Macular corneal dystrophy, also known as Fehr corneal dystrophy, is a rare pathological condition affecting the

stroma of cornea The stroma of the cornea (or substantia propria) is a fibrous, tough, unyielding, perfectly transparent and the thickest layer of the cornea of the eye. It is between Bowman's layer anteriorly, and Descemet's membrane posteriorly. At its centre, ...

first described by Arthur Groenouw in 1890.Groenouw A. Knötchenförmige Hornhauttrübungen (noduli corneae). Arch Augenheilkunde. 1890;21:281–289. Signs are usually noticed in the first decade of life and progress afterwards, with opacities developing in the cornea and attacks of pain. This gradual opacification leads to

visual impairment Visual or vision impairment (VI or VIP) is the partial or total inability of visual perception. In the absence of treatment such as corrective eyewear, assistive devices, and medical treatment, visual impairment may cause the individual difficul ...

often requiring keratoplasty in the later decades of life.

Epidemiology

While Macular Corneal Dystrophy is found throughout the world, countries with the highest prevalence include

Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...

Saudi Arabia Saudi Arabia, officially the Kingdom of Saudi Arabia (KSA), is a country in West Asia. Located in the centre of the Middle East, it covers the bulk of the Arabian Peninsula and has a land area of about , making it the List of Asian countries ...

India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...

, and the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. In Iceland, MCD accounts for almost one-third of all corneal grafts performed. Estimates from Claims Data in the United States place the prevalence of MCD at 9.7 per million, which represents less than 1% of corneal dystrophies.

Pathophysiology

Macular Corneal Dystrophy is an

autosomal recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the Phenotype, effect of a different variant of the same gene on Homologous chromosome, the other copy of the chromosome. The firs ...

genetic disorder caused by mutations in the carbohydrate sulfotransferase gene (CHST6), resulting in abnormal

proteoglycan Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan (GAG) chain(s). The point of attachment is a serine (Ser) residue to w ...

synthesis. The accumulation of abnormal glycosaminogycans in the corneal epithelium and stroma leads to progressive opacification of the cornea and subsequent loss of visual acuity. There are three variants of MCD characterized by

immunophenotype Immunophenotyping is a technique used to study the protein expressed by cells. This technique is commonly used in basic science research and laboratory diagnostic purpose. This can be done on tissue section (fresh or fixed tissue), cell suspensio ...

: * Type 1: no detectable keratan sulfate in either the serum or cornea * Type 1A: keratan sulfate is absent in the serum but stroma shows immunoreactivity to keratan sulfate antibodies * Type 2: normal amounts of keratan sulfate in the serum and stroma These three variants are clinically and histopathologically indistinguishable.

Signs and Symptoms

The first signs of MCD are cloudy regions that appear on the cornea during adolescence, although opacification may be noticed as early as the first decade. These minute, gray, punctate opacities will over time merge into larger areas, causing the entire corneal stroma to become opaque. Ultimately this results in severe visual impairment, generally before the 5th decade of life. While some individuals remain asymptomatic, initial symptoms typically consist of painful attacks with

photophobia Photophobia is a medical symptom of abnormal intolerance to visual perception of light. As a medical symptom, photophobia is not a morbid fear or phobia, but an experience of discomfort or pain to the eyes due to light exposure or by presence o ...

, foreign body sensations, and recurrent erosions. Corneal sensitivity is also reduced.

Diagnosis

Histopathological staining shows characteristic alcian blue-positive deposits. Various imaging modalities, including

confocal microscopy Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM) or laser scanning confocal microscopy (LSCM), is an optical imaging technique for increasing optical resolution and contrast (vision), contrast of a micrograph by me ...

and ocular coherence tomography, can provide information about the changes within the cornea and may be suitable replacements for tissue biopsy and excision.

Treatment and Prognosis

When visual acuity is impacted, various forms of keratoplasty are often indicated. While

corneal transplant Corneal transplantation, also known as corneal grafting, is a surgical procedure where a damaged or diseased cornea is replaced by Corneal button, donated corneal tissue (the graft). When the entire cornea is replaced it is known as penetrating ...

has traditionally been the standard treatment, less-invasive surgical techniques such as deep anterior lamellar keratoplasty and photo-therapeutic keratectomy are increasingly playing a role in management of MCD. While post-operative prognosis is favorable, reoccurrences may occur. Various gene therapies, including enzyme replacement therapy and gene-targeting therapy, remain a potential future treatment modality for MCD.

References

External links

{{Human corneal dystrophy Disorders of sclera and cornea