Age-Related Macular Degeneration
Globally, age-related macular degeneration (AMD) is the leading cause of permanent vision loss in adults over 60. Due to ageing of the global population, it has been suggested that by 2040 an estimated 288 million people will have at least partially lost their vision due to this disorder.
It’s called ‘age-related’ as it usually occurs in people who are older than 50, and is more likely in those who are 65 or older. There are some rare diseases that can mimic macular degeneration in children and adolescents, but these variants affect very few.
And it is called ‘macular’ because this is the name of the centre of the retina. The macular is responsible for your central vision. As you read these words, it is the macula that is receiving the light and processing the image.
Amongst Australians, early AMD affects approximately 15% of the population, and the advanced forms affect <1% of the population (1, 3). AMD is the leading cause of permanent impairment of central vision among people aged 65 years and older. Central vision is what we use whenever we look directly at something. This includes reading, watching television, and recognising someone’s face. For this reason, unfortunately AMD often has a severe impact on a person’s quality of life.
The eye is made up of three layers or ‘tunics’:
1. The outer layer, or fibrous tunic. This consists of the sclera (the white of the eye) and the cornea (the transparent eye covering);
2. The middle layer, or vascular tunic, which comprises the iris (the membrane that is coloured and contains the pupil), the choroid (the pigmented vascular layer of the eyeball between the retina and the sclera). and the ciliary body (the part of the eye that connects the iris to the choroid);
3. The inner layer, called the nervous tunic, which consists of a layer of photoreceptors and neurons called the retina. The retina lines the back two-thirds of the eyeball. The central are of the retina is called the macula. This area of the retina is specialised to see objects in very high detail.
The macula contains contains a high concentration of cone photoreceptors, packed densely together. These cones enhance colour vision, and allow us to see fine details. When the macular photoreceptors become damaged, central vision loss occurs, which is the chief characteristic of AMD.
The macular is coloured yellow because of the pigmented antioxidants lutein, zeaxanthin, and meso-zeaxanthin.
• Meso-zeaxanthin is most highly concentrated in the middle of the macula
• Zeaxanthin in the mid-periphery of the macular, and
• Lutein in the periphery of the retina.
This yellow colour of these pigments are very important as they absorb (block) harmful blue and violet wavelengths of light from reaching the photoreceptors. The yellow pigment therefore acts like a pair of sunglasses inside the eye, shielding the retina from damaging light.
Causes of AMD
A cardinal feature of AMD is the presence of “drusen” in the macular. Drusen are always present in cases of AMD, even though they probably don’t actually cause AMD.
What are drusen?
Drusen are tiny yellow deposits of abnormal material beneath the retina. They look like pale yellow dots scattered around the macular. Drusen are made from similar material fatty plaques in arteries – fats, cholesterol, and protein. Drusen are essentially the waste products of the retina. When you are younger, drusen are normally eliminated at the rate that they are formed. However, when the retina is aged and sick, it is unable to process its waste material efficiently. These waste products build up and begin accumulating around the macula as drusen.
Drusen are toxic to the retina. As the drusen accumulate, they cause stress to the retinal cells. This causes the retinal cells to deteriorate and die, which adds to the pile of drusen. The result is a vicious cycle, whereby drusen triggers retinal cell injury, which results in more drusen forming. The retinal cells that are most susceptible to injury from drusen are the photoreceptors (these cells capture the image) and the Retinal Pigment Epithelium.
Having a few small drusen does not mean that you have AMD. Most people over 50 have some small drusen, and this is considered a normal ageing change.
However, a person is considered to have AMD if they have a large number of drusen, drusen that are medium or large in size, or evidence of retinal cell death. Drusen are more commonly found in people over 60, more common in women than men, and more common in smokers than non-smokers. The size of drusen is calculated by your eye specialist comparing them to the width of your retinal veins, or measuring the drusen on a laser scan.
Two types of AMD
There are two primary types of macular degeneration: wet and dry. The dry form is much more common: about 90% of AMD cases are ‘dry’.
