Implantation of Stimulation Electrodes in the Subretinal Space to Demonstrate Cortical Responses in Yucatan Minipig in the Course of Visual Prosthesis Development

Purpose

During the course of the development of visual prostheses, subretinal stimulation films were implanted in micropigs in order to prove the feasibility of subretinal electrical stimulation with subsequent cortical response. One aim was to demonstrate that epidural recording of visual evoked potentials is possible in the micropig.

Methods

Film-bound stimulation electrode arrays were placed in the subretinal space of micropigs. This enabled the retina to be stimulated subretinally. Since conventional visual evoked potential (VEP) measuring is virtually impossible in the pig from the neurosurgical point of view, epidural recording electrode arrays were positioned over the visual cortex as permanent electrodes.

Results

The feasibility of temporary implantation of film-bound stimulation electrode arrays was successfully demonstrated in the micropig model. On stimulation with monopolar voltage pulses (1000 to 3000 mV), reproducible epidural VEP measurements (5 to 10 μV) were detected.

Conclusions

The feasibility of subretinal stimulation of the retina was demonstrated in a retinal model that is similar to the human retina. This animal model therefore offers a suitable means of studying the tolerability of stimulation situations in the course of visual prosthesis development.

Transfer characteristics of subretinal visual implants: corneally recorded implant responses

PURPOSE

The subretinal Alpha IMS visual implant is a CE-approved medical device for restoration of visual functions in blind patients with end-stage outer retina degeneration. We present a method to test the function of the implant objectively in vivo using standard electroretinographic equipment and to assess the devices' parameter range for an optimal perception.

METHODS

Subretinal implant Alpha IMS (Retina Implant AG, Reutlingen, Germany) consists of 1500 photodiode-amplifier-electrode units and is implanted surgically into the subretinal space in blind retinitis pigmentosa patients. The voltages that regulate the amplifiers' sensitivity (V _gl) and gain (V _bias), related to the perception of contrast and brightness, respectively, are adjusted manually on a handheld power supply device. Corneally recorded implant responses (CRIR) to full-field illumination with long duration flashes in various implant settings for brightness gain (V _bias) and amplifiers' sensitivity (V _gl) are measured using electroretinographic setup with a Ganzfeld bowl in a protocol of increasing stimulus luminances up to 1000 cd/m².

RESULTS

CRIRs are a meaningful tool for assessing the transfer characteristic curves of the electronic implant in vivo monitoring the implants' voltage output as a function of log luminance in a sigmoidal shape. Changing the amplifiers' sensitivity (V _gl) shifts the curve left or right along the log luminance axis. Adjustment of the gain (V _bias) changes the maximal output. Contrast perception is only possible within the luminance range of the increasing slope of the function.

CONCLUSIONS

The technical function of subretinal visual implants can be measured objectively using a standard electroretinographic setup. CRIRs help the patient to optimise the perception by adjusting the gain and luminance range of the device and are a useful tool for clinicians to objectively assess the function of subretinal visual implants in vivo.

[The role of the vitreous body in diseases of neighboring structures]

BACKGROUND

The vitreoretinal interface is not merely the space between the vitreous body and the retina but it is also a site for highly complex pathologies with the vitreous body exerting an influence on all the neighbouring structures.

METHODS

A literature search was performed in Pubmed and current book articles

RESULTS

This review article highlights the role of the vitreous body in vitreomacular adhesion and traction, in the development of macular holes and epiretinal membranes as well as its role in age-related macular degeneration. In the retinal periphery the vitreous structures play a pivotal role in retinal tears and detachment as well as in diabetic and other proliferative vitreoretinopathies. The role of the vitreous bodyin the emergence of various forms of cataract is often underestimated.

DISCUSSION

Vitreo-etinal surgeons should thoroughly understand the pathophysiological relationship between the vitreous body and the neighboring structures, especially in the era of medical vitreolysis.

In vivo biocompatibility of a new cyanine dye for ILM peeling

PURPOSE

To investigate the biocompatibility of the new cyanine dye: 3,3'-Di-(4-sulfobutyl)-1,1,1',1'-tetramethyl-di-1H-benz[e]indocarbocyanine (DSS) as a vital dye for intraocular application in an in vivo rat model and to evaluate the effects of this dye on retinal structure and function.

