OM t of c.iii - Vision Research Coordinating Center - Washington ...

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2/1/99 Chapter 1 Background page 1-14 optically corrected. Spectacles cannot adequately correct the irregular astigmatism present in keratoconus, so before contact lenses, surgery was the only recourse. After diagnosing keratoconus and establishing a need for improved vision beyond that which can be achieved with spectacles, rigid contact lenses are the method of choice for optimizing vision and managing the irregular astigmatism (corneal distortion) associated with keratoconus (Mandell, 1988). The ultimate objective is to provide the best vision, comfort and wearing time possible while minimizing any adverse effect on corneal physiology from the contact lens. Scleral contact lenses were used a century ago to treat corneal distortion and keratoconus and were made of glass and later of a plastic, polymethylmethacrylate (PMMA) (Gould, 1970). From approximately 1950 through the mid-1970s, PMMA contact lenses provided the most common management of keratoconus and the fitting methods used today were developed (Hall, 1963; Voss and Liberatore, 1962; Soper and Jarrett, 1972). More recently, soft contact lenses have been used in the earliest stage of the disease (Koliopoulos and Tragakis, 1981; Lundh, 1978). Because of the inability of soft contact lenses to correct significant amounts of irregular astigmatism, gas permeable rigid corneal lenses are required for mild, moderate, and advanced keratoconus (Raber, 1986; Rosenthal, 1986; Belin et al., 1988; Gasset and Lobo, 1975; Mackie, 1977; Buxton, 1978; Mobilia and Foster, 1979; Raber, 1983; Maguen et al., 1983; Cohen and Parlato, 1986). These lenses may be fitted flat (touching the center or apex of the cornea ) or steep (not touching, or clearing the corneal apex). The piggyback approach, a rigid lens fitted over an underlying soft lens (Baldone, 1984) is currently only used in very complicated, advanced stages of the disease due to excessive corneal edema and inconvenience of lens care systems. While contact lenses provide improved vision, they also have the potential to damage the cornea. Scarring of the central cornea may occur in keratoconus without contact lens wear (Krachmer et al., 1984), but it is possible that contact lenses can hasten axial scarring (Korb et al., 1982). Whether by causing hypoxia, entrapment of biodebris and tears, or physical trauma, the contact lens may induce deep central corneal abrasions. Due to the loss of epithelial layers, disruption of the anterior limiting membrane and Bowman’s membrane, and disruption of the regular anterior stroma collagen structure (Pataa et al., 1970), the central cornea may become scarred—and thus the vision reduced—secondary to contact lens wear in keratoconus. 1.6.1 Fitting Methods: Flat Lenses vs. Steep Lenses in Keratoconus The major techniques for fitting rigid lenses in keratoconus (as described by Korb et al., 1982) are:
2/1/99 Chapter 1 Background page 1-15 1. Flat, with primary lens support on the apex of the cornea (Hall, 1963), where the central optic zone of the lens actually touches or “bears on” the central corneal epithelium 2. Steep, with lens support and bearing directed off the apex and onto the paracentral cornea, with clearance (vaulting) of the apex of the cornea (Voss and Liberatore, 1962); and 3. Divided support, or “three-point touch,” with lens support and bearing shared between the corneal apex and the paracentral cornea. Most optometrists continue to use central corneal touch fitting since it is easier and appears to provide better vision, comfort, and wearing time. We have shown in a prospective survey of almost 1,600 keratoconus patients that 75% of the patients wearing rigid contact lenses are fitted with apical touch (Edrington et al., 1991). In addition, it has been argued (Kemmetmuller, 1962) that flat lenses delay the need for surgery by physically keeping the shape of the conus relatively flat. In contrast, it has been argued that the flat fitting method, which the majority of practitioners use today, may actually increase the risk of corneal compromise in the form of scarring. However, only one prospective randomized clinical trial has compared central corneal touch fitting to central corneal clearance fitting in keratoconus (Korb et al., 1982), and the results are inconclusive as to whether contact lenses influence the disease course directly. 1.7 Tissue Changes in Keratoconus Keratoconus corneas are structurally and biochemically different from normal corneas. The distribution of stromal fibers is uneven. The results of a number of investigations appear to indicate that there is an increase in intercellular substance and a decrease of total collagen accompanied by an increase of structural glycoproteins (Robert et al., 1970; Critchfield et al, 1988; Yue et al, 1984). However, others argue that correction for age negates these observations (Zimmermann et al., 1988). Teng (1963) demonstrated early changes in keratoconus in the basal epithelium indicating enzymes may be released from fragile basal epithelial cells which may cause fragmentation, fibrillation and liquefaction of the anterior stroma with invasion of fibroblasts and resultant scarring. Fibrin and fibrinogen are decreased in the epithelial basement membrane in keratoconus (Millin et al, 1986). In scarred areas of keratoconus corneas, collagen type III predominates (Maumenee, 1978; Newsome et al, 1981). Some studies have found high amounts of lysinonorleucine in keratoconus corneas indicating abnormal collagen cross linking (Cannon and Foster, 1978), while others have not
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