Losing one of your senses is a frightening prospect for most people, but for many people, it is an unfortunate reality. Millions of people in the US face the prospect of irreversible changes to their ability to see the world around them.

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Worldwide, there are an estimated 39 million people living with blindness. Around 82% are aged 50 and above.

As you read this passage, the fact that you have the ability to see and read might not even cross your mind. There are a lot of people who are not as fortunate. According to the World Health Organization (WHO), 285 million people are estimated to be visually impaired worldwide. Of these, 39 million are blind and 246 million have low vision.

A recent poll conducted by Research!America and the Alliance for Eye and Vision Research (AEVR) found that Americans regarded the loss of vision as potentially having the biggest impact on everyday life, ranking it alongside cancer, Alzheimer's disease and HIV as one of the top four "worst things that could happen to you."

Unfortunately, losing eyesight is a common problem, be it due to the process of aging or the development of a specific condition. The good news is that 80% of visual impairment can be prevented or cured, but what of the remaining 20%?

In this Spotlight article, we take a brief look at retinal degeneration disorders - a group of related conditions that are presently without a cure. What treatments are currently being developed? And can researchers envision a future where sight can be restored to all patients?

Curable and incurable disorders

Most frequently, visual impairment is caused by uncorrected refractive errors (43%) or cataracts (33%). Refractive errors include myopia (short-sightedness), hyperopia (far-sightedness) and astigmatism, whereby the cornea or lens does not have a perfectly curved shape.

When visual impairment is caused by these problems, often treatment is readily available. Refractive errors can be corrected with glasses, contact lenses or refractive surgery. Cataracts - the clouding of the lens - are commonly treated with a surgical procedure that is among the most frequently carried out in the US.

While 80% of visual impairment can be prevented or cured, there remains 20% of cases for which there is currently no way of curing. A range of conditions exists where those who develop them are faced with a gradual loss of vision until their impairment is so severe that they are effectively blind.

Retinal degeneration disorders have no cure. These diseases break down the retina, the layer of tissue found at the back of the eye containing cells that detect light entering the organ.

There are a number of these degenerative diseases, including retinitis pigmentosa, macular degeneration and Usher syndrome. In particular, age-related macular degeneration is the leading cause of blindness in the developed world.

Medical News Today asked Dr. Raymond Iezzi, an ophthalmology consultant with the Mayo Clinic, what the biggest obstacles were to finding a cure for retinal degeneration disorders. He told us that scientists and clinicians face many challenges in developing treatments as there are several hundred biochemical abnormalities underlying theses disorders.

"Further," he added, "while there are several patterns of retinal degeneration, each is treated differently depending on the cells affected as well as the stage and severity of their degeneration."

When retinal degeneration conditions were first diagnosed, they were all labeled as retinitis pigmentosa. As knowledge in this area has improved, scientists have become aware that there is a variety of different related conditions, each affecting different areas of the retina with their own specific mechanisms.

In patients whose vision is still good, therapeutic approaches can be directed at neuroprotection or gene therapy.

"By protecting cells within the retina from death associated with the underlying biochemical disorder, we may preserve sight among large populations of patients," explained Dr. Iezzi. "A robust neuroprotection strategy would prevent cell death and vision loss, regardless of the underlying biochemical abnormality."

Gene therapy focuses instead on correcting the biochemical abnormalities that lead to the death of retina cells. This approach is highly specific, and Dr. Iezzi told MNT that several hundred treatments would need to be developed in order to treat the full range of retinal degenerative diseases.

Restoring and protecting vision with stem cells

The eye lends itself to experimental treatments, being easy to operate on and often protected from inflammatory responses that could disrupt therapies. Due to its accessibility, surgeons can also easily observe and track how new treatments are progressing.

Dr. Iezzi stated that at Mayo Clinic they are currently working on new methods to grow stem cells from a patient's own tissue samples; a regenerative approach that could one day lead to the restoration of sight to people that have lost it.

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Macular degeneration is a major cause of partial blindness among people aged over 50.

Embryonic stem cells could be used to build new retinal pigmented epithelial cells - cells that nourish retinal visual cells and absorb light - that could be transplanted into a patient.

Doing this could slow or prevent the loss of the visual cells, and while deriving new visual cells from embryonic stem cells could lead to even more pronounced results, researchers have found it more difficult to successfully derive these cells and transplant them into the retina.

