Scleritis is an inflammatory disease that affects the sclera; it may be localized, nodular, or diffuse.  It may involve the anterior (visible segment) and/or posterior segments of the eye and manifests with redness of the eye and severe eye pain. [2, 3] Isolated posterior scleritis will not present with redness of the visible portion of the eye and may or may not present with pain.
The 4 types of anterior scleritis are as follows:
- Diffuse anterior scleritis: This is characterized by widespread inflammation of the anterior portion of the sclera. This is the most common form of anterior scleritis as well as the most benign.
- Nodular anterior scleritis: This type is characterized by one or more erythematous, immovable, tender inflamed nodules on the anterior sclera. Approximately 20% of cases progress to necrotizing scleritis.
- Necrotizing anterior scleritis with inflammation: This form frequently accompanies serious systemic collagen vascular disorders including rheumatoid arthritis.  Pain with this condition is usually extreme, and damage to the sclera is often marked. Necrotizing anterior scleritis with corneal inflammation is also known as sclerokeratitis.
- Necrotizing anterior scleritis without inflammation: This type most frequently occurs in patients with long-standing rheumatoid arthritis; it is due to the formation of a rheumatoid nodule in the sclera and is notable for its absence of symptoms. Necrotizing anterior scleritis without inflammation is also known as scleromalacia perforans.
Necrotizing anterior scleritis is the most severe form and most common form of scleritis with vision-threatening complications and resultant permanent visual loss. 
Posterior scleritis occurs much less frequently than anterior scleritis, but they may occur concurrently. It has been reported to mimic orbital cellulitis. 
In cases of non-necrotizing scleritis, vision is often maintained unless complications such as uveitis occur. 
The examiner must be cognizant of conditions that masquerade as scleritis such as toxoplasmosis-induced posterior uveitis and chronic lymphocytic leukemia. [8, 9] Episcleritis often presents similarly to scleritis, but inflammation and erythema is isolated to the episclera, which lies between the sclera and the conjunctiva.  Episcleritis has a more benign course and does not cause any visual changes or permanent impairment; however, recurrence is common.
The sclera, which consists of collagen and elastic connective tissue, provides a tough protective casing around the eye. Enzymatic degradation of collagen fibrils and invasion of inflammatory cells, including T cells and macrophages, appear to play an important role.
The thickness of the sclera varies from 0.3-1.2 mm. Healthy sclera is consistently white. Inflammation, the principal pathology affecting the sclera, is frequently part of a general inflammatory reaction associated with a systemic immune-mediated collagen vascular disease. [11, 12, 13]
Inflammation of the sclera can progress to ischemia and necrosis, eventually leading to scleral thinning and perforation of the globe. Necrotizing anterior scleritis represents a particularly destructive form of scleritis.
Scleritis is an uncommon disease. Well-defined incidence rates are hard to find. The prevalence is estimated to be 6 cases per 10,000 population. Of patients diagnosed with scleritis, anterior scleritis is demonstrated in 94% of patients, as opposed to posterior scleritis, which is diagnosed only 6% of the time.
An increased incidence of scleritis has been reported in patients taking bisphosphonates, which are commonly used in the management of osteoporosis. 
No particular geographic distribution has been noted.
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Morbidity arises from primary scleritis and associated systemic disease.
Giant pigment epithelial tear and retinal detachment has been reported in a patient with scleritis. 
A significant percentage of patients with concurrent scleritis and collagen vascular disease die within 5 years.
In 15% of cases, scleritis is the presenting manifestation of collagen vascular disorder and may precede additional symptoms by one to several months.
No published information is available on racial differences.
The female-to-male ratio is approximately 1.6:1.
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Cases have been reported in patients ranging from 11-87 years of age. Mean age for all types of scleritis is 52 years.
Episcleritis tends to affect younger patients.
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