unstable contact lens fit (even with good vision and tolerance)
progressive thinning to the periphery approaching the limbus, requiring a very large graft (with increased risk)
PK is still the most widely performed surgical procedure for keratoconus, and the prognosis is excellent. Some surgeons prefer DALK for keratoconus, as it leaves the host’s endothelium untouched, decreasing rejection episodes, and wound integrity with DALK is superior. See Chapter 15 for further discussion of PK and DALK.
Hydrops is treated conservatively with topical hypertonic agents and patching or a soft contact lens for several months. A cycloplegic agent may be needed for relief of ciliary pain. Aqueous suppressants may decrease the flow of fluid into the cornea. Hydrops is not an indication for immediate surgery. Intracameral injection of gas may be helpful in the resolution of hydrops.
Kiliç A, Colin J. Advances in the surgical treatment of keratoconus. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2012, module 2.
Pellucid Marginal Degeneration
Pellucid marginal degeneration is uncommon, nonhereditary, and bilateral. Pellucid (ie, transparent) inferior, peripheral corneal thinning takes place in the absence of inflammation. The etiology is unknown.
CLINICAL FINDINGS Protrusion of the cornea occurs above the band of thinning (Fig 10-29). At times, a clear distinction between pellucid marginal degeneration and keratoconus is not possible. A cornea with keratoconus will show protrusion at the point of maximal thinning, but pellucid marginal degeneration can be superior or inferior (Fig 10-30; see also Fig 10-31) and will show protrusion above the area of maximum thinning. No vascularization or lipid deposition occurs, but posterior stromal scarring has been noted within the thinned area. Pellucid marginal degeneration is diagnosed in most patients between 20 and 40 years of age, and men and women are affected equally. Decreased vision results from high irregular astigmatism. Acute hydrops has been reported, and, though rare, spontaneous corneal perforation has also occurred.
Figure 10-29 Pellucid marginal degeneration. (Courtesy of Vincent P. deLuise, MD.)
Figure 10-30 Topography of pellucid marginal degeneration. Note the inferior steepening and “lobster claw” pattern.
MANAGEMENT Treatment consists of contact lenses early in the disease, although lens fitting is more difficult in pellucid marginal degeneration than in keratoconus. Hybrid (gas-permeable contact lenses with a soft lens “skirt”) or scleral lenses may be options. Eventually, PK may be required to restore vision. Because of the location of the thinning, the grafts tend to be large and close to the limbus, making surgery technically more difficult and the graft more prone to rejection. Wedge resection and lamellar tectonic grafts have been advocated as alternative or adjunctive procedures. Collagen crosslinking may also be considered for some of these patients.
Belin MW, Asota IM, Ambrosio R Jr, Khachikian SS. What’s in a name: keratoconus, pellucid marginal degeneration, and related thinning disorders. Am J Ophthalmol. 2011;152(2):157–162.
Rasheed K, Rabinowitz YS. Surgical treatment of advanced pellucid marginal degeneration. Ophthalmology. 2000;107(10):1836–1840.
Keratoglobus
Keratoglobus is a very rare, bilateral, noninflammatory condition that differs from keratoconus and pellucid marginal degeneration in typically being present at birth. It is usually not hereditary. Keratoglobus is similar in appearance to keratoconus but manifests as a globular rather than a conical deformation of the cornea (Fig 10-31).
Figure 10-31 The presence of corneal thinning and the type of contour abnormality can be helpful in recognizing the type of ectatic disorder. A, Keratoconus. B, Pellucid marginal degeneration. C, Keratoglobus. (Reproduced with permission from Krachmer
JH, Mannis MJ, Holland EJ, eds. Cornea. 2nd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2005:956.)
PATHOLOGY Keratoglobus is strongly associated with blue sclerae and Ehlers-Danlos syndrome type VI (see Chapter 9), and it may represent a defect in collagen synthesis. Histologically, it is characterized by an absent or fragmented Bowman layer, thinned stroma with normal lamellar organization, and a thin Descemet membrane. Unlike keratoconus, keratoglobus is not associated with atopy and hard contact lens wear.
CLINICAL FINDINGS Both corneas have a globular shape with a very deep anterior chamber. The corneal curvature may be as steep as 50–60 D, and generalized thinning appears, especially in the midperiphery; this is in contrast to keratoconus, which has maximal thinning at or near the apex of the protrusion. Spontaneous rupture of the Descemet membrane and corneal hydrops can occur, but iron lines, stress lines, and anterior scarring are not seen. The corneal diameter may be slightly increased. Fleischer rings are usually not present, but prominent folds and areas of thickening in the Descemet membrane are common.
MANAGEMENT Contact lenses, especially scleral lenses, may be of benefit. The prognosis for PK is much poorer in keratoglobus than in the other ectasias. A lamellar tectonic graft followed by PK could be considered in cases requiring intervention to maintain functional vision. Spontaneous corneal rupture has been reported, so patients must be counseled regarding the importance of protective eyewear. High myopia is treated with spectacles to prevent amblyopia.
Feder RS, Kshettry P. Noninflammatory ectatic disorders. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 3rd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2011:865–887.