Understanding AMD

What is age-related macular degeneration?
How does AMD affect your sight?
What is dry (atrophic) macular degeneration?
What is wet (exudative) macular degeneration?
What causes AMD?

 

What is age-related macular degeneration?

Age-related macular degeneration is a disease causing progressive damage to the macula, the central area of the retina. It is the most common cause of visual impairment in people over the age of sixty-five. There are two types of macular degeneration; wet and dry.

In AMD, the extent of the degeneration may be different in each eye and may not progress at the same rate.

It is helpful to know a little about the eye and how it works in order to understand the effect AMD has on the vision and how it can be treated.

Anatomy of a normal eye

The cornea forms the clear window into the eye. The iris, which is the coloured part of the eye with the black pupil in the middle, is behind the cornea. The lens lies behind the iris.

The wall of the eye is formed by three layers, the retina, the choroid and the sclera

The retina is the light-sensitive nerve tissue that lines the inner wall of the eye. Rays of light enter the eye, passing through the cornea, pupil and lens before focusing on to the retina. The retina contains photoreceptors which convert light into electrical impulses. In the healthy eye these impulses are sent via the optic nerve to the brain, where sight is interpreted as clear, bright, colourful images. The retina can be likened to photographic film in a camera.

The retina lies on a layer of supporting tissue known as retinal pigment epithelium or RPE. The RPE is important as it nourishes the photoreceptors and removes their waste products.

The macula is a small area at the centre of the retina. It is very important as it is responsible for our central vision. It allows us to see fine detail for activities such as reading, recognising faces, watching television and driving. It also enables us to see colour.

The choroid is the underlying vascular (blood vessel) layer of the eye from which the retina receives oxygen and nutrients.

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How does AMD affect your sight?

The earliest sign of macular degeneration is often the appearance of small yellow deposits called drusen, which form under the retina.

The presence of drusen does not necessarily mean that a person will have visual problems. Drusen can be found in younger people who do not have macular degeneration.

Over time, however, the drusen can become larger and increase in number. The pigment layer of the eye can also be disturbed, leading to a “patchy” appearance (atrophy). When drusen or atrophy are accompanied by a decrease in vision, the term macular degeneration is used.

Common symptoms of AMD are:

  • blurred vision;

  • distortion of straight lines;

  • a dark spot in central vision;

  • objects appearing the wrong shape, size or colour;

  • objects moving or disappearing;

  • difficulty in bright sunlight and dim lighting.
Example of vision with age-related macular degeneration

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What is dry (atrophic) macular degeneration?

Drusen are accumulated waste products from the retina which, as they get larger, stop the flow of nutrients to the retina. Drusen cause the layer of supporting cells beneath the retina to become very thin. When cells in this layer (the retinal pigment epithelium) die, the overlying macula degenerates and loses its visual function.

This type of degeneration is called dry macular degeneration; there is no leaking of fluid or blood.

Over time, sometimes many years, the disease process worsens, with more of the macula being affected. This degeneration is called geographic atrophy and may be considered the end stage of the dry form of the disease.

The dry type is much more common, but with many people maintaining useful vision.

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What is wet (exudative) macular degeneration?

AMD patients start with the dry form. Ten per cent of these people will go on to develop the wet form of the disease. Although the wet type is much less common it tends to be more aggressive, causing greater and more rapid visual loss.

In response to the retinal degeneration, new blood vessels from the choroid (the deeper vascular layer of the eye) sometimes develop beneath the retina, like weeds growing up through the cracks in a pavement. This process is called neovascularisation.

Neovascularisation does not help the retina because the vessels are very fragile and easily leak or bleed. Eventually this process results in the formation of a disc-shaped grey scar in the middle of the visual field.

The scar may take some time to form, but once established all retinal tissue in the area is destroyed, leaving a large central blind spot. Macular degeneration cannot be reversed once this has occurred.

There are two types of neovascularisation, the difference being apparent on special tests such as fluorescein angiography (which will be explained later). One is called “classic”, in which the new vessels develop in a clear and well-defined manner. The second form of wet AMD is called “occult”, where the new vessels are hidden in deeper layers and are poorly defined.

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What causes AMD?

Although the cause of AMD is currently not fully understood, a number of risk factors have been identified.

  • Age; this is the main risk factor for developing AMD. As you age, your risk of developing the condition increases.

  • Genetics; although there is rarely a strong hereditary pattern, it is known that people with a family history of macular degeneration have an increased chance of developing AMD.

  • Smoking; in a number of studies, smoking has been linked to development of AMD. It has been shown that stopping smoking can reduce the risk.

  • High blood pressure; studies have shown that high blood pressure is linked to the likelihood of developing AMD.

  • Nutrition; the fragile cells of the macula are highly susceptible to damage from oxygen-charged molecules called free-radicals. It has been shown that people who have a low intake of antioxidants (nutrients that fight the damaging effects of free-radicals) may be at an increased risk of developing AMD. Excessive alcohol consumption may also deplete the levels of antioxidants in the body.

  • Sunlight; the cells of the macula are highly sensitive to sunlight and cell damage from the sun can lead to deterioration of the macula. The use of good quality sunglasses helps to protect the eyes from this risk.

  • Gender; women are more susceptible to developing AMD than men.

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