|Graefe's Archive for Clinical and Experimental Ophthalmology
Incorporating German Journal of Ophthalmology
|© Springer-Verlag 2004|
Helmut G. Sachs1 , Thomas Schanze2, Marcus Wilms2, Andreas Rentzos2, Ursula Brunner1, Florian Gekeler3 and Lutz Hesse4, 5
|(1)||University Eye Clinic, University of Regensburg, Franz Josef Strauß Allee 11, 93042 Regensburg, Germany|
|(2)||Applied Physics-NeuroPhysics Group, Department of Physics, Philipps University Marburg, Marburg, Germany|
|(3)||Department of Neuroophthalmology, University Eye Hospital, University of Tuebingen, Tuebingen, Germany|
|(4)||Department of Ophthalmology, Philipps University Marburg, Marburg, Germany|
|(5)||Klinikum am Gesundbrunnen-Augenklinik, SLK-Kliniken Heilbronn GmbH, Heilbronn, Germany|
|Helmut G. Sachs
Received: 10 March 2004 Revised: 8 August 2004 Accepted: 13 September 2004 Published online: 1 December 2004
Millions of people worldwide suffer photoreceptor loss due to degenerative retinal diseases . In Germany alone, 17,000 people become blind every year , and to date there is no effective treatment or cure. About 50% of all cases of blindness are caused by damage to the retina , and this population includes patients with macular degeneration and retinitis pigmentosa—conditions that cause progressive degeneration of the outer retina.
A number of approaches, including gene therapy and pharmacological measures, are currently being pursued in the hope of preventing blindness [2, 29]. However, once vision is lost, only two approaches have shown promise for restoring vision: retinal transplantation  and bioelectronic visual prostheses [1, 3–9, 11–14, 16–18, 20, 21, 23–26, 28, 30–32, 34–37].
In this latter category, two kinds of retinal implants are under development. Epiretinal implants are designed to stimulate retinal ganglion cells with an electrode array that is implanted on top of the inner limiting membrane. Subretinal implants are inserted between the pigment epithelium layer and the outer layer of the retina and attempt to stimulate intact remaining retinal cells by photodiode-generated currents. Previous experiments have shown that microphotodiode arrays (MPDAs) do not deliver sufficient energy to stimulate the retina .
The objective of the present study was to implant polyimide film electrodes subretinally in anaesthetised cats and to record the intracortical responses evoked by subretinal electrical stimulation in order to draw conclusions concerning the feasibility of this approach.
All surgical and electrophysiological procedures were performed in Marburg with anaesthetised cats. The procedures complied with the guidelines of the European Communities Council Directive (86/609/EEC) and were approved by an official German Animal Care and Use Committee. In addition, the Principles of laboratory animal care (NIH publication No. 85-23, revised 1985), the OPRR Public Health Service Policy on the Humane Care and Use of Laboratory Animals (revised 1986), the U.S. Animal Welfare Act, as amended, and the Association for Research in Vision and Ophthalmology (ARVO) guidelines were followed.
Three adult cats (body weight 3–4 kg) received atropine sulphate (0.03–0.05 mg/kg) to reduce salivation. Anaesthesia was induced by intramuscular injection of a mixture of ketamine hydrochloride (Ketanest, 10–20 mg/kg) and xylazine hydrochloride (Rompun, 1–2 mg/kg). During eye surgery the cats were fixed on their back in a modified Horsely–Clarke support and anaesthesia was maintained with ketamine hydrochloride (Ketanest, 1–10 mg/kg). Local anaesthetics (lidocaine, bupivacaine) were used in addition. The level of anaesthesia was controlled by monitoring body respiration, ECG, and reflexes. Body temperature was maintained at about 38°C.
For cortical recording the cats were positioned in a standard Horsely–Clarke support. Anaesthesia was maintained by ventilation with N2O/O2 (70%/30%) and halothane (0.3–0.8%) (n=1) or isoflurane (0.5–1.5%) (n=2). Continuous monitoring of rectal temperature (38°C), end-expiratory CO2 (3.8–4.2%), ECG, EEG, and reflexes was used to control the level of anaesthesia. In addition to general anaesthesia, local anaesthetics (lidocaine) were applied during the skull surgery required to insert up to seven fibre electrodes into the primary visual cortex.
