Your Complete Guide to AMD Testing
Doctors have access to a whole range of tests that help diagnose and monitor age-related macular degeneration. We've got the lowdown on what you can expect.
In just the past few decades, the variety and sophistication of the ways we test for age-related macular degeneration (AMD) have grown tremendously. And while there is no cure for the condition—or rather, there is no cure yet—early detection of both the "dry" and "wet" forms of the disease can help doctors and patients reliably manage AMD, and in some cases slow its progression.
No one knows exactly what sparks the process behind the more common, dry form of macular degeneration. (We do know that the sudden emergence of abnormal blood vessels that leak into the retina causes wet AMD, but why they erupt in the first place is not thoroughly understood.) As for those most at risk, the data are clear: Caucasians and people who are over 50 years old, are overweight, smoke cigarettes, have high blood pressure, and/or have a family history of the disease should be especially vigilant. Scientists have identified genetic variants that are at higher risk for developing AMD. Symptoms include, but are not limited to, an onset of blurry eyesight, a dimming of colors, straight lines appearing wavy, and blind spots in one's central field of vision. In addition, distortion in vision; either straight lines appearing wavy or sometimes images appear as if one is “looking through water.” as well as blind spots are in the center of the vision. When reading, letters in words seem to be missing. Those with macular degeneration will also complain of difficulty adapting in the dark.
If you or someone you love is at risk for AMD, knowing the sorts of tests available for assessing that risk, or for diagnosing the condition itself, can go a long way toward managing irrational (and even rational) fears. Here are the tests to keep in mind.
AMD: Risk-Factor Tests
While the following are not considered diagnostic tools, they can help doctors and patient alike assess the risk some people face of developing AMD.
Macular Pigment Optical Density Test: MPOD is a non-invasive way to determine the level of two naturally occurring pigments, lutein and zeaxanthin, in our eyes. Both pigments help block "blue light"—for example, from digital devices—which might damage eyes over time. (The National Institutes of Health notes that while there have been “many studies on blue light's effects on the retina,” the data “are still being debated.”) Low levels of either or both pigments, meanwhile, might indicate an increased risk of AMD.
Family History: Knowledge of one's family history of AMD (or lack of it) can help shape the course of testing and, conceivably, treatment. "The risk of AMD is greatly increased," according to the National Institutes of Health, "by having an affected first-degree relative … [and] AMD patients should advise siblings and children to seek prompt ophthalmological advice" if they develop symptoms. In other words, if a parent or sibling has been diagnosed with AMD, other siblings or children—and their doctors—should be made aware so they can act, and test, accordingly.
AMD: Visual-Performance Tests
In early stages of AMD, visual acuity is rarely affected. Nevertheless, these tests can help physicians and patients monitor potential progression of the disease.
Amsler Grid: Named for the Swiss ophthalmologist who devised it, the Amsler grid has been a standard test for AMD and other central-vision ailments since the 1940s. It consists of a simple grid of horizontal and vertical lines, with a prominent dot in the middle. Depending on what a patient sees—straight lines vs. wavy/curvy—the grid can help detect, or at least suggest the presence of, disturbances in the retina and macula. (In wet AMD, particularly, even a minuscule leakage of blood or other fluid from swollen or compromised vessels can interfere with central vision, distorting images, causing straight lines to appear wavy or missing, etc.)
The grid is also, as Jayanth Sridhar, M.D., describes it, "beautiful in its simplicity." For Dr. Sridhar, an assistant professor of clinical ophthalmology at the Bascom Palmer Eye Institute of the University of Miami's Miller School of Medicine, the Amsler is more than a mere checkerboard. "It is a remarkably reliable, and remarkably sensitive, test," he says. "Of course, one of the issues around Amsler is that its efficacy depends on the user's interpretation of what he or she sees. But I think almost anyone can look at straight lines and tell whether they look curved or not."
Contrast Sensitivity: Difficulty distinguishing between objects or textures with low contrast (black coffee in a dark mug, for example) is often an early symptom of AMD. Contrast sensitivity testing, of which there are a few different types, essentially gauges how well or how poorly the eyes distinguish between fine light increments, according to a study in the journal Investigative Ophthalmology & Visual Science. The Pelli-Robson test, for example, features a chart with capital letters arrayed along horizontal lines, with the contrast of each letter and its background decreasing at each step.
ERG: Early AMD can also bring about seemingly contradictory symptoms, such as an increased sensitivity to glare and/or an increased difficulty reading or functioning in dim light. An electroretinography (ERG) test, or electroretinogram, can help gauge light receptivity by measuring the electrical response of the eyes’ light-sensitive cells—the rods and cones. During the test the doctor will dilate the patient's eyes with eye drops and numb them with anesthetic drops. With a flashing light, the doctor then measures the retina's electrical response to light, thus determining the relative light sensitivity in the eye.
