Phosphene thresholds elicited by transcorneal electrical stimulation in healthy subjects and patients with retinal diseases

PURPOSE

To evaluate electrically evoked phosphene thresholds (EPTs) in healthy subjects and in patients with retinal disease and to assess repeatability and possible correlations with common ophthalmologic tests.

METHODS

In all, 117 individuals participated: healthy subjects (n = 20) and patients with retinitis pigmentosa (RP, n = 30), Stargardt's disease (STG, n = 14), retinal artery occlusion (RAO, n = 20), nonarteritic anterior ischemic optic neuropathy (NAION, n = 16), and primary open-angle glaucoma (POAG, n = 17). EPTs were determined at 3, 6, 9, 20, 40, 60, and 80 Hz with 5 + 5-ms biphasic current pulses using DTL electrodes. Subjects were examined twice (test-retest range: 1-6 weeks). An empirical model was developed to describe the current-frequency relationship of EPTs. Visual acuity, visual field (kinetic + static), electrophysiology (RP, RAO, STG: Ganzfeld-electroretinography [ERG]/multifocal-ERG; POAG: pattern-ERG; NAION: VEP), slit-lamp biomicroscopy, fundus examination, and tonometry were assessed.

RESULTS

EPTS varied between disease groups (20 HZ: healthy subjects: 0.062 ± 0.038 mA; STG: 0.102 ± 0.097 mA; POAG: 0.127 ± 0.09 mA; NAION: 0.244 ± 0.126 mA; RP: 0.371 ± 0.223 mA; RAO: 0.988 ± 1.142 mA). In all groups EPTs were lowest at 20 Hz. In patients with retinal diseases and across all frequencies EPTs were significantly higher than those in healthy subjects, except in STG at 20 Hz (P = 0.09) and 40 Hz (P = 0.17). Test-retest difference at 20 Hz was 0.006 mA in the healthy group and 0.003-0.04 mA in disease groups.

CONCLUSIONS

Considering the fast, safe, and reliable practicability of EPT testing, this test might be used more often under clinical circumstances. Determination of EPTs could be potentially useful in elucidation of the progress of ophthalmologic diseases, either in addition to standard clinical assessment or under conditions in which these standard tests cannot be used meaningfully. (ClinicalTrials.gov number, NCT00804102.).

[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.

Positioning of electronic subretinal implants in blind retinitis pigmentosa patients through multimodal assessment of retinal structures

PURPOSE

To optimize methods for positioning subretinal visual implants, customizing their cable length, guiding them to the predetermined retinal position, and evaluating their performance.

METHODS

Ten eyes of 10 patients (6 male, 4 female, mean age 46.4 years) were investigated before implantation of a subretinal visual implant. The structural characteristics of the retina as well as the ocular dimensions were determined. Topographic images of the prospective implantation site were subdivided into grids of squares. Each square received a weighted score for suitability. The sum of the scores was calculated, and the region with the highest score was chosen for the implant. In each case, the implant's power supply cable length was calculated by means of magnetic resonance imaging. The planned and achieved positions before and after implantation were compared.

RESULTS

The mean light sensitivity ratio between the area actually covered by the chip and that of the planned position was 90.8% with an SD of 11.4%. In two cases with almost perfect positioning, the computed ratio was 100%. Measurements showed that to achieve a 95% sensitivity rate the difference between the planned and achieved chip position must be less than 1.7 mm. Preoperative calculations of the intraocular cable length proved accurate in all cases.

CONCLUSIONS

Preoperative evaluation of retinal structures and eye morphology is useful for guiding a retinal implant to the designated area. It is a meaningful tool for planning and performing retinal chip implantation, and it optimizes personalized implantation. (ClinicalTrials.gov numbers, NCT00515814, NCT01024803.).

Structural and functional changes of the human macula during acute exposure to high altitude

Background

This study aimed to quantify structural and functional changes at the macula during acute exposure to high altitude and to assess their structure/function relationship. This work is related to the Tuebingen High Altitude Ophthalmology (THAO) study.

