- The sclera is the tough, opaque, fibrous outer layer of the eye commonly referred to as the “white of the eye.” It forms the posterior five-sixths of the outer tunic of the eyeball and serves as a protective shell that maintains the shape of the eye while providing an attachment site for the extraocular muscles.
- The sclera is continuous anteriorly with the cornea, which is the transparent portion of the eye that allows light to enter, and posteriorly with the optic nerve sheath. Its white, non-transparent nature contrasts sharply with the cornea, emphasizing its role in structural integrity rather than visual function.
- Composed primarily of dense connective tissue, the sclera is made up of collagen fibers, elastic fibers, and fibroblasts. These components are irregularly arranged, which is why the sclera is opaque, unlike the regularly aligned collagen in the cornea that allows for transparency. The thickness of the sclera varies throughout the eye, being thickest at the posterior pole near the optic nerve (around 1 mm) and thinnest just behind the insertion points of the extraocular muscles (as little as 0.3 mm). This structural variability supports both flexibility and resilience under pressure from intraocular forces.
- Although the sclera is generally avascular (lacking its own blood vessels), it is surrounded and nourished by adjacent vascular layers such as the episclera and the choroid. The episclera lies just beneath the conjunctiva and contains small blood vessels that may become visible during inflammation or infection, such as in episcleritis. Nerve fibers from the trigeminal nerve innervate the sclera, contributing to the sensation of eye pain when it is inflamed or injured.
- Functionally, the sclera acts as a shield for the delicate internal components of the eye, protecting them from mechanical trauma and helping maintain a consistent intraocular pressure. It also provides anchorage for the extraocular muscles, enabling precise control of eye movement. The tensegrity of the sclera helps maintain the proper shape and alignment of the optical elements of the eye, which is crucial for accurate focusing and vision.
- Scleral abnormalities or conditions can impact eye health significantly. Common disorders include scleritis, a painful and potentially vision-threatening inflammation often associated with autoimmune diseases, and episcleritis, a more benign and superficial condition. Scleral thinning or degeneration can occur in chronic inflammatory conditions or as a complication of surgery or trauma. In some cases, the sclera may appear bluish, especially in children or individuals with connective tissue disorders like osteogenesis imperfecta, due to the underlying pigmented choroid showing through the thin scleral tissue.
- In medical practice, the sclera’s visibility through the palpebral fissure makes it a useful diagnostic area. Yellowing of the sclera, known as scleral icterus, is a clinical sign of jaundice and may indicate liver dysfunction or hemolysis. Additionally, certain contact lenses and drug delivery systems are designed to interface with the sclera, taking advantage of its durability and non-sensitive surface.
