Sclera is an avascular, dense fibrous tissue. It forms the posterior 5/6th part of the outer coat of the eyeball. Developmentally, it is derived from the neural crest. Its primary function is to protect the eye and maintain the shape of the eyeball.
Anteriorly, it is continuous with cornea at the limbus. The anterior most sclera near the limbus is marked by an indentation (furrow) on the inner surface called scleral sulcus. Posteriorly, it ends at the optic nerve canal. Its outer surface is covered by Tenon’s capsule which is separated from the sclera by an episcleral tissue. Its inner surface is separated from the choroid by the suprachoroidal space.
The thickness of sclera varies from one place to the other. It is thickest (1 mm) near the optic nerve and thinnest (0.3 mm) at the insertion of extraocular muscles. At limbus, the thickness of sclera is 0.83 mm.
Histologically, sclera can be divided into three parts from outward to inward (Fig. 7.1):
•Scleral stroma (sclera proper).
It is a thin, loose layer of connective tissue overlying the sclera and situated below the Tenon’s capsule. It is a densely vascularized layer. Apart from the vessels and nerves, it contains collagen, fibroblasts, and occasional melanocytes. Anteriorly, it is supplied by anterior ciliary arteries, while the posterior part of episcleral tissue is supplied by posterior ciliary artery.
■Scleral Stroma (Sclera Proper)
It is composed of collagen bundles, elastic fibers, fibroblasts, and ground substances (proteoglycans and glycoproteins). The collagen fibers in sclera are of varying sizes and are irregularly arranged. Due to this irregularity of collagen fibers, the sclera is opaque (cornea is transparent due to uniform orientation of collagen fibers). The fibroblasts in sclera play an important role in the synthesis of collagen, proteoglycans, and glycoproteins.
It is the inner most layer of sclera. The connective tissue of this layer is loosely arranged than the rest of the sclera and contains an abundance of melanocytes, mostly migrated from the choroid. It is separated from the choroid by a potential space known as suprachoroidal space.
The sclera is perforated by many nerves and vessels. Posteriorly, it is pierced by optic nerve and blends with dural and arachnoid coverings of optic nerve at its exit. The sclera is modified into a sieve-like membrane (lamina cribrosa) through which the optic nerve fibers pass. It is also penetrated by 8 to 20 short posterior ciliary arteries in a ring (circle of Zinn) which are accompanied by short ciliary nerves. Little anterior to these, two long posterior ciliary arteries and nerves also pierce the sclera to enter the eye ball. Posterior to the equator, it is perforated by four vortex veins which drain the veins of uveal tract. Anteriorly, the anterior ciliary arteries and veins penetrate the sclera 3 to 4 mm away from the limbus (Fig. 7.2).
It is almost avascular and gets its nourishment from the episclera and choroid.
Sclera is supplied by branches from long and short posterior ciliary nerves.
▃Inflammation of Sclera
The inflammation of sclera may be:
•Superficial or episcleritis.
•Deep or scleritis.
It is a benign, self-limiting but recurrent inflammatory disease affecting the episclera which lies between the Tenon’s capsule and the sclera proper.
Age and sex– It commonly affects young females.
Most of the time it is idiopathic. It may be associated with:
•Connective tissue diseases (rheumatoid arthritis, systemic lupus erythematosus [SLE], and relapsing polychondritis).
•Systemic vasculitic diseases (polyarteritis nodosa)
•Dermatologic disease (Rosacea)
•Metabolic disease (gout).
It is often regarded as a hypersensitivity reaction to an endogenous toxin.
The inflammatory response is localized to the superficial episcleral vascular network. Histopathology shows a nongranulomatous inflammation with vascular dilatation and perivascular lymphocytic infiltration of subconjunctival and episcleral tissues.
Simple or Diffuse Episcleritis
The large episcleral vessels run radially beneath the conjunctiva. Therefore, engorgement of these vessels in episcleritis results in sectoral or diffuse redness of one or both eyes. The pain is unusual, but if present, it is localized to the eye itself. It is the most common type and presents with sudden redness and generalized discomfort (Fig. 7.3a).
Nodular or Focal Episcleritis
It has a less acute onset and more prolonged course than simple episcleritis. It presents with a circumscribed nodule situated 2 or 3 mm from the limbus. The nodule is hard, tender, immovable, and often temporal in location. The overlying conjunctiva moves freely over it. It is traversed by deeper episcleral vessels which impart a bright red or salmon pink color to it. It is usually transient but has a tendency to recur. The nodule never ulcerates and leaves a slate-colored scar (Fig. 7.3b).
Occasionally, a fleeting but frequently repeated episcleritis (episcleritis periodica fugax) may be seen.
It must be differentiated from scleritis, conjunctivitis, and foreign body or granuloma.
•The episcleritis is differentiated from scleritis (Table 7.1) by:
•Phenylephrine 2.5% eye drops blanch conjunctival vessels, allowing the differentiation of conjunctivitis and episcleritis.
•The nodular episcleritis may be confused with congestion due to foreign body or granuloma which must be ruled out as the causes for the episcleral nodule.
The recurrent attacks of episcleritis may require systemic evaluation. The basic tests to order include:
Table 7.1 Differentiating features of episcleritis and scleritis
•Instillation of phenylephrine (10%) blanches the conjunctival and episcleral vessels. Therefore, patient’s eye redness improves after phenylephrine instillation.
•Diffuse or sectoral bright red or pink bulbar injection.
•Instillation of phenylephrine (10%) does not blanch the deep scleral vessels.
•In scleritis, there is violaceous hue.