Dry Age-Related Macular Degeneration Causes

Age-related macular degeneration (AMD or ARMD) is the most common cause of vision loss in those aged over 50. It causes a gradual loss of central (but not peripheral) vision. Central vision is needed for detailed work and for things like reading and driving.

The disease does not lead to complete blindness. Visual loss can occur within months, or over many years, depending on the type and severity of AMD. There are two main types of AMD – ‘wet’ and ‘dry’. ‘Wet’ AMD is most severe but more treatable.

Visual loss caused by AMD cannot normally be reversed. New medicines are an exciting development for wet AMD as they may halt or delay the progression of visual loss.

Causes of macular degeneration

Age-related macular degeneration (AMD) is a deterioration or breakdown of the eye’s macula. The macula is a small area in the retina — the light-sensitive tissue lining the back of the eye. The macula is the part of the retina that is responsible for your central vision, allowing you to see fine details clearly.

Eye anatomy diagramThe macula makes up only a small part of the retina, yet it is much more sensitive to detail than the rest of the retina (called the peripheral retina). The macula is what allows you to thread a needle, read small print, and read street signs. The peripheral retina gives you side (or peripheral) vision. If someone is standing off to one side of your vision, your peripheral retina helps you know that person is there by allowing you to see their general shape.

Many older people develop macular degeneration as part of the body’s natural aging process. There are different kinds of macular problems, but the most common is age-related macular degeneration.

One symptom of macular degeneration is dark areas in your central vision.

With macular degeneration, you may have symptoms such as blurriness, dark areas or distortion in your central vision, and perhaps permanent loss of your central vision. It usually does not affect your side, or peripheral vision.

For example, with advanced macular degeneration, you could see the outline of a clock, yet may not be able to see the hands of the clock to tell what time it is.

Causes of macular degeneration include the formation of deposits called drusen under the retina, and in some cases, the growth of abnormal blood vessels under the retina. With or without treatment, macular degeneration alone almost never causes total blindness.

People with more advanced cases of macular degeneration continue to have useful vision using their side, or peripheral vision. In many cases, macular degeneration’s impact on your vision can be minimal.

When macular degeneration does lead to loss of vision, it usually begins in just one eye, though it may affect the other eye later.

Many people are not aware that they have macular degeneration until they have a noticeable vision problem or until it is detected during an eye examination.

What is age-related macular degeneration?

AMD is a condition that occurs when cells in the macula degenerate. This occurs with partial breakdown of the RPE and the cells become damaged and die. Damage to the macula affects your central vision which is needed for reading, writing, driving, recognising people’s faces and doing other fine tasks.

The rest of the retina is used for peripheral vision – the ‘side’ vision which is not focused. Therefore, without a macula you can still see enough to get about, be aware of objects and people, and be independent.

However, the loss of central vision will severely affect normal sight. There are two types – ‘dry’ and ‘wet’ AMD – described below.

Who gets age-related macular degeneration?

AMD is the most common form of macular degeneration and develops in older people. There are other rare types of macular degeneration which occur in younger people. AMD can affect anyone. It is the most common cause of severe sight problems (visual impairment) in the UK, and indeed in the developed world.

It becomes more common with increasing age, as the name suggests. It is rare under the age of 60. If you develop wet AMD (see below) in one eye the risk of developing wet AMD in the second eye is about 1 in 4.

About 5 in 100 people aged over 65 and about 1 in 8 people aged over 80 have AMD severe enough to cause serious visual loss. About twice as many women over the age of 75 have AMD compared with men of the same age.

The two types of age-related macular degeneration

This is the most common form and occurs in 9 in 10 cases. In this type the cells in the RPE of the macula gradually become thin (they ‘atrophy’) and degenerate. This layer of cells is crucial for the function of the rods and cones which then also degenerate and die.

Typically, dry AMD is a very gradual process as the number of cells affected increases. It usually takes several years for vision to become seriously affected. Many people with dry AMD do not totally lose their reading vision.