Visual Field: According to the American Academy of Ophthalmology, “Visual field testing is one way your ophthalmologist measures how much vision you have in either eye and how much vision loss may have occurred over time.” The Visual field test basically maps blind spots. Those with macular degeneration will have blind spots in the center of their vision. The patient will look into a large bowl called a perimeter and press a button when he/she sees flashing lights. The computer will track where a patient misses the flashing lights and map out the blind spots.
AMD: Diagnostic Tests
Direct, observational tests of the eye itself are still the standard for determining the presence of, or progression of, AMD
Back-of-the-Eye Exam: As with the ERG, this exam involves dilation of the pupils, so that the physician, using a special viewing instrument, can examine the back of the eye. The doctor is looking for evidence of drusen—tiny, yellow, fatty-protein deposits that form under the retina and, in large numbers, can damage light-sensitive cells. People with macular degeneration often have significant numbers of drusen, especially of the "soft" variety, which tend to be larger and potentially more threatening than “hard” drusen.
“An abundance of soft drusen tend to indicate higher risk, while smaller, hard drusen tend to be lower risk,” notes Priyatham Mettu, M.D., an ophthalmologist and retinal specialist at Duke Eye Center in Durham, N.C. Dr. Mettu also points out that the “soft” and “hard” characterizations of drusen are less about the physical make-up of these deposits and more about their appearance. Hard drusen are visually better defined or more clearly delineated, while soft drusen are less well-defined, with irregular, amorphously shaped borders.
Fluorescein Angiography: Colored dye injected into a vein in the arm travels to the blood vessels in the eye and highlights them. A doctor or technician uses a specially designed camera to take pictures as the dye journeys to the vessels. These pictures show if a patient has experienced abnormalities in blood vessels (their shape or size) or other parts of the retina (typically newly formed blood vessels that leak fluid underneath the retina), which might indicate wet AMD.
Optical Coherence Tomography: OCT is a noninvasive test that returns detailed, cross-sectional pictures of the retina, and indentifies areas of thickening, swelling, or thinning of the macula and other retinal parts. It is also, from a technical perspective, one of the wonders of the ophthalmologist's diagnostic toolkit. OCT has become the gold standard in the diagnosis of AMD.
"Twenty or thirty years ago our understanding of AMD came primarily from direct examination: looking inside the eye and taking photos that we could then study," says Dr. Mettu. But OCT "has revolutionized the way we characterize the disease by offering high-res, microscopic-level visualizations of the structures and cross sections of the retina. That allows us to see disease earlier and evaluate its relative severity.
"In fact, we're only now beginning to appreciate the power of our imaging technologies, both for screenings and for monitoring existing disease. What's really exciting is that OCT and other imaging methods lend themselves quite well to computational sciences and artificial intelligence algorithms that might detect disease in an automated way. This will allow us to screen for disease across higher volumes of patients, and we'll be able to do it more efficiently and less expensively."
Whether it's an old-school Amsler grid or a cutting-edge OCT machine, the tests available for detecting and monitoring age-related macular degeneration are there for a reason: so patients will make use of them. See your eye doctor early, and often.
Causes and risk factors for AMD: Mayo Clinic. (2019). “Wet Macular Degeneration.” https://www.mayoclinic.org/diseases-conditions/wet-macular-degeneration/symptoms-causes/syc-20351107
Blue light and risk factors: International Journal of Ophthalmology. (2018). “Research Progress on the Effect and Prevention of Blue Light on Eyes.” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6288536/
Family history and AMD risk: British Journal of Ophthalmology. (2012). “Age-related Macular Degeneration: The Importance of Family History as a Risk Factor.” https://www.ncbi.nlm.nih.gov/pubmed/21865200
Contrast impairment with AMD: Investigative Ophthalmology & Visual Science. (2015). “Contrast Sensitivity in Patients with Macular Degeneration.” https://iovs.arvojournals.org/article.aspx?articleid=2331984
Visual field testing: American Academy of Ophthalmology. (2019). “Visual Field Test.” https://www.aao.org/eye-health/tips-prevention/visual-field-testing
Fluorescein angiography and wet AMD: Review of Ophthalmology. (2008). “Fluorescein Angiography in Neovascular AMD.” https://www.reviewofophthalmology.com/article/fluorescein-angiography-in-neovascular-amd
Optical coherence tomography: American Academy of Ophthalmology. (2019). “What Is Optical Coherence Tomography?” https://www.aao.org/eye-health/treatments/what-is-optical-coherence-tomography