinal pigment epithelium and photoreceptor cells. The low-level current had no adverse effects on retinal structure and function. This method should be generally applicable for other AAV serotypes and may have broad application in both basic research and clinical studies.

Transpalpebral Electrical Stimulation as a Novel Therapeutic Approach to Decrease Intraocular Pressure for Open-Angle Glaucoma: A Pilot Study

Purpose

To determine the effect on intraocular pressure of transpalpebral specific exogenous voltages in a cohort of open-angle glaucoma patients.

Methods

This is a prospective, comparative, and experimental pilot study. The electrical stimuli applied consisted of 10 Hz, biphasic, nonrectangular current pulses (100 μA) delivered from an isolated constant current stimulator. At intake, baseline IOP measurements were obtained from each eye. The measurement was repeated before and after microstimulation until the end of the treatment.

Results

Seventy-eight eyes of 46 patients diagnosed with POAG were studied: 58 eyes with maximum tolerated medical treatment and 20 eyes without treatment (naïve). The mean baseline IOP on the treated POAG group was 19.25 mmHg ± 4.71. Baseline IOP on the naïve group was 20.38 mmHg ± 3.28. At the four-month follow-up visit, the mean IOP value on the treatment group was 14.41 mmHg ± 2.06 (P < 0.0001). The obtained mean IOP measurement on the treatment-naïve group was 15.29 mmHg ± 2.28 (P < 0.0001).

Conclusions

The hypotensive response obtained using transpalpebral electrical stimulation on POAG patients, both on treatment-naïve patients and on patients receiving maximum tolerable treatment, was statistically significant when comparing basal IOP measurements to those obtained at the four-month follow-up visit.

Neuroprotective strategies for retinal disease

Diseases that affect the eye, including photoreceptor degeneration, diabetic retinopathy, and glaucoma, affect 11.8 million people in the US, resulting in vision loss and blindness. Loss of sight affects patient quality of life and puts an economic burden both on individuals and the greater healthcare system. Despite the urgent need for treatments, few effective options currently exist in the clinic. Here, we review research on promising neuroprotective strategies that promote neuronal survival with the potential to protect against vision loss and retinal cell death. Due to the large number of neuroprotective strategies, we restricted our review to approaches that we had direct experience with in the laboratory. We focus on drugs that target survival pathways, including bile acids like UDCA and TUDCA, steroid hormones like progesterone, therapies that target retinal dopamine, and neurotrophic factors. In addition, we review rehabilitative methods that increase endogenous repair mechanisms, including exercise and electrical stimulation therapies. For each approach, we provide background on the neuroprotective strategy, including history of use in other diseases; describe potential mechanisms of action; review the body of research performed in the retina thus far, both in animals and in humans; and discuss considerations when translating each treatment to the clinic and to the retina, including which therapies show the most promise for each retinal disease. Despite the high incidence of retinal diseases and the complexity of mechanisms involved, several promising neuroprotective treatments provide hope to prevent blindness. We discuss attractive candidates here with the goal of furthering retinal research in critical areas to rapidly translate neuroprotective strategies into the clinic.

Transcorneal electrical stimulation for the treatment of retinitis pigmentosa: results from the TESOLAUK trial

Objective

To explore the impact of weekly transcorneal electrical stimulation (TES) over a 6-month period as a treatment for retinitis pigmentosa (RP).

Methods and analysis

A prospective open-label observational trial was carried out assessing weekly TES in participants with RP for a period of 6 months followed by observation for a further 6 months. Clinical examination and investigations were carried out at 3 monthly intervals for a total of 12 months. The primary outcome measure explored safety through a descriptive analysis of adverse effects with secondary outcome measures evaluating structural and functional efficacy.

Results

Seven male and seven female participants with RP aged 18–80 years were recruited. TES was well tolerated with no serious adverse events reported. Two participants reported transient foreign body sensation and one participant had discomfort underneath the skin electrode. Following 6 months of TES, best-corrected visual acuity increased by 1.1±1.4 letters in the control arm and 0.93±1.4 letters in the treated arm. Central microperimetry threshold sensitivity rose by 0.02±0.5 decibels (dB) and 0.37±0.4 dB and Goldmann visual field volume by 0.16±0.09 steradians (sr) vs 0.22±0.12 sr for the control and treated eye, respectively. There was no statistical significance seen between eyes following the treatment or observation period.

Conclusion

This small open-label clinical trial showed that TES was safe and well tolerated in patients with RP. Visual function measurements at 6 months demonstrated no significant difference between the control and treated eyes. The results justify a larger clinical trial over a longer period of time in order to identify any treatment effect.

