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Overview
Macular degeneration is the progressive
deterioration of the macula, the
small central area of the retina. The
central macula, the fovea, is responsible for fine-detail vision and has the
highest concentration of color receptors (i.e.,
cone cells).
The most common type of macular degeneration is
called age-related macular
degeneration (AMD), because it usually
develops in patients over the age of 55. A rare
form of macular generation, called juvenile macular
degeneration (JMD), occurs in younger
patients, including infants and children. JMD is
an inherited disorder caused by mutated genes.
Incidence and Prevalence
Macular degeneration is diagnosed every 3 minutes
in the United States. It occurs in about 10% of
people over the age of 50, and about 33% of people
over 75. AMD is most common in Caucasians of
European decent and is more prevalent in women.
Every year 1.2 million people with macular
degeneration lose part of their central vision,
and 200,000 suffer complete loss of central vision
in one or both eyes.
Types
Age-Related Macular Degeneration
Because AMD primarily affects central vision,
patients usually do not lose vision completely,
even at very advanced stages. This disorder can
make it difficult to read, drive, work at a
computer, and perform other activities that
require clear central vision. AMD occurs in two
forms, dry and wet.
Dry (atrophic)  This
form of AMD accounts for 85–90% of all cases. The
earliest sign of AMD is the development of waste
material deposits, called drusen, that appear as
tiny orange or yellow dots among the retinal
epithelial (RPE) cells. These deposits are
initially tiny and few in number, but they may
grow larger and become more numerous. The presence
of drusen does not necessarily signal vision loss,
and many people with drusen continue to have good
vision for decades. As dry AMD progresses, mild to
moderate visual acuity loss may occur.
Over time, patches of RPE cells may die, leaving
"bare" spots. This is called geographic atrophy
and results in vision loss in the affected areas
of the retina. If these patches become large and
involve the fovea, visual acuity can deteriorate
to the point of legal blindness. Geographic
atrophy is a severe form of dry macular
degeneration.
Wet (vascular) The
wet form accounts for approximately 10% of cases
but is responsible for the vast majority of
severe, AMD-related vision loss. Vascular macular
degeneration begins as the dry form and progresses
to the wet form when abnormal blood vessels
develop. In wet AMD, abnormal blood vessel growth
is triggered by mechanisms that are not completely
understood. The new vessels are very delicate,
break easily, and bleed and leak fluid into
surrounding tissue. This can damage the macula
very quickly and may cause central vision loss in
a short time. The risk for progression from dry to
wet AMD is approximately 14-87% over 5 years and
depends on many factors.
Causes and Risk Factors
Age-related macular degeneration may be influenced
by a combination of environmental and genetic
factors. Factors that may predispose a person to
develop AMD include conditions and behaviors that
interfere with the blood supply to the macula:
-
Hypertension
(constricts blood vessels)
-
Arteriosclerosis (thickening of arterial walls
due to plaque deposits)
-
Diet low in antioxidants and high in saturated
fat (increases the tendency for fatty deposits
to stick to vessel walls)
-
Hypercholesterolemia (excess
cholesterol in the blood)
-
Smoking (constricts blood vessels)
Because the condition often runs in families, AMD
may be hereditary.
Signs and Symptoms
The main symptom is central vision loss, which is
gradual in dry AMD and sudden in wet AMD. Other
symptoms include
-
blurry or fuzzy vision;
-
dark, empty spots in the center of vision;
-
difficulty reading or performing detail work;
and
-
seeing straight lines as wavy or bent (e.g.,
telephone poles, sentences on a page).
Diagnosis
Vision testing, Amsler grid test, retinal exam,
and fluorescein angiography are used to diagnose
macular degeneration. Visual acuity is
tested using the standard eye chart, which
features black letters on a white background. The
chart measures vision at various distances and can
detect vision loss due to AMD.
The Amsler grid test assesses distorted or
reduced vision and small irregularities in the
central field of vision. The grid consists of
evenly spaced horizontal and vertical lines
printed on black or white paper, with a small dot
in the center. Using one eye at a time, the
patient stares at the dot, which limits the image
to the macula. Distortion of the grid lines or
blank areas may indicate a change that requires a
thorough retinal examination.
The retinal exam is performed with a slit
lamp microscope, which enables the doctor to
examine different parts of the eye under
magnification. After instilling drops to dilate
the pupil the microscope is used to detect drusen
and abnormal areas. If the exam reveals
abnormalities that suggest neovascularization,
such as fluid or blood beneath the retina, other
testing may be required.
Fluorescein angiography
determines the presence and location of
neovascularization. A small amount of dye is
administered intravenously and photographs of the
retina are taken as the dye passes through the
blood vessels.
Treatment
Dry Macular Degeneration
There is no treatment for dry macular
degeneration. Quality of life can be maintained by
the use of eyeglasses, magnifying glasses, closed
circuit television, audio books, and reading
material in large print.
Wet Macular Degeneration
Ophthalmic surgeons use laser photocoagulation
to treat leakage that results from
neovascularization in wet AMD. Using flourescein
angiography to pinpoint neovascularization, the
laser beam burns abnormal blood vessels to seal
the leakage. By slowing or stopping the leakage,
the progression of macular degeneration is also
slowed or stopped.
Unfortunately, only about one-tenth of patients
with wet AMD are candidates for this procedure. In
most cases, the abnormal blood vessels are located
beneath the fovea (area of the macula responsible
for detailed vision), and laser photocoagulation
in that area would result in immediate and
permanent vision loss. The leakage recurrence rate
is about 50% with this procedure.
Photodynamic therapy (PDT)
is a minimally invasive outpatient procedure.
Fluorescein angiography is used to determine which
patients are candidates for this treatment.
The procedure involves a light-activated drug
called verteporfin (Visudyne®) and low-intensity,
or nonthermal, laser light. The eye is number with
eyedrops and a special contact lens is placed on
the eye. The light-activated drug is administered
intravenously. The laser light is directed through
the contact lens to the affected area of the
retina for approximate a minute and a half. When
clots are formed successfully by the activation of
the drug in the abnormal blood vessels, leakage
stops.
Patients may require subsequent treatments as
often as every 3 months for the first year,
because neovascularization can recur. After the
first year, treatment is required less frequently.
Side Effects After
treatment, the skin and eyes are sensitive to
bright light. Patients are advised to avoid
exposure to direct sunlight or bright light for 5
days. Indoors, use curtains or shades to block out
direct sunlight and avoid exposure to sunlight
from skylights. If one cannot avoid going
outdoors, the skin and eyes should be protected
with a long-sleeved shirt or blouse, slacks,
gloves, socks and shoes, sunglasses, and a
wide-brimmed hat. Sunscreens do not protect the
skin from photosensitivity reactions (e.g.,
sunburn) caused by verteporfin.
Prevention
Modifying environment risk factors may prevent the
progression of macular degeneration:
-
Consume a diet high in vitamins C and E, the
mineral zinc, and lutein and zeaxanthin (plant
antioxidants). High doses of antioxidants may
lower the risk of progression from dry to wet
AMD.
-
Reduce saturated fats and cholesterol.
-
Avoid smoking and exposure to second-hand smoke.
-
Monitor vision daily using the Amsler grid to
detect changes that may require further
examination.
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