Glaucoma In Your Dog Or Cat
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Ron Hines DVM PhD
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When the pressure of the fluid in the front chamber of your pet's eye rises - it is developing Glaucoma. Any condition that interferes with the normal circulation of fluid in the eye can result in glaucoma.
Glaucoma is a progressive disease that will, in most cases, eventually destroy your pet's vision. Veterinarians in general practice can delay this as best they can with medication. But if you want to make every effort to preserve your pet's vision as long as possible, have your regular veterinarian refer you to a board-certified veterinary ophthalmologist as soon as glaucoma is suspected. If glaucoma does take your pet's vision, remember that dogs and cats with limited or no vision remain just as happy.
What Are The Signs Of A Glaucoma?
The first sign that a pet is developing glaucoma is often an enlarged pupil that doesn't constrict normally in bright light. The blood vessels on the affected eye are also often very inflamed or bloodshot.
Often, the problem is mistaken for conjunctivitis or an eye allergy. When these pets are given common steroid-containing eye drops, their eyes do look better - temporarily. Veterinarians see so many eye irritations that steroid-containing eye drops are frequently dispensed. However, it is wise to have the intraoccular pressure of your pet's eye checked (with a tonometer) whenever there is even a slight suspicion that the problem might be more than simple conjunctivitis. This intraoccular pressure test is the only way glaucoma can be diagnosed early. By the time the eye actually enlarges, vision has been permanently lost.
If a developing glaucoma is not treated immediately, the pet's eye will begin to look cloudy. By this time, vision is usually lost and can not be restored. This can all occur extremely fast - sometimes in less than one day. Ophthalmologists have a "30-30 rule". If the pressure in your pet's eye stays above 30mm for thirty hours the pet will loose vision in that eye.
What Are The Types Of Glaucoma That Develop In Dogs And Cats?
Glaucomas are divided as to being primary or secondary and as to being open angle or closed ( narrow) angle.
Primary glaucoma is an inherited problem. It is due to abnormal anatomy of the eye. We see it most often in Cocker Spaniels, Basset Hounds, Australian shepherds, Chows, Shar peis, Labradors and Nordic breeds. But any breed can be affected. Glaucoma of this type usually begins in one eye - but eventually both eyes are involved and the result is complete loss of the pet's vision. It is usually no more than 8 months until the second eye is involved. It is much less frequent in cats. Pets with this problem should not be bred.
Secondary glaucoma occurs when something occurs in the eye that prevents normal fluid flow and drainage. This can be a cataract, inflammation (uveitis) or trauma to the eye. In cats, uveitis is the most common cause. Uveitis in cats is often due to FIP .
Glaucoma is also divided into open angle and closed or narrow angle glaucoma. Narrow angle glaucoma is the most common in dogs although beagles have a high incidence of the open form.
How Is Glaucoma Diagnosed?
There is only one way to diagnose a glaucoma early enough so that there might be something that might help. It is by using a small device called a tonometer that measures pressure within the pet's eye.
This can be done in your veterinarian's office. There are two types of tonometers. The popular Schiotz type, rests on your pet's cornea and measures the amount of indentation that a given weight will cause in the cornea. The more the pressure within the eye, the less the cornea will indent. The type that blasts a jet of air against the cornea to measure the same effect is less commonly used.
Checking eye pressure in your pet is not painful. A drop of local anesthetic is placed in the animal's eye and its head must be held steady. The whole test only takes a minute or two.
Normal pressure (IOP, intra-ocular pressure) varies depending on the technique and apparatus, but generally, a reading greater than 25 indicates glaucoma is present. A pressure of 50, permanently damages the optic nerve and retina. So with a reading of 50 or more, vision has already been permanently lost - or soon will be.
If the test indicates that glaucoma is present, a second test, called gonioscopy determines if the glaucoma is the wide angle or narrow angle type.
What Happens As Glaucoma Develops?
Pets that develop the rarer form, open angle glaucoma, develop it slowly over a matter of months.
But with more common narrow angle glaucoma, pressure within the eye rises suddenly. This increased pressure quickly destroys the pet's retina and optic nerve.
Either form is quite painful. The pet may paw at the eye or rub its head along the floor. The blood vessels of the white portion (sclera) stand out prominently and the normally-clear portion (cornea) may become cloudy or bluish. Bright light will bother your pet. Some pets loose their appetite and become depressed due to the pain, but many are more stoic.
As the problem progresses, the affected eye will appear larger and the pupil will not contract in light. By the time this occurs, it is too late to save vision.
This makes glaucoma an emergency. If the pressure in the has risen significantly vision can be permanently lost within less than a day.
What Treatments Are Available For My Pet?
Unfortunately, medical treatment of glaucoma works much better in people than it does in dogs and cats. You probably know of someone with glaucoma whose vision has been preserved for many years with topical and oral medications. But dogs and cats have different types of glaucoma than we do. And their types do not respond nearly as well to medicine. In many cases, surgery to relieve the pressure is the only option that might benefit your pet.
How your pet is treated depends on the stage of the problem. The goals of treatment are:
1) Reduce the pressure within the eye
2) Reduce the amount of fluid that the eye produces
3) Increase the amount of fluid drainage
4) Provide pain relief for your pet
Medications that suppress fluid production within the eye:
These are also called aqueous formation suppressors and carbonic anhydrase inhibitors. They include Acetazolamide (Diamox), Dichlorphenamide (Daranide), dorzolamide HCl (Trusopt), and brinzolamide (Azopt). Timolol, a beta-adrenergic receptor blocker is commonly used for this purpose. Some (eg Timolol-dorzolamide) are used in combination.
Medications that constrict the pupil:
These are also called miotics. They relieve pain and allow fluid in the eye to drain. Pilocarpine is the miotic most often used in the treatment of canine glaucoma; Physostigmine and Demacarium Bromide are others.
Other Medications that encourage fluid to flow out of the eye:
These are also called uveosclaeral outflow enhancers. Two are Brimonidine Tartrate (Alphagan) and Latanoprost
These medications are used in glaucoma emergences to attempt to lower intraocular pressure rapidly. Mannitol, Glycerol and Urea are the ones generally used.
These medications are given to try to protect the vision-producing cells within the eye from death when pressure within the eye rises. This is still an experimental therapy. The medications are generally giving to promote blood supply to these cells. Some of these drugs are in a class called calcium channel blockers.
Because current medications have been so disappointing in halting glaucoma, a number of surgical techniques are commonly used. These techniques try to destroy the cells in the front section of the eye that are producing the fluid (cilliary bodies) and to open channels for the fluid to leave the eye.
There are many variations of this surgery with various complicated names. Once you have located a veterinary ophthalmologist in your area, that person will tell you which of these procedures they are most confident in using. The skill and experience of the ophthalmic surgeon in performing the procedure is more important than the type of procedure they use. New techniques and materials are always being explored. You can read about one here.
When no vision remains in your pet's eye, the goal of the surgery is to reduce pain and preserve the eye structure. Preserving the structure of a blind eye is for the benefit of the owner - not the pet.
The best alternative to extensive cosmetic surgery is to remove the pet's eye. Your regular veterinarian should be willing to do this, or to send you to a colegue in general practice who will. But all other procedures are best done by a board certified veterinary ophthalmologist.