Electrical stimulation was performed in three cats with computer-generated sequences of voltage waveforms and fast voltage–current converters with zero offset current . The applied stimulation currents consisted of short charge-balanced rectangular impulses (400 s) with amplitudes ranging from 10 to 100 A. The currents were applied by the subretinally placed film electrodes against a distant large reference electrode located at the rectal body temperature sensor.
Cortical recordings in the primary visual cortex were performed with up to seven quartz-isolated platinum–tungsten electrodes (80 m diameter) placed in locations corresponding to retinal stimulation sites using an Eckhorn electrode drive . Three types of signals were extracted: (1) single unit activity; (2) multiple unit activity; and (3) local field potentials. Alternatively, broadband cortical signals were band-passed (1–4000 Hz) and stored on hard disk/CD for subsequent data analysis. For details see ref. .
Averages of the cortical responses evoked by focal electrical retinal stimulation were computed in order to assess electrical retinal stimulation. Cortical activation distributions were then computed according to the method described by Schanze et al. . To estimate the temporal resolution of cortical responses to retinal stimulation by a waveform approach, all values were renormalised to a retinal eccentricity of 2° visual angle. In this case the duration of the first cortical excitatory deflection was estimated.
All three implantations were performed successfully. In all three feline left eyes operated, the film electrodes were completely implanted subretinally in the area centralis.
By comparison with human eyes, the feline retina is considerably more difficult to detach surgically. The partial detachment required for subretinal placement of the stimulation film was achieved in all three eyes. Manipulation of the subretinal film for submacular placement caused unintentional perforation of the retina out of the subretinal space in one animal. Satisfactory submacular placement was achieved in this case by retracting and readvancing the film. Once the film is in a subretinal position and has been stabilised with PFCL, the extraocular part of the stimulation film can be carefully fixed to the sclera without causing dislocation. This permits further manipulation of extraocular film components during the stimulation procedure, such as connecting the film to the stimulation unit with a microplug. During all three experiments neither displacement of the film nor any other adverse events, especially inflammatory reactions, were detected over a 12-h period. At the end of the experiments the stimulation unit was disconnected and the films were mobilised and withdrawn. PFCL was removed and the sclerostomy, conjunctiva and canthotomy were sealed. All cats regained consciousness without problems within 2 h after skull closure. No adverse events were detected for the operated eyes during the 3-month follow-up period.
The ocular surgery for implanting subretinal film-bound stimulation arrays in feline eyes was developed by applying surgical techniques and instruments normally used in humans for routine pars plana vitrectomy and buckle surgery.
Low-threshold stimulation is important for the safety of retinal tissue. Stimulation thresholds around 50 A were measured in our experiments approximately 2 to 5 h after retinal electrode implantation. The surgical trauma might be an explanation for the relative high thresholds of around 50 A. A successful retinal stimulation may have a lower threshold in an intact retina or might be assumed if the retina were allowed to recover for a longer period. The observed differences in the animals may be caused by different retinal conditions shortly after surgery and seems to be worth looking at in further experiments. The calculated charge densities were approx. 2 mC/cm2 for the platinum-coated titanium nitrite electrode with its fractal surface geometry and are therefore in a reasonable range.
The measured spatial and temporal resolutions of cortical responses evoked by subretinal electrical stimulation were in the same range as those reported for epiretinal stimulation with fibre or polyimide film electrodes [16, 27].
These experiments have demonstrated the feasibility of film-bound subretinal electrical stimulation. The model suggests a variety of possible further experiments to determine electrical and technical parameters in subretinal visual prosthesis development. Because of the accumulated wealth of knowledge concerning the feline visual system and the availability of the model, it appears reasonable to use this experimental design to collect extensive and essential data in preparation for electrical subretinal stimulation experiments in humans.
