Abstract and Introduction
Abstract
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.
Introduction
Retinal implants elicit visual perceptions by electrical stimulation of retinal cells. They are primarily designed for patients suffering from degenerative retinal disease such as retinitis pigmentosa (RP) where outer retinal cells deteriorate while inner retinal cells stay intact a longer time and can be used to transmit electronically generated signals to the brain. Our group has proposed a device based on microphotodiodes which are implanted into the subretinal space and deliver electrical pulses by transforming light energy into electrical energy. However, this energy is insufficient to excite retinal cells even under bright light. Therefore, additional energy has to be supplied to such implants, for example via a cable connection. To this purpose, our group has developed a compound visual prosthesis with connections from a subretinal microphotodiode array (MPDA) to an extracorporeal connector in the retro-auricular space via a transchoroidally, trans-sclerally implanted polyimide foil. A transcutaneous cable passage to an extracorporeal connector can certainly only be a transitional step, but for the limited duration of our pilot study (implantation was granted for 4 weeks) a direct cable connection offered us several important advantages. More importantly, this allowed us to directly assess parameters such as charge transfer and impedance of individual subretinal electrodes, which are paramount for the understanding of the interaction of electrodes and neural tissue. To our knowledge, these data have not been gathered before from subretinal electrical stimulation in long-term experiments. In addition to that, several control parameters of MPDA function were directly measurable and adjustable in these first experiments which proved to be crucial for optimisation of these functions.
For implantation of a prosthesis with a transchoroidal, trans-scleral connection, our group has established a surgical approach in animal tests. The experience gained from these successful experiments led to optimisation of the technique that would be suitable for humans. Of high importance were tension relief points along the implant's extraocular course to prevent transduction of movements of the extraocular to the subretinal parts which had resulted in frequent retinal tears and subsequent detachments in research animals.
We report here on the success of the extraocular surgical approach to connect the subretinal device via a transchoroidal and transcutaneous cable to an extracorporeal connector in seven human volunteers suffering from RP.