Visual
impairment
[Macular degeneration]
Cataract
Glaucoma
Diabetic retinopathy
Macular Degeneration
What is Macular Degeneration?
Age-related maculopathy, also
called macular degeneration, is a term describing disease of the macula,
the most central part of the retina at the back of the eye. When we look
directly at someone or at text in a book or at a work of art, we are using
our macula. When the macula is damaged, it is like having a dark, grayish
or distorted patch in the centre of everything you look at. You can't
drive, read or recognise the faces of your friends.
Although macular degeneration is most frequently seen in the older age groups, forms of this condition can also affect young people. These cases most commonly occur in younger women (typically aged in their 20's or 30's, who are also often short-sighted) and the condition may follow a childhood or adolescent infection or inflammation. Other forms may be seen in people who have high myopia or certain inherited conditions, such as pseudoxanthoma elasticum.
The common form in older people is termed age-related maculopathy (ARM). This term is generally restricted to the condition occurring in people aged over 50. There are two recognised late forms; a more frequent neovascular ("wet") type in which new blood vessels break through the retina, then bleed or leak and subsequently cause a scar, and a less frequent atrophic ("dry") type in which critical cells at the macula slowly die off.
These late forms have well-recognised
precursor or early signs that are usually present for many years, before
vision becomes affected. There are two main signs that indicate that a
person is at risk of vision loss from macular degeneration;
1) large drusen (white or cream-coloured spots in the macular area) that
often develop a "soft" appearance.
2) visible pigment changes at the macula, including clumps of increased pigment or areas in which the pigment layer has become thinned.
When signs of early ARM are present, vision is mostly normal or only slightly affected. Very typical and relatively sudden visual symptoms are described by people who develop progression from the early to late stage of ARM. These symptoms include distortion of straight lines or change in the shape of objects (also called metamorphopsia), or the onset of a dark or greyish patch seen in the centre of the field of vision of one eye (also called a scotoma). Most people describe a sudden worsening of the vision in the affected eye.
Progression to the neovascular stage occurs suddenly, often with a bleed in the retina. Over time, the haemorrhage clears, but is replaced by a scar which causes irreversible damage to the macula with resulting poor vision.
Evidence suggests that over half of people who develop late stage macular degeneration in one eye will subsequently develop these signs in their second eye.
Over years, macular degeneration scars can enlarge and involve a large area of the central retina, This produces a large blank patch in the central vision in which any details of images or faces are invisible.
ARM is now the leading cause of blindness in Australia, being responsible for over two thirds of new cases of blindness in people aged over 50, with all other eye diseases together making up the other one third. Australian research has been at the forefront of efforts to unravel the causes of this disease. The Blue Mountains Eye Study (BMES) indicates that around 100,000 Australians currently have either neovascular or atrophic macular degeneration, including more than 30,000 Australians who are legally blind in both eyes from this disease.
Risk Factors
Familial
Apart from increasing age, two dominant risk factors have been identified.
The first is a genetic risk. Information from the Rotterdam
Study suggests that if your brother or sister has macular degeneration,
then your lifetime risk is probably around 50%. On the other hand, if
you have no family history, then your lifetime risk is much less, around
12%.
Smoking
Second, although only relatively few older people are still smoking, those
who do are much more likely to develop macular degeneration. In the BMES
current smokers were shown to have four times the risk of developing macular
degeneration than people who had never smoked or those who had given up
smoking. Pooled
data from 3 continents found a similar risk. BMES data also indicate
that smokers develop the disease about 10 years earlier than non-smokers.
A second BMES report estimated that there are around 8,000 Australians whose blindness from macular degeneration was caused by their smoking. Smoking may thus cause or contribute to around 20% of new blindness in people over 50. This is a massive proportion of blindness due to an avoidable risk factor. This information from the BMES was taken up by the Australian government and used as one of two themes for the Australian National Quit Campaign, on smoking and blindness. Centre for Vision Research staff have called for a new cigarette pack warning: "Smoking causes blindness".
Nutrition
Nutritional factors may also increase or decrease a person's risk of developing
age-related maculopathy. Although several studies have suggested that
higher intakes of certain antioxidants in the diet may protect against
age-related maculopathy, this could not be demonstrated in the BMES.
However, a publication from the Age-Related Eye Disease Study has shown that taking supplements containing high levels of antioxidants and zinc significantly reduced the risk of advanced age-related macular degeneration in persons with a moderate level of early stage lesions.
Other studies have suggested that specific carotenoids that contribute to macular pigment density (lutein) may be useful. A number of worldwide studies are underway to evaluate carotenoids such as lutein.
Data from the BMES also indicate a link with dietary fat consumption. The risk was increased with higher dietary fat and was lower in people who said that they consumed fish regularly. These findings matched similar data from the Beaver Dam Eye Study, based in Wisconsin, USA
Hormonal
Hormonal factors may also be important in women. The Rotterdam Study indicated
a higher risk of macular degeneration in women who had an early menopause,
while the Eye
Disease Case Control Study reported a lower risk of macular degeneration
in women who used hormone replacement therapy.
Vascular
It is difficult to study vascular risk factors in a condition like macular
degeneration, which occurs in people living to an older age. However,
recent information indicates that vascular diseases (long standing hypertension
and underlying large vessel disease, particularly affecting carotid arteries)
may cause a significant increase in risk. The smoking and dietary fat
findings from the BMES support a vascular basis, although these may be
shared risk factors. Data linking carotid disease to macular degeneration
has been shown from the Rotterdam and Beaver Dam Studies.
Skin and eye colour
Macular degeneration is more frequently seen in whites than in blacks
or asian races. Among whites, blue eyes have been associated with a higher
risk in a number of studies including the BMES.
Environmental
Although some studies have linked sunlight or UV exposure to macular degeneration,
this was not able to be demonstrated in a large
case-control study conducted in Newcastle or in the BMES. Nevertheless,
both these studies have suggested that people with skin that is more sensitive
to sunlight may have a higher risk.
