Preferential Hyperacuity Perimeter
What is AMD?
Age-related macular degeneration (AMD) is a serious and currently untreatable disease that is the leading cause of acquired blindness among aging Americans. Approximately ten million Americans show early signs of AMD and a half million people or more may have significant vision loss from late-stage AMD. The macula is the part of the eye responsible for turning light into fine color images in the brain, which allows people to read, drive and perform other daily activities. Though the exact cause of damage to the macula is unknown, a breakdown in the macular area can lead to a loss in people’s central vision.
What are the types of AMD and why should I care?
On the basis of its clinical features and disease progress, AMD can be divided into two major forms:
“Dry” (atrophic) MD is more common. It occurs when the tissues of the macula thin due to aging and results in gradual vision loss.
“Wet” (exudative) MD is less common and results from the formation of abnormal blood vessels underneath the retina. These new blood vessels then leak fluid or blood which blurs central vision. This can cause very quick and severe vision loss. There are close to 200,000 new cases of wet macular degeneration diagnosed every year in the United States.
Regardless of the type and form, you must remember that the macula is responsible for “central vision,” or images seen straight-ahead. Damage to the macula can cause blind spots, blurred or distorted vision, and the loss of ability to see fine details. In severe, uncontrolled or untreated cases, it may even lead to blindness. In fact, AMD is a major cause of vision loss in the United States for people over the age of 60.
How did doctors used to check for AMD?
The key to successfully treating or deferring age-related macular degeneration (AMD) is to catch at its earliest possible stage, and always in its dry stage before it becomes wet AMD. For many years, this was done with a slit lamp and precise plotting of anomalies and tracking them over time. Digital photography advances made software possible that could track the size and locations of the AMD macula, but could not make any earlier predictions or findings. Similarly and traditionally, macular degeneration can be detected with a test called an Amsler grid. This is a poster showing a grid of vertical and horizontal lines with a dot in the centre. A person with macular degeneration may see wavy lines or missing areas of the grid while focusing on the dot in the centre. When wet macular degeneration is suspected after an eye exam, a fluorescein angiography is done to see the blood vessels in the eye more clearly. For this test, fluorescein (a non-toxic dye) is injected in to a vein and a special light is used to see the macula.
What is Preferential Hyperacuity Perimeter (PHP)?
The key to successfully treating or deferring age-related macular degeneration (AMD) is to catch at its earliest possible stage, and always in its dry stage before it becomes wet AMD. For many years, this was done with a slit lamp and precise plotting of anomalies and tracking them over time. Digital photography advances made software possible that could track the size and locations of the AMD macula, but could not make any earlier predictions or findings. Then Preferential Hyperacuity Perimeter (PHP) technology came along, and has allowed doctors to find abnormalities half the size they could before.
PHP Technology is based on the well studied phenomenon of hyperacuity. Hyperacuity, also known as Vernier acuity, is the ability to detect misalignment of an object’s location relative to other objects in space. This ability to detect misalignments is in the magnitude of 3-6 seconds of arc, which is 10 times better than the resolution ability in the fovea (around 30- 60 seconds of arc). Hyperacuity is unaffected by a patient’s age and is highly resistant to retinal image degradation, and is thus suitable for assessing retinal function in patients with cataracts as well
Why is PHP better than other diagnostic techniques?
PHP is significantly better than many other diagnostic methods used for detecting AMD for a number of reasons such as simplicity, accuracy and fewer chances of error. Preferential Hyperacuity Perimeter provides an unsurpassed level of sensitivity and specificity in detecting recent onset Choroidal Neovascularization (CNV) and differentiating it from the intermediate (dry) state of AMD. The visual field analyzer monitors macular function by measuring relative photoreceptor field location and utilizing the phenomenon of hyperacuity, the ability to detect the misalignment of an object’s location relative to the other objects in space.
If I am detected with early signs of AMD, would I still maintain vision?
The answer to this question is a both yes and no. For example, while there's little that can be done for dry macular degeneration, the disease progresses very slowly, and will probably never completely black out the central vision. Many people with this condition live full lives without serious disability. Some studies have suggested a link between poor nutrition and faster degeneration of the macula. According to this evidence, fruit and dark green vegetables like spinach can slow the disease and contribute to better outcomes. For some people, a doctor will recommend a daily supplement that contains zinc, copper, vitamin E, vitamin D, and beta-carotene or vitamin A.
As for the wet AMD, there is no cure, but treatment may help to slow it down. Laser surgery destroys tiny, newly grown blood vessels that may be bleeding into the macula. Photodynamic therapy may also be used. This involves injecting a medication called verteporfin* into a vein. Then, a light is used to activate the medication to close abnormal blood vessels. Anti-VEGF medMedications injected into the eye, such as ranibizumab or pegaptanib, may be used to slow down the growth of blood vessels. Daily supplements may also be recommended.
What are anti-VEGF injections? How do they work to stop the AMD process?
Most recently, the FDA approved ranibizumab injection therapy, trade name Lucentis™, as a treatment for neovascular AMD. Lucentis™ is another VEGF inhibitor administered through 6 intravitreal injection. Like other VGEF inhibitors, this drug binds to receptor binding sites, preventing interaction of VEGF-A with its receptors on the surface of endothelial cells. This action reduces cell proliferation and neovascularization and helps show down or stop the AMD process in some (not all) patients.
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