Electrifying Results: Restoring Partial Color Vision in Retinitis Pigmentosa Patients with Retinal Implants

(Posted on Thursday, December 21, 2023)

This story is part of a series on the current progression in Regenerative Medicine. This piece is part of a series dedicated to the eye and improvements in restoring vision. 

In 1999, I defined regenerative medicine as the collection of interventions that restore tissues and organs damaged by disease, injured by trauma, or worn by time to normal function. I include a full spectrum of chemical, gene, and protein-based medicines, cell-based therapies, and biomechanical interventions that achieve that goal.

Color blindness is a visual deficiency that impairs the ability to distinguish specific colors. This condition can become an independent ailment or a result of retinal diseases such as retinitis pigmentosa (RP). It is a degenerative disease that progressively destroys the light-sensitive cells in the retina, leading to varying levels of vision loss. 

Retinitis pigmentosa is a disease that can cause total blindness over time. The disease affects the cells in the retina that detect light and color and can lead to color blindness. As the disease progresses, the person’s vision can become more limited, resulting in “tunnel vision,” where only a tiny circle in the center of their vision is visible. Color blindness and other visual impairments caused by retinitis pigmentosa can also occur.

Retinal implants have recently gained attention as a potential treatment for RP-induced color blindness. These implants restore vision by bypassing the non-functioning photoreceptor cells and stimulating the remaining functional cells. While they are not perfect solutions for individuals with color blindness as a primary disorder, they show promising results in treating retinitis pigmentosa, which can cause it in its advanced stages.

What is a Retinal Implant?

Retinal implants have emerged as a revolutionary treatment option for various retinal diseases, including retinitis pigmentosa (RP), an inherited disorder that causes blindness. These devices can replace the functions of deteriorated phototransducing cells present in the retina.

Two major types of retinal implants have been developed: bioelectronic and photovoltaic. Bioelectronic implants capture light from an image, transform it into electrical impulses, and transmit it to the remaining cells in the retina. This type of implant is designed to stimulate the inner retinal neurons and has shown promising results in restoring partial vision in patients. These bioelectronic implants convert visual information into electrical signals that the brain can interpret, bypassing the damaged cells in the retina.

The second type of implant, photovoltaic retinal prostheses, employs light as the source of stimulation. These prostheses can potentially restore high-resolution responses to single-pixel stimulation in blind retinas. Photovoltaic implants use tiny solar cells to power up the implant and stimulate the retina, which enables color vision. Photovoltaic implants also involve fewer electrical wires and lower energy requirements, which allows them to operate for longer durations without disturbing the patient’s comfort levels.

Regarding color blindness, photovoltaic retinal implants have the edge over bioelectronic implants. As the latter relies on detecting electrical impulses and transmitting them to the brain, it may not be as effective in restoring normal color vision in those with vision impairments. On the other hand, the photovoltaic implant efficiently converts light energy into electrical energy, stimulating the remaining healthy cells in the retina and restoring color vision entirely.

Feasibility of Retinal Implants for Color Blindness Due to Retinitis Pigmentosa

Numerous studies have explored the feasibility of using electrical stimulation strategies to provide chromatic information to patients to partially restore color perception. These studies have shown promising results and could significantly improve the quality of life.

A: This image shows the placement of an intraocular implant in Subject 3’s eye, with a close-up of the retinal area near the electrode array. B-C: These fundus images are from the implant eye of Subject 4 (B) and Subject 5 (C), who reported blue/purple sensations at higher stimulation frequencies. D-E: These images are from the implanted eye of Subject 6 (D) and Subject 7 (E), who did not report blue/purple sensation at higher stimulation frequencies. F: The perceptual threshold of 19 electrodes in blue-sensing subjects versus 15 in non-blue-sensing subjects. This suggests that individual differences in perception may be related to electrode placement.

A study published in ScienceDirect found that electrical stimulation of the retina of blind RP patients resulted in the restoration of partial color vision, specifically along the blue-yellow axis. The researchers used a retinal implant that delivered controlled electrical currents to the residual inner retina, which allowed the perception of chromatic content. The study involved 12 patients; all reported seeing blue-yellow colors after activating the retinal implant. Although the restored vision was imperfect, the researchers were encouraged by the results and believed this technology could improve prosthetic eyes for RP patients.

The second study, published in Nature, explored using a photovoltaic retinal prosthesis to restore high-resolution responses in blind retinas. The researchers created a photovoltaic pixel measuring 0.1mm², implanted onto the retina to stimulate the remaining cells. The study involved five blind patients. All reported seeing flashes of light when the photovoltaic pixel was activated. The researchers found that the responses to the single-pixel stimulation were high-resolution, indicating that the photovoltaic prosthesis could be an effective treatment option for RP patients.

However, despite the potential benefits of retinal implants in partially restoring color perception in RP patients, the current success rate is still limited. Certain device risks associated with retinal implants include overstimulation, which could cause damage to the retina, and the delamination of implanted components. Additionally, previous retinal implants have produced poor results, and wearers are still considered legally blind.

Further research and development are needed to improve the effectiveness of retinal implants for color blindness in RP patients. New implant technologies, such as optogenetic approaches, which use photosensitive proteins to restore light sensitivity to retinal cells, may offer alternative or complementary approaches to current retinal implants.

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