Introduction
In general, most corneal dystrophies are autosomal dominant, bilateral disorders that primarily affect one or more layers of an otherwise normal cornea, progress slowly after their appearance in the first or second decade, and are not associated with any systemic disease. Historically, corneal dystrophies were categorized by the involved corneal layer. The distinctive clinical appearance of most corneal dystrophies allows accurate diagnosis based on clinical grounds. Transmission electron microscopy is the most accurate method for histopathological diagnosis. Genetic studies on corneal dystrophies, however, also provide insight at a basic molecular level and are assisting in refining the classification systems. Several corneal dystrophies are closely related at the molecular level with different phenotypes resulting from mutations within the same gene. For example, the BIGH3 gene on chromosome 5q31 is associated with granular corneal dystrophy (types I, II, and III), lattice corneal dystrophy (types I, IIIA, IIIA-like, and IV), and corneal dystrophy of Bowman’s layer (types I and II).
The anterior corneal dystrophies involve Bowman’s layer and involve the epithelium as well. Some also may involve the anterior stroma. This categorization, however, is arbitrary because many of the stromal dystrophies also involve Bowman’s layer and the epithelium. The genetic understanding of these disorders makes this classification even less appropriate, and many of these conditions will ultimately be better classified and named by their specific biochemical defects ( Table 4.20.1 ).
Anterior Basement Membrane Dystrophy
Introduction
Anterior basement membrane dystrophy (ABMD), also termed epithelial basement membrane dystrophy (EBMD), map-dot-fingerprint corneal dystrophy , and Cogan’s microcystic dystrophy , is the most common anterior corneal dystrophy.
Epidemiology and Pathogenesis
ABMD has been reported to occur in approximately 3%–6% of the general population but is much more common over age 40 years. Familial patterns have been described for ABMD, but many patients with the disorder will not have any known family history, which has caused some debate as to whether ABMD is actually a corneal dystrophy or a degenerative change. Point mutations in the TGFBI/BIGH3 gene have been reported in two families showing autosomal dominant inheritance.
Ocular Manifestations
The most common clinical manifestations of ABMD are recurrent erosion syndrome (RES) and blurry or distorted vision. Most patients with RES describe mild pain on awakening, subsiding within minutes to hours, but larger erosions can cause severe pain, which may take hours to days to resolve. Patients also note blurred vision or occasionally monocular diplopia or image ghosting. Although ABMD is often thought to be the most common cause for RES, other conditions (including trauma), can cause RES. In some cases, both entities are present, and the diagnosis of ABMD is made by carefully examining the unaffected eye after trauma. Patients with ABMD and a history of a traumatic abrasion are even more likely to develop RES. The clinical symptoms of ABMD overlap considerably with recurrent erosions secondary to trauma. In both conditions, recurrent erosions are felt to be secondary to friability of the superficial epithelium caused by poor adhesion of epithelial cells to each other and to the underlying basement membrane.
In patients with blurred or distorted vision, which is thought to be related to central ABMD changes, corneal topography can be very helpful in identifying irregular astigmatism as the source of the visual problem. In addition, a trial with a soft bandage contact lens can be a helpful diagnostic or therapeutic tool.
ABMD is characterized by several changes apparent in the anterior cornea on slit-lamp examination. They often are seen best with a dilated pupil and retroillumination. Cases may be subtle and can be detected by looking for negative staining using cobalt blue filter after the application of fluorescein ( Fig. 4.19.1 ). Corneal findings seen with ABMD include intra-epithelial microcysts (dots) ( Fig. 4.19.2 ), map-like grayish patches, and fingerprint parallel lines ( Fig. 4.19.3 ). The condition is referred to as Cogan’s microcystic dystrophy when only microcysts are present. Confocal microscopy is helpful in corneas with a history of erosions that appear to be normal on clinical examination. With confocal microscopy, corneas with recurrent erosions or ABMD showed deposits in basal epithelial cells, subbasal microfolds and streaks, damaged subbasal nerves, or altered morphology of the anterior stroma.
Pathology
The clinical appearance of ABMD corresponds well with the pathology. Specimens from affected patients show protrusions of sheets (maps) or rows of thickened basement membrane (fingerprints) into the superficial epithelium as well as mounds of thickened basement membrane beneath the epithelium. Microcysts of degenerated cellular material accumulate within the epithelium (dots). A bilaminate subepithelial layer of fibrogranular material often occurs.
