Degeneration of a tissue is defined as a deterioration that results in impaired structure and function. These changes occur at the cellular level and are caused by biomolecular alterations induced by aging, as well as a wide variety of adverse external and internal influences. Traditionally, degenerations have been classified as involutional (age-related) or non-involutional, depending on type of deposition (hyaline, amyloid, lipid, calcific), and by anatomic location (anterior, posterior, central or peripheral). Degenerations may be unilateral or bilateral, often with asymmetric involvement. Heredity does not play a direct role in these processes. The noninflammatory ectatic diseases of the cornea discussed in this chapter include keratoconus, pellucid marginal degeneration, keratoglobus, and posterior keratoconus. An exceptional degeneration unreported in literature (presumed alimentary cuprum keratopathy) is presented in this chapter. Pseudogerontoxon is one of the characteristic degenerations commonly seen in Middle Eastern populations.
TopCorneal Degenerations
Arcus senilis/Cornea senilis, after the fourth decade, and arcus juvenilis, before the fourth decade, represents a degenerative change resulting in lipid deposition in the peripheral cornea. Lipid deposition usually starts as a gray to yellow arc, firstly developing in the inferior, and then the superior cornea (Francois et al, 1973; Ritfkind et al, 1972). The deposition gradually progresses and forms a complete ring (Fig 1). Corneal arcus has no visual significance; thus, no treatment is required. Nevertheless, patients under the age of 40 with corneal arcus have an increased risk of coronary artery disease and should be evaluated for hyperlipoproteinemia.
Figure 1. Arcus senilis. Dense, complete ring is seen
The limbal girdle of Vogt is a symmetrical, yellowish-white band, which is located parallel to the interpalpebral limbus (Sugar et al, 1960). The depositions are located beneath the epithelial layer, adjacent to the Bowman's layer. Histologically, the deposits are calcific in nature, or they can represent a focal area of hyaline accumulation (Fig 2). This degeneration is an incidental finding and is asymptomatic. No treatment is required.
Iron lines can be found when there are alterations of the ocular surface that produce focal elevations and depressions (Loh et al, 2009). The most common iron line is the Hudson-Stahli line, usually located at the juncture of the lower and middle thirds of the cornea (Rose et al, 1968). Iron lines can be seen after glaucoma surgery (Ferry's line) (Ferry et al, 1968). located anterior to the filtering bleb. Iron lines have been observed at the edge of the pterygium (Stocker's line) and the base of the cone in the keratoconus (Fleischer's ring, Fig 3). The less common type is the Coat's ring, a circular or oval-shaped deposition associated with history of previous metallic foreign bodies (Fig 4). Iron lines represent an incidental finding and are asymptomatic.
Figure 3. Fleischer’s ring is seen at the base of the cone
Figure 4. Coats ring after intracorneal foreign body removal