Vitelliform macular dystrophy


Vitelliform macular dystrophy

Description, Causes and Risk Factors:

Alternative Name: Vitelliform retinal dystrophy.

Vitelliform macular dystrophy is a genetic eye disorder that can cause progressive vision loss. This disorder affects the retina, the specialized light-sensitive tissue that lines the back of the eye. Specifically, vitelliform macular dystrophy disrupts cells in a small area near the center of the retina called the macula. The macula is responsible for sharp central vision, which is needed for detailed tasks such as reading, driving, and recognizing faces.

Vitelliform macular dystrophy causes a fatty yellow pigment (lipofuscin) to build up in cells underlying the macula. Over time, the abnormal accumulation of this substance can damage cells that are critical for clear central vision. As a result, people with this disorder often lose their central vision, and their eyesight may become blurry or distorted. Vitelliform macular dystrophy typically does not affect side (peripheral) vision or the ability to see at night.

Researchers have described two forms of vitelliform macular dystrophy with similar features. The early-onset form usually appears in childhood; the onset of symptoms and the severity of vision loss vary widely. The adult-onset form begins later, usually in mid-adulthood, and tends to cause vision loss that worsens slowly over time. The two forms of vitelliform macular dystrophy each have characteristic changes in the macula that can be detected during an eye examination.

Mutations in the BEST1 and PRPH2 genes cause vitelliform macular dystrophy. The BEST1, located on chromosome 11 (11q13). This gene regulates the production of (codes for) the protein bestrophin-1. The precise function of bestrophin-1 remains unclear, but recent studies have shown that the protein is multi-functional. Bestrophin-1 can act both as a calcium-activating chloride ion channel, regulating the passage of fluids and ions through the retinal pigment epithelium, and as a bicarbonate channel. Bestrophin-1 is normally localized in the outermost (basolateral) membrane of the pigment epithelium. Many BEST1 mutations cause faulty protein localization, which may impair the function of the protein.

A BEST1 mutation will lead to accumulation of the yellow waste product lipofuscin in the macula, and sometimes in the periphery of the retina. This deposit impairs function and sometimes causes cell death in the retinal pigment epithelium, a tissue that supports the adjacent retinal visual cells. A non-functioning pigment epithelium will lead to visual cell death. As the central part of the retina is affected, mainly visual acuity and color vision are impaired.

BEST1 mutations have also been identified in individuals with other retinal disorders, such as autosomal-dominant vitreoretinochoroidopathy (ADVIRC).

The PRPH2 gene provides instructions for making a protein called peripherin 2. This protein is essential for the normal function of light-sensing (photoreceptor) cells in the retina. Mutations in the PRPH2 gene cause vision loss by disrupting structures in these cells that contain light-sensing pigments. It is unclear why PRPH2 mutations affect only central vision in people with adult-onset vitelliform macular dystrophy.

Symptoms:

The first signs of the disorder may manifest at any time from about the age of four up to 60. In most cases the onset occurs before the age of 40. The primary symptom is progressive loss of visual acuity. Color vision may also be affected. Some individuals have a BEST1 mutation without ever developing any vision problems.

One eye often remains relatively unaffected. In the more affected eye, extensive vision loss may occur if blood vessels develop from the choroid, which is a vascular tissue located beneath the retina, or if degeneration of the retinal pigment epithelium (RPE) is severe. As peripheral vision is rarely affected, visual orientation is maintained throughout the person's Lifetime.

The degree of visual impairment may vary greatly among individuals. In many people visual loss remains moderate, although changes may occur during the progression of the disorder. Vision may deteriorate, but periods of improvement may also occur.

Diagnosis:

The following examinations are used to establish the diagnosis:

Fundoscopic examination. Eye fundus abnormalities are usually bilateral, but in some cases only one eye is affected. In vitelliform macular dystrophy the fundus of the eye may have a normal appearance. Visible lesions range from yellow, egg yolk-like accumulations in the central macula (vitelliform lesions) to a pseudohypopon, in which the yellow deposit (lipofuscin) has spread beneath the retina.

An eye microscope and a magnifying lens can be used to examine the retina. Ocular coherence tomography(OCT) is also used. This technology offers cross-sectional images of the retina, and may show the yellow deposit between the retina and the pigment epithelium in the macula. Fluid in and/or under the retina is also visible. Spectral-domain optical coherence tomography (SD-OCT) makes it possible to detect defects in the layers of cones and the pigment epithelium. Late stages of the disorder, associated with RPE degeneration, may be difficult to differentiate from other types of macular degeneration.

Electrooculography (EOG) is an electrophysiological technique that measures the resting potential between the cornea and the pigment epithelium. The measurement is carried out in dark and light conditions, with electrodes placed on each side of the eyes. An EOG will usually detect abnormalities associated with vitelliform macular degeneration, even in individuals who carry the mutation but remain asymptomatic.

Full-field electroretinography (ERG) is used to examine electrical activity in the retina. The test result of ERG examinations in individuals with vitelliform macular dystrophy were previously often described as normal, but recent studies indicate that ERG may be affected, for example in individuals with pronounced macular atrophy or biallelic BEST1 mutations.

Multifocal electroretinogram (mfERG) is used to identify impaired macular function.

DNA analysis can be used to identify BEST1 mutations. At the time of diagnosis, the family should be offered genetic counselling. Normal results on eye fundus examination do not exclude the possibility of being a carrier of the disorder. A DNA analysis is therefore often required to establish the diagnosis.

Treatment:

Individuals with the disorder should be regularly monitored by an ophthalmologist, who will also assess vision and the need for visual aids. As chamber angle defects as well as glaucoma may occur, the chamber angles should be examined using gonioscopy, and eye pressure should be measured. Abnormal blood vessels developing from the choroid (choroidal neovascularization) can be treated by injecting medication that inhibits vascular growth.

Visual rehabilitation are carried out in cooperation between an Ophthalmologist & vision resource centre. Interventions are planned to meet the current needs of the individual. Pedagogical, technical, psychological and social support are adapted to age and Lifestyle. Visual aids may be provided, such as magnifiers, special eye wear for close-up work, enhanced-vision TV systems and computers, and training in using these aids is offered. Compensating techniques are also taught, such as Braille and using a cane to facilitate mobility.

It is important to offer Psychological support, both at the time of diagnosis and later. Children and young people should also be offered psychological support, adapted to their individual needs.

Having a visual impairment may affect both a person's social life and choice of occupation. Much can be done to support the individual and compensate as much as possible for functional limitations.

NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.

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