 Glaucoma
is the elevation of pressure inside the eye, known as
intraocular pressure (IOP) beyond a specific point at
which vision is compromised or is no longer possible.
Glaucoma is a frequent cause of blindness in humans and
animals.
To understand glaucoma, it is
necessary to understand how the normal flow of intraocular
fluid maintains normal intraocular pressure. The fluid
inside the eye is called the aqueous humor, which is
produced in the ciliary body, which is located behind the
iris. This fluid flows through the pupil and drains from the
eye through a sieve-like network located at the junction of
the cornea and the iris called the iridocorneal cleft or
drainage angle. The aqueous humor is produced and drains
from the eye at approximately the same rate, resulting in a
stable pressure inside the eye of 15 to 20 mmHg (millimeters
of mercury). Glaucoma occurs as a consequence of inadequate
outflow of aqueous humor and a subsequent buildup of
pressure inside the eye. The resulting high pressure damages
the optic nerve and results in blindness.
There are two categories of
glaucoma. Primary glaucoma occurs without any other
ocular cause. Secondary glaucoma occurs when some
other inciting cause is present. Primary glaucoma is known
to occur in certain purebred breeds of dogs and is thought
to be inherited. The following breeds are known to be at
risk for developing glaucoma:
|
Afghan
Akita
Alaskan Malamute
Basset Hound
Beagle
Border Collie
Boston Terrier
Bouvier Des Flanders
Cairn Terrier
Cardigan Welsh Corgi
Chihuahua
Cocker Spaniel
Dalmation |
Dandie Dinmont Terrier
English Springer Spaniel
Giant Schnauzer
Great Dane
Maltese
Manchester Terrier
Miniature Pinscher
Norfolk Terrier
Norwegian Elkhound
Norwich Terrier
Pembroke Welsh Corgi
Poodle
|
Saluki
Samoyed
Scottish Terrier
Sealyham Terrier
Shih Tzu
Siberian Husky
Smooth-Coated Fox Terrier
Tibetan Terrier
Welsh Springer Spaniel
Welsh Terrier
West Highland White
Terrier
Whippet
Wire-Haired Fox Terrier.
|
Many purebred cats such as the
Persian, Siamese and some Domestic short hairs
are also subject to glaucoma.
Secondary glaucoma is the
result of some intraocular condition that interferes with
the natural flow of ocular fluid. Diseases that commonly
cause secondary glaucoma include ocular inflammation (uveitis),
lens dislocation, intraocular tumors and injury to the eye.
Glaucoma results in blindness
by blocking the nerve impulse through the optic nerve. Once
the optic nerve has been permanently damaged, there can be
no restoration of vision. With early aggressive and
appropriate surgical intervention and then medical therapy,
your pet's vision can sometimes be maintained. Frequently
with extreme elevations of pressure, the eye becomes
permanently blind and painful. The aim of therapy at that
point is to keep your pet pain-free and maintain a cosmetic
appearance to the eye.
Diagnosis of Glaucoma:
The diagnosis of glaucoma is
based on history, clinical signs, measurement of intraocular
pressure (tonometry) and gonioscopy. Clinical signs of
glaucoma include some or all of the following:
Ø
Excessive
tearing
Ø
Green or yellow
eye discharge,
Ø
Reddened eye, or
an eye that suddenly looks blue,
Ø
Non responsive
dilated pupil
Ø
Excessive
sleeping, hiding, or irritability
People with glaucoma often
report a constant headache that medication will not help. In
later stages of glaucoma, the eye becomes enlarged.
A variety of techniques can be
used to estimate intraocular pressure. We have recently acquired
an electronic device called a Tono-Vet which makes measuring
intra ocular pressure very simple and painless for your pet.
Early detection of glaucoma gives us the best chance of
maintaining sight in the eye. Therefore, we recommend that all
pets be checked for glaucoma durin their yearly physical exam.
Treatment of Glaucoma:
After the initial diagnosis of
glaucoma is made, your pet is aggressively treated with
medication if there is any hope of saving vision. This will
require a period of hospitalization. During periods of
hospitalization, medication may be given directly into the vein
to help reduce the intraocular pressure. Additional drugs
commonly used include those that are aimed at increasing the
outflow of aqueous humor and/or suppressing its production.
These drugs include pilocarpine, timolol, epinephrine, some
newer synthetic epinephrine-like drops and combinations of these
drugs. Yet more medications, known as carbonic anhydrase
inhibitors, are aimed at reducing the production of aqueous
humor. Examples of these medications are Daranide(R) and
Neptazane(R).
If we get the pressure under
control with medication and the eye is still visual,
there are a number of surgical techniques that can be performed
by board certified ophthalmologists, which may control the
pressure permanently. They include laser based techniques and
cyclocryothermy (freezing technique) which aim to reduce the
production of aqueous fluid by selectively destroying parts of
the ciliary body where the aqueous is produced.
If the IOP cannot be normalized
and/or the eye is no longer visual, we can consider the
following options:
Enucleation
The blind, painful eye may be
surgically removed or enucleated. After enucleation, the skin is
stitched shut and the hair will soon re-grow over the surgery
site. This solution offers the least complications and most
owners and pets are very happy with the outcome.. One main
advantage of enucleation is that it gives the opportunity for
the veterinary pathologist to examine the eye to determine the
cause of the glaucoma if there was any uncertainty over this
point. This knowledge may help in assessing the risk of the
development of glaucoma in the opposite eye.
Evisceration and Implantation of a
Silicon Prosthesis
An alternative surgical procedure
is the implantation of a silicone implant within the eye. This
is called an intraocular prosthesis. The technique involves
surgically removing of the contents of the eye, leaving the
outer shell or sclera, and implanting a silicone implant within
the walls of the eye. The shape of the eye is maintained and the
eye moves normally. Following surgery, minimal care is needed
and the eye is maintained in a relatively normal cosmetic
appearance while being free of pain. Complications of this
technique are that corneal ulceration may occasionally occur
following surgery. In some cases scarring of the cornea results
in a gray appearance.
Ciliary Ablation by Intravitreal
Injection of Gentamycin
Another technique used to control
glaucoma is the injection of gentamycin (an antibiotic) into the
inside of the eye. This drug in high concentrations result in a
killing effect on the ciliary body resulting in the reduction or
cessation of the aqueous humor production.. A brief anesthetic
is required and the antibiotic is injected into the eye through
the white of the eye or sclera. Complications of this
technique are generalized shrinking of the eye, return of the
glaucoma at a later time and occasionally chronic pain. This
technique is generally only recommended in quite elderly pets
where the other choices are not acceptable to the client.
Conclusion:
Glaucoma is a common disease in
pets, which can often be treated with early detection. Close
observation of your pet’s eyes and yearly glaucoma screenings
give us the best chances of preventing and/or successfully
treating this disease. |
