The sclera comprises a group of tissues that closely resemble the tough, fibrous tissue seen in the joints of vertebrates. Only the vertebrate population has a sclera, the invertebrates demonstrating an ocellus , which differs by being strengthened by cartilage and bone. Much like a hip joint, the ocular globe rests in a soft tissue socket covered with collagen rich tissue simulating synovia . Because of this unique anatomical relationship, it is subject to many of the same inflammations and disorders that often befall the joints of the human body. Because it is tough and fibrous it resists deformation and provides a protective covering for the intraocular contents, though it is not without risk of perforation or penetration.
The sclera is opaque and appears white due to the high (68%) water content and disorderly array of transverse and oblique collagen (75%) fibers that make up its coat. (Figure1). It is this same array that lends the strength and rigidity to the globe despite its constant movement and pull from the extraocular muscles. Unlike cornea, which has a very orderly array of parallel bundles that are layered on top of one another, the sclera is designed for strength and not clarity. If the water content of sclera is reduced below 40%, it begins to appear clear like the cornea. The sclera may assume pigmentation such as the yellow discoloration of aging due to lipid deposition or blue, grey or brown spots due to melanin, the example of excess being the Nevus of Ota .
The circular shell created by the sclera ranges in size from 20 to 24 mm with an average being 22 mm and a radius of curvature of approximately 12 mm. Gradations on either end of the extreme are seen in conditions such as microphthalmia and megalophthalmos . It is generally one mm thick, and is thinner in the equator and under the insertions of the recti muscles. It is an open sphere posteriorly due to the scleral foramen required for optic nerve penetration, and anteriorly due to the cornea. Actually, the cornea and the sclera are one and separated by the vague transition known as the limbus . Anatomically, the limbus is a zone which ends with Bowman's membrane anteriorly, and the canal of Schlemm posteriorly. The surgical limbus usually refers to the corneoscleral junction, a blue zone, which becomes evident upon removal of the overlying conjunctiva.
The sclera does not demand much in terms of blood supply or perfusion of nutrients, due to its relatively low metabolic need. As a result, the healing of wounds, either surgical or from trauma, can take months or years to complete, if they heal at all. The sclera is penetrated by numerous small, arteries, veins and nerve fibres which pass through openings known as emissaria . The interior surface of the sclera is brown, due to melanocytes which form an inner coating. The inner side of the sclera is nourished by the choroid .
The scleral structure is protected by a thin fibrovascular surface coating known as the episclera . The role of the episclera is to provide a matrix for an extensive arteriolar plexus that feeds these tissues above and below. On top of this structure lays Tenon's capsule , an avascular layer of collagen tissue, which acts like a lubricating coating to the underlying tissue. The conjunctiva, a mucus membrane coating, overlies the entire structure. All of the tissues in contact with the avascular sclera are subject to respond with inflammation.
The diseases which affect the sclera are collagen destructive, painful, chronic, result in marked inflammation, and are often localized manifestations of disease processes affecting the entire body. The episclera is affected by acute processes which are usually short lived.
Episcleritis is a common inflammatory condition seen in younger patients. The majority of etiologies remain elusive. Associated generalized conditions include allergy, collagen-vascular disease and infection, but these probably comprise less than thirty percent of cases. The pathology of episcleritis demonstrates non-specific inflammatory cells with lymphocytes. Most patients have an acute onset with redness, aching, and a sense of warmth. Pain can be significant and usually is localized to the inflamed section of the eye, either nasal or temporal. The overlying conjunctiva can also be swollen and red, revealing the crisscrossing pattern of the underlying congested episcleral vessels. Simple episcleritis is self-limited, resolving within weeks. It may respond to topical steroid and decongestant-astringent eyedrops.
In nodular episcleritis, swelling is localized to one small section of the globe, creating the appearance of a true nodule, which is freely mobile over the underlying sclera. Its course is much more ominous and rarely resolves without continued therapy, sometimes requiring oral steroids and anti-inflammatory medications. Recurrent attacks of episcleritis are quite common.
i. With inflammation
ii. Without inflammation
Scleritis is a much more severe and chronic disease. Its onset can be quite rapid and startling, leading to severe pain, loss of vision and even globe perforation. It is usually seen in the older patient, over the age of forty. Early diagnosis and treatment are important to the successful management of the disease. Unlike episcleritis, which usually does not affect the underlying sclera, in scleritis, the episclera is always involved in the inflammatory process. Thus the patient presents with a red eye that can rapidly progress to a more fulminant course. Often, both eyes are involved. Almost half of the patients in our practice with a diagnosis of scleritis had a underlying connective tissue disease. The most common diagnosis is rheumatoid arthritis in the severe necrotizing form, which is less often associated with diffuse anterior or nodular scleritis.