Wet AMD may also be called neovascular or exudative AMD. It occurs in about 1 in 10 cases. However, it is likely to cause severe visual loss over quite a short time – sometimes just months.

Very occasionally, if there is a bleed (haemorrhage) from a new blood vessel, this visual loss can occur suddenly, within hours or days. In wet AMD, in addition to the retinal pigment cells degenerating, new tiny blood vessels grow from the tiny blood vessels in the choroid. This is called choroidal neovascularisation.

The new vessels break through Bruch’s membrane and into the macular part of the retina. These vessels are not normal. They are fragile and tend to leak blood and fluid. This can damage the rods and cones, and cause scarring in the macula, causing further vision loss.

Both wet and dry AMD are further classified according to severity. Early, intermediate or advanced types refer to the degree of damage to the macula. 6 in 10 cases of intermediate/advanced AMD are due to wet AMD.

What causes age-related macular degeneration?

In people with AMD the cells of the RPE do not work so well with advancing age. They gradually fail to take enough nutrients to the rods and cones, and do not clear waste materials and byproducts made by the rods and cones either.

As a result, tiny abnormal deposits called drusen develop under the retina. In time, the retinal pigment cells and their nearby rods and cones degenerate, stop working and die. This is the dry type of AMD.

In other cases, something also triggers new blood vessels to develop from the choroid to cause the wet form of AMD. The trigger is not known. It may be that some waste products which are not cleared from the RPE may stimulate new blood vessels to grow in an attempt to clear the waste.

The exact reason why cells of the RPE stop working properly in people with AMD is not known. Certain risk factors increase the risk of developing AMD. These include:

  • Smoking tobacco.
  • Possibly, high blood pressure (inconclusive evidence).
  • A family history of AMD. (AMD is not a straightforward hereditary condition. However, your risk of developing AMD is increased if it occurs in other family members.)
  • Sunlight. This has yet to be proven, but laboratory studies suggest that the retina is damaged by sunlight rays (UVA and UVB rays).
  • AMD seems to be more common in people from white (Caucasian) racial backgrounds than from other racial groups.

What are the symptoms of age-related macular degeneration?

The main early symptom is blurring of central vision despite using your usual glasses. In the early stages of the condition you may notice that:

  • You need brighter light to read by.
  • Words in a book or newspaper may become blurred.
  • Colours appear less bright.
  • You have difficulty recognising faces.
  • One specific early symptom to be aware of is visual distortion. Typically, straight lines appear wavy or crooked. For example, the lines on a piece of graph paper, or the lines between tiles in a bathroom, or the border of any other straight object, etc.
  • A ‘blind spot’ then develops in the middle of your visual field. This tends to become larger over time as more and more rods and cones degenerate in the macula.
  • Visual hallucinations are common in people with severe visual loss of any cause. Visual hallucinations (also called Charles Bonnet syndrome) can occur if you have severe AMD. People see different images, from simple patterns to more detailed pictures. The experience can be upsetting but is less frightening if you are aware that it can happen in AMD. Importantly, it does not mean you are developing a serious mental illness. If you do develop visual hallucinations they typically improve by 18 months but in some people they last for years.
  • AMD is painless. Symptoms of dry AMD tend to take 5-10 years to become severe. However, severe visual loss due to wet AMD can develop more quickly.

Always see a doctor or optometrist promptly if you develop visual loss or visual distortion. This is not only the case if you are worried about AMD. Other sight-threatening conditions can occur suddenly with visual loss, such as a detached retina. Peripheral vision is not affected with AMD and so it does not cause total blindness.

Note: if the vision of one eye only is affected, you may not notice any symptoms, as the other good eye often compensates. When both eyes are affected you are more likely to notice symptoms. Older people should have regular eye checks to check each eye separately for early AMD (and to check for other eye conditions such as glaucoma).

How is age-related macular degeneration diagnosed?

If you develop symptoms suggestive of AMD, your doctor or optician (optometrist) will refer you to an eye specialist (ophthalmologist). This should be done urgently, especially if there is any suggestion of wet AMD.

