Sclera Definition: White of The Eyeball
The sclera is commonly known as the white of the eye. It surrounds your entire eyeball, except for the cornea (the clear covering in the front of your eye). The cornea is continuous with the sclera. The area where the sclera meets the cornea is called the limbus.
The sclera forms more than 80 percent of the surface area of the eyeball. Several layers comprise the sclera. These include (in order from outermost to innermost layer):
- Episclera
- Stroma
- Lamina fusca
- Endothelium
The bulbar conjunctiva covers the front part of your sclera. This is a thin layer of tissue with blood vessels.
The word "sclera" comes from the greek "sklēros" which means hard. The plural is sclerae.
Purpose & Function of The Sclera
The sclera is a tough, fibrous, opaque tissue. It, along with the intraocular pressure (IOP) of the eye helps maintain the shape of your eyeball. The sclera also protects the inner structures of your eye from trauma.
Most of your eyeball is filled with a gel-like fluid called the vitreous humor. Your extraocular muscles, which control your eye movements, also attach firmly to the sclera.
Sclera Problems
Several conditions can affect the sclera. Some problems may appear to affect the sclera but actually affect the overlying bulbar conjunctiva. The appearance can be misleading because the bulbar conjunctiva is mostly transparent.
Sclera conditions include:
Episcleritis
Episcleritis is an inflammation of the outermost layer of the sclera. Symptoms include:
- Redness
- Mild pain
- Swelling of the conjunctiva
- Eyelid swelling
- Raised nodules on the eye.
Most cases are sectoral, meaning only a section of the sclera is affected. In diffuse cases, the entire white part of the eye can be affected. About 26 to 36 percent of those with episcleritis have an associated systemic condition such as rheumatoid arthritis, lupus, Crohn’s disease, Behcet’s disease, or gout.
Scleritis
Scleritis is inflammation of the sclera. Its presentation can be similar to episcleritis. But it also causes significant pain, light sensitivity, and possibly vision loss.
Scleritis appears as a deeper red (almost violet) than episcleritis. Approximately 30 percent of people with scleritis have collagen vascular diseases. These include rheumatoid arthritis, lupus, and ankylosing spondylitis.
This condition is more severe than episcleritis and may cause permanent damage to the eye if left untreated.
Scleromalacia Perforans
Scleromalacia perforans is a rare and severe condition. Your immune system attacks the tissue of the sclera, causing significant thinning and bulging.
Because of thinning, the underlying uveal tissue is visible. This gives the sclera a bluish appearance. In rare cases, the eye can rupture. Scleromalacia perforans is often associated with older women with long-standing rheumatoid arthritis.
Scleral Icterus (Jaundice)
Scleral icterus is jaundice that affects the eyes. It gives the whites of the eyes a yellow appearance. The name is not entirely accurate because it affects the conjunctiva, not the sclera. It is sometimes known as conjunctival icterus.
However, many doctors still refer to this condition as scleral icterus. This condition occurs when your body has high bilirubin levels. Bilirubin is a yellowish substance that is produced from the breakdown of red blood cells. Liver disease or other serious health conditions can cause high bilirubin levels.
Scleral Plaques
Scleral plaques are blue-gray calcified plaques that appear on the sclera. They are related to old age. They are benign and do not cause any symptoms.
Blue Sclera
Blue sclera occurs when the sclera is thinned. This exposes the dark, uveal tissue underneath, which is where the sclera gets its blue appearance.
The cause may be congenital (present at birth). Osteogenesis imperfecta can cause it, along with bones that break easily. Blue sclera is also associated with Marfan syndrome, Ehlers-Danlos syndrome, and Loeys-Dietz syndrome. Acquired blue sclera (developed later in life) may be a sign of severe iron deficiency.
Types of Congenital Scleral Abnormalities
There are other conditions present at birth that may affect the sclera. Some of these are harmless. Others are more severe and may affect vision or eye health.
Ocular melanosis
Ocular melanosis is the pigmentation of the conjunctiva overlying the sclera. They typically look like flat, brown spots on the white part of the eye, similar to a freckle on the skin. Melanosis can be congenital or acquired.
There are two acquired forms, primary and secondary acquired melanosis.
The primary form is more common in light-skinned individuals and may grow in size. In rare cases, primary acquired melanosis may develop into malignant (cancerous) melanoma.
The secondary form is associated with hormonal changes, radiation exposure, chemical exposure, or metabolic disorders. People with darker skin and hair are more likely to have secondary acquired melanosis. This form rarely develops into melanomas.
Nevus of Ota (Oculodermal Melanocytosis)
Nevus of Ota is a congenital condition. It causes hyperpigmentation of the eye, eyelids, and other surrounding areas. The pigment often appears blue or brown.
Typically, the nevus does not require treatment. Some people desire laser therapy to lighten the spots for cosmetic reasons. Nevus of Ota is associated with a higher risk of glaucoma and, in rare cases, melanoma of the eye.
Ectasia (Staphyloma)
Ectasia is a congenital thinning and bulging of the sclera.
The thinning usually occurs near the limbus and also affects the cornea. The staphyloma often presents with a blue appearance. Staphylomas can also develop on the back of the eye, particularly in people with high myopia (nearsightedness).
Colobomas
Colobomas occur when there is some tissue missing in the eye at birth.
This results in the incomplete formation of a particular structure of the eye. Colobomas can affect the eyelid, iris, lens, optic nerve, choroid, and retina. Optic nerve, choroidal, or retinal colobomas can cause scleral thinning and staphylomas.
Types of Sclera Procedures & Surgeries
Scleral Expansion
Scleral expansion is a somewhat controversial procedure designed to treat presbyopia. Presbyopia is the loss of the ability to focus up close with age.
The surgery involves inserting four plastic pieces into the sclera, between the eye muscles. This technology is still improving, and some researchers feel that this procedure can become the gold standard in presbyopia treatment.
Scleral Buckle
A scleral buckle procedure is performed to repair a retinal detachment.
During the surgery, the doctor uses cryopexy (freezing therapy) to seal your retina to the eye. Then, the surgeon places a silicone or plastic band around your eyeball (this is the scleral buckle). This helps the retina stay in place. The buckle remains in your eye permanently.