METHODS

DSS at a concentration of 0.5% was applied via intravitreal injections to adult Brown Norway rats with BSS serving as a control. Retinal toxicity was assessed 7 days later by means of retinal ganglion cell (RGC) counts, light microscopy, optical coherence tomography (OCT), and electroretinography (ERG).

RESULTS

No significant decrease in RGC numbers was observed. No structural changes of the central retina were observed either in vivo (OCT) or under light microscopy. ERGs detected a temporary reduction of retinal function 7 days after injection; this was no longer evident 14 days after injection.

CONCLUSIONS

DSS showed good biocompatibility in a well-established experimental in vivo setting and may be usable for intraocular surgery as an alternative to other cyanine dyes. In contrast to indocyanine green, it additionally offers fluorescence in the visual spectrum. Further studies with other animal models are needed before translation into clinical application.

[Therapeutic approaches for retinitis pigmentosa]

BACKGROUND

Retinitis pigmentosa (RP) is a clinically and genetically heterogeneous group of hereditary retinal disorders, which lead to progressive loss of vision and finally blindness. Yet there is no approved therapy. Advances in unravelling underlying genetic disorders and pathophysiological mechanisms offer new therapeutic approaches of which some are summarised in this review.

METHODS

We performed a systematic literature research for defined key words in PubMed.

RESULTS

New approaches to therapy for RP include: gene therapy, pharmacological treatment, neuroprotection, electrical stimulation, retinal prostheses, retinal transplantation and optogenetic therapy.

CONCLUSIONS

Recently there have been advances in new approaches for therapy of dystrophic retinal diseases. Advances in the different approaches are being made at different rates. Although there is no approved therapy yet, the future for treating RP at least in some patients looks promising.

[What can blind patients see in daily life with the subretinal Alpha IMS implant? Current overview from the clinical trial in Tübingen]

The subretinal visual implant is a scientific research approach to restore partial vision in end-stage hereditary retinal diseases by replacing the function of the degenerated photoreceptors by microelectronic chips. In a clinical trial in Tübingen these implants were tested on voluntary blind patients. By using the implants in daily living the patients reported valuable visual information. The subretinal microchip mediates subjectively useful visual information in near as well as in distant vision.

[Transcorneal electrostimulation]

Stimulation with weak electrical currents as a potential therapy for ophthalmic diseases has been propagated for a long time and encountered mixed acceptance but has recently regained interest due to observations in retinal implant projects. In these projects involving patients with retinitis pigmentosa a neuroprotective effect of weak, even subthreshold, electrical stimulation was found which led to improvement in visual functions even in areas distant from the implant. Results of several animal studies clearly substantiate a neuroprotective effect in degenerative retinal diseases and traumatic optic neuropathies. Studies in patients have so far been published for nonarteritic anterior ischemic and traumatic optic neuropathy, retinal artery occlusion and Stargardt's disease. These studies were mostly small and of short duration so that effects did not reach statistical significance. A controlled, randomized study in patients with retinitis pigmentosa has recently been published where positive tendencies and several statistically significant improvements in visual function were observed in one subgroup. Electrical stimulation further deserves further attention as several companies currently offer devices which should enable patients to self-administer electrical stimulation. This article gives an overview of the scientific background of electrical stimulation and critically assesses data from animal and human studies with focus on retinitis pigmentosa.

Extraocular surgery for implantation of an active subretinal visual prosthesis with external connections: feasibility and outcome in seven patients

BACKGROUND

Due to low energy levels in microphotodiode-based subretinal visual prostheses, an external power supply is mandatory. We report on the surgical feasibility and the functional outcome of the extraocular part of an approach to connect a subretinal prosthesis to an extracorporeal connector in the retro-auricular space via a trans-scleral, transchoroidal cable.

METHODS

Seven volunteers with retinitis pigmentosa received an active subretinal implant; energy was supplied by gold wires on a trans-sclerally, transchoroidally implanted polyimide foil leading to the lateral orbital rim where it was fixated and connected to a silicone cable. The cable was implanted subperiostally beneath the temporal muscle using a trocar to the retro-auricular space where it penetrated the skin for connection to a stimulator. To avoid subretinal movement of the implant, three tension relief points have been introduced.