Methodology/Principal Findings

Spectral domain optical coherence tomography and microperimetry were used to quantify changes of central retinal structure and function in 14 healthy subjects during acute exposure to high altitude (4559 m). High-resolution volume scans and fundus-controlled microperimetry of the posterior pole were performed in addition to best-corrected visual acuity (BCVA) measurements and assessment of acute mountain sickness. Analysis of measurements at altitude vs. baseline revealed increased total retinal thickness (TRT) in all four outer ETDRS grid subfields during acute altitude exposure (TRT_outer = 2.80±1.00 μm; mean change±95%CI). This change was inverted towards the inner four subfields (TRT_inner = −1.89±0.97 μm) with significant reduction of TRT in the fovea (TRT_foveal = −6.62±0.90 μm) at altitude. BCVA revealed no significant difference compared to baseline (0.06±0.08 logMAR). Microperimetry showed stable mean sensitivity in all but the foveal subfield (MS_foveal = −1.12±0.68 dB). At baseline recordings before and >2 weeks after high altitude exposure, all subjects showed equal levels with no sign of persisting structural or functional sequels.

Conclusions/Significance

During acute exposure to high altitude central retinal thickness is subject to minor, yet statistically significant changes. These alterations describe a function of eccentricity with an increase in regions with relatively higher retinal nerve fiber content and vascular arcades. However, these changes did not correlate with measures of central retinal function or acute mountain sickness. For the first time a quantitative approach has been used to assess these changes during acute, non-acclimatized high altitude exposure.

Effects of combined ketamine/xylazine anesthesia on light induced retinal degeneration in rats

Objectives

To explore the effect of ketamine-xylazine anesthesia on light-induced retinal degeneration in rats.

Methods

Rats were anesthetized with ketamine and xylazine (100 and 5 mg, respectively) for 1 h, followed by a recovery phase of 2 h before exposure to 16,000 lux of environmental illumination for 2 h. Functional assessment by electroretinography (ERG) and morphological assessment by in vivo imaging (optical coherence tomography), histology (hematoxylin/eosin staining, TUNEL assay) and immunohistochemistry (GFAP and rhodopsin staining) were performed at baseline (ERG), 36 h, 7 d and 14 d post-treatment. Non-anesthetized animals treated with light damage served as controls.

Results

Ketamine-xylazine pre-treatment preserved retinal function and protected against light-induced retinal degeneration. In vivo retinal imaging demonstrated a significant increase of outer nuclear layer (ONL) thickness in the non-anesthetized group at 36 h (p<0.01) and significant reduction one week (p<0.01) after light damage. In contrast, ketamine-xylazine pre-treated animals showed no significant alteration of total retinal or ONL thickness at either time point (p>0.05), indicating a stabilizing and/or protective effect with regard to phototoxicity. Histology confirmed light-induced photoreceptor cell death and Müller cells gliosis in non-anesthetized rats, especially in the superior hemiretina, while ketamine-xylazine treated rats showed reduced photoreceptor cell death (TUNEL staining: p<0.001 after 7 d), thicker ONL and longer IS/OS. Fourteen days after light damage, a reduction of standard flash induced a-wave amplitudes and a-wave slopes (p = 0.01) and significant alterations in parameters of the scotopic sensitivity function (e.g. Vmax of the Naka Rushton fit p = 0.03) were observed in non-treated vs. ketamine-xylazine treated animals.

Conclusions

Our results suggest that pre-treatment with ketamine-xylazine anesthesia protects retinas against light damage, reducing photoreceptor cell death. These data support the notion that anesthesia with ketamine-xylazine provides neuroprotective effects in light-induced cell damage.

Tempol protects against intravitreous indocyanine green-induced retinal damage in rats

PURPOSE

Indocyanine green (ICG) has been widely used as a vital dye for macular surgery. However, ICG can be toxic to retinal cells. Here we evaluate whether tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl), a free radical scavenger, can protect against ICG-induced retinal damage in rats.

METHODS

Brown Norway rats received intravitreal injections of ICG 0.5 % or BSS as controls. Tempol (20 mg/kg BW) or PBS as a control was administered intraperitoneally 24 h and 30 min before ICG and once daily for 7 consecutive days. Tempol was detected in the retina using electron paramagnetic resonance (EPR) spectroscopy. One week after ICG injections, the effects of tempol on retinal toxicity were assessed by retinal ganglion cell (RGC) back-labeling and by light microscopy. Electroretinography (ERG) was performed after 1 and 2 weeks.