Treatment
The majority of patients with recurrent corneal erosions will respond to conventional forms of therapy, such as topical lubricants, patching, debridement, or bandage soft contact lenses. Topical hyperosmotic solutions are well tolerated and appear to be effective in treating recurrent corneal erosion in some cases. Therapy with a combination of medications that inhibit metalloproteinase-9 (topical corticosteroid and oral doxycycline) may produce rapid resolution and help prevent further recurrence in cases unresponsive to conventional therapies.
In some cases, surgical intervention will be required. Anterior stromal micropuncture, superficial keratectomy, excimer laser ablation (phototherapeutic keratectomy [PTK]), and diamond burr (DB) superficial keratectomy have all been used. Anterior stromal puncture was the first to be described, and presumably stimulates more secure epithelial adhesion to the underlying stroma. The use of 23- or 25-gauge needles instead of 30-gauge needles may decrease the risk of corneal perforation. Micropuncture should be used with caution for erosions in the visual axis because central corneal scarring can lead to decreased vision.
PTK is a safe and effective procedure for recurrent corneal erosions (trauma and ABMD) refractory to conventional treatment as well as to treat visually significant ABMD. In the great majority of patients, visual fluctuation and monocular diplopia or “ghost images” resolved. A hyperopic shift can be an adverse side effect in some individual cases. It is possible to combine PTK with PRK to manage the hyperopic shift in select patients. ABMD has been reported as a risk for increasing complication rates after laser-assisted in situ keratomileusis (LASIK) including epithelial defects and epithelial ingrowth, so these patients may be better candidates for photorefractive keratectomy (PRK).
DB superficial keratectomy appears to be an effective and safe method of treating recurrent erosions (both posttraumatic and ABMD). Little refractive shift occurs with this method. As an alternative to DB, an Amoils epithelial scrubber may be used to debride the epithelium and buff Bowman’s layer. The DB procedure is simple, office-based, and inexpensive, so it may be the preferred technique in some settings.
Epidemiology and Pathogenesis
ABMD has been reported to occur in approximately 3%–6% of the general population but is much more common over age 40 years. Familial patterns have been described for ABMD, but many patients with the disorder will not have any known family history, which has caused some debate as to whether ABMD is actually a corneal dystrophy or a degenerative change. Point mutations in the TGFBI/BIGH3 gene have been reported in two families showing autosomal dominant inheritance.
Ocular Manifestations
The most common clinical manifestations of ABMD are recurrent erosion syndrome (RES) and blurry or distorted vision. Most patients with RES describe mild pain on awakening, subsiding within minutes to hours, but larger erosions can cause severe pain, which may take hours to days to resolve. Patients also note blurred vision or occasionally monocular diplopia or image ghosting. Although ABMD is often thought to be the most common cause for RES, other conditions (including trauma), can cause RES. In some cases, both entities are present, and the diagnosis of ABMD is made by carefully examining the unaffected eye after trauma. Patients with ABMD and a history of a traumatic abrasion are even more likely to develop RES. The clinical symptoms of ABMD overlap considerably with recurrent erosions secondary to trauma. In both conditions, recurrent erosions are felt to be secondary to friability of the superficial epithelium caused by poor adhesion of epithelial cells to each other and to the underlying basement membrane.
In patients with blurred or distorted vision, which is thought to be related to central ABMD changes, corneal topography can be very helpful in identifying irregular astigmatism as the source of the visual problem. In addition, a trial with a soft bandage contact lens can be a helpful diagnostic or therapeutic tool.
ABMD is characterized by several changes apparent in the anterior cornea on slit-lamp examination. They often are seen best with a dilated pupil and retroillumination. Cases may be subtle and can be detected by looking for negative staining using cobalt blue filter after the application of fluorescein ( Fig. 4.19.1 ). Corneal findings seen with ABMD include intra-epithelial microcysts (dots) ( Fig. 4.19.2 ), map-like grayish patches, and fingerprint parallel lines ( Fig. 4.19.3 ). The condition is referred to as Cogan’s microcystic dystrophy when only microcysts are present. Confocal microscopy is helpful in corneas with a history of erosions that appear to be normal on clinical examination. With confocal microscopy, corneas with recurrent erosions or ABMD showed deposits in basal epithelial cells, subbasal microfolds and streaks, damaged subbasal nerves, or altered morphology of the anterior stroma.
Pathology
The clinical appearance of ABMD corresponds well with the pathology. Specimens from affected patients show protrusions of sheets (maps) or rows of thickened basement membrane (fingerprints) into the superficial epithelium as well as mounds of thickened basement membrane beneath the epithelium. Microcysts of degenerated cellular material accumulate within the epithelium (dots). A bilaminate subepithelial layer of fibrogranular material often occurs.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
