How an injury to one eye can cause disease in the fellow eye

How an injury to one eye can cause disease in the fellow eye
Photo: https://www.cbsnews.com/

When we discuss lazy eye, we oftentimes mention several consequences that amblyopia has on the quality of our lives. Troubles reading, learning, seeing depth, etc., yet there is one larger reason why we should strive to the well-developed vision of both eyes. Just imagine a scenario, when the lazy eye gets so serious that it shuts off completely. Now, what happens if you injure your only functioning eye?

The immune system protects us against pathogens

The human immune system is crucial for our protection against pathogens. The immune cells in our bodies are trained to recognize the proteins that are foreign to our bodies, hence being able to act and protect us against viruses, bacteria, fungi, and protozoa. Should they be able to recognize what is foreign, they also must know the proteins that are native to us. Indeed, it is exactly what happens in our bodies during the maturation of our immune system.

Virtually all our proteins are presented to the immune cells so that they take note and do not act against them. In certain circumstances, however, due to various, frequently unidentified reasons, our immune system can act against proteins that are native to our bodies – referred to as autoimmune diseases.

The eye is an immune-privileged organ

Whilst most of our proteins are known to the immune cells of our bodies there are certain exceptions to that. Certain organs and tissues of our body, namely the brain, testes, placenta, fetus and the eye are limited by the normal immune response – and are referred to as immune-privileged organs and tissues. This makes them an ideal environment for studying certain mechanisms of treatment. On the other hand, though, this also makes them prone to certain conditions that we are about to discuss next. The eye, as an immune-privileged organ, possesses certain proteins that are not known to our immune cells. In normal physiological conditions, it is all well and good as due to the immune privilege of the eye immune cells cannot act against these proteins. However, in some pathological conditions, those proteins are exposed to the immune cells and the disease occurs.

Sympathetic ophthalmia

The disease known as sympathetic ophthalmia is a rare, bilateral, and vision-threatening condition that occurs due to trauma (or rarely surgery) in one eye. During the injury to the eye, previously unexposed proteins of the eye are exposed to the immune cells. In some rare instances, the immune system reacts in the fashion that leads to the immune reaction (inflammation) in the fellow (non-traumatized) eye. The autoimmune inflammatory reaction in the fellow (or both) eyes can happen days, weeks, months, years, or even decades after the initial injury. It affects different structures of the eye including, but not limited to, the optic nerve and retina.

What exactly triggers it and why it only occurs in a small minority of ocular trauma cases remains poorly understood, as do the ways to prevent it. It is clear, though, that the immune privilege of the eyes is somewhat lost when the proteins of the eye are exposed to the immune cells during trauma to the eye. The anti-inflammatory treatment should be initiated as soon as possible, although the prognosis is generally poor as only about half of patients will end up with 20/40 or better vision.

Perhaps in every bad there literally is good, as Louise Braille, the inventor of a modern Braille writing system for visually impaired is believed to had lost his vision in both his eyes due to sympathetic ophthalmia 2 years after he had suffered an injury to one of his eyes. Without him, we probably would not know the Braille as we do today.

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