Introduction
Degenerations of the cornea are common conditions that, in most cases, have relatively little effect on ocular function and vision. These conditions occur with increasing age, as a result of past inflammation, and with long-term toxic effects of environmental exposure. Unlike corneal dystrophies, corneal degenerations are not inherited, may be unilateral or bilateral, and often are associated with corneal vascularization. Degenerations tend to involve the peripheral cornea and may overlap the limbus and conjunctiva.
The conditions that occur in the corneal periphery are discussed first, followed by the conditions that occur more centrally. This is an arbitrary division as many conditions, such as spheroidal degeneration or band keratopathy, can be found in either or both locations.
Corneal Arcus (Arcus Senilis)
Corneal arcus presents as a gray-to-white, occasionally yellow, band of peripheral corneal opacification. It consists of fine dots, has a clear zone (clear interval of Vogt) between it and the limbus, and has a diffuse central border with a sharper peripheral border ( Fig. 4.22.1 ). It begins superiorly and inferiorly and spreads to involve the entire periphery. Deposits begin in the deep stroma with progression to involve the superficial stroma. Arcus is almost always bilateral but can be asymmetric in unilateral carotid vascular disease (decreased arcus) or chronic ocular hypotony (increased arcus).
Arcus is the most common corneal degeneration. In men, the frequency increases with age and occurs in essentially all men older than 80 years of age. The presentation is delayed by 10 years in women.
Arcus deposits consist of extracellular corticosteroid esters of lipoproteins, mostly of low density. Lipid material leaks from limbal capillaries with central flow being limited because of a functional barrier to the flow of large molecules in the cornea.
Strong evidence exists for an association with increased plasma cholesterol and low-density lipoprotein cholesterol, particularly when it occurs in men younger than 50 years (arcus juvenilis). Men with arcus juvenilis have a fourfold increased relative risk of mortality from coronary heart disease and cardiovascular disease. Arcus in young men, therefore, is a useful clinical indication for the need for lipid and cardiovascular evaluation. Young patients who have arcus have an increased risk for type IIa dyslipoproteinemia but a decreased risk for type IV. In older patients, arcus does not correlate with mortality.
Lipid Keratopathy
Lipid keratopathy may be peripheral, central, or diffuse and is similar in appearance to arcus. It occurs mainly in a secondary form, but rarely may be seen in a primary form. Primary lipid keratopathy has features of a corneal dystrophy, is usually bilateral, and the central lipid, often with cholesterol crystals, may severely decrease vision.
Secondary lipid keratopathy appears as a white or yellow stromal deposit separated by a narrow, clear zone from corneal stromal neovascularization ( Fig. 4.22.2 ). It often is denser than arcus and may appear as a circular deposit at the end of long-standing stromal vessels. It can follow corneal edema, as in hydrops. Histological evaluation shows that the material consists of intra- and extracellular lipids.
Lipid deposition may occur secondary to systemic lipid processing disorders. Defects in esterification of cholesterol and in lipoprotein scavenging have been implicated in lecithin cholesterol acetyltransferase (LCAT) deficiency, Fish-eye disease, and Tangier disease. Disorders of high-density lipoprotein function appear to allow accumulation of cholesterol centrally within the cornea.
Vogt’s White Limbal Girdle
Vogt was the first to describe two types of limbal girdle—white, arc-like opacities in the cornea central to the limbus in the 3 o’clock and 9 o’clock positions. Type I is a mild, early form of calcific band keratopathy. Type II lacks a peripheral clear zone and consists of fine, white radial lines, located nasally more often than temporally ( Fig. 4.22.3 ). The prevalence of this condition increases with age to essentially 100% in those older than 80 years of age.
Histologically, Vogt’s limbal girdle type II is made up of hyperelastotic and hyaline deposits peripheral to Bowman’s layer, similar to those seen in a pingueculum and pterygium.
Senile Corneal Furrow Degeneration
Furrow degeneration is a painless bilateral thinning of peripheral cornea. A peripheral corneal furrow can occur between corneal arcus and the limbus in older adults. Often the clear area may appear to be furrowed, but it was considered to be falsely thinned by Vogt. Rarely, true thinning with no inflammation, vascularization, or induced corneal astigmatism can occur in this region. Furrow degeneration does not require any therapy, but the location and degree of thinning should be evaluated when considering location for cataract incisions.
