The sclera or sclera is a part of the eye and spans a large portion of the eyeball. It has predominantly a protective function.
What is the sclera?
The sclera spans almost the entire eye and shimmers white through the conjunctiva. For this reason, it is less commonly referred to as the white skin of the eye. A thin sclera can cause a slightly bluish tint to the eye. This phenomenon occurs especially in infants. As the area cribrosa, the sclera begins at the point of entry of the optic nerve at the back of the eyeball. At this point, the sclera has fine openings through which several blood vessels pass. In addition, the sclera and Tenon’s capsule join at this point. The Tenon capsule demarcates the sclera from the outside and separates it from the surrounding fatty tissue. As a result, the Tenon capsule allows the eye to move freely in different directions. At the front of the eye, the sclera surrounds the ocular cornea, forming a bulge called the scleral bulge or sulcus sclerae.
Anatomy and structure
The sclera is composed of several layers: In the center is the substantia propria, which consists of collagen connective tissue. It is tensioned and held in shape by the internal pressure of the eye. Above the substantia propria lies the lamina episcleralis as the second layer. It is permeated by numerous blood vessels and thus plays an important role in the supply of nutrients and oxygen. Towards the inside, the substantia propria is surrounded by another tissue layer, the lamina fusca. The lamina fusca is very thin and contains pigments. In addition, the lamina fusca provides the connection to the choroid below the sclera, through which the majority of the blood vessels of the eyeball flow.
Function and Tasks
The main function of the sclera is to protect the eye. It shields the eye from mechanical impact and direct sunlight and provides stability. The sclera primarily protects the choroid beneath it, which contains numerous delicate blood vessels. In order not to obstruct the blood supply, there are openings in the sclera for blood vessels or connecting veins to pass through. This is especially the case in the anterior part of the eye, where the sclera forms the scleral bulge on the cornea. Numerous blood vessels run through the scleral bulge at the junction between the cornea and the sclera. They play an important role in supplying nutrients to the other parts of the eye. The sclera also has a function as an indicator of the general state of health: various diseases can be inferred from its coloration. In the case of liver disease or infection with jaundice, the otherwise white sclera turns white-yellowish to deep yellow. This discoloration is not a disease of the eye, but an early sign of another disease. After treatment of the cause, the sclera turns white again. Diseases that typically cause yellowish discoloration of the sclera other than jaundice are hepatitis, alcohol disorders, and malnutrition or undernutrition. The substance bilirubin is responsible for the yellow discoloration. It is formed during the breakdown of red hemoglobin, which colors the blood red. Dark spots in the sclera provide evidence of the tyrosine metabolic disease alkaptonuria.
Diseases
In the majority of cases, the typical diseases of the sclera are inflammations. Physicians commonly refer to these inflammations as scleritis. If only the uppermost layer of the sclera is inflamed, it is episcleritis-named after the outermost layer of the sclera, the lamina episcleralis. Scleritis is usually triggered by another disease that affects other parts of the human body. Autoimmune diseases such as rheumatism or gout are among the most common triggers. Here, for example, the body’s immune system mistakes the collagen connective tissue of the sclera for a potentially harmful substance and attacks it. The symptoms of inflammation, such as swelling, redness and itching, are a result of this attack by the body’s own immune system. In rarer cases, inflammation of the sclera may be the result of a spatially restricted infection. Micro-injuries to the eye can trigger such a local infection when bacteria enter the wound. Other infectious diseases may also appear as a result of inflammation of the sclera. Lyme disease can also cause corresponding tissue damage as Lyme disease.Lyme disease is an infectious disease caused by bacteria in humans and some animals. The most common carriers of these bacteria are ticks, and less frequently certain types of mosquitoes. In the case of shingles (herpes zoster), infection with the corresponding virus causes inflammation in affected areas of the body. If the sclera or another part of the eye or face is affected, physicians refer to the infection as zoster ophthalmicus. Infection of the eye with the herpes zoster virus carries the risk of permanent blindness because the cornea can become cloudy or otherwise damaged as the disease progresses. Syphilis can also lead to inflammation of the sclera. This sexually transmitted disease was a widespread and feared infectious disease from the Middle Ages to modern times. However, syphilis can be treated well nowadays with the help of antibiotics. In addition, blood poisoning (sepsis) can also lead to inflammation of the sclera. Blood poisoning is a so-called systemic inflammatory reaction that attacks numerous organs at the same time.