RESULTS

All implantations were performed as planned without complications, and no serious adverse events occurred in the postoperative period. Fixation of the implants was stable throughout the entire study duration of 4 weeks; permanent skin penetration proved to be uncomplicated. Motility was minimally restricted in downgaze and ab-/adduction. Explantation was uneventful.

CONCLUSION

The above-described procedure provides a method for stable fixation of a subretinal device with a trans-scleral, transchoroidal cable connection to an extracorporeal connector.

Creatine supplementation in Parkinson disease: a placebo-controlled randomized pilot trial

Mitochondrial dysfunction plays a major role in the pathogenesis of Parkinson disease (PD). Creatine (Cr) is an ergogenic compound that exerts neuroprotective effects in animal models of PD. We conducted a 2-year placebo-controlled randomized clinical trial on the effect of Cr in 60 patients with PD. Cr improved patient mood and led to a smaller dose increase of dopaminergic therapy but had no effect on overall Unified Parkinson's Disease Rating Scale scores or dopamine transporter SPECT.

Stand des subretinalen Implantatprojekts

The development of a subretinal prosthesis has come to a stage where human trials are forthcoming. Subretinal prostheses are designed to replace degenerated photoreceptors in diseases such as retinitis pigmentosa or age-related macular degeneration. Microphotodiode arrays are implanted between retinal pigment epithelium and retina. Our group has collected convincing evidence for the principle feasibility of a subretinal prosthesis. Animal experiments have shown that subretinal electrical stimulation can successfully elicit spatially ordered responses in the visual cortex; visual acuity is estimated to reach 0.25 degrees of visual angle. Histological long-term examinations have demonstrated that the retina tolerates a subretinal implant well and also that the implant itself sustains the ocular environments. Surgical procedures have been successfully developed to implant complex subretinal devices.

Implantation of stimulation electrodes in the subretinal space to demonstrate cortical responses in Yucatan minipig in the course of visual prosthesis development

PURPOSE

During the course of the development of visual prostheses, subretinal stimulation films were implanted in micropigs in order to prove the feasibility of subretinal electrical stimulation with subsequent cortical response. One aim was to demonstrate that epidural recording of visual evoked potentials is possible in the micropig.

METHODS

Film-bound stimulation electrode arrays were placed in the subretinal space of micropigs. This enabled the retina to be stimulated subretinally. Since conventional visual evoked potential (VEP) measuring is virtually impossible in the pig from the neurosurgical point of view, epidural recording electrode arrays were positioned over the visual cortex as permanent electrodes.

RESULTS

The feasibility of temporary implantation of film-bound stimulation electrode arrays was successfully demonstrated in the micropig model. On stimulation with monopolar voltage pulses (1000 to 3000 mV), reproducible epidural VEP measurements (5 to 10 micronV) were detected.

CONCLUSIONS

The feasibility of subretinal stimulation of the retina was demonstrated in a retinal model that is similar to the human retina. This animal model therefore offers a suitable means of studying the tolerability of stimulation situations in the course of visual prosthesis development.

Sensitivity and specificity of ELISA and immunoblot for diagnosing neurocysticercosis

In patients with neurocysticercosis (NCC), clinical manifestations and the results of neuroimaging procedures vary widely and often do not facilitate a definite diagnosis. In order to determine the value of immunodiagnosis for NCC, 222 serum and cerebrospinal fluid samples from patients with NCC and healthy subjects were examined. The samples represented patients from various endemic regions, those with other neurological disorders from an endemic area (Mexico), persons with various helminth infections other than NCC, and a group of healthy volunteers. All specimens were tested by enzyme-linked immunosorbent assay and immunoblot for the presence of Taenia solium-specific antibodies. The sensitivities of the enzyme-linked immunosorbent assay and the immunoblot test in NCC patients were almost identical (80% and 81.7%, respectively). For both tests, the sensitivity was higher when cerebrospinal fluid (86%) was tested compared with serum (75%). The overall specificity of enzyme-linked immunosorbent assay was only 75.3% because of frequent false-positive results in patients with other helminth infections, especially in those with echinococcosis. The specificity (99.4%) of the immunoblot test was clearly superior. It is concluded that enzyme-linked immunosorbent assay as a screening method and immunoblot as a confirmatory test contribute considerably to the diagnosis of NCC.

