Retinal implants and prostheses are implantable devices intended to replace phototransduction in the eyes of patients with severe retinal disorders. Modern retinal implants are approximately 1cm in diameter, or about the size of a five-pence coin, and contain about 10,000 tiny electrodes. The gadget is surgically connected to the retina in the posterior segment of the eye.
All retinal implants are based on the replacement of rod and cone photoreceptor function in individuals with outer retinal degeneration. This principle is accomplished by stimulating the bipolar and ganglion cells of the retina's secondary neurons with the electrodes.
The system consists of five components: a digital camera, a video processing unit, a radio transmitter, a radio receiver, and a retinal implant. The camera catches the Image and sends it to an external processing unit worn on the body. The processing unit subsequently processes the data and transmits it via an outer wire to an implanted radio receiver. The receiver then transfers the signal to the retinal implant, which relays the impulses to the visual cortex, where the image is produced.
The eyes are the primary organ of visual perception; they take in the world's colors, contrasts, shapes, and motions. When someone loses their visual sense, they lose a connection to their surroundings. The two most common disorders that destroy retinal function and effects this connection are retinitis pigmentosa and macular degeneration. In the healthy eye, photoreceptors found in the outer layers of the retina contain a light-sensitive pigment that initiates the phototransduction cascade to generate neural impulses. Damage to these photoreceptor cells is the underlying mechanism in these disorders where, however, the inner retinal layers remain intact. Retinal implants collect and process incoming light and then communicate the information as electrical impulses to the surviving inner retinal layers for visual function. With the help of these modern retinal implants, patients can perceive light in a limited grayscale, facilitating their ability to complete routine visual tasks and even travel autonomously.
We can distinguish two types of implants based on the implantation site inside the posterior segment. The device could be positioned in the epiretinal area on the ganglion cell surface of the vitreous cavity or in the subretinal space containing the degenerated photoreceptors.
● Epiretinal implants are designed to interact with ganglion cells of the retina as they sit on the innermost layer of the retina and have several advantages:
● Less disruptive to the retina: The prosthesis touches the retina on an inner surface that is accessible from the vitreous, and the vitreous cavity provides space for surgical operations, thereby lowering the danger of mechanical retinal damage.
● Dissipation of heat: In addition to choroidal perfusion, fluid in the vitreous cavity serves as an additional heat sink, enhancing the removal of heat generated by the implant's electronics and reducing thermal dangers associated with continuous use.
● Direct stimulation to the ganglion cells: Its proximity makes it easier for the device to directly stimulate ganglion cells, which could be advantageous in cases of extensive retinal degeneration in which the inner retinal circuitry has been changed as it bypasses all other retinal layers to stimulate the ganglion cells layer directly.
Among the potential disadvantages of the epiretinal prosthesis are the following:
● Surgical Limitation: The challenge of implanting the device near the retina contains a definite risk.
●Distorted retinotopic map: It could occur due to the distorted visual perceptions caused by the unintended activation of the passing axons.
●Less accurate Image: When a person with such an implant changes the direction of their gaze, they perceive the item to be in motion, even though their eye movements do not alter the transmitted Image on the retina. Therefore, patients with these implants are advised to scan the visual field by turning their heads instead of using their natural eye movements.
Subretinal implants are placed between the pigment epithelium and the photoreceptor layer, directly activating retinal cells and depending on the regular processing of the inner and middle retinal layers.
Its advantages are:
●More straightforward design: It is beneficial over an epiretinal implant due in part to its simpler construction. Microphotodiodes on a single chip perform the light acquisition, processing, and stimulation processes.
●Accurate Image: Subretinal implants allow patients to use their natural eye movements. Because the incident light pattern on the micro photodiodes is a direct mirror of the desired Image, retinotopic stimulation via subretinal implants is intrinsically more accurate.
Its disadvantages are:
● Need for an external source: Subretinal implants frequently include an external power source to enhance the effect of incoming light.
● Thermal Injury: The subretinal space's compactness imposes retinal thermal injury caused by implant-generated heat.
● Limited utility: Subretinal implants require undamaged inner and middle retinal layers; hence, retinal disorders extending beyond the outer photoreceptor layer are ineffective candidates.
Patients with retinal disorders, such as retinitis pigmentosa or age-related macular degeneration, are ideal candidates for retinal implants. As discussed, These disorders cause blindness by damaging photoreceptor cells in the outer retinal layer while leaving the inner and middle retinal layers unaffected.
To qualify as an excellent candidate for retinal implant surgery:
● The patient should have an intact ganglion cell layer of the inner retina
● The patient should have an appropriate amount of residual vision
● The patient's family should be committed to their rehabilitation
● The patient should lack conditions like diabetes and glaucoma affecting his vision.
Author: Dr. Muhammad Saad, Resident Ophthalmologist at Al-Shifa Trust Eye Hospital in Rawalpindi, Pakistan