Restoring sight via a whole-eye transplant is not yet a medical reality. In late 2023, surgeons performed the world’s first such procedure. While the transplanted eye has shown signs of health, like direct blood flow, the patient has not regained vision. This highlights a key distinction: transplanting an entire eye is currently impossible, but transplanting specific parts of the eye is a common and successful practice.
The Challenge of the Optic Nerve
The primary obstacle to a whole-eye transplant is reconnecting the optic nerve to the brain. The optic nerve is a complex bundle of over a million nerve fibers that transmit visual information from the retina to the brain. When severed during transplantation, this link is broken.
The core difficulty is that central nervous system nerves, including the optic nerve, do not naturally regenerate when cut. Unlike nerves in the limbs, these axons do not regrow to re-establish precise connections within the brain. No reliable method currently exists to make these fibers regrow and map themselves correctly.
Another challenge is ensuring the donor eye’s viability. The retina has a high metabolic demand, and interrupting its blood supply can cause irreversible damage. While surgeons have successfully restored blood flow in a transplanted eye, doing so quickly enough to prevent harm remains a hurdle.
What Eye Transplants Are Possible Today
While a whole-eye transplant for restoring vision is not yet feasible, transplanting specific eye tissues is a well-established field. The most common procedure is the cornea transplant, or keratoplasty. The cornea is the transparent outer layer at the front of the eye that helps focus light. This procedure has been performed for over a century and helps thousands of people each year.
The success of corneal transplants is due to the cornea’s biology. It lacks blood vessels, a feature known as avascularity. This means immune cells, which travel through the bloodstream and attack foreign tissue, have limited access to the transplanted cornea. This reduces the risk of immune rejection compared to other organ transplants.
Corneal transplants are used to treat conditions that cause the cornea to become cloudy or misshapen, distorting vision. These include:
- Damage from an injury
- Scarring from infections
- Diseases like keratoconus, where the cornea bulges outward
- Fuchs’ dystrophy, which affects the cornea’s inner layer
Surgeons can perform a full-thickness transplant or replace only the specific diseased layers. Other, less frequent procedures involve transplanting the sclera (the white part of the eye) or amniotic membrane to repair damage.
The Path Toward Whole-Eye Transplantation
Making a vision-restoring whole-eye transplant a reality is an active area of medical research focused on optic nerve regeneration. The recent surgery that kept a transplanted human eye viable provided insights into how the organ survives without vision being restored. This case serves as a platform for future research.
Much of the progress is being made in animal models, where researchers have had some success stimulating nerve fiber regrowth in rodents. These studies explore strategies like using neurotrophic substances (nerve growth factors) to encourage severed axons to regrow. Gene therapy is another avenue, where viruses deliver genetic material into retinal cells to switch on growth programs that are dormant in adults.
Researchers are also tackling immune rejection, which remains a threat even if nerve regeneration becomes possible. Developing more effective immunosuppressant medications is part of this effort. The goal is to create a protocol that protects the transplanted eye from the immune system long enough for nerve reconnection to occur. These steps in medicine and immunology are paving the way forward.
Alternatives to Transplantation for Vision Restoration
While scientists work toward whole-eye transplantation, other technologies are available or in development to restore sight. These alternatives bypass the need for organ donation and nerve regeneration by working with the patient’s existing visual system. They offer hope for individuals with conditions that a transplant could not fix.
One technology is the retinal prosthesis, or “bionic eye,” designed for people with specific forms of blindness like retinitis pigmentosa. This system uses a camera on glasses to capture images, which are converted into electrical pulses. These pulses are sent to a retinal implant, stimulating the remaining healthy cells to send signals to the brain. This allows the user to perceive patterns of light and shapes.
Stem cell therapy targets diseases that cause the death of retinal cells, like age-related macular degeneration. The approach involves cultivating stem cells in a lab and guiding them to become retinal cells. These new cells are then transplanted into the eye to replace the ones lost to disease, aiming to rebuild damaged tissue and improve function.
Gene therapy is also showing promise for inherited retinal diseases caused by a single faulty gene. In this treatment, a corrected version of the gene is delivered into the retinal cells. This allows them to function properly, halting or even reversing vision loss.