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Corneal dystrophies are rare eye disorders that result in clouding of the cornea, the transparent outer layer of the eye. There are many different types and are defined by the layer of the cornea that is affected.
What are corneal dystrophies?
Corneal dystrophies usually affect both eyes equally causing cloudy deposits, loss of sight and pain and grittiness in the eye. Cloudiness in the cornea and eye pain can develop at different ages, depending on the type.
The build-up of opaque deposits in the cornea can take years or even decades but may eventually become so bad that a corneal transplant is needed to restore vision.
Keratoconus [Link to condition page on keratoconus] is the most common corneal dystrophy. Like other corneal dystrophies, many cases of keratoconus are inherited but a few can result from eye injury, infection or develop because of underlying health problems.
Corneal dystrophies: how they affect corneal structure and function.
The cornea is the outermost lens of the eye and acts as a barrier between the eye and the outside world. It has 5 layers and any one of these can develop cloudy deposits in different forms of corneal dystrophy:
- The epithelium is the surface layer; it has thousands of tiny nerve endings, and an extremely smooth surface.
- The Bowman’s layer is a thin, transparent, collagen protein fibre layer directly below the epithelium; this can become scarred after any damage.
- The stroma accounts for 90% of the cornea’s thickness and is mainly water with some collagen, and is strong, elastic and transparent.
- Descemet’s membrane is a thin, strong collagen layer between the stroma and the endothelium.
- The endothelium is the inner, extremely thin, corneal layer that pumps fluid out of the stroma; if damaged, it cannot be repaired and this causes the cornea to swell and become hazy.
Main types of corneal dystrophies
Corneal dystrophies are currently classified according to the layer of corneal tissue that becomes abnormal.
Epithelial and sub-epithelial corneal dystrophies:
- Epithelial basement membrane dystrophy, also called map-dot-fingerprint dystrophy or Cogan’s dystrophy. This is the most common epithelial dystrophy and it causes pain, grittiness and blurred vision, which usually begins around the age of 30. Treatable with antibiotic drops and ointments and a bandage soft contact lens.
- Meesmann’s corneal dystrophy: a very rare condition that does not often affect sight but that creates the feeling of grit in the eyes. It can be diagnosed in a baby under one but symptoms often do not start until the early 20s, or even middle age.
- Bowman’s layer corneal dystrophies:
- Reis-Bucklers corneal dystrophy: symptoms usually start in children who inherit it from one of their parents. Their vision becomes hazy because the corneal surface becomes very irregular. The eyes are sensitive to light and feel gritty; if damage to the corneal worsens, a corneal transplant may be recommended.
- Thiel-Behnke corneal dystrophy: symptoms begin in childhood or during the teen years. The epithelial surface of the cornea becomes irregular but sight loss is caused by white deposits in the Bowman’s layer.
- Stromal corneal dystrophies:
- Granular dystrophy: signs of dot-like deposits appear in the stroma of the cornea before the age of 20 but these do not start to affect vision for perhaps 30 years. Can be treated by a corneal transplant but the deposits can recur within 5 years.
- Macular dystrophy: this can develop in the teens, with cloudy areas appearing in both corneas; a corneal transplant is often needed before the age of 30. The deposits can also recur but this takes much longer than in granular dystrophy; 20 years on average.
- Lattice corneal dystrophy: another corneal disease that occurs in relatively young people. Sight loss is less of a problem than the gritty feeling and pain that occurs in the eyes. A corneal graft is offered if vision loss is significant, but the donor cornea may become affected in 5-10 years. Other treatment aims to reduce the pain.
- Descemet’s membrane and endothelial corneal dystrophies:
- Fuchs’ endothelial corneal dystrophy: this is the most common of the corneal dystrophies that affect the deeper layers of the cornea. Fuchs’ endothelial dystrophy tends to occur more often in women than in men but its genetics are complex. In someone who is affected, the endothelial cells deteriorate, causing the cornea to take on water. This causes distorted vision that worsens throughout the day, the eyes become painful and blisters form on the surface of the cornea. A soft bandage contact lens can be used to protect the epithelium and a corneal transplant is often advised. The condition does not recur after corneal transplantation, so this treatment is extremely successful.
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