DEGENERATIVE CORNEAL CONDITIONS

KERATOCONUS

Keratoconus is an uncommon degenerative bilateral disease that may be inherited as an autosomal recessive or autosomal dominant trait. Unilateral cases of unknown cause occur rarely. Symptoms appear in the second decade of life. The disease affects all races. Keratoconus has been associated with a number of diseases, including Down's syndrome, atopic dermatitis, retinitis pigmentosa, aniridia, vernal catarrh, Marfan's syndrome, Apert's syndrome, and Ehlers-Danlos syndrome. Pathologically, there are disruptive changes in Bowman's layer with keratocyte degeneration and ruptures in Descemet's membrane.

Blurred vision is the only symptom. Many patients present with rapidly increasing myopic astigmatism. Signs include cone-shaped cornea (Figure 6-11); linear narrow folds centrally in Descemet's membrane (Vogt's lines), which are pathognomonic; an iron ring around the base of the cone (Fleischer's ring); and, in extreme cases, indentation of the lower lid by the cornea when the patient looks down (Munson's sign). There is an irregular or scissor reflex on retinoscopy and a distorted corneal reflection with Placido's disk or the keratoscope-all of which often being more obvious in the early stages than the other corneal signs. Color-coded topography units provide more accurate information on the degree of corneal distortion (Figure 2-26). Generally, the fundi cannot be clearly seen because of corneal astigmatism.

Acute hydrops of the cornea may occur, manifested by sudden diminution of vision associated with central corneal edema. This arises as a consequence of rupture of Descemet's membrane and may be triggered by the patient rubbing the eye. The condition may be mistaken for extreme thinning with impending perforation. Acute hydrops usually clears gradually without treatment but often leaves apical scarring.

Rigid contact lenses will markedly improve vision in the early stages by correcting irregular astigmatism. Keratoconus is one of the most common indications for penetrating keratoplasty. Surgery is indicated when a contact lens can no longer be effectively worn or when peripheral thinning will affect the surgery.

Keratoconus is often slowly progressive between the ages of 20 and 60, although an arrest in progression of the keratoconus may occur at any time. If a corneal transplant is done before extreme corneal thinning occurs, the prognosis is excellent; about 80-95% obtain good best-corrected sight.

CORNEAL DEGENERATION

The corneal degenerations are a rare group of slowly progressive, bilateral, degenerative disorders that usually appear in the second or third decades of life. Some are hereditary. Other cases follow ocular inflammatory disease, and some are of unknown cause.

Marginal Degeneration of the Cornea

A. Terrien's Disease:

This is a rare bilateral symmetric degeneration characterized by marginal thinning of the upper nasal quadrants of the cornea. Males are more commonly affected than females, and the condition occurs more frequently in the third and fourth decades. There are no symptoms except for mild irritation during occasional inflammatory episodes, and the condition is slowly progressive. The clinical picture consists of marginal thinning and peripheral vascularization with lipid deposition. Perforation is a known complication, especially from trauma. Tectonic (structural) keratoplasty may be required. Histopathologic studies of affected corneas have revealed vascularized connective tissue with fibrillary degeneration and fatty infiltration of collagen fibers.

Because the course of progression is slow and the central cornea is spared, the prognosis is good.

B. Band (Calcific) Keratopathy:

(Figure 6-12.) This disorder is characterized by the deposition of calcium salts in the anterior layers of the cornea. The keratopathy is usually limited to the interpalpebral area and appears as a band. The calcium deposits are noted in the basement membrane, Bowman's layer, and anterior stromal lamellas. A clear margin separates the calcific band from the limbus, and clear holes may be seen in the band, giving the Swiss cheese appearance. Symptoms include irritation, injection, and blurring of vision.

Calcific band keratopathy has been described in a number of inflammatory, metabolic, and degenerative conditions. It is characteristically associated with juvenile rheumatoid arthritis. It has been described in long-standing inflammatory conditions of the eye, glaucoma, and chronic cyclitis. Band keratopathy may also be associated with hyperparathyroidism, vitamin D intoxication, sarcoidosis, and leprosy. Treatment consists of removal of the corneal epithelium by curettage under topical anesthesia followed by irrigation of the cornea with a sterile 0.01-molar solution of ethylenediaminetetraacetic acid (EDTA) or application of EDTA with a cotton applicator. The excimer laser has shown particular value in the treatment of band keratoplasty, or the band can be removed surgically.

Climatic Droplet Keratopathy (Labrador Keratopathy, Spheroid Degeneration of the Cornea) (  Figure 6-13)

Climatic droplet keratopathy affects mainly men who work out of doors. The corneal degeneration is thought to be caused by exposure to ultraviolet light and is characterized in the early stages by fine subepithelial yellow droplets in the peripheral cornea. As the disease advances, the droplets become central, with subsequent corneal clouding causing blurred vision. Treatment in advanced cases is by corneal transplantation.

Salzmann's Nodular Degeneration

This disorder is always preceded by corneal inflammation, particularly phlyctenular keratoconjunctivitis or trachoma. Symptoms include redness, irritation, and blurring of vision. There is degeneration of the superficial cornea that involves the stroma, Bowman's layer, and epithelium with superficial whitish-gray elevated nodules sometimes occurring in chains.

Corneal transplantation is rarely required; superficial lamellar keratectomy or phototherapeutic (laser) keratectomy can result in visual improvement.

Rigid contact lenses will significantly improve visual acuity in most cases.