Studies on the feasibility of a subretinal visual prosthesis: data from Yucatan micropig and rabbit

BACKGROUND

To estimate the feasibility of the subretinal concept of a visual prosthesis, animal models and prototypes, each representing a certain aspect of the final prosthesis, were utilised to test for requirements for such a medical device: (1) the ability to elicit--by electrical stimulation--event-related central activity in the central visual system, and (2) the long-term biocompatibility and biostability of the implant within the subretinal space.

METHODS

(1) In rabbit and Yucatan minipig, cortical evoked potentials were recorded with chronically implanted epidural electrodes during stimulation with light flashes as well as during electrical stimulation in the subretinal space. Voltage pulses ranging from -3 V to +3 V were applied via an acutely implanted electrode array on a wired prototype. (2) For biocompatibility studies a silicon-based micro-photodiode array (MPDA) was used that closely resembled the design and composition of the final prosthesis. Fourteen months after implantation, angiography was performed and the histological findings of the retina in the immediate vicinity of the implant were evaluated.

RESULTS

(1) In both rabbit and minipig, subretinal electrical stimulation resulted in evoked cortical potentials that were comparable to visual evoked potentials. The lowest threshold levels for the subretinal stimulation were 0.6 V for rabbits and 2 V for minipigs. (2) Long-term stability of an implanted MPDA and its biocompatibility were proven for a postoperative period of 14 months.

CONCLUSIONS

Data from animal experiments with certain prototypes of the final prosthesis suggest the feasibility of the concept of a subretinal visual prosthesis: Both requirements were met: (1) the functioning of the subretinal stimulation and (2) the biocompatibility of the MPDA implant.

Changes in contrast sensitivity induced by defocus and their possible relations to emmetropization in the chicken

PURPOSE

To test whether the level of contrast adaptation (CA) relates to refractive development in the chicken. (CA refers to a spatial frequency-selective increase of suprathreshold contrast sensitivity after exposure to low-contrast patterns).

METHODS

CA was determined in individual chicks by comparing their optomotor gain in response to drifting low-contrast stripe patterns before and after treatment with spectacle lenses. The amount of CA was compared with the loss of contrast predicted from defocus at the tested spatial frequency. The reversion of CA and recovery from deprivation myopia were studied while the retinal image features were controlled by forcing the animals to watch spatially filtered digital video clips.

RESULTS

CA was induced by wearing positive and negative lenses for 1.5 hours, both without and with cycloplegia, but was less pronounced in the case of positive lenses when accommodation was intact. The amount of CA at a tested spatial frequency was predicted from the loss of contrast calculated from the modulation transfer function for a defocused optical system. Watching low-pass-filtered video clips induced deprivation myopia and inhibited recovery from it. It also prevented the reversal of CA that was previously induced by deprivation. Both recovery from deprivation myopia and recovery from CA occurred with sharp video clips, although less so than with normal visual exposure.

CONCLUSIONS

CA changes with retinal image sharpness and occurs even when accommodation is intact. Because CA correlates with myopia induced by frosted occluders, negative lenses, and low-pass-filtered video clips, and its reversal correlates with recovery from myopia, it is possible that shifts in CA may represent a signal related to refractive error development.

Subretinales Mikrophotodioden-Array als Ersatz für degenerierte Photorezeptoren?

A survey is given on the status of developments, concerning a subretinal electronic microphotodiode array that aims at replacing degenerated photoreceptors. Various prototypes have been developed, tested, and implanted in various experimental animals up to 18 months. The fact that electrical responses were recorded from the visual cortex of pigs after electrical stimulation by subretinal electrodes and the fact that responses are also recorded in-vitro in degenerated rat retinae, shows the feasibility of this approach. However, there are a number of open questions concerning the biocompatibility, the long-time stability, and the type of transmitted image to be solved before application in patients can be considered.

Physiologische Funktionsprüfungen von Retinaimplantaten an Tiermodellen

Retinal implants can--by electrical stimulation--create visual impressions in people with certain kinds of degenerative retinal diseases (e.g. Retinitis Pigmentosa). Electrically evoked potentials in the retina must be transferred into the visual cortex in an orderly manner, a prerequisite for any kind of form- and movement-perception. In the current developmental stage the difficult investigations are performed in various animal models: isolated retinae of intact chicken and of RCS-rats (a model for Retinitis Pigmentosa), as well as in anesthetised rabbits, pigs and cats with intact retinae. Our investigations show that spatially selective ganglion-cell responses can be recorded following focal electrical stimulation, in healthy and as well in degenerated retinae. Registration of activities in area 17 of the visual cortex demonstrate that electrical retinal stimulation can indeed activate it.

