Who would not appreciate a contact lens that connects to the internet and shows you all the information in real time like in the TV series Altered Carbon [1]? Nevertheless, some people have retinal implants or bionic eyes, because they cannot see without them. For example, individuals with neuronal degeneration of the retina have the possibility to get retinal implants to restore some of their vision. There are two diseases that cause the majority of retinal degeneration: retinitis pigmentosa (RP) and age related macular degeneration (ARMD).
Retinal Implants Can Restore Basic Greyscale Vision
RP is caused by mutations in more than 50 different genes, and thus can be inherited. Here, the translation of light to electrical signals by the rod photoreceptor cells is disrupted. The symptoms develop gradually, starting in most cases start with night blindness and loss of the peripheral vision. The symptoms affect both eyes equally. The disease progresses to affect the peripheral visual field and then the central area. Fortunately, only a few patients become completely blind [2].
In contrast to RP, in ARMD the central visual field is affected first. Macular degeneration mostly affects older people and does not lead to complete blindness. Nevertheless, loss of vision in the central visual field greatly affects the quality of life [3].
Fortunately, we live in the 21st century, so our research on biomedical devices is already quite advanced. Now, patients have the possibility to get retinal implants. These implants mostly work with glasses and a build-in external camera. The camera sends the information to a mini computer. Here the information is processed and send to a retinal implant via cable (see figure). The implant electrically stimulates the nerve fibres similarly to functioning photoreceptors. The current implants are able to restore basic greyscale light perception, so that patients are even able to identify objects [4].
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| Source: https://bit.ly/2I7HRyk |
The Bionic Eye
Argus II, also known as the "Bionic Eye", is the oldest model with an external camera. It has been approved for clinical application in Europe and the US. This device helps patients see patterns of light, which they learn to interpret in time [5].
Similar to the Argus II is the Boston Retinal Implant. The only difference is the amount of electrodes in the retinal implant. The Boston Retinal Implant has a higher resolution due to higher number of electrodes. This model is still being tested in animals and not yet in clinical studies [6].
The Intelligent Medical Implant (IMI) is also comparable to the Argus II. The difference and advantage to the Argus II is that the IMI can be individually calibrated upon implantation. This allows adjustment of visual perception for each patient. The IMI is not available for clinical applications yet, but is in clinical trial in Europe [7].
One disadvantage of all three models are the transscleral cables that connect the outside electronics with the retinal implant. Here, the risk of infections due to long term implantation is higher as for example with a different model, the Epi-Ret 3. This model is also a combination of an external camera and a retinal implant, but there is no external case or transscleral cables. The information is send wirelessly to the retinal implant, where it is translated to electrical stimulation. This is an advantage due to the missing transscleral cables, eliminating problems like infections or other postoperative complications. Epi-ret 3 is currently in clinical trials in Europe [8].
Getting Closer to Natural Vision
A very different model to the ones mentioned so far is the Alpha-IMS. The Alpha-IMS works without external cameras but instead has a retinal implant that consists of a 1500-pixel multiphotodiode array mimicing thereby very closely the natural vision. The light sensitive photodiode responds to light and sends a signal to a subdermal power control. This power control can be charged wirelessly by a handheld device that also adjusts the light sensitivity [9]. The model is currently under clinical trial in Europe and Hong Kong. Unfortunately, there have been cases with corrosion of the implant and subretinal bleeding [10, 11].
In 2014 a review analysed these devices and looked at the visual performance in patients [4]. They showed that the Alpha-IMS and Argus II have the best results, when it comes to visual performance. The best recorded visual acuity to date is 20/1260 of the Argus II [12]. Compared to perfect 20/20 vision, patients remain nearly blind, but they are able to see significantly more than without the implants.
The Alpha-IMS may have a better resolution in a smaller more focused central vision, but the Argus II has a bigger field of view. Similar to the Argus II, the Boston Retinal Implant has an array with more electrodes, and in theory this would give a higher resolution. However, there is still no data to back this up. In addition, these larger devices might create more heat.
In conclusion, there are good options for people with retinal degeneration. Though the presented retinal implants still have limited resolution and biocompatibility, future research will help improving these features. Unfortunately, for the privileged people with intact vision, retinal implants with a wifi connection (still) remain science fiction.
Larissa Kraus
PhD Student, AG Holtkamp
[1] Altered Carbon, Netflix, 2018
[2] https://bit.ly/2r9gZ5C
[3] https://bit.ly/1dBK8yC
[4] Chuang et al., Br. J. Ophthalmol, 2014
[5] https://bit.ly/1PwBZGY
[6] Rizzo, J. Neuro-Ophthalmology, 2011
[7] Keserü et al., Acta Ophthalmol, 2012
[8] Klauke et al., Investig. Opthalmology Vis, 2011
[9] Kusnyerik et al., Investig. Opthalmology Vis 2012
[10] Stingl et al., Proc. R. Soc. B Biol. Sci., 2013
[11] Zrenner, Sci. Transl. Med., 2013
[12] Humayun et al., Ophthalmology, 2012
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