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Definition of
Glaucoma
Glaucoma is a group of eye diseases that damage the
optic nerve. The optic nerve is the main nerve to the
eye (located in the back of the eye) that is responsible
for transmitting electrical impulses to the brain.
Damage usually occurs as a result of elevated pressure
of the fluid (aqueous humor) in the eye. This damage
results in gradual visual changes and then loss of
vision.
Description of Glaucoma
The eye is protected by the sclera (the white covering
of the eye). At the front of the eye is a clear window
(the cornea) that lets light in. The iris is the colored
part of the eye that controls the amount of light that
can enter the eye by automatically opening and closing
the pupil (the opening of the eye).
The lens (the portion of the eye lying behind the pupil)
focuses light onto the retina (the innermost layer of
the eye and the sensitive organ upon where light rays
are focused). Nerve fibers and blood vessels in the
retina gather together to form the optic nerve which
carries images to the brain. The place where the optic
nerve leaves the eye is called the optic disc.
The front part of the eye is filled with aqueous humor
that nourishes the lens, iris and cornea, as well as
maintains the shape of the eye. This fluid is
continuously produced every hour by ciliary tissues.
The fluid circulates from behind the iris through the
opening at the center of the pupil and into the spaces
between the iris and cornea. Since this fluid is
constantly produced, it must be drained. The fluid
drains through an area called the drainage angle
(located where the iris and cornea meet) to the
trabecular meshwork (the valve that regulates pressure
within the eye) to a channel (called the canal of
Schlemm), and then to a system of small veins outside of
the eye, called the drainage canals.
When there is a problem in the drainage, the fluid
builds up in the eye and presses against the optic nerve
(called intraocular pressure or IOP), resulting in
glaucoma.
This increased fluid pressure actually pushes the optic
nerve back into a "cupped" or concave shape. If the
intraocular pressure remains too high for too long, the
extra pressure damages parts of the optic nerve.
This damage appears as gradual visual changes and then
loss of vision. The early visual changes are very slight
and do not affect the central vision the center portion
of what is seen when looking straight ahead or when
reading. Certain parts of the peripheral vision (the
top, sides and bottom) are affected first. Glaucoma
usually occurs in both eyes, but extra fluid pressure
first begins to build up in one eye.
There are many types of glaucoma, but the two most
common types are open-angle glaucoma and closed angle
(angle-closure) glaucoma.
Open-angle glaucoma, (also called primary open-angle
glaucoma and chronic glaucoma) accounts for 90 percent
of all glaucoma cases and occurs when the trabecular
meshwork becomes blocked and the fluid can't get to the
normal drainage canals. This blockage results in fluid
build-up and intraocular pressure. The fluid build-up
happens gradually.
Closed angle glaucoma, (also called acute glaucoma or
angle closure glaucoma), accounts for about 9 percent of
all glaucoma cases and occurs when the opening between
the cornea and iris narrows, such that the fluid cannot
get to the trabecular meshwork and normal drainage
channels. This narrowing results in fluid build-up and
intraocular pressure. The fluid build-up happens very
quickly.
Other less common glaucomas include:
* normal tension glaucoma: optic nerve is damaged even
though intraocular pressure is consistently within a
normal range
* congenital glaucoma: incorrect or incomplete
development of the eye's drainage canals during the
prenatal period
* childhood glaucoma: open-angle glaucoma in infancy,
childhood or adolescence
* secondary glaucoma: occurs as a result of eye injury,
inflammation or tumor, or in advanced cases of cataracts
or diabetes |