The pathology of diffuse scleritis involves the anterior areas of the sclera which are penetrated by the vessels. This is presumably due to the circulating antibody-antigen complexes that incite this type of inflammatory process. The underlying vascular pattern can be distorted. Granulomatous scleritis is associated with a fibrinoid necrosis typical of granulation tissue, including lymphocytes, plasma cells and giant cells destroying scleral fibers. Nodular scleritis demonstrates multiple nodules which are more discrete. Necrotizing scleritis is the most serious form of the disease which can lead to scleral thinning and perforation. In women, necrotizing scleritis can be seen without inflammation and is often associated with polyarticular rheumatoid disease.
Scleritis can be posterior, which can be difficult to diagnose unless the anterior sclera is involved, and is often associated with exudative retinal detachment. Associated edema and hemorrhages of the retina or optic nerve can be seen. Treatment requires topical, local and systemic anti-inflammatory medication. It is not uncommon for the disease to progress to scleral thinning, uveitis, loss of vision, cataract and glaucoma.
c. Progressive Myopia
Degenerations of the sclera occur commonly in response to aging and disease. As we age, the eyes take on a lackluster appearance due to the reduction in tear flow and a loss of episcleral tissue. Lipids get deposited between the scleral fibers, and the globe can assume a yellow appearance. Frequently there are also deposits of calcium causing whitish plaques. Following severe inflammations of the sclera, amyloid deposition may occur.
If progressive myopia is considered within the context of a disease, then the thinning of the scleral wall may be considered the underlying pathological defect. Why this occurs is still not fully known, however, genetic predisposition, eye-rubbing and prolonged exposure to nighttime lighting have all been suggested as causes.
5. Staphyloma and Ectasia
Ectasias occur when the sclera distorts outward. When uvea is involved it is referred to as staphyloma. Staphylomas can occur in response to inflammation. The location of the staphyloma predisposes it to pathologic risk. Anterior and intercalary staphylomas are frequently associated with retinal detachment and glaucoma.
6. Granulomatous Inflammations
The etiology of most cases of granulomatous inflammation is not known. Connective tissue diseases are common in a third of cases. In exogenous inflammations, the causative agent may be mycotic or pyogenic . Steroid treatment in these disorders would be contraindicated. Infections in the area of retinal detachment scleral bands, stitch abscesses and foreign bodies can all lead to granulomatous inflammation. We have seen cactus thorns and insect parts as the underlying causative agent in traumatic inflammations.
In endogenous inflammation , the causative factor can be due to infectious agents including, herpes, syphilis, and tuberculosis, though true infection of the sclera is rare. More often, nonspecific granulomatous scleritis can occur and as with other forms of scleritis, is seen most commonly in the connective tissue disorders.
True vasculitis of the sclera does not occur, as the sclera itself is devoid of its own vascular supply. The sclera can be affected secondarily in vasculitis, as in associated inflammatory diseases of the uvea or secondary to panophthalmitis.
Metabolic disease can affect the sclera. Deposition of uric acid crystals, as seen in gout , can lead to the appearance of an episcleritis or scleritis. Cystine can be deposited in cystinosis , although no known signs of inflammation are detected. Homogentisic acid metabolism, altered in alkaptonuria , can lead to pigmentation of the sclera. High levels of circulating calcium , and defects in fat metabolism can lead to depositions in the sclera. The most familiar deposition is the abnormal accumulation of billirubin as seen in the icterus of jaundice.
There are no known primary tumors of the sclera. Metastatic disease is the most common involvement of the sclera, as can be seen in breast or lung cancer in its late stages. The sclera, because of it's tough fibrous structure, prevents the spread of intraocular tumors. Unfortunately, tumors which affect the uveal tract, such as melanomas, myleomas and sarcoid, can spread through the vascular conduits and nerve bundle perforations of the sclera.
10. Pigmented Abnormalities
The thinning of the sclera that can be seen in newborns and young children, can give rise to a bluish tinge due to the appearance of the underlying uvea and choroid through the transparent sclera. Melanotic pigmentation of the sclera is often seen in the pigmented races as a normal occurrence. Technically, this is not congenital, as it appears following birth. Small cuffs of melanin can be seen surrounding the vascular channels. When this normal pigmentation is associated with a neural vascular bundle it is known as an Axenfeld's loop . Appearance in a white individual of pigmentation of the sclera, may represent intraocular disease, such as malignant melanoma .