Background

Microelectronic retinal implants are an example of research underway to restore some vision to those with severe vision loss by replacing a defect or missing link along the visual pathway. Microelectronic retinal implants in development include subretinal devices, designed to replace photoreceptors in the retina, and epiretinal devices, designed to communicate directly with the ganglion and bipolar cells. The purpose of both types of implants is to restore some vision by electrically stimulating functional neurons in the retina. The implants are being developed to help people with degenerative diseases of the retina such as retinitis pigmentosa and macular degeneration. All retinal implants require an intact optic nerve pathway to allow them to function.

Research into the results of electrical stimulation of the retinal surface showed that sensation to light could be produced1-5 and that retinal neurons are preserved after death of photoreceptors in retinitis pigmentosa.6,7 Devices have been tested for feasibility and biocompatibility in animals.8-11

Technology

There have been two approaches to creating microelectronic retinal implants. The subretinal device or implant is a microelectrode array powered by approximately 3,500 microscopic solar cells. The implant, called the Artificial Silicon Retina™, is 2 mm in diameter and 0.001 inch in thickness. Dr. Alan Chow, an ophthalmologist, and Dr. Vincent Chow, co-founders of Optobionics™ Corporation [Wheaton, IL] developed the implant. Preliminary research was conducted in conjunction with Dr. Neal Peachey and his research group at the Edward Hines, Jr. Veterans’ Administration Medical Center in Chicago. The premise of the design is that current generated by the device in response to light stimulation will alter the membrane potential of overlying neurons and thereby activate the visual system. 9

The epiretinal implant differs from this approach. A research team at the Wilmer Eye Institute at Johns Hopkins University is working on an epiretinal device that will consist of several subsystems. The components will include a camera for image acquisition, image processing electronics, a telemetry system to provide power and data to the implanted subsystems, implanted electronics for signal decoding and stimulus generation, and an electrode array for delivery of charge to the retina. 12 Researchers at North Carolina State University and University of North Carolina, Chapel Hill, are members of this retinal implant project. 13

A research team from the Massachusetts Institute of Technology and Harvard University are working on an epiretinal device. Their project includes the use of a miniature laser to provide power and signal to the implanted photodiode array. The array will convert incident light into electrical current. Another component of the implant, the stimulator chip, will control the distribution of current. 10,14

Other research teams in the United States participating in various aspects of developing epiretinal implants include Wayne State University and Kresge Eye Institute15, and at Second Sight, an Alfred E. Mann company in California. 16 Two German research consortiums, directed by Drs. R. Eckmiller in Bonn and E. Zrenner in Tubingen, are working on both types of intraocular retinal implants. 17,18 A team of researchers in Japan is working on retinal implants as part of Japan’s Neuroinformatics Research in Vision project. 19

Developers of retinal implants hope that the devices will restore the ability to distinguish between light and dark, and to see dim outlines and shapes for patients blind from retinal degenerative diseases.

Results

There are no reports in the peer-reviewed literature of retinal implants in humans.
According to a company report, two patients had smaller versions of the Optobionics™ Corporation’s microelectronic retinal device implanted on June 28, 2000, at the University of Illinois at Chicago Medical Center.20 These two patients and a third who received an implant the following day, had lost almost all their vision because of retinitis pigmentosa. Their surgery was undertaken as a first step in the feasibility and safety studies in humans required by the US Food and Drug Administration before any clinical trials can be undertaken. The third patient received an implant the following day at a nearby hospital. There were no reported complications from the surgery.

Questions Regarding Clinical Application

  • Will these devices produce usable vision?
  • What are the surgical risks, and the long-term biocompatibility risks?
  • What are the advantages and disadvantages of epiretinal and subretinal implant location?
  • Will the brain learn to interpret the artificial electrical input and improve the induced sensory perception, as happened with cochlear implants?

Expert Comments

Charles P. Wilkinson, MD, Bethesda, MD: It is important to stress that all efforts are VERY preliminary, and that patients should not expect a genuine breakthrough for some time, if ever.