ARCUS SENILIS (Corneal Annulus, Anterior Embryotoxon)

Arcus senilis is an extremely common, bilateral, benign peripheral corneal degeneration that may occur at any age but is far more common in elderly people as part of the aging process. Arcus senilis in people under age 50 is often associated with hyper- cholesterolemia; blood lipid studies should be done.

Pathologically, lipid droplets involve the entire corneal thickness but are more concentrated in the superficial and deep layers, being relatively sparse in the corneal stroma.

There are no symptoms. Clinically, arcus senilis appears as a hazy gray ring about 2 mm in width and with a clear space between it and the limbus (Figure 6-14). No treatment is necessary, and there are no complications.

HEREDITARY CORNEAL DYSTROPHIES

This is a group of rare hereditary disorders of the cornea of unknown cause characterized by bilateral abnormal deposition of substances and associated with alteration in the normal corneal architecture that may or may not interfere with vision. These corneal dystrophies usually manifest themselves during the first or second decade but sometimes later. They may be stationary or slowly progressive throughout life. Corneal transplantation, when indicated, improves vision in most patients with hereditary corneal dystrophy.

Anatomically, corneal dystrophies may be classified as epithelial, stromal, and posterior limiting membrane dystrophies.

Epithelial Corneal Dystrophies

A. Meesman's Dystrophy:

This slowly progressive disorder is characterized by microcystic areas in the epithelium. The onset is in early childhood (first 1-2 years of life). The main symptom is slight irritation, and vision is slightly affected. The inheritance is autosomal dominant

B. Anterior Membrane Dystrophies (Cogan's, Map-Dot-Fingerprint):

Map or fingerprint patterns are seen at the level of the epithelial basement membrane. Debris, cysts, and dots also may be noted. Recurrent erosion is common. Vision usually is not significantly affected. In Cogan's dystrophy, intraepithelial opacities are seen in the pupillary area

C. Others:

Reis-Bücklers dystrophy is a dominantly inherited dystrophy affecting primarily Bowman's layer. The disease begins within the first decade of life with symptoms of recurrent erosion. Opacification of Bowman's layer gradually occurs and the epithelium is irregular. No vascularization is usually noted. Vision may be markedly reduced.

Vortex dystrophy, or cornea verticillata, is characterized by pigmented lines occurring in Bowman's layer or the underlying stroma and spreading over the entire corneal surface. Visual acuity is not markedly affected. Such a pattern of radiating pigmented lines may also be seen in patients treated with chlorpromazine, chloroquine, indomethacin, or amiodarone as well as in Fabry's disease.

Stromal Corneal Dystrophies

There are three primary types of stromal corneal dystrophies:

A. Granular Dystrophy:

This usually asymp-tomatic, slowly progressive corneal dystrophy most often begins in early childhood. The lesions consist of central, fine, whitish "granular" lesions in the stroma of the cornea. The epithelium and Bowman's layer may be affected late in the disease. Visual acuity is slightly reduced. Histologically, the cornea shows uniform deposition of hyaline material. Corneal transplant is not needed except in very severe and late cases. The inheritance is autosomal dominant

B. Macular Dystrophy:

This type of stromal corneal dystrophy is manifested by a dense gray central opacity that starts in Bowman's layer. The opacity tends to spread toward the periphery and later involves the deeper stromal layers. Recurrent corneal erosion may occur, and vision is severely impaired. Histologic examination shows deposition of acid mucopolysaccharide in the stroma and degeneration of Bowman's layer. Penetrating keratoplasty is often required.

The inheritance is autosomal recessive.

C. Lattice Dystrophy:

Lattice dystrophy starts as fine, branching linear opacities in Bowman's layer in the central area and spreads to the periphery. The deep stroma may become involved, but the process does not reach Descemet's membrane. Recurrent erosion may occur. Histologic examination reveals amyloid deposits in the collagen fibers. Penetrating keratoplasty is common, as is recurrence of the dystrophy in the graft. The hereditary pattern for lattice dystrophy is autosomal dominant.

Posterior Corneal Dystrophies

A. Fuchs' Dystrophy:

This disorder begins in the third or fourth decade and is slowly progressive throughout life. Women are more commonly affected than men. There are central wart-like deposits on Descemet's membrane, thickening of Descemet's membrane, and defects of size and shape of the endothelial cells. Decompensation of the endothelium occurs and leads to edema of the corneal stroma and epithelium, causing blurring of vision. Corneal haze is slowly progressive. Histologic examination of the cornea reveals the wart-like excrescences over Descemet's membrane that are secreted by the endothelial cells. Thinning and pigmentation of the endothelium and thickening of Descemet's membrane are characteristics. Penetrating keratoplasty, often combined with extracapsular lens extraction and a posterior lens implant, is often needed. Cataract surgery alone can trigger endothelial decompensation in advanced disease

B. Posterior Polymorphous Dystrophy:

This is a common disorder with onset in early childhood. Polymorphous plaques of calcium crystals are observed in the deep stromal layers. Vesicular lesions may be seen in the endothelium. Edema occurs in the deep stroma. The condition is asymptomatic in most cases, but in severe cases epithelial and total stromal edema may occur. The inheritance is autosomal dominant.

10.1036/1535-8860.ch6

Customer Privacy Policy Copyright ©2002-2003 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Notice. Additional credits and copyright information. For further information about this site contact tech_support@accessmedicine.com. Last modified: October 30, 2002 .