Ahmed Hassane
Introduction
Principally, corneal dystrophies are a set of conditions that impact corneal transparency and distorts corneal structure (1–3). They are commonly bilateral, progressive, and differs from corneal degeneration as they are often inherited through autosomal dominant or recessive modes as well as x-linked modes (4). Several genes have been implicated in corneal dystrophies and will be elaborated on in the description of dystrophies below.
As mentioned previously, Corneal dystrophies are progressive and patients may often be asymptomatic. However, typical symptoms that should raise suspicion include dry eyes, recurrent corneal erosions, decreased visual acuity, and photophobia (5). Females are more likely to develop corneal dystrophies than males, with the typical age range of patients being between 40-70 years old (5).
The ICD3 classification is currently the preferred system used for categorising corneal dystrophies, commonly used in the United States and in most places in Europe. Revised in 2015, Edition 2 of the ICD3 groups the dystrophies based on location (6).
Management of corneal dystrophies depend on severity of symptoms and require specialist input. Mildly symptomatic dystrophies do not warrant management. However, in those with progressive disease and/or poor vision, surgical options may be of use. Techniques commonly involved in the management of dystrophies include deep lamellar endothelial keratoplasty, penetrating keratoplasty, or the area of disease is ablated using a photo-therapeutic keratectomy (7,8).
Corneal dystrophies are a vast topic, but this paper will primarily focus on the subtypes of the anterior corneal dystrophies and its clinical features.
Clinical features
Corneal dystrophies can be broken down according to the layer of the cornea that it affects (6):
Epithelial and subepithelial dystrophies
Meesman corneal dystrophy:
The characteristic feature of Meesman corneal dystrophy is the development of intraepithelial cysts across the epithelium. These contain deteriorated epithelial cells that are periodic acid-schiff positive. Cysts develop in the first decade of life, however symptoms tend to develop in adulthood. Typical symptoms for Meesman corneal dystrophy include foreign body sensation, photophobia and tearing (2,5).
Subepithelial mucinous corneal dystrophy:
This type of dystrophy is characterised by a build-up of chondroitin 4-sulfate and dermatan sulfate located anterior to the Bowmans layer. It is periodic acid-shciff positive and alcian blue positive (9).
Epithelial basement membrane dystrophies:
More commonly known as “map-dot-finger print” dystrophy, epithelial basement membrane is the most common epithelial dystrophy (10). The colloquial term was derived from the descriptive pattern of the dystrophy: increased sheets of basement membrane invading the corneal epithelium (maps), parallel lines of thickened basement membrane (fingerprint) and epithelial cells confined in the extra sheets (dot) (2,3,10). The treatment of this dystrophy is primarily symptomatic, however severe cases require a surgical approach.
Epithelial-stromal dystrophies
Reis Buckler
Reis Buckler Dystrophy affects the bowmans layer of the cornea as well as the superficial stroma, and is characterised by an asymmetrical ring-shaped pattern of spots (11). On light microscopy, mutated growth factor beta induced protein can be observed in the superficial stroma. The TGFB1 gene has been implicated for Reis buckler dystrophy.
Theil Behnke dystrophy
Also affecting the bowmans layer, Theil Behnke is typified by opacities that are grouped in a honeycomb shaped pattern with an apparent clear zone at the corneoscleral limbus (5,12). Due to the degeneration of the bowmans layer, aspects of the corneal surface can seem raised. This particular condition typically presents later in life compared to reis buckler.
Lattice corneal dystrophy
Lattice corneal dystrophy (LCD) can be split into type 1 and type 2 (13). Type 1 usually occurs in adolescence and is the more common out the two. Linear and branching opacities can be observed and tends to affect the central stroma. LCD 2 differs from type 1 in that there is systemic involvement from amyloidosis and the peripheral stroma is affected rather than the central layers (3). LCD 2 appears later on life. Dysfunction in the GSN gene is linked with LCD2 while TGFB1 has a higher associated with LCD 1.
Granual dystrophy type 1 and 2
Granular dystrophy usually appears in young children is has a typical opaque “crumb-like” opacities underneath the bowman layer centrally (2,5,14). While it is difficult to differentiate between the two types, Granular type 1 occurs more frequently in Europe while granular type 2 usually is seen in Japanese and Korean patients. Granular type 2 has fewer opacities and can often appear similar to lattice corneal dystrophies.
Stromal Dystrophies
Macula corneal dystrophy
Macular corneal dystrophy presents with discrete opacities creating a hazy cornea (15). These opacities affect both the central and peripheral cornea and histologically, glycosaminoglycans and fibrillogranular material can be seen. Vacuoles can also be observed in the Descemet’s membrane. Mutation in the CHST6 gene, which is involved in the formation of keratan chains on lumican (a protein that is partly responsible for corneal transparency (16)), leads to macular dystrophy (5,15). A key feature of macular dystrophy is the absence of keratan sulfate and thinner cornea.
Schnyder corneal dystrophy
This is characterised by a white-yellow ring-shaped opacity which is composed of needle shaped cholesterol crystals on histology in the bowman and anterior stromal layers (2,17). Schnyder dystrophy has been linked to hypercholesteremia and thus lipid deposits can also be observed on histology (5).
Congenital stromal dystrophy
An extremely rare dystrophy, a feathery clouding of the stroma can be observed which can eventually provide difficulty examining the endothelial layer (5). Abnormal collagen filaments dividing collagen lamella can be seen on electron microscopy. Open angle glaucoma and strabismus has been linked to this dystrophy in the literature (2).
Fleck corneal dystrophy
As the name suggests, opacities resembling flecks can be observed across the stroma. The lesions are non-progressive and asymptomatic and usually affects the deeper stromal layers. The PIP5K3 gene mutation leads to this dystrophy (4).
Posterior amorphous corneal dystrophy
This dystrophy primarily affects the posterior stroma with sheet like opacities. Symptoms start in Infancy (2,3).
Central cloudy dystrophy of francois:
A rare dystrophy, this is characterised by “crocodile” appearance of opacities that are polygonal and cloudy in appearance (18). This is primarily different to posterior crocodile shagreen due to the inheritance nature of this dystrophy.
References
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