Thomas A. Weingeist, MD, PhD, University of Iowa: The questions regarding clinical application listed above are critical. As with the development of the cochlear implant we must be satisfied to move ahead incrementally step by step. Vision is not the perception of light alone nor is recognition of sound the same as hearing. With advances in computer technology, miniaturization of electrodes, and creation of improved microchips and computers, the prospect exists that future devices will be developed that will enable some blind patients useful vision. A multitude of approaches is likely to be taken to solve the problem. There is no expectation that a solution will occur soon. Even if it were possible to attach a miniature television camera to an individual’s head how practical would such a device be? Would it be acceptable? A great deal of scientific work must be done to achieve even this level of success. In the meantime physicians and patients should remain hopeful, but not be unrealistic or deceived by the hype that often accompanies each scientific disclosure.

Future Research

Retinal implants are at a very early stage of development. Additional basic research to prove the concept and clinical trials to demonstrate safety and effectiveness will probably take several years to complete.

*This information is designed to inform ophthalmologists and their patients of current clinical developments in a summary fashion, and does not reflect a position or policy of the American Academy of Ophthalmology or its Board of Trustees. This information is time-limited, and based solely on a summary review of articles available as of July 2000. The material provided is informational only, and is not intended to be a basis for diagnosis, treatment or any other clinical application.

References

Humayun MS, de Juan E Jr, Weiland JD, et al. Pattern electrical stimulation of the human retina. Vision Res 1999; 39:2569-76.

Humayun MS, de Juan E Jr, Dagnelie G, et al. Visual perception elicited by electrical stimulation of retina in blind humans. Arch Ophthalmol 1996;114:40-6.

Humayun MS, de Juan E Jr. Artificial vision. Eye 1998; 12:605-7.

Weiland JD, Humayun MS, Dagnelie G, et al. Understanding the origin of visual percepts elicited by electrical stimulation of the human retina. Graefes Arch Clin Exp Ophthalmol 1999; 237:1007-13.

Chow AY, Chow VY. Subretinal electrical stimulation of the rabbit retina. Neurosci Lett 1997; 225:13-6.

Santos A, Humayun MS, de Juan E Jr, et al. Preservation of the inner retina in retinitis pigmentosa. A morphometric analysis. Arch Ophthalmol 1997; 115:511-5.

Stone JL, Barlow WE, Humayun MS, et al. Morphometric analysis of macular photoreceptors and ganglion cells in retinas with retinitis pigmentosa. Arch Ophthalmol 1992; 110:1634-9.

Majji AB, Humayun MS, Weiland JD, et al. Long-term histological and electrophysiological results of an inactive epiretinal electrode array implantation in dogs. Invest Ophthalmol Vis Sci 1999; 40:2073-81.

Peachey NS, Chow AY. Subretinal implantation of semiconductor-based photodiodes: progress and challenges. J Rehabil Res Dev 1999; 36:371-6.

Rizzo JF, Wyatt JL. Prospects for a visual prosthesis. Neuroscientist 1997; 3:251-62.

Peyman G, Chow AY, Liang C, et al. Subretinal semiconductor microphotodiode array. Ophthalmic Surg Lasers 1998; 29:234-41.

Intraocular Retinal Prosthesis Group. Accessed July 17, 2000.

The Retinal Prosthesis Project at North Carolina State University. Accessed July 24, 2000.

The Retinal Implant Project. Harvard-MIT Collaboration. Accessed July 17, 2000.

Ligon Research Center of Vision. Accessed July 24, 2000.

The Alfred E. Mann Foundation for Scientific Research. Accessed July 24, 2000.

Eckmiller R. Learning retina implants with epiretinal contacts. Ophthalmic Res 1997; 29:281-9.

Zrenner E, Miliczek KD, Gabel VP, et al. The development of subretinal microphotodiodes for replacement degenerated photoreceptors. Ophthalmic Res 1997; 29:269-80.

Hybrid Retinal Implant. Accessed July 24, 2000.

Optobionics™ Corporation. First silicon chip artificial retinas implanted in blind patients. June 30, 2000.

Further information
The Foundation Fighting Blindness.
Intraocular Retinal Prostheses Group.
The Retinal Implant Project. Harvard-MIT Collaboration.
Ligon Research Center of Vision.
Second Sight, Robert J. Greenberg, President.
Optobionics™ Corporation.
EPI-RET Consortium, Germany.
Subretinal Implant Project, Germany.