RESULTS

ICG administration reduced RGC numbers by 17 % (1,943 ± 45 vs. 2,342 ± 31 RGCs/mm(2)). Tempol treatment rescued RGCs in a significant manner (2,258 ± 36, p < 0.01) and diminished morphological changes detected by light microscopy. ICG-injected eyes showed a significant reduction of ERG potentials only in PBS-treated animals (V(max) 530 ± 145 µV vs. 779 ± 179 µV, p = 0.0052), but not in the tempol-treated group.

CONCLUSIONS

Tempol significantly attenuates ICG-induced toxicity in rat retinas and may therefore be considered for further evaluation as accompanying treatment in ICG-assisted chromovitrectomy.

Electroretinographic findings associated with panretinal photocoagulation (PRP) versus PRP plus intravitreal ranibizumab treatment for high-risk proliferative diabetic retinopathy

To evaluate changes in electroretinographic (ERG) findings after panretinal photocoagulation (PRP) compared to PRP plus intravitreal injection of ranibizumab (IVR) in eyes with high-risk proliferative diabetic retinopathy (PDR). Patients with high-risk PDR and no prior laser treatment were assigned randomly to receive PRP (PRP group; n = 9) or PRP plus IVR (PRPplus group; n = 11). PRP was administered in two sessions (weeks 0 and 2), and IVR was administered at the end of the first laser session (week 0) in the PRPplus group. Standardized ophthalmic evaluations including (ETDRS) best-corrected visual acuity (BCVA), and fluorescein angiography to measure area of fluorescein leakage (FLA), were performed at baseline and at weeks 16 (±2), 32 (±2) and 48 (±2). ERG was measured according to ISCEV standards at baseline and at week 48 (±2). At 48 weeks, 2,400-3,000 laser spots had been placed in eyes in the PRP group, while only 1,400-1,800 spots had been placed in the PRPplus group. Compared to baseline, there was a statistically significant (P < 0.05) FLA reduction observed at all study visits in both groups, with the reduction observed in the PRPplus group significantly larger than that in the PRP group at week 48. ROD b-wave amplitude was significantly reduced to 46 ± 5% (P < 0.05) of baseline in the PRP group and 64 ± 6% (P < 0.05) in the PRPplus group. This reduction was significantly larger in the PRP group than in the PRPplus group (P = 0.024; t Test). Similar results were observed for the dark-adapted Combined Response (CR) b-wave amplitude, with a reduction at 48 weeks compared to baseline of 45 ± 4% in the PRP group and 62 ± 5% in the PRPplus group; the reduction in CR b-wave amplitude was significantly larger in the PRP group than in the PRPplus group (P = 0.0094). CR a-wave, oscillatory potentials, cone single flash, and 30 Hz flicker responses showed statistically significant within-group reductions, but no differences in between-group analyses. These results suggest that treating high-risk PDR with PRP plus IVR is effective for PDR control, and permits the use of less extensive PRP which, in turn, induces less retinal functional loss, in particular for rod-driven post-receptoral responses, than treatment with PRP alone.

[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.

Assessment of "non-recordable" electroretinograms by 9 Hz flicker stimulation under scotopic conditions

To refine methods of electroretinographical (ERG) recording for the analysis of low retinal potentials under scotopic conditions in advanced retinal degenerative diseases. Standard Ganzfeld ERG equipment (Diagnosys LLC, Cambridge, UK) was used in 27 healthy volunteers (mean age 28 ± SD 8.5 years) to define the stimulation protocol. The protocol was then applied in clinical routine and 992 recordings were obtained from patients (mean age 40.6 ± 18.3 years) over a period of 5 years. A blue stimulus with a flicker frequency of 9 Hz was specified under scotopic conditions to preferentially record rod-driven responses. A range of stimulus strengths (0.0000012-6.32 scot. cd s/m² and 6-14 ms flash duration) was tested for maximal amplitudes and interference between rods and cones. Analysis of results was done by standard Fourier Transformation and assessment of signal-to-noise ratio. Optimized stimulus parameters were found to be a time-integrated luminance of 0.012 scot. cd s/m² using a blue (470 nm) flash of 10 ms duration at a repetition frequency of 9 Hz. Characteristic stimulus strength versus amplitude curves and tests with stimuli of red or green wavelength suggest a predominant rod-system response. The 9 Hz response was found statistically distinguishable from noise in 38% of patients with otherwise non-recordable rod responses according to International Society for Clinical Electrophysiology of Vision standards. Thus, we believe this protocol can be used to record ERG potentials in patients with advanced retinal diseases and in the evaluation of potential treatments for these patients. The ease of implementation in clinical routine and of statistical evaluation providing an observer-independent evaluation may further facilitate its employment.