Terrien’s Marginal Corneal Degeneration
This condition is described in Chapter 4.17 .
Peripheral Corneal Guttae
The corneal endothelium undergoes degeneration with age, as manifested by a decreasing endothelial cell density and thickening of the posterior, nonbanded layer of Descemet’s membrane. Degenerating endothelial cells produce localized nodular thickening of Descemet’s membrane, known as guttae . The relationship of central guttae to Fuchs’ corneal endothelial dystrophy (FD) is discussed elsewhere (see Chapter 4.21 ). Peripheral guttae, known as Hassall–Henle warts, are visible in normal adult corneas and are thought to be truly degenerative and unrelated to FD. They are not associated with functional corneal changes.
Calcific Band Keratopathy
Band keratopathy is a common corneal degeneration that can occur at any age, presenting in the central or peripheral cornea. It most commonly occurs secondary to chronic corneal diseases, particularly uveitis, advanced glaucoma, keratitis, or trauma; primary idiopathic forms rarely occur. Band keratopathy can present secondarily with elevated serum calcium or phosphate in systemic diseases, including sarcoidosis, hyperparathyroidism, vitamin D toxicity, metastatic neoplasm to bone, and chronic renal failure with secondary hyperparathyroidism. Researchers have described a toxic form resulting from mercurial preservatives in pilocarpine and an acute form after intracameral tissue plasminogen activator injection. In children, band keratopathy may be the presenting sign of chronic uveitis as a result of juvenile idiopathic arthritis. Band keratopathy may occur after localized corneal damage from intraocular silicone oil and phosphate forms of corticosteroids.
Histologically, calcium is deposited as the hydroxyapatite salt in the epithelial basement membrane, basal epithelium, Bowman’s layer, and anterior stroma. The mechanism of calcium deposition in the cornea is unknown but occurs primarily in the exposed area of the cornea. Deposition of calcium may result from the precipitate tears leave when they evaporate or because the exposed cornea is at a lower pH than other areas. The most severely affected areas are the middle and inferior thirds of the cornea, which are the areas of greatest exposure to the atmosphere.
Corneal calcium deposits as a horizontal band and begins near the corneal periphery ( Fig. 4.22.4 ). As the deposits move centrally, the band has clear void circles where Bowman’s layer is traversed by nerve endings. The deposits begin as a gray haze and can become densely white with a rough, pebbly surface.
Patients can have symptoms of pain, foreign body sensation, recurrent corneal erosions, and decreased vision. The rate of development and progression of disease is variable. If band keratopathy causes persistent pain or decreased vision, corneal surgery is indicated. The surgery involves removing the epithelium over the deposits, applying 0.05 mol/L disodium ethylenediaminetetraacetic acid to chelate the calcium and dissolve it, and using a brush or surgical blade to remove remaining calcium. The process can take a few minutes to an hour, depending on the density of the calcium. Excimer laser phototherapeutic keratectomy also may be used to remove the calcium deposits.
Spheroidal Degeneration
Spheroidal degeneration may occur in the cornea or the conjunctiva. Other names for this degeneration include climatic droplet keratopathy, hyaline degeneration, and local designations, such as Labrador keratopathy. Spheroidal degeneration occurs as a primary corneal form, a secondary corneal form, and a conjunctival form. The frequency of this degeneration increases with age and varies with geographical location—occurring most often in areas that have high sunlight exposure (snow or sand) and high winds. It is twice as prevalent in men as in women. The prevalence varies from 6% in England to greater than 60% in Labrador. It is thought to be a result of ultraviolet light exposure and may be associated with blue-light exposure. Other risk factors are drying of the cornea and repeated corneal trauma. Secondary forms can occur with corneal scars, lattice dystrophy, and glaucoma.
Spheroidal degeneration presents as fine droplets, yellow or golden in color, beneath the epithelium ( Fig. 4.22.5 ). The droplets appear oily, although they are not composed of lipid. In the primary form, droplets begin peripherally and advance centrally between the palpebral fissures. As the condition advances, the droplets become larger, more nodular, and more opaque lifting the central corneal epithelium. Three stages of the primary form have been described:
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Grade I—fine shiny droplets are present only peripherally without symptoms.
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Grade II—the central cornea is involved and vision ≥20/100 (6/30).
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Grade III—there are large corneal nodules and vision ≥20/200 (6/60).
