- 1.
What are corneal dystrophies?
Corneal dystrophies are bilateral, inherited, noninflammatory, commonly progressive alterations of the cornea that are usually not associated with any other systemic condition. Most corneal dystrophies are autosomal dominant disorders occurring after birth. Because each dystrophy may exhibit a spectrum of clinical manifestations, examining multiple family members frequently aids in establishing the diagnosis.
- 2.
How do degenerations differ from dystrophies?
In contrast to dystrophies, degenerations are unilateral or bilateral aging changes that are not inherited. They are also not associated with systemic disease.
- 3.
Discuss the general anatomic classification of corneal dystrophies.
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Anterior membrane dystrophies include disorders affecting the corneal epithelium, epithelial basement membrane ( Fig. 12-1 ), and Bowman’s layer.
Figure 12-1
Typical mare’s tail sign in epithelial basement membrane dystrophy.
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Stromal dystrophies occur anywhere in the stromal layer of the cornea between Bowman’s layer and Descemet’s membrane.
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Posterior membrane dystrophies are primarily abnormalities of the endothelium and Descemet’s membrane.
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- 1.
Corneal dystrophies are always bilateral.
- 2.
They are inherited.
- 3.
They may occur shortly after birth.
- 4.
What is the International Committee for Classification of Corneal Dystrophies?
The International Committee for Classification of Corneal Dystrophies (IC3D) was created in 2008 to study what genetic analyses had brought to light and the relations between genetic abnormalities and their phenotypic description available at the time. Members of The Cornea Society assigned a category number from 1 to 4 to each one of the known dystrophies, reflecting the “level of evidence” of its existence. All dystrophies were given a name, alternative names, and eponyms; their Mendelian inheritance in humans, inheritance, genetic locus, and gene; their onset, signs, symptoms, and course; their light microscopy, transmission electron microscopy, immunohistochemistry, and confocal microscopy results; and a category.
All anterior membrane dystrophies are autosomal dominant. Examples are Meesmann’s juvenile epithelial dystrophy, epithelial basement membrane dystrophy, and corneal dystrophies of Bowman’s layer.
- 5.
Which is the most common anterior membrane dystrophy? Which is strictly epithelial?
Epithelial basement membrane dystrophy is by far the most common anterior membrane dystrophy. In fact, it has the highest prevalence of all of the corneal dystrophies. Areas of extra basement membrane result in maplike and/or fingerprint changes as well as intraepithelial microcysts. Five percent of otherwise normal corneas have been observed to have such changes.
Second in prevalence are the corneal dystrophies of the Bowman’s layer (CDBs): Reis-Bücklers (CDB-I) and Thiel-Behnke honeycomb-shaped dystrophy (CDB-II). These disorders consist of gray reticular opacities beneath the epithelium.
Meesmann’s dystrophy is the rarest of the three and is strictly epithelial. This disorder, noted in the first few years of life, presents as a bilaterally symmetric pattern of microcysts or vesicles seen strictly in the epithelial layer of the cornea, usually in the interpalpebral fissure.
- 6.
What are the most common presenting symptoms of anterior membrane dystrophies?
First are the symptoms associated with corneal erosions—pain, foreign body sensation, photophobia, and tearing, especially with opening of the lids during sleep or upon awakening in the morning. Erosions are most common in the setting of epithelial basement membrane dystrophy. The second symptom is blurred vision secondary to either irregularity of the surface, seen in epithelial basement membrane dystrophy, or corneal clouding, frequently seen in the dystrophies of the Bowman’s layer or Meesmann’s dystrophy
- 7.
Discuss treatment options for recurrent corneal erosions associated with anterior membrane dystrophies.
The conservative approach includes the generous use of lubricating eyedrops during the day and ointments at night. Some physicians advocate the use of topical steroids to stabilize the basement membrane, and others advocate hypertonic saline, especially in ointment form at night to dehydrate the epithelium and aid in its attachment to the underlying layers. Patching, either conventional or with collagen or bandage contact lenses, hypothetically decreases the mechanical effect of lid movement on the already weakened corneal epithelium. Recalcitrant cases may require surgical intervention.
- 1.
Recurrent corneal erosions may be associated with anterior membrane and stromal dystrophies.
- 2.
They have common symptoms: pain, blurred vision, and photophobia.