A placebo-controlled crossover trial of creatine in mitochondrial diseases

To test the efficacy and safety of creatine (Cr) monohydrate in mitochondrial diseases, 16 patients with chronic progressive external ophthalmoplegia or mitochondrial myopathy were randomized in a crossover design to receive double-blind placebo or 20 g Cr/day for 4 weeks. Cr was well tolerated, but there were no significant effects with regard to exercise performance, eye movements, or activities of daily life. The power of this pilot study was limited and future multicenter trials are needed.

Genome-wide, large-scale production of mutant mice by ENU mutagenesis

In the post-genome era, the mouse will have a major role as a model system for functional genome analysis. This requires a large number of mutants similar to the collections available from other model organisms such as Drosophila melanogaster and Caenorhabditis elegans. Here we report on a systematic, genome-wide, mutagenesis screen in mice. As part of the German Human Genome Project, we have undertaken a large-scale ENU-mutagenesis screen for dominant mutations and a limited screen for recessive mutations. In screening over 14,000 mice for a large number of clinically relevant parameters, we recovered 182 mouse mutants for a variety of phenotypes. In addition, 247 variant mouse mutants are currently in genetic confirmation testing and will result in additional new mutant lines. This mutagenesis screen, along with the screen described in the accompanying paper, leads to a significant increase in the number of mouse models available to the scientific community. Our mutant lines are freely accessible to non-commercial users (for information, see http://www.gsf.de/ieg/groups/enu-mouse.html).

Diabetes insipidus, diabetes mellitus, optic atrophy and deafness (DIDMOAD) caused by mutations in a novel gene (wolframin) coding for a predicted transmembrane protein

Wolfram syndrome is an autosomal recessive disorder characterized by juvenile diabetes mellitus, diabetes insipidus, optic atrophy and a number of neurological symptoms including deafness, ataxia and peripheral neuropathy. Mitochondrial DNA deletions have been described in a few patients and a locus has been mapped to 4p16 by linkage analysis. Susceptibility to psychiatric illness is reported to be high in affected individuals and increased in heterozygous carriers in Wolfram syndrome families. We screened four candidate genes in a refined critical linkage interval covered by an unfinished genomic sequence of 600 kb. One of these genes, subsequently named wolframin, codes for a predicted transmembrane protein which was expressed in various tissues, including brain and pancreas, and carried loss-of-function mutations in both alleles in Wolfram syndrome patients.

Measurement of astigmatism by automated infrared photoretinoscopy

BACKGROUND

There are basically two possibilities to measure cylindrical refractive errors by eccentric photorefraction. The first is to determine the size and the tilt of the light crescent in the subject's pupil. Sphere, cylinder, and axis can be obtained from two pictures with the knife edge at two different orientations by using equations derived by Wesemann et al. In natural eyes, the procedure has limitations because undetermined factors (not considered in the theory) affect size, shape, and intensity of the light crescent. A second possibility is to perform eccentric photorefraction separately in at least three different meridians.

METHODS

We have tested the power of the second possibility. The three critical parameters (sphere, cylinder, and axis) were calculated from Euler's law, which describes curvatures (or refractions) at any given angle. The procedure relied only on empirical calibration and not on a theoretical treatment of the optics. Therefore, it was not necessary to identify all factors that determine the path of light.

RESULTS

The procedure compared favorably with subjective refractive (first population: students, age 26-30 years, N = 7 (14 eyes); correlations: sphere, r = 0.983; cylinder, r = 0.867; axis, r = 0.935) and with a Canon R-1 Autorefractor (second population: children, age 4-14 years, N = 48 (96 eyes); correlations: sphere, r = 0.955; cylinder, r = 0.600; axis, r = 0.846).

CONCLUSIONS

Because it is fast, the technique may be suitable for screening in children. The refractions in the different meridians are performed in real time (25 to 30 Hz) and a single reading (the average from 4-6 refractions in each of the 6 meridians) is obtained in 1-2 s. It constitutes a major improvement to commercially available videorefractors which use measurements only in two meridians in conjunction with the formula by Wesemann et al., although it is still not precise enough to permit spectacle prescription.