Electrical Stimulation as a Means for Improving Vision

Evolving research has provided evidence that noninvasive electrical stimulation (ES) of the eye may be a promising therapy for either preserving or restoring vision in several retinal and optic nerve diseases. In this review, we focus on minimally invasive strategies for the delivery of ES and accordingly summarize the current literature on transcorneal, transorbital, and transpalpebral ES in both animal experiments and clinical studies. Various mechanisms are believed to underlie the effects of ES, including increased production of neurotrophic agents, improved chorioretinal blood circulation, and inhibition of proinflammatory cytokines. Different animal models have demonstrated favorable effects of ES on both the retina and the optic nerve. Promising effects of ES have also been demonstrated in clinical studies; however, all current studies have a lack of randomization and/or a control group (sham). There is thus a pressing need for a deeper understanding of the underlying mechanisms that govern clinical success and optimization of stimulation parameters in animal studies. In addition, such research should be followed by large, prospective, clinical studies to explore the full potential of ES. Through this review, we aim to provide insight to guide future research on ES as a potential therapy for improving vision.

Electrical brain stimulation induces dendritic stripping but improves survival of silent neurons after optic nerve damage

Repetitive transorbital alternating current stimulation (rtACS) improves vision in patients with chronic visual impairments and an acute treatment increased survival of retinal neurons after optic nerve crush (ONC) in rodent models of visual system injury. However, despite this protection no functional recovery could be detected in rats, which was interpreted as evidence of “silent survivor” cells. We now analysed the mechanisms underlying this “silent survival” effect. Using in vivo microscopy of the retina we investigated the survival and morphology of fluorescent neurons before and after ONC in animals receiving rtACS or sham treatment. One week after the crush, more neurons survived in the rtACS-treated group compared to sham-treated controls. In vivo imaging further revealed that in the initial post-ONC period, rtACS induced dendritic pruning in surviving neurons. In contrast, dendrites in untreated retinae degenerated slowly after the axonal trauma and neurons died. The complete loss of visual evoked potentials supports the hypothesis that cell signalling is abolished in the surviving neurons. Despite this evidence of “silencing”, intracellular free calcium imaging showed that the cells were still viable. We propose that early after trauma, complete dendritic stripping following rtACS protects neurons from excitotoxic cell death by silencing them.

Non-invasive electrical brain stimulation: from acute to late-stage treatment of central nervous system damage

Non-invasive brain current stimulation (NIBS) is a promising and versatile tool for inducing neuroplasticity, protection and functional rehabilitation of damaged neuronal systems. It is technically simple, requires no surgery, and has significant beneficial effects. However, there are various technical approaches for NIBS which influence neuronal networks in significantly different ways. Transcranial direct current stimulation (tDCS), alternating current stimulation (ACS) and repetitive transcranial magnetic stimulation (rTMS) all have been applied to modulate brain activity in animal experiments under normal and pathological conditions. Also clinical trials have shown that tDCS, rTMS and ACS induce significant behavioural effects and can – depending on the parameters chosen – enhance or decrease brain excitability and influence performance and learning as well as rehabilitation and protective mechanisms. The diverse phaenomena and partially opposing effects of NIBS are not yet fully understood and mechanisms of action need to be explored further in order to select appropriate parameters for a given task, such as current type and strength, timing, distribution of current densities and electrode position. In this review, we will discuss the various parameters which need to be considered when designing a NIBS protocol and will put them into context with the envisaged applications in experimental neurobiology and medicine such as vision restoration, motor rehabilitation and cognitive enhancement.

Transcorneal electrical stimulation promotes survival of retinal ganglion cells after optic nerve transection in rats accompanied by reduced microglial activation and TNF-α expression

Microglial activation plays a crucial role in the pathological processes of various retinal and optic nerve diseases. TNF-α is a pro-inflammatory cytokine that is rapidly upregulated and promotes retinal ganglion cells (RGCs) death after optic nerve injury. However, the cellular source of TNF-α after optic nerve injury remains unclear. Thus, we aimed to determine the changes of retinal microglial activation in a rat model of optic nerve transection (ONT) after transcorneal electrical stimulation (TES). Furthermore, we assessed TNF-α expression after ONT and evaluated the effects of TES on TNF-α production. Rats were divided into 2 control groups receiving a sham surgery procedure, 2 ONT+Sham TES groups, and 2 ONT+TES groups. The rats were sacrificed on day 7 or 14 after ONT. RGCs were retrogradely labelled by Fluorogold (FG) 7 days before ONT, one TES group and corresponding controls were stimulated on day 0, 4, and the second were stimulated on day 0, 4, 7, 10. Whole-mount immunohistofluorescence, quantification of RGCs and microglia, and western blot analysis were performed on day 7 and 14 after ONT. TES significantly increased RGCs survival on day 7 and 14 after ONT, which was accompanied by reduced microglia on day 7, but not 14. TNF-α was co-localized with ameboid microglia and significantly increased on day 7 and 14 after ONT. TES significantly reduced TNF-α production on day 7 and 14 after ONT. Our study demonstrated that TES promotes RGCs survival after ONT accompanied by reduced microglial activation and microglia-derived TNF-α production.

