Wednesday, August 18, 2010

Some info on Glaucoma

By Mayo Clinic staff

Movement of fluid in the eye
               Open-angle glaucoma
                                 Angle-closure glaucoma

Glaucoma is not just one eye disease, but a group of eye
conditions resulting in optic nerve damage, which causes
loss of vision. Abnormally high pressure inside your eye
(intraocular pressure) usually, but not always, causes this

Glaucoma is the second leading cause of blindness.
Sometimes called the silent thief of sight, glaucoma can
damage your vision so gradually you may not notice any
loss of vision until the disease is at an advanced stage.
The most common type of glaucoma, primary open-angle
glaucoma, has no noticeable signs or symptoms except
gradual vision loss.

Early diagnosis and treatment can minimize or prevent
optic nerve damage and limit glaucoma-related vision loss.
It's important to get your eyes examined regularly, and make
sure your eye doctor measures your intraocular

Movement of fluid in the eye

Aqueous humor continuously circulates from behind the iris
into the anterior chamber at the front of the eye. It exits where
the iris and the cornea meet. The fluid filters through a spongy
tissue containing microscopic channels (trabecular meshwork)
before passing into a larger channel called Schlemm's canal,
where it eventually merges with the bloodstream. ]

Angle-closure glaucoma

In angle-closure glaucoma, the angle formed by the
cornea and the iris closes. In this illustration, the iris
is plastered against the trabecular meshwork, which
prevents the aqueous humor from reaching the drainage
channels (see black arrow). This can lead to a rapid
 increase in intraocular pressure, a serious medical

 Open-angle glaucoma
In this illustration the trabecular
meshwork is partially blocked,
impairing the flow of aqueous
fluid out of the eye (see black
arrow). This blockage of the
trabecular meshwork leads to
a gradual increase in intraocular
pressure. Within months or years,
an elevated pressure can result in
vision loss

 Anatomy of the eye

Your eye is a complex and
compact structure measuring
about 1 inch (2.5 centimeters)
in diameter. It receives
millions of pieces of information
about the outside world,
which are quickly processed by
your brain.
Is It Early-Stage Glaucoma? New Tests May
Provide the Answer

Loss of vision in people with glaucoma is due
to a progressive loss of retinal ganglion cells --
the cells that transmit visual information from
the retina to the brain through the optic nerve.
Doctors use a visual field test, also known as
perimetry, to detect the resulting functional vision
loss. The problem: Researchers now know that at
 least 50% of retinal ganglion cells are lost before
vision problems are identified on standard visual
field tests.
Some of the efforts to more accurately identify
early-stage glaucoma have focused on methods
that can better determine whether functional visual
loss has occurred.

Functional changes. By testing your visual field,
your doctor can learn whether your peripheral
vision is being lost from glaucoma. During a
computerized visual field test, you place your
chin on a stand in front of a computerized screen.
Whenever you see a flash of light appear, you
press a button. At the end of this test, your
doctor receives a printout of your field of vision.

•The most commonly used and best-studied test
displays white lights on a white background, but
some researchers have demonstrated that short-
wavelength automated perimetry (SWAP), which
uses blue lights on a yellow background, may
detect defects from glaucoma earlier.

•Another method of checking peripheral vision is
electroencephalography (EEG) using multifocal
visual evoked potentials (mfVEP). During VEP testing,
special sensors (electrodes) are placed on your scalp
to record your brain's response to visual stimuli, such
as flashing lights that flicker from black on white to white
on black on a video screen. In a person with normal vision,
repeated stimulation of the visual field evokes changes on
the EEG. But if there are no EEG changes, the brain is not
receiving signals from the eyes, suggesting loss of vision.
Multifocal VEP records separate responses from
multiple visual field locations.
•Another test, frequency doubling technology (FDT)
perimetry, has a unique approach to evaluating peripheral
vision in glaucoma. This test is based on an optical illusion
that can occur when viewing patterns of black and white
parallel bars. For some people, when the pattern is flashed
on the screen at a high speed, the number of bars appears
to double. Some research suggests that this illusion arises
from specific retinal ganglion cells, which have a high
tendency to get damaged in glaucoma.

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