Age-related macular degeneration or AMD is a disease of the macula, an area in the centre of the retina.
The retina is the light-detecting membrane at the back of the eye. The macula contains a particularly high density of light receptors, especially cones (the receptors that detect colour) and so the macula is particularly important for seeing fine detail and objects directly in front of us. It plays a vital role in helping people to read, write, drive and perform other detailed tasks. It also enables us to recognise faces and see colour.
Dry AMD is the most common form of the condition. Light receptor cells in the retina function less well as a person gets older. The cells don’t take in enough vital nutrients and become less efficient at clearing by-products of cell functioning. This causes tiny abnormal deposits, called drusen, to build up under the retina, making it uneven.
In time, retinal cells degenerate and die causing sight loss. This occurs very gradually over many years. Dry AMD has three stages:
-> Early AMD – small to medium drusen with no symptoms or loss of vision.
-> Intermediate AMD – early symptoms such as a blurred spot in the centre of vision, and more light needed for reading.
-> Advanced AMD – more extensive destruction of light sensitive cells and worsening vision.
Wet AMD accounts for ten to 15 per cent of cases. It’s also known as ‘neovascular AMD’ because it involves the growth of new blood vessels behind the retina. These new blood vessels are very fragile and so may leak fluid or blood, lifting up and distorting the retina. This results in distortion of vision and scarring that causes rapid visual loss. Everyone who develops wet AMD has the dry form first. Wet AMD can develop very quickly and it’s not possible to predict who’ll develop wet AMD or when it will occur.
It’s unclear what causes AMD, but it’s probably a combination of genetic vulnerability combined with environmental factors. Those with a family history of the condition are at increased risk, as they may carry the same genes.
AMD becomes more likely as a person ages. Over time we’re exposed to more and more environmental insults or other illnesses which aggravate the AMD disease process and contribute to damage to the cells in the macula.
Both eyes are usually affected, although one eye may be affected before the other. The good eye usually compensates for the affected eye and for many years this can disguise the fact that there’s a problem. There’s no pain or redness of the eyes.
As the macula is in the middle of the retina its central vision that’s affected, and patients retain some residual vision. However this only provides the edges of what a person is trying to look at, and the focus (and colour strength) in this area is poor. Whatever the person tries to bring to the centre of their vision, to see more clearly, simply becomes dark or lost.
Any activity that requires detailed, clear vision is compromised, and in the late stages of the disease sight loss is so severe that patients are offered registration as partially sighted or blind.
In developed countries it’s estimated that one in 50 people over 50 years of age, and up to one in five people over the age of 85 have AMD. Wet AMD is less common (about one in ten cases of AMD, which amounts to 26,000 people diagnosed with wet AMD every year in the UK) but it’s more critical because it can progress so rapidly.
A person is at greater risk of developing AMD if someone in the family has the condition. Lifestyle factors also are important – smokers are three times more likely to develop AMD (probably because this causes oxidation damage to the light-receptor cells, or damages their blood and oxygen supply), and high blood pressure, being overweight, having small eyes (linked to long sight), and blue iris pigmentation all increase the risk. It’s also more common in women than men.
Currently, there’s no reliably proven medical treatment for dry AMD. However, not smoking and eating a healthy diet may help to slow the rate of deterioration, which is generally very gradual. A regular dietary supplement of vitamins C and E, and zinc may help to slow down visual loss for some people who already have established AMD.
There has been particular interest recently in two carotenoid dietary substances known as lutein and zeaxanthin, which in some cases seem to reduce the development and progression of AMD. Although studies don’t consistently demonstrate a benefit, people at high risk of AMD may want to consider taking supplements of these substances.
Other treatments, such as a blood filtering process called Rheopheresis, are currently being studied.
Additional lighting and magnifiers can help those with dry AMD to make better use of their residual sight.
When wet AMD develops, urgent treatment is usually advised because the disease can progress rapidly. Treatment for wet AMD aims to control the formation of new, leaky blood vessels. Recent medical breakthroughs such as new drug treatments are providing much better results. In most cases, treatments can prevent further visual loss, and in some cases bring some improvement of sight. As a result, some older treatments such as laser photocoagulation are no longer used.