12/21/2009 - Articles

Age-Related Macular Degeneration (AMD) - An Overview

By: Bruce OD Rosenthal, Low Vision Lighthouse International, Chair of the AMD Alliance, and Chief

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AMD is the leading cause of blindness in people over 50 years old in the Western world. Here's an overview of the disease, authored by Dr Bruce Rosenthal, Adjunct Professor and Chief of Low Vision Service at the Mt. Sinai Hospital, and Chief of the Low Vision Programs, Lighthouse International, a leading resource worldwide on vision impairment and vision rehabilitation.

Introduction

AMD is the leading cause of blindness in people over 50 years old in the Western world. 1 It is a degenerative eye disease that affects the macula. The macula is the central part of the retina at the back of the eye and responsible for the clear central vision needed for daily activities like reading or driving. AMD causes the progressive loss of central vision, leaving only peripheral vision intact. It usually starts in one eye but there is a high risk (over 40%) that the disease will affect the other eye within 5 years. 2

In the early stages of the disease, the transport of nutrients and waste by the retinal pigment epithelium (RPE) slows down. In the healthy eye, a layer of RPE supplies the photoreceptors with these nutrients and pumps out the waste products created as the photoreceptors convert light into nerve signals. As waste products build up under the retina, they form yellowish deposits called drusen. These are common - most people develop at least one druse by the time they are 50 years old. As the build-up of waste material continues, the drusen enlarge. In some people, this leads to stretching of the overlying RPE cells. Eventually the RPE cells will die and since the photoreceptors depend on RPE for survival, they too degenerate. This represents one of two types of macular degeneration, the 'dry' form; however this can lead to a more aggressive form, called 'wet' AMD.

Dry AMD: accounts for 85-90% of all cases. Dry macular degeneration rarely causes severe visual impairment.

Wet AMD: this is the most serious form, accounting for 10-15% of all cases; approximately 500,000 new cases are diagnosed annually worldwide. Wet AMD is caused by a growth of abnormal blood vessels under the central part of the retina or macula. The proliferation of these new blood vessels is called choroidal neovascularisation, or CNV. The vessels leak fluid and blood, which leads to the development of scar tissue, destroying the central part of the retina. This results in a deterioration of sight over a period of months to years. 3

Symptoms and Diagnosis

The severe loss of central vision can be rapid. Many patients will be legally blind within 2 years of diagnosis 3 (visual acuity equal to or worse than a Snellen equivalent of 20/200, which is classified as legal blindness when it occurs in the better-seeing eye 4 ).

Peripheral vision usually remains normal, but the individual will have difficulty seeing at a distance or doing detailed work like sewing or reading fine print. Faces may begin to blur, and it becomes harder to distinguish colors. Distortion that can cause edges or lines to appear wavy may accompany or precede the blurred vision. Those with more advanced forms of AMD may begin to see dark or empty spaces that block the center of vision.

A key problem of AMD is that patients at an early stage of the disease may incorrectly connect the symptoms of vision loss to the normal aging process or to development of a cataract. Therefore early diagnosis is key, because once vision is lost, it cannot be reclaimed.

People over 50 should see an eye care professional every second year. The physician will examine the eyes to determine if a patient has macular degeneration. There are several different tests; one of them is the Amsler Grid. This test may be helpful in determining areas of distorted or reduced vision - common symptoms of AMD. However, the Amsler Grid is not a substitute for regularly scheduled eye tests.

To administer the test, the patient holds the Amsler Grid at eye level at a comfortable reading distance, covering one eye at a time and then focusing on the dot in the center. (If patients wear any type of glasses, then they should wear them during the test.) If the patient notices any wavy, broken or distorted lines, or any blurred or missing areas of vision, an eye care provider should be contacted immediately.

Causes and Risk Factors

It is not generally known what causes AMD. The only clinically proven risk factor is smoking. 4 Several studies have identified possible additional risk factors:

Age - AMD is present in 0.2% of the age group 55-64 years, rising to 13% in people over 85 years.

Cataracts

Eye Color - AMD is more common in people with blue eyes. This may be related to damage associated with exposure to ultraviolet light. Blue-eyed people may have less protective pigment in their eyes.

Farsightedness / Severe Hyperopia - Many people are farsighted and need glasses to see objects that are near. Severe hyperopia is not common and is related to a strong distortion of the shape of the eye. This may have an effect on the retina.

Gender - AMD is more common in women than in men. This may be partially explained by the fact that women live longer than men.