Advances in implantable bionic devices for blindness: a review

Since the 1950s, vision researchers have been working towards the ambitious goal of restoring a functional level of vision to the blind via electrical stimulation of the visual pathways. Groups based in Australia, USA, Germany, France and Japan report progress in the translation of retinal visual prosthetics from the experimental to clinical domains, with two retinal visual prostheses having recently received regulatory approval for clinical use. Regulatory approval for cortical visual prostheses is yet to be obtained; however, several groups report plans to conduct clinical trials in the near future, building upon the seminal clinical studies of Brindley and Dobelle. In this review, we discuss the general principles of visual prostheses employing electrical stimulation of the visual pathways, focusing on the retina and visual cortex as the two most extensively studied stimulation sites. We also discuss the surgical and functional outcomes reported to date for retinal and cortical prostheses, concluding with a brief discussion of novel developments in this field and an outlook for the future.

Repetitive Transcorneal Alternating Current Stimulation Reduces Brain Idling State After Long-term Vision Loss

BACKGROUND

Deafferentation of visual system structures following brain or optic nerve injury leaves cortical areas deprived of visual input. Deprived cortical areas have a reduced sensory information processing and are characterized with localized enhanced or synchronized rhythms believed to represent an "idling state".

OBJECTIVE/HYPOTHESIS

We hypothesized that cortical idling can be modified with transcorneal alternating current stimulation (tACS) known to modulate cortical oscillations and thus change the functional state of the deafferented areas.

METHODS

tACS was applied in rat model of severe optic nerve crush using a protocol similar to our clinical studies (200 μA, 2-8 Hz) for 5 treatment days right after the lesion and at the chronic stage (3 months later). EEG and VEP were recorded over the visual cortices. In vivo confocal neuroimaging of the retina and histology of the optic nerves were performed.

RESULTS

Morphological investigations showed massive retinal ganglion cells death and degeneration of the optic nerves after crush. Visual loss was associated with increased EEG spectral power and lower coherence, indicating an "idling state". Stimulation induced a significant decrease of EEG power towards normal values. These effects were especially pronounced in the chronic stage.

CONCLUSION

Our results suggest that alternating current injected via the eye is able to modulate visually deprived brain areas and thus reduce cortical idling.

Proteomic study of retinal proteins associated with transcorneal electric stimulation in rats

Background. To investigate how transcorneal electric stimulation (TES) affects the retina, by identifying those proteins up- and downregulated by transcorneal electric stimulation (TES) in the retina of rats. Methods. Adult Wistar rats received TES on the left eyes at different electrical currents while the right eyes received no treatment and served as controls. After TES, the eye was enucleated and the retina was isolated. The retinas were analyzed by proteomics. Results. Proteomics showed that twenty-five proteins were upregulated by TES. The identified proteins included cellular signaling proteins, proteins associated with neuronal transmission, metabolic proteins, immunological factors, and structural proteins. Conclusions. TES induced changes in expression of various functional proteins in the retina.

The role of electrical stimulation therapy in ophthalmic diseases

INTRODUCTION

Electrical stimulation therapy (EST) involves the use of a low-intensity electrical current in the treatment of neuromuscular conditions. During the recent two decades, EST has emerged as a potential neuroprotective strategy in certain ophthalmic diseases, aided by a lack of effective management for these conditions.

PURPOSE

The aim of this review is to summarize and discuss current available evidence for the use of EST in ophthalmic diseases in the laboratory setting and in human trials.

METHODS

The compilation and review of published English-language reports on the use of EST in human ophthalmic disease and animal models of ophthalmic disease.

