Age-related Macular Degeneration

Age-related macular degeneration, or AMD for short, can strike fear into the bravest hearts. These words can conjure up images of total blindness, complete loss of independence, white walking sticks, and Labrador guide dogs (which should be considered a perk of vision impairment, really). While permanent vision loss isn’t generally a topic to laugh at, the fear factor of AMD often arises from a public misunderstanding of what it actually is. Asking the right questions can help to allay these fears.

 
What is this macula you speak of?
The macula is an anatomical structure of the retina in the eye. The retina is a sensory layer of tissue, comprised of its own ten layers, at the back of the eye. After passing through various other structures at the front of the eyeball, light falls onto this retina and the nerve impulses are then carried via the optic nerve to the visual cortex of the brain for interpretation.

The macula is a special part of the retina because of a slightly different structure. It is optimized for acuity (sharp vision) and color vision because it contains the highest density of a certain type of retinal cell known as cone photoreceptors. The other type of retinal cell, known as rod photoreceptors, exist outside of the macula region and have been found to perform better in dim lighting than cones but aren’t so good at discerning color or fine detail.

 
I didn’t ask about rod photoreceptors, tell me more about the macula.
Ahh, ok then. The macula controls central vision, so when you are looking directly at something, such as the letters on a chart with the optometrist or ophthalmologist, you’re receiving this information onto the macula. When the macula is affected by AMD it is therefore the central vision that begins to blur or become patchy.

It is thought that AMD is due, at a very basic level, to the macula’s high energy demand. After cone photoreceptors receive light and pass on the neural signal they need to regenerate. A layer of cells known as the retinal pigment epithelium (RPE) is responsible for supporting the regeneration of these photoreceptors by removing the waste material produced during metabolism. Over time and with age, the cells of the RPE decrease in number and accumulate metabolic debris, which makes them increasingly ineffective at removing the waste produced by the constant renewal process of the photoreceptors.  It is also thought that exposure to oxidative stress, another metabolic by-product, further compromises the function of RPE cells, which in turn, signals these cells to apoptose (a nice medical word for “die”). RPE cells contain antioxidants to counter the effects of oxidative stress but the concentration of these antioxidants has been shown to decrease with age.

 
How do I know if I have macular degeneration? Will it make me totally blind? Sometimes I can’t read road signs from one hundred miles away. I have AMD, I just know it!
Don’t worry. No one except cyborgs can read road signs from a hundred miles away.

Because macular degeneration, by definition, affects the macula of the eye only, even in the worst case of AMD the peripheral vision remains intact. A patient with AMD may experience difficulties with tasks that require being able to visually discern fine detail, such as reading or writing. While driving often involves peripheral vision, the state of California has a visual acuity standard of 20/40 to hold a driver’s license, which means AMD can also impact the ability to drive.

Of course, having difficulties with any of these tasks does not indicate a definite diagnosis of AMD as other diseases can also mimic the effects of early AMD, such as those fingerprint smudges on your reading glasses. The most definitive way of determining the presence of AMD is to schedule an eye test with your eye care practitioner, either an optometrist or ophthalmologist.

AMD investigations will involve a visual acuity test (reading letters on a chart) and viewing the actual macula with a glorified microscope known as a slit lamp. Optical coherence tomography has also been shown to be extremely useful in the diagnosis of AMD as this particular imaging technique is able to visualise the lower layers of the retina, including the RPE.

One of the signs of AMD as seen by your eye care practitioner is the presence of drusen, which is a little lump of waste material accumulated underneath the RPE. The size and number of drusen deposits have been shown to correlate with the risk of AMD progression. Changes in pigment around the macula is also another sign of AMD, such as dropout and loss of the RPE cells or an accumulation of these cells appearing as a darkly pigmented spot. In advanced AMD there may be total loss of the RPE in one area, an end-stage state known as geographic atrophy. A certain type of AMD commonly known as wet AMD involves the development of new fragile blood vessels around the macula, which can result in bleeding and scarring of the retina. Until these new blood vessels develop, the AMD is classified as dry. In a lot of cases, AMD never progresses to the wet form.

The signs and symptoms of AMD may be found in only one eye and in many situations where AMD is present in both eyes, may be more progressed in one eye than the other.

 
How do I know if I’m at risk of AMD? Should I sign up for a guide dog puppy now?
As can be deduced by the name, one of the risk factors of AMD is increasing age. Other factors that increase the risk of developing AMD include:

  • Tobacco smoking
  • Hypertension and high cholesterol
  • Caucasian ethnicity
  • Family history of AMD

As you can see, two of these factors are modifiable and within your control while the other two are not.

 
If I ever do get AMD, what can be done about it?
Unfortunately there is no current treatment for the dry form of AMD. There is a nutritional supplement available based on a study called AREDS2 (age-related eye disease study) that has been shown to slow the progression of intermediate to late AMD. This supplement contains lutein, zeaxanthin, copper, zinc, and vitamins C and E. Not everyone will benefit from taking this supplement, such as those with only early AMD, so it is best to see your eye care practitioner before you start popping pills. The wet form of AMD is treated with the use of intravitreal injections (meaning the needle goes into your eye – yes, into your eye) with a medication that prevents the new blood vessels from developing. Studies have shown that some patients actually experience an improvement in their visual acuity following these treatments, instead of just maintaining their current vision.

If you are concerned about AMD remember that some risk factors can be reduced or eliminated altogether (unfortunately disowning your biological parents doesn’t work) and a diagnosis of AMD doesn’t mean complete loss of independence though a guide dog puppy might be cute. Hopefully by the end of this article you’re feeling a bit more educated about AMD, but if you are getting really worried just keep calm and see your eye doctor.

 

References
Age-related macular degeneration and the aging eye: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2682379/
State of California DMV – Vision Requirements for Driving C Class Vehicles: https://www.dmv.ca.gov/portal/dmv/?1dmy&urile=wcm:path:/dmv_content_en/dmv/pubs/brochures/fast_facts/ffdl14
Risk Factors for AMD: http://www.visionaware.org/info/your-eye-condition/age-related-macular-degeneration-amd/risk-factors-for-amd/125

 

 


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