GLAUCOMA IN DIABETICS
Glaucoma is a group of diseases
characterized by damage to the optic nerve as a result of high pressure
in the eye. This high pressure can be due to a number of different
causes.
This
drawing depicts a cross-section through the anterior chamber of the eye,
the chamber formed by the cornea in front and the iris in back. The
aqueous fluid that fills the anterior chamber is constantly being made
by the epithelium of the ciliary body, behind the iris. This aqueous
then moves into the anterior chamber through the pupil (white arrows).
The fluid leaves the anterior chamber through a filtering apparatus
called the trabecular meshwork (TM). After passing through the
trabecular meshwork, the fluid is picked up by the bloodstream and
carried away.
When
the aqueous fluid cannot leave the eye as rapidly as it is made, the
pressure in the eye goes up, and the high pressure damages the optic
nerve, resulting in loss of portions of the visual field and eventual
blindness. Glaucoma is usually categorized as open-angle or closed-angle
glaucoma. In open-angle
glaucoma, the angle formed by the iris and the cornea remains open, and
the elevated pressure is caused by blockage of aqueous flow due to
microscopic changes at the trabecular meshwork. In closed-angle
glaucoma, the iris is mechanically pushed or pulled forward, thereby
closing off the angle and blocking the flow of aqueous. Diabetics may
develop either open- angle or closed-angle glaucoma. In addition to the
more common types of glaucoma (which affect patients regardless of
whether or not they have diabetes), there are some glaucomas that can
occur as a result of diabetic eye changes.
In
patients who have had a vitreous
hemorrhage, some of the blood may travel forward from the vitreous into
the anterior chamber. Red blood cells that have been in the eye for
several weeks lose their hemoglobin and turn white. These "ghost
cells" plug up the openings in the trabecular meshwork and
gradually reduce the drainage efficiency of the meshwork. When enough of
the meshwork has been blocked, the pressure in the eye begins to
increase. This is referred to as ghost cell glaucoma. The pressure can
be controlled for a short time with traditional anti-glaucoma eye drops.
Some of these drops lower the pressure by decreasing the production of
aqueous, which also decreases the flow of aqueous and slows the clearing
of ghost cells from the eye. Eventually, if there is enough vitreous
blood, the meshwork is overwhelmed and the pressure goes up despite the
medications. The definitive treatment for ghost cell glaucoma is a
vitrectomy to remove the blood that is the source of the ghost cells.
In proliferative diabetic
retinopathy, patients develop new blood vessels growing on the retinal
surface, probably due to vasoproliferative growth factors. Abnormal new
vessels may also grow on the surface of the iris, presumably because of
diffusion of growth factors from the retina forward into the anterior
chamber. The new vessels on the iris surface are called rubiosis.
Just like neovascularization on the retina, rubiosis
tends to bleed and tends to create scar tissue. As scar tissue from rubiosis
in the anterior chamber angle contracts, it closes off the angle and
blocks the entry of fluid into the trabecular meshwork. This results in
high pressure in the eye, known as neovascular glaucoma.
Since the stimulus for the growth of rubiosis
is retinal ischemia, treatment of the rubiosis
is aimed at reducing that ischemia. Panretinal Photocoagulation
(PRP), in addition to causing regression of retinal neovascularization,
also causes regression of rubiosis.
Treatment of rubiosis
is an urgent concern, since the closure of the angle by scar tissue may
take only a few days or weeks. If the laser is not done in time, the
angle may be closed despite the regression
of the rubiosis.
Surgery is then required to create a new drainage for the fluid in hopes
of preventing blindness from optic nerve damage. The success rate for
this type of surgery is usually quite low if the vessels continue to
grow, since the rubiosis
can block off the new drainage site. It is important, therefore, to stop
the growth of rubiosis
as soon as possible.
The Country Hills Eye Center Home Page
Dr. Scott C. Richards Main Page
For information regarding the
Country Hills Eye Center phyician
specializing in retinal and diabetic eye diseases
www.countryhillseyecenter.eyemd.org
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