On anatomical grounds, the cat may not be the ideal model for the development of surgical techniques in connection with subretinal implants. However, the feline model does provide extensive additional information if interest is focused not only on the surgical aspects but also on the desired function of a stable implant. Therefore the feline model should not be rejected in the context of visual prosthesis development.
Our results further indicate that short focal subretinal stimulation evokes localised spatio-temporal cortical responses. While coarse restoration of vision should be feasible with the subretinal implant, further experiments are necessary to determine the potential and limitations of this visual prosthesis approach.
M, Girvin JP, Hambrecht FT, Kufta CV, Loeb GE, Schmidt EM (1990) Visual
sensations produced by intracortical microstimulation of the human
occipital cortex. Med Biol Eng Comput 28:257–259
EL, Rosner B, Sandberg MA, Hayes KC, Nicholson BW, Weigel-DiFranco C,
Willet W (1993) Vitamin A supplementation for retinitis pigmentosa.
Arch Ophthalmol 111:1456–1459
|3.||Brindley GS (1973) Sensory effects of electrical stimulation of the visual and paravisual cortex in man. In: Jung R (ed) Handbook of sensory physiology, vol. 7, sect 3B. Springer, Berlin Heidelberg New York, pp 583–594|
GS, Lewin WS (1968) The sensations produced by electrical stimulation
of the visual cortex. J Physiol (Lond) 196:479–493
|5.||Chow AY, Chow VY (1997) Subretinal electrical stimulation of the rabbit retina. Neurosci Lett 225:13–16
AY, Pardue MT, Chow VY, Peyman GA, Liang C, Perlman JI, Peachy NS
(2001) Implantation of silicon chip microphotodiode arrays into the cat
subretinal space. IEEE Trans Neural Syst Rehabil Eng 9:86–95
|7.||Dobelle WH (2000) Artificial vision for the blind by connecting a television camera to the visual cortex. ASAIO J 46:3–9
WH, Mladejovsky MG, Girvin JP (1974) Artificial vision for the blind:
electrical stimulation of visual cortex offers hope for a functional
prosthesis. Science 183:440–444
|9.||Dobelle WH, Mladejovsky MG, Evans JK, Roberts TS, Girvin JP (1976) Braille reading by a blind volunteer by visual cortex stimulation. Nature 259:111–112
R, Thomas U (1993) A new method for the insertion of multiple
microprobes into neural and muscular tissue, including fiber
electrodes, fine wires, needles and microsensors. J Neurosci Methods
|11.||Eckmiller R (1995) Towards retina implants for improvement of vision in humans with retinitis pigmentosa—challenges and first results. In: Proc WCNN 95, Washington DC. INNS Press, New Jersey, pp 228–233|
|12.||Eckmiller R (1997) Learning retina implants with epiretinal contacts. Ophthalmic Res 29:281–289
|13.||Eckmiller R, Eckhorn R et al (1994) Final report of the feasibility study for a neurotechnology program. In: Eckmiller R (ed) Neurotechnology report. BMBF, Bonn, Germany|
|14.||Girvin J (1988) Current status of artificial vision by electrocortical stimulation. Can J Neurol Sci 15:58–62
H, Kobuch K, Kohler K, Nisch W, Sachs H, Stelzle M (2002) Biostability
of micro-photodiode arrays for subretinal implantation. Biomaterials
L, Schanze T, Wilms M, Eger M (2000) Implantation of retina stimulation
electrodes and recording of electrical stimulation responses in the
visual cortex of the cat. Graefes Arch Clin Exp Ophthalmol 238:840–845
M, Propst R, de Juan E, McCormick K, Hickingbotham D (1994) Bipolar
surface electrical stimulation of the vertebrate retina. Arch