The ophthalmologist may ask you to look at a special piece of paper with horizontal and vertical lines to check your visual fields. If you find that any section of the lines is missing or distorted, then AMD is a possible cause of the visual problem.

The ophthalmologist will examine the back of your eye with a slit lamp microscope. This is a magnifying piece of equipment where the ophthalmologist examines your retinae through what look like binoculars. Digital photographs can be taken of the retinae. The ophthalmologist will look for the typical changes that occur with dry AMD and wet AMD.

Another test called ocular coherence tomography is becoming more commonly used. This is a non-invasive test that uses special light rays to scan the retina. It can give very detailed ‘3D’ information about the macula, and can show if the macula is thickened or abnormal.

This test is useful when there is doubt about whether AMD is the wet or dry form. It is also a useful test to assess and monitor the results of any treatment.

If wet AMD is diagnosed or suspected, then a further test called fluorescein angiography may be done. For this test a dye is injected into a vein in your arm.

Then, by looking into your eyes with a magnifier and taking pictures with a special camera, the ophthalmologist can see where any dye leaks into the macula from the abnormal leaky blood vessels. This test can give an indication of the extent and severity of the condition.

Is there any treatment for age-related macular degeneration?

For the more common dry AMD, there is no specific treatment yet. There are, however, certain things that can be done to maximise the sight you do have and to improve your eye health.

Low vision rehabilitation and low vision services are offered by hospital eye departments and information can be found from the Macular Disease Society and the Royal National Institute of Blind People (RNIB).

Stopping smoking and protecting the eyes from the sun’s rays by wearing sunglasses are important. A healthy balanced diet rich in antioxidants may be beneficial, as may the addition of dietary supplements (see below for details). Remember that in this type of AMD the visual loss tends to be gradual, over 5-10 years or so.

For the less common wet AMD, treatment may halt or delay the progression of visual loss in some people. Newer treatments may even be able to reverse some of the visual loss. Treatments which may be considered include treatment with anti-vascular endothelial growth factor (anti-VEGF) medicines, photodynamic therapy and laser photocoagulation.

Photodynamic therapy

This is a technique that was developed in the late 1990s. A medicine called verteporfin is injected into a vein in the arm. Within a few minutes the verteporfin binds to proteins in the newly formed abnormal blood vessels in the macula.

A light at a special wavelength is then shone into the eye for just over a minute. Verteporfin is a photosensitive medicine. This means that when light is shone at the blood vessels coated with verteporfin, the verteporfin activates and causes damage, destroying the abnormally growing blood vessels (neither damaging the nearby rods and cones, nor any normal blood vessels).

Photodynamic therapy is only suitable for some cases. It depends on exactly where the new blood vessels are growing and their extent. It does not work in all cases although the success rate in treated people is high.

Success means that the visual loss is prevented from getting worse – it does not restore any lost vision. Treatment usually needs to be repeated every few months to continue to suppress newly growing blood vessels. The main advantage that this method has over laser photocoagulation is that there is less damage to the normal retina.

Laser photocoagulation

This is a technique where a fine laser is ‘fired’ at the tiny new blood vessels that are forming. This destroys the developing new blood vessels which helps to prevent the condition from getting worse. People undergoing this treatment will develop a permanent black or grey patch affecting their vision and no sight is restored.

Laser photocoagulation is only suitable for a small number of cases. It depends on exactly where the new blood vessels are growing, as the laser may also damage the rods and cones.

New blood vessels growing very close to the fovea may not be suitable because of the risk of severe visual loss arising from laser damage or scarring due to laser treatment.

Other treatments

Treatments such as radiation therapy, other medicines and surgery to the retina are being investigated. For example, a surgical technique where a part of the peripheral retina is grafted into the diseased macular area is being investigated.

The value of these newer treatments is not clear. The treatment of macular degeneration is an active area of research and treatment may well improve in the near future.

Source & Other Info: Patient.co.uk and Get Eye Smart



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