Dry AMD develops when drusen collects in and around the macula. This causes thinning and atrophy (cell death) of the macula. Large areas of retinal atrophy are called ‘geographic atrophy’. This is because the patches of cell death tend to have irregularly-shaped borders, just like the borders of countries on a geography map. As the retinal cells atrophy (die), dark pigmentation can occur. Therefore advanced dry AMD typically causes the macular to have a speckled appearance of patchy pale and dark areas.
Vision loss from dry AMD is slow. Vision change typically occurs over many years. Many patients with dry AMD will maintain reasonably good vision for the rest of their life.
Importantly, peripheral vision is not affected by macular degeneration. Therefore patients with AMD never go completely blind. Even in severe AMD, patients can usually safely navigate independently because they still have good peripheral vision.
In some cases, dry AMD can lead to wet macular degeneration (see below). This progression is not inevitable and most patients with dry AMD will not progress to the wet form. There are numerous examination features an ophthalmologist can look for to help prognosticate a patient’s risk of converting to wet AMD.
Three stages of dry AMD have been identified:
Early stage of dry AMD
Here, medium-sized drusen are present. Most patients will have normal vision or only trace distortion of central vision.
Intermediate stage of dry AMD
Here, one or more large drusen are present, or there are medium-sized drusen accompanied by pigmentation. Symptoms at this stage may include distortion of central vision, a blurred spot affecting central vision, and the need more light for reading.
If you reach this stage of dry AMD, you are at higher risk of developing the advanced stage, or even wet AMD
Advanced stage of dry AMD
The advanced stage of dry AMD is known as geographic atrophy. This is characterised by large areas of photoreceptor death. Patients with geographic atrophy have dense blind spots that correspond to these patches of cell death.
Wet AMD is also known as neovascular AMD or choroidal neovascularization. This condition affects only about 10% of those who suffer AMD.
Wet AMD develops when new, abnormal blood vessels grow into the macular. The process of new blood vessels forming is called “neovascularisation”, hence the term “neovascular AMD”. These blood vessels are unhealthy and they have thin walls that tear and leak easily. The vessels tend to leak fluid and blood, both of which are toxic to the retina. As the retina swells, the photoreceptors are knocked out of alignment and a patients central vision becomes very distorted.
It is thought that the accumulation of drusen around the macula may encourage the growth of these new blood vessels into the macular.
Wet AMD causes rapid and severe damage of central vision. It typically progresses over days to weeks. If the wet AMD is not treated promptly, the vision loss is often permanent.
Note that the onset of wet AMD is typically sudden. If you have AMD and you notice that your vision in one eye is changing – straight lines suddenly appear curved and wavy, or there is a dark spot in your central vision – then you need to see an ophthalmologist immediately. The sooner wet AMD is treated, the better the visual outcome.
Relationship between wet and dry AMD
Wet AMD can result from the progression of dry AMD, although in rare cases, it may occur before significant dry AMD has even developed.
Symptoms of AMD
It is possible to have AMD for some time before symptoms are experienced. Sometimes, only one eye is affected, although most commonly it will affect both eyes.
As a progressive and degenerative disease, AMD gets worse over time. It can progress quickly or slowly. However, there are various treatments that act to slow its progression and minimise the ongoing damage to the retina. It is important to note that the earlier treatment is instigated, the better the final outcome.
This is why it is important to get your eyes checked by an optometrist or ophthalmologist at least once a year if you are over 40.
How do you know if you are developing AMD?
The following symptoms are common:
• Blurred central vision. Central vision is what you need for reading, sewing, cooking, looking at faces, and driving. Over time, this blurred area may become larger, or you may develop some blind spots.
• Straight lines start to appear curved, blurry or wavy. The medical term for this is “metamorphopsia”. This is usually the earliest symptom of AMD.
• Trouble seeing in dim light.
• Colours may seem to fade, becoming less vivid or bright.
Symptoms of Dry AMD
If you have early forms of dry AMD, you may not notice any symptoms at all. Most patients with dry AMD are therefore more likely to receive a diagnosis as an incidental finding when they see their eye specialist, rather than because of any particular symptoms that they have.
How is AMD diagnosed?
The key to slowing or preventing vision loss is getting your eyes checked regularly. In particular, people with the following characteristics should get a complete eye examination, and then follow up ever one or two years. Regular checking is important.
If you have cardiovascular issues, or are:
- Aged 60 or older
- A smoker