Heredity - The American Academy of Ophthalmology (AAO) recommends that patients who have blood relatives with AMD have their eyes checked every two years. (source: http://www.aao.org/ )

High Blood Pressure - Patients with dry AMD on anti-hypertensive drug therapy coupled with high serum cholesterol levels and low serum carotenoid are at a greater risk for developing wet AMD. 5

Hormone Replacement Therapy (HRT) - Postmenopausal women who do not undergo estrogen therapy are also at a greater risk of developing AMD. 7

Race - AMD is more common in Caucasians than other races. This may be partially due to the pigment in the eye or eye colour. 5

Smoking

Sun Exposure

Prevalence and Incidence

Worldwide, approximately 25-30 million individuals are affected by some form of AMD. Due to the demographic development of the population this number is expected to triple over the next 25 years. 6

In North America alone, approximately 200,000 new cases of wet AMD are diagnosed each year. It is estimated that 6 million Americans have vision loss from AMD. Based on currently available census data, it is also estimated that another 13-15 million Americans have pre-symptomatic signs of AMD.

Treatment Options

There is currently no treatment for dry AMD. However, there are two clinically-proven treatments for some forms of wet AMD:

Photodynamic Therapy (PDT)
Photodynamic Therapy is a two-step procedure using a non-thermal laser light in conjunction with a special light-sensitive drug, verteporfin (Visudyne®). The drug is injected in the patient's arm and concentrates in the abnormal blood vessels in the eye. Then the non-thermal laser is shone into the eye and destroys the abnormal blood vessels, sparing normal, healthy tissue.

PDT is a multi-course treatment. After containing the disease during the first year of treatment, vision is stabilized. 7 Treatment may be applied again at 3-month follow up visits if evidence of CNV leakage is observed on the fluorescein angiogram. A reduction in the need to treat is seen over the years. PDT using verteporfin is effective in patients with various lesion sizes and lesion compositions. 8 However, patients will benefit more from the therapy if the disease is detected early, when the lesions are most likely to be smaller.

In a clinical study being conducted at a low vision center, it has been found that patients who are treated with PDT retain their visual function longer than people who are candidates for PDT but have not undergone treatment. The findings show a stabilization of the visual acuity (ETDRS) as well as the contrast sensitivity function (Pelli-Robson Test). 9

The ultimate outcome of the stabilization is that functional vision is retained for longer periods of time, allowing the individual to continue working as well as to live independently.

Laser photocoagulation (LPC)
In laser photocoagulation, a heat-producing laser is aimed at the AMD-causing blood vessels. The laser heats the vessel walls, causing their structure to change, and thereby helping to seal the leaks.

Unlike PDT, which targets the newly grown blood vessels by sticking to them and so only treats the affected parts of the eye, LPC acts wherever the laser is directed. This means that cells surrounding the blood vessels are also heated and can therefore be damaged; because of this it is only advisable to use LPC on AMD lesions that are not near the macula (the area which gives us our central vision).

A list of key patient support groups around the world can be reached by clicking here.

Links

Footnotes

1.

Age related macular degeneration. NM. Bressler, SB. Bressler, SI. Fine, Arch Ophthalmol., 1996, vol. 114, pp. 400--412

 

2.

Risk factors for choroidal neovascularisation in the second eye of patients with juxtafoveal or subfoveal choroidal neovascularisation secondary to age-related macular degeneration.  Macular Photocoagulation Study Group, Arch Ophthalmol, 1997, vol. 115, pp. 741--747

 

3.

Natural course of choroidal neovascular membranes within the foveal avascular zone in senile macular degeneration. SB. Bressler,  et al., Am J Ophthalmol, 1982, vol. 93, pp. 157--163

 

4.

Risk Factors for Age-related Macular Degeneration. Pooled Findings from Three Continents.  Smith,  Wayne,  et al., Ophthalmology, 2001, vol. 108, pp. 697--704

 

5.

Age-related macular degeneration and risk factors for the development of choroidal neovascularisation in the fellow eye. DJ. Pieramici, SB. Bressler, Curr Opin Ophthalmol, 1998, vol. 9, pp. 38--46

 

6.

Global data on blindness. B. Thylefors, AD. Ngrel, R. Pararajasegaram, KY. Dadzie, Bull World Health Organ, 1995, vol. 73, pp. 115--121

 

7.

Verteporfin Therapy for Subfoveal Choroidal Neovascularization in Age-Related Macular Degeneration. NM. Bressler,  et al., Three-Year Results of an Open-Label Extension of 2 Randomized Clinical Trials TAP Report No. 5. Arch Ophthalmol, 2002, vol. 120, pp. 1307--1314

 

8.

Guidelines for using verteporfin (Visudyne®) in photodynamic therapy to treat choroidal neovascularization due to age-related macular degeneration and other causes. JJ. Arnold,  et al., Retina, 2002, vol. 22, pp. 6--18

 

9. Unpublished ongoing study by B.Rosenthal, Lighthouse Low Vision Service, Lighthouse International, NY, USA.

Created on: 01/20/2003
Reviewed on: 12/21/2009

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