- 3.
Recurrent corneal erosions are frequently amenable to medical therapy with lubrication and hyperosmotic agents.
- 4.
They can be treated surgically with mechanical or laser keratectomy or stromal puncture.
- 8.
Discuss the role of surgery in the treatment of anterior membrane dystrophies.
In the setting of recalcitrant corneal erosions, mechanical debridement of the loose epithelium and basement membrane or anterior stromal puncture, together with the use of a bandage lens, may aid in reepithelialization of the surface and adherence of the epithelium to the underlying layers. Mechanical debridement also may be used to remove an irregular epithelial basement membrane if an associated visual decline is noted. Typically these patients do not have symptoms of corneal erosion but complain of blurred vision. Topography reveals marked irregularity of the Placido rings. Removal of the abnormal epithelium and basement membrane can restore normal anterior corneal anatomy paralleled by improvement in vision. For the Bowman’s layer dystrophies a more aggressive superficial or lamellar keratectomy may be required. This may be microkeratome assisted. Lamellar keratoplasty may also be considered.
- 9.
Do lasers have a role?
The yttrium–aluminum–garnet laser has been used instead of a needle to accomplish anterior stromal puncture but does not offer a clear advantage. The excimer laser has been used for treatment of recurrent erosions associated with basement membrane dystrophies and for removal of deeper layers in conditions such as Reis-Bücklers and Thiel-Behnke dystrophies (phototherapeutic keratectomy). Although in the first instance the excimer laser may not offer a clear advantage over debridement, in the second it has supplanted manual lamellar keratectomy as the treatment of choice. Microkeratome-assisted lamellar keratectomy may be equally effective.
- 10.
What controversy surrounds the dystrophies affecting the Bowman’s layer?
Until recently there has been some confusion over dystrophies affecting the Bowman’s layer, because they present with two different sets of characteristics, but historically they have been lumped under Reis-Bücklers dystrophy. The first set was described by Reis in 1917 and later by Bücklers in 1949 and the second by Thiel and Behnke in 1967. Küchle et al. divided the Bowman’s membrane dystrophies into two classifications: corneal dystrophy of the Bowman’s layer type I and type II. Type I is synonymous with the original Reis-Bücklers dystrophy and equivalent to what also has been described as superficial variant of granular dystrophy. Type II is honeycomb-shaped and is also known as the Thiel-Behnke corneal dystrophy. The two dystrophies have slightly different characteristics on light microscopy. Transmission electron microscopy, on the other hand, differentiates them unequivocally.
- 11.
Describe the inheritance patterns of the stromal dystrophies.
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Autosomal dominant: Granular (Groenouw type I; Fig. 12-2 ), lattice, Avellino granular–lattice, Schnyder’s crystalline, fleck, central cloudy dystrophy of François, pre-Descemet, congenital hereditary (stromal), and posterior amorphous dystrophies
Figure 12-2
Slit lamp appearance of granular dystrophy.
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Autosomal recessive: Macular (Groenouw type II) and possibly gelatinous droplike dystrophies
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- 12.
Match the stromal dystrophy with the histochemical stain for the accumulated substance.
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Granular: Masson trichrome stains hyaline
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Lattice: Congo red stains amyloid (amyloid deposits exhibit polarized light birefringence and dichroism)
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Macular: Alcian blue stains mucopolysaccharides (glycosaminoglycans)
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Lattice and macular dystrophies also stain with periodic acid–Schiff stain.
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- 13.
Describe the clinical features of the three major stromal dystrophies.
See Table 12-1 .
Table 12-1
Clinical Features of the Three Major Stromal Dystrophies
Feature
Age of Onset
Granular dystrophy
Lattice dystrophy
Macular dystrophy
Deposits
First decade
First decade
First decade
Symptoms
Third decade or none
Second decade
First decade
Decreased vision
Fourth or fifth decade
Second or third decade
First or second decade
Erosions
Uncommon
Frequent
Common
Corneal thickness
Normal
Normal
Thinned
Opacities
Discrete with sharp borders and clear intervening stroma early but becoming hazy later, not extending to limbus
Refractile lines and subepithelial spots, diffuse central haze, not extending to limbus except in advanced cases
Indistinct margins with hazy stroma between, extending to limbus; central lesions more anterior and peripheral lesions more posterior 
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