Quantification of optic disc edema during exposure to high altitude shows no correlation to acute mountain sickness

Background

The study aimed to quantify changes of the optic nerve head (ONH) during exposure to high altitude and to assess a correlation with acute mountain sickness (AMS). This work is related to the Tuebingen High Altitude Ophthalmology (THAO) study.

Methodology/Principal Findings

A confocal scanning laser ophthalmoscope (cSLO, Heidelberg Retina Tomograph, HRT3®) was used to quantify changes at the ONH in 18 healthy participants before, during and after rapid ascent to high altitude (4559 m). Slitlamp biomicroscopy was used for clinical optic disc evaluation; AMS was assessed with Lake Louise (LL) and AMS-cerebral (AMS-c) scores; oxygen saturation (SpO₂) and heart rate (HR) were monitored. These parameters were used to correlate with changes at the ONH. After the first night spent at high altitude, incidence of AMS was 55% and presence of clinical optic disc edema (ODE) 79%. Key stereometric parameters of the HRT3® used to describe ODE (mean retinal nerve fiber layer [RNFL] thickness, RNFL cross sectional area, optic disc rim volume and maximum contour elevation) changed significantly at high altitude compared to baseline (p<0.05) and were consistent with clinically described ODE. All changes were reversible in all participants after descent. There was no significant correlation between parameters of ODE and AMS, SpO₂ or HR.

Conclusions/Significance

Exposure to high altitude leads to reversible ODE in the majority of healthy subjects. However, these changes did not correlate with AMS or basic physiologic parameters such as SpO₂ and HR. For the first time, a quantitative approach has been used to assess these changes during acute, non-acclimatized high altitude exposure. In conclusion, ODE presents a reaction of the body to high altitude exposure unrelated to AMS.

Transcorneal electrical stimulation for patients with retinitis pigmentosa: a prospective, randomized, sham-controlled exploratory study

PURPOSE

To assess the safety of transcorneal electrical stimulation (TES) and explore its efficacy in various subjective and objective parameters of visual function in patients with retinitis pigmentosa (RP).

METHODS

Twenty-four patients in this prospective, randomized, partially blinded, good-clinical-practice study underwent TES (5-ms biphasic pulses; 20 Hz; DTL electrodes) 30 minutes per week for 6 consecutive weeks. The patients were randomly assigned to one of three groups: sham, 66%, or 150% of individual electrical phosphene threshold (EPT). Visual acuity (VA), visual field (VF; kinetic, static), electroretinography (Ganzfeld, multifocal), dark-adaptation (DA), color discrimination, and EPTs were assessed at all visits or four times, according to the study plan.

RESULTS

TES using DTL electrodes was tolerated well; all patients finished the study. Two adverse (foreign body sensation), but no serious adverse events were encountered. There was a tendency for most functional parameters to improve (8/18) or to remain constant (8/18) in the 150% group. VF area and scotopic b-wave amplitude reached statistical significance (P < 0.027 and P < 0.001, respectively). Only desaturated color discrimination and VF mean sensitivity decreased. There was no obvious trend in the 66% group.

CONCLUSIONS

TES was found to be safe in RP patients. Positive trends were discovered, but due to the small sample size of this exploratory study, statistical significance was reached only for VF area and scotopic b-wave amplitude. Further studies with larger sample sizes and longer duration are needed to confirm the findings and to define optimal stimulation parameters. (ClinicalTrials.gov number, NCT00804102.).