RESULTS

From published reports, research work on the use of EST in ophthalmic diseases began in the last 20 years. Different methods of electrical stimulation have been devised, with varying levels of invasiveness. Results from human trials have favored earlier and repeated treatment after insults to the optic nerve, while EST has shown transient effectiveness in degenerative diseases of photoreceptors. Patients also reported no serious adverse effects from EST in the clinical trials. Results from animal studies have further confirmed survival benefits of EST in retinal cell survival, with the underlying mechanism likely multifactorial, but involving Müller cell modulation.

CONCLUSIONS

Results from human and animal studies have demonstrated the relevance and potential effectiveness of EST in ophthalmic disease. However, optimal disease and species-specific stimulation settings need to be defined.

Effects of steroid administration and transcorneal electrical stimulation on the anatomic and electrophysiologic deterioration of nonarteritic ischemic optic neuropathy in a rodent model

PURPOSE

To elucidate the effectiveness of steroid administration and transcorneal electrical stimulation (TES) on anatomic changes and visual function in a rodent model of nonarteritic ischemic optic neuropathy (rNAION).

METHODS

Methylprednisolone (20 mg/kg) was injected through a central venous catheter twice a day for 3 days. TES was delivered with biphasic square pulses of 1 ms/phase, 100 μA of current, and 20 Hz of frequency for 60 min 3 h after induction on the 1st, 4th, 7th, 14th, and 28th days.

RESULTS

Intravenous infusion of methylprednisolone significantly decreased the degree of acute disc edema but did not preserve the inner retinal thinning, decreasing the amplitude of scotopic threshold responses (STR) and decreasing retinal ganglion cell (RGC) numbers in rNAION. TES preserved the decreasing STR amplitude and the decreasing RGC numbers in rNAION.

CONCLUSION

Steroids are effective for reducing disc edema in the acute stage in rNAION. TES is effective for preserving decreasing RGC numbers and function in the chronic stage of rNAION.

Transcorneal electrical stimulation alters morphology and survival of retinal ganglion cells after optic nerve damage

Traumatic optic nerve injury leads to retrograde death of retinal ganglion cells (RGCs), but transcorneal electrical stimulation (TES) can increase the cell survival rate. To understand the mechanisms and to further define the TES-induced effects we monitored in living animals RGC morphology and survival after optic nerve crush (ONC) in real time by using in vivo confocal neuroimaging (ICON) of the retina. ONC was performed in rats and ICON was performed before crush and on post-lesion days 3, 7 and 15 which allowed us to repeatedly record RGC number and size. TES or sham-stimulation were performed immediately after the crush and on post-injury day 11. Three days after ONC we detected a higher percentage of surviving RGCs in the TES group as compared to sham-treated controls. However, the difference was below significance level on day 7 and disappeared completely by day 15. The death rate was more variable amongst the TES-treated rats than in the control group. Morphological analysis revealed that average cell size changed significantly in the control group but not in stimulated animals and the morphological alterations of surviving neurons were smaller in TES-treated compared to control cells. In conclusion, TES delays post-traumatic cell death significantly. Moreover, we found "responder animals" which also benefited in the long-term from the treatment. Our in vivo cellular imaging results provide evidence that TES reduces ONC-associated neuronal swelling and shrinkage especially in RGCs which survived long-term. Further studies are now needed to determine the differences of responders vs. non-responders.

Transcorneal alternating current stimulation induces EEG "aftereffects" only in rats with an intact visual system but not after severe optic nerve damage

Noninvasive alternating current stimulation can induce vision restoration in patients with chronic optic nerve damage and results in electroencephalogram (EEG) aftereffects. To better understand the mechanisms of action, we studied such EEG "aftereffects" of transcorneal alternating current stimulation (tACS) at the chronic posttraumatic state in rats. EEG baseline was recorded from visual cortex under ketamine/xylazine narcosis of healthy rats and rats with chronic severe optic nerve crush. One week later, both groups were again anesthetized and stimulated transcorneally twice for 12 min each time. tACS-induced changes were compared with baseline EEG. Over the course of 65 min narcosis baseline EEG revealed a shift from a dominant delta power to theta. This shift was significantly delayed in lesioned animals compared with healthy controls. tACS applied during the late narcosis stage in normal rats led to significantly increased theta power with a parallel shift of the dominating peak to higher frequency which outlasted the stimulation period by 15 min (aftereffects). EEG in lesioned rats was not significantly changed. In rodents, tACS can induce neuroplasticity as shown by EEG aftereffects that outlast the stimulation period. But this requires a minimal level of brain activation because aftereffects are not seen when tACS is applied during deep anesthesia and not when applied to animals after severe optic nerve damage. We conclude that tACS is only effective to induce cortical plasticity when the the retina can be excited.