Biology of AMD

What Is Age-Related Macular Degeneration (AMD)?

As a chronic, progressive disease, age-related macular degeneration (AMD) is the leading cause of adult blindness in developed countries. It occurs when fatty deposits accumulate in the retina and block absorption of nutrients, such as vitamin A, necessary for normal cell function.

AMD affects the part of the retina responsible for central vision, called the macula. As the macula deteriorates, it becomes increasingly difficult to read, recognize faces, and see in dim light.

What Causes Age-Related Macular Degeneration?

Cholesterol deposits accumulate in the macula years before drusen are visible, making drusen the tip of the iceberg in AMD pathology.

Before drusen are visible, an invisible layer of cholesterol builds up between the pigmented layer of the retina (RPE) and the elastic layer of Bruch’s membrane. These cholesterol deposits — basal laminar (BLamD) and basal linear (BLinD) — cause oxidative stress and inflammation, hindering nutrient transportation to photoreceptor cells. As photoreceptor cells die, it becomes harder for the eyes to adjust to darkness, and night vision declines. This dark adaptation impairment is the first sign of AMD.

Prior to pivotal research conducted at the University of Alabama Birmingham, the Alabama Study on Age-Related Macular Degeneration (ALSTAR), AMD was staged as early, intermediate, and advanced. Thanks to the groundbreaking findings of this investigation, we can identify a subclinical stage of AMD. 

Risk Factors for AMD

While no one knows exactly what causes age-related macular degeneration, there are a few factors that put older adults at risk for developing this condition. 

• Age: Age is the greatest risk factor for AMD, especially for those over 50 years of age.
• Family history: Individuals with a family history of AMD are more prone to this disease. Several genes related to the development of AMD have been identified, showing that the disease has a hereditary component.
• Race: Caucasians have the greatest likelihood of developing AMD. 
• Smoking: Smokers, ex-smokers and people regularly exposed to smoke are more likely to get AMD than those who never smoked.
• Obesity: Being overweight increases your risk of AMD, but those who are obese have a greater chance of progressing to the more severe form of the disease.
• Cardiovascular disease: Conditions that affect your heart and blood vessels may put you at a higher risk of AMD. 

Types of AMD

There are two types of age-related macular degeneration. The more common form is dry AMD. Wet AMD is less frequent but much more serious. complicated.

Non-Exudative (“Dry”) AMD

Nearly 85% of those who develop age-related macular degeneration have the dry form. Dry AMD causes you to slowly lose your central vision as the macula gets thinner and tiny clumps of drusen begin to grow drusen accumulate.

Exudative (“Wet”) AMD

In exudative AMD, abnormal blood vessels grow under the retina to aid nutrient transport to the macula (choroidal neovascularization or CNV). These vessels may leak fluids such as blood, potentially scarring the macula. Wet AMD causes you to lose your vision faster than dry AMD.

Stages and Symptoms of AMD

From subclinical to advanced AMD, all stages of the disease are the manifestation of the same underlying pathology

AMD, especially early on, can progress gradually and without pain. Even when early symptoms are detected, they may not be connected to AMD until after the condition has advanced. For this reason, it’s essential to be aware of what the subclinical, early, intermediate and late stages of age-related macular degeneration entail as well as the symptoms associated with these stages.

Subclinical AMD

Diagnosing the earliest stages of AMD can present a challenge when patients often have good corrected visual acuity and the retina appears normal upon clinical examination. Consequently, a routine vision test may not raise any suspicion of the disease.

Subtle yet progressive, night vision difficulty is the earliest symptom of AMD. Often presented as problems seeing at night or reading in dim light, impaired dark adaptation function is the first biomarker of AMD — with impairment taking place at a subclinical level at least three years before drusen are visible.

No pigmentary abnormalities are apparent upon examination at this stage, and if drusen are detected, these are <63 μm.

Early AMD

Most people do not experience adverse symptoms or vision loss in the early stage of AMD, but night vision problems are often reported. Though no pigmentary abnormalities are apparent upon examination, medium-sized drusen (>63 μm and ≤125 μm) are present. Symptoms of increasingly impaired dark adaptation include:

• A need for brighter light.
• Difficulty reading in dim light.
• Difficulty seeing at night.

Intermediate AMD

At this stage, some people may see a blurred spot in the center of their vision. In addition to night vision difficulties, these patients may also have issues with contrast sensitivity. Some pigmentary abnormalities may be observed and at least one druse >125 μm is present.

Other intermediate AMD symptoms could include:

• Visual distortions.
• Blurriness of printed words.
• Less intense colors.
• Difficulty seeing at night.
• Difficulty with low light.
• Limited central vision.

Advanced AMD

Insidious by nature, the symptoms experienced early in the disease may dramatically worsen. At this stage, vision loss becomes noticeable due to geographic atrophy (GA) or choroidal neovascularization (CNV). 

Additional signs that you may have late-stage AMD may include:

• Facial recognition difficulties.
• Worsened visual acuity.
• Distorted vision.
• Central vision loss.

From subclinical to advanced AMD, all stages of the disease are the manifestation of the same underlying pathology.

Learn More about AMD

Even with annual eye exams and the latest imaging tools, AMD patients are still under-diagnosed. Take action to preserve the vision of your patients with the help of AdaptDx Pro^®. The AdaptDx Pro has the power to measure dark adaptation, which can be useful in detecting AMD. With treatment, your patients may delay or reduce the risk of vision loss.

See how dark adaptation testing with the AdaptDx Pro can help enhance your diagnostic ability. Contact MacuLogix today for more information.