Subretinal electronic chips allow blind patients to read letters and combine them to words

A light-sensitive, externally powered microchip was surgically implanted subretinally near the macular region of volunteers blind from hereditary retinal dystrophy. The implant contains an array of 1500 active microphotodiodes ('chip'), each with its own amplifier and local stimulation electrode. At the implant's tip, another array of 16 wire-connected electrodes allows light-independent direct stimulation and testing of the neuron-electrode interface. Visual scenes are projected naturally through the eye's lens onto the chip under the transparent retina. The chip generates a corresponding pattern of 38 × 40 pixels, each releasing light-intensity-dependent electric stimulation pulses. Subsequently, three previously blind persons could locate bright objects on a dark table, two of whom could discern grating patterns. One of these patients was able to correctly describe and name objects like a fork or knife on a table, geometric patterns, different kinds of fruit and discern shades of grey with only 15 per cent contrast. Without a training period, the regained visual functions enabled him to localize and approach persons in a room freely and to read large letters as complete words after several years of blindness. These results demonstrate for the first time that subretinal micro-electrode arrays with 1500 photodiodes can create detailed meaningful visual perception in previously blind individuals.

[Vision restoration with implants in retinal degenerations]

Up until now there has been no available treatment for diseases causing the permanent impairment of retinal photoreceptors. Currently the development of the retinal prostheses is the earliest to promise a result that can be implemented in the clinical treatment of these patients. Implants with different operating principles and in various stages of progress are presented in details, highlighting the characteristics, as well as the Hungarian aspects of the development. This survey intends to provide an overview on retinal prostheses, implantable in case of degenerative diseases of the retina, by reviewing and assessing the papers published in relevant journals and based on personal experience. Developments in microelectronics in recent years made it possible and proved to be feasible to replace the degenerated elements in the retina with electrical stimulation. Multiple comparable approaches are running simultaneously. Two types of these implants are directly stimulating the remaining living cells in the retina. Hitherto the finest resolution has been achieved with the subretinal implants. Although the epiretinal implant offer lower resolution, but requires shorter surgery for implantation. Retinal implants in certain retinal diseases are proved to be capable of generating vision-like experiences. A number of types of retinal implants can be expected to appear in clinical practice a few years after the successful conclusion of clinical trials.

Restoration of useful vision up to letter recognition capabilities using subretinal microphotodiodes

Our group has developed a subretinal microphotodiode array for restoration of vision. In a clinical pilot study the array has been implanted in 11 patients suffering from photoreceptor degenerations. Here we present promising results from some of those patients where the retinal tissue above the chip was functional and the implant fulfilled its expected function. A spatial resolution of approximately 0.3 cycles/degree could be achieved with fine stripe patterns. In one subject where the implant had been placed directly under the macular region of the retina a visual acuity of 20/1000 could be measured. Artificially restored visual acuity of this quality has not been reported previously. Finally, we present images illustrating an approximation of how the visual perceptions might have appeared to the subjects, based on a mathematical model and patient reports.

Novel COL4A1 mutations associated with HANAC syndrome: a role for the triple helical CB3[IV] domain

The COL4A1 gene encodes the α1-chain of type IV collagen, which is ubiquitously expressed in basement membranes. Mutations in COL4A1 have been reported in autosomal-dominant porencephaly and in patients with symptomatic small vessel brain disease, inconstantly associated with eye defects. We have previously reported three COL4A1 mutations associated with a systemic phenotype that we called HANAC (Hereditary Angiopathy, Nephropathy, Aneurysms, and Cramps). We carried out a clinical and genetic study of three families presenting with characteristic features of HANAC syndrome. Common systemic signs included arterial retinal tortuosity and muscle cramps, with a variable combination of small vessel brain disease, Raynaud phenomena, and kidney defects. Three novel COL4A1 missense substitutions are described, which affect highly conserved glycine residues within the collagenous domain of the protein. All six known mutations associated with the HANAC phenotype are localized within the CB3[IV] fragment of COL4A1, which encompasses major integrin-binding sites. Our results confirm that HANAC syndrome is a distinct clinical entity within the COL4A1-related disorders, which is characterized by systemic involvement and usually asymptomatic brain disease. The restricted distribution of COL4A1 mutations within the CB3[IV] region is a characteristic of the reports of patients with HANAC, which suggests that abnormal cell-type IV collagen interactions may underlie the systemic defects observed in this syndrome.