Mouse studies have previously shown that this technique can work and that transplanted cells can integrate fully with the retina, restoring vision to the animals.

There are two types of photoreceptive cells in the retina - rod cells and cone cells. While rod cells are stimulated by light over a wide range of intensities, perceiving shape, size and brightness, it is the cone cells that perceive color and fine detail.

Researchers have managed to derive rod cells from embryonic stem cells and are currently working on deriving cone cells and transplanting them into animals. If these trials prove successful, the next step could be human trials.

Without adequate neuroprotection, however, newly transplanted cells may be just as vulnerable to retinal degeneration disorders as the cells they replace. This problem underlies the importance of research into gene therapy aiming to correct the biochemical abnormalities that lead to cell death.

There is a form of treatment, however, that has led to the restoration of vision in patients with advanced forms of retinal degeneration disorders. Just as prosthetics can be used to restore function to individuals that have lost limbs, so too can individuals who have lost their vision use retinal prostheses.

The miracle of retinal prosthesis

"In patients who have already lost their sight, our therapeutic goal is to restore vision," said Dr. Iezzi. "This has been successfully accomplished via the Argus II retinal prosthesis in patients with advanced retinitis pigmentosa."

Last month, MNT reported on the story of Allen Zderad, a man who was effectively blind but is now able to make out the outlines of objects and people thanks to his new retinal prosthesis.

Dr. Iezzi was the ophthalmologist that put Mr. Zderad forward for the procedure, making him the 15th man in the US to receive the life-changing device. He is now able to navigate through crowded environments - such as shopping centers - without the use of a cane.

A camera connected to a pair of glasses transmits visual information to a small chip attached to the back of the eye via a small computer worn in a belt pack. The chip can send light signals directly to the optic nerve, bypassing the damaged retina and providing the patient with visual information in the form of flashes of light.

Dr. Iezzi describes what the retinal prosthesis provides as "artificial vision," unlike any form of vision that patients will have experienced before. While this form of vision could be considered basic compared with what normal-sighted people are used to, it is a marked improvement for many without sight.

As he used his retinal prosthesis for the first time, Mr. Zderad described the artificial vision as "crude, but significant."

"While this technology restores rudimentary vision, eventually as these devices improve, we may one day be able to treat patients with advanced macular disease such as Stargardt macular dystrophy or age-related macular degeneration," Dr. Iezzi suggested.

The future prospects of such treatment are truly exciting, but it is likely to be some time before the benefits of this technological breakthrough are widely experienced. Currently, the device costs $144,000, and the WHO estimate around 90% of the world's visually impaired live in low-income settings.

Keeping your eyes healthy

Many people will be fortunate enough not to develop a retinal degeneration disorder, but that does not mean that their eyes are invulnerable to harm. A large amount of visual impairment can be prevented, but what measures can be taken to ensure that this happens?

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Regular comprehensive eye exams can detect eye disorders before any vision is lost.

The National Eye Institute (NEI) suggest a number of simple steps that can be taken to ensure your eyes are as healthy as they can be:

  • Do not smoke. Smoking has been associated with increased risks of developing cataracts, optic nerve damage and age-related macular degeneration
  • Eat a balanced diet and maintain a healthy weight
  • Clean your hands and contact lenses properly to reduce the risk of eye infection
  • Know you family's eye health history. Many serious eye disorders such as retinitis pigmentosa are hereditary and knowing your risk of developing such conditions can improve the level of treatment available
  • Have a comprehensive eye exam. The only way to be completely sure of the health of the eyes is to visit an eye care professional.

WHO state that over the past 20 years there has been significant progress in preventing and curing visual impairment in many countries around the world. In 2013, the World Health Assembly approved the 2014-19 Action Plan for the universal access to eye health. The aim of this plan is to reduce avoidable visual impairments by 25% by 2019.

While many people believe that the loss of vision is one of the health problems that could have the biggest impact on everyday life, it is encouraging to think that visual impairment is no longer as absolute a prospect as it once was.

Thanks to prosthetics, people who have lost legs can walk around unaided, with most people unaware that they have any limbs missing. It is not too hard to now envisage a future world in which people could be cured of blindness, with any hints of their previous disability remaining invisible.