MS, de Juan E, Dagnelie G, Greenberg RJ, Propst RH, Phillips DH (1996)
Visual perception elicited by electrical stimulation of retina in blind
humans. Arch Ophthalmol 114:40–46
MS, Prince M, de Juan E Jr, Barron Y, Moskowitz M, Klock IB, Milam AH
(1999) Morphometric analysis of the extramacular retina from postmortem
eyes with retinitis pigmentosa. Invest Ophthalmol Vis Sci 40:143–148
MS, Weiland JD, Fujii GY, Greenberg R, Williamson R, Little J, Mech B,
Cimmarusti V, Van Boemel G, Dagnelie G, de Juan E Jr (2003) Visual
perception in a blind subject with a chronic microelectronic retinal
prosthesis. Vis Res 43:2573–2581
|21.||Ito N, Shirahata A, Yagi T, Matsushima T, Kawase K, Watanabe M, Uchikawa Y (1997) Development of artificial retina using cultured neural cells and photoelectric device: a study on electric current with membrane model. Proceedings of The 4th International Conference on Neural Information Processing (ICONIP 97), pp 124–127|
|22.||Krumpazsky HG, Klauss V (1996) Epidemiology of blindness and eye disease. Ophthalmologica 210:1–84
|23.||Normann RA, Maynard EM, Rousche PJ, Warren DJ (1999) A neural interface for a cortical vision prosthesis. Vis Res 39:2577–2587
|24.||Normann RA, Maynard EM, Guillory KS, Warren DJ (1996) Cortical implants for the blind. IEEE Spectrum 33:54–59
|25.||Rizzo JF, Wyatt J (1997) Prospects for a visual prosthesis. Neuroscientist 3:251–262|
|26.||Rizzo JF, Loewenstein J, Kelly SK, Shire DB, Herndon T, Wyatt JL (1999) Electrical stimulation of human retina with a microfabricated electrode array. Invest Ophthalmol Vis Sci 40:S783|
T, Wilms M, Eger M, Hesse L, Eckhorn R (2002) Activation zones in cat
visual cortex evoked by electrical retina stimulation. Graefes Arch
Clin Exp Ophthalmol 240:947–954
|28.||Schmidt EM, Bak MJ, Hambrecht FT, Kufta CV, ORourke
DK, Vallabhanath P (1996) Feasibility of a visual prosthesis for the
blind based on intracortical microstimulation of the visual cortex.
|29.||Sharma S (1999) Ophthaproblem. Ocular ischemic syndrome. Can Fam Physician 45:901, 909
|30.||Stieglitz T, Blau C, Beutel H, Keller R, Meyer JU (1997) Konzeption und Entwicklung von flexiblen Stimulatorstrukturen innerhalb eines Retina Implant Systems [Conception and development of flexible stimulator structures within a retinal implant system]. Biomed Tech (Berl) 42[Suppl]:458–459|
|31.||Tassiker GE (1956) US patent 2,760,783|
C, Raftopoulos C, Mortimer JT, Delbeke J, Pins D, Michaux G, Vanlierde
A, Parrini S, Wanet-Defalque MC (1998) Visual sensations produced by
optic nerve stimulation using an implanted self-sizing spiral cuff
electrode. Brain Res 813:181–186
|33.||Wilms M (2001) Electrical receptive fields and cortical activation spread in response to electrical retina stimulation. Assessment of spatio-temporal resolution for a retina implant. PhD thesis, Department of Physics, University of Marburg, Germany|
|34.||Wyatt J, Rizzo J (1996) Ocular implants for the blind. IEEE Spectrum 33:47–53
|35.||Zrenner E (2002) Will retinal implants restore vision? Science 295:1022–1025
E, Miliczek KD, Gabel VP, Graf HG, Guenther E, Haemmerle H, Hoefflinger
B, Kohler K, Nisch W, Schubert M, Stett A, Weiss S (1997) The
development of subretinal microphotodiodes for replacement of
degenerated photoreceptors. Ophthalmic Res 29:269–280
E, Stett A, Weiss S, Aramant RB, Guenther E, Kohler K, Miliczek KD,
Seiler MJ, Haemmerle H (1999) Can subretinal microphotodiodes
successfully replace degenerated photoreceptors. Vis Res 39:2555–2567