Ron Hines DVM PhD 4/25/06

When the pressure of the fluid (aqueous humor) in the forward chamber of the eye rises we call the condition Glaucoma. Approximately one-half of one percent of all dogs in the United States develop this problem. It is much rarer in cats. In animals and man, pressure increases because the normal channels through which fluid leaves the eye become obstructed. As pressure in the eye increases, the cells of vision in the retina on the posterior surface of the eye, become damaged and vision is lost. In dogs with glaucoma, intraocular pressure rises above 30milimeters Hg (of mercury). The mean intraocular pressure of normal cats is 18.7±3.6.

Two forms of Glaucoma can occur, primary and secondary. Primary glaucoma is due to an inherited abnormal angle at the point where the iris meets the cornea. This abnormal angle obstructs the exit of fluid from the eye. Because it is genetic, the second eye often becomes affected within 6-12 month of the first. We suspect primary glaucoma when we can find no other causes for the reduction in intraocular fluid flow. Middle-aged and older dogs and female dogs and are most at risk of primary glaucoma. While human primary glaucoma is termed open angle glaucoma (beagle dogs get this), canine primary glaucoma is usually closed angle. Dog breeds commonly affected with closed angle glaucoma are spaniels, toy poodles, Boston terriers, dalmatians basset hounds and huskies. Persian cats are the most susceptible to glaucoma. Glaucoma is not a disease that can be cured. We can, however, slow the progress of the disease with medications. Because primary glaucoma has a genetic component, pets with this condition should not be bred.

Secondary glaucoma usually affects only one eye. This is the most common form in cats. Secondary glaucoma occurs due to inflammation of structures within the eye. There are many causes for such inflammation. It can be due to infectious or autoimmune disease, trauma or cancer. Scaring (synechia) and distortion at the corneal/iris angle where the fluid leaves, impedes fluid exit. Fluid pressure then increases as it does in primary glaucoma. Secondary glaucoma can also occur when the lens is jarred loose from its attachments or when it moves due to degenerative changes in its attachments (zonules).

Although advanced glaucoma can be diagnosed on sight, early cases require the use of an instrument called a Schiotz tonometer. Using this instrument, the normal pressure in the canine and feline eye is 15-25milimeters of mercury (Hg) with less that 5mm difference between eyes. By the time the eye is grossly enlarged vision has been lost and no form of treatment will allow its return.

An early sign of glaucoma is enlargement of the blood vessels of the sclera or whites of the eyes along with impaired vision or blindness. At an early stage, blindness may be reversible. Once blindness is 3-5 days old it is usually irreversible. These eyes often show a bluish, diffuse cloudiness of the cornea. The pupils of these eyes are dilated (mydriasis) because light stops at a damaged retina and can not reach the optic centers of the brain.

Treatment of Glaucoma

If the glaucoma is newly formed pressure must be reduced in the eye as quickly as possible. Even then, the outlook for vision in the eye is poor. Once glaucoma has been present for a month or so, the object of treatment is to make the dog or cat as comfortable as possible. Luckily, dogs and cats are not as dependent on their vision as people and seem to be perfectly happy if their eyes do not hurt. The pet’s acute sense of smell, touch and hearing readily make up for the loss of vision.

In dogs, very early in the disease the intravenous administration of osmotic mannitol (0.5-1gm/pound over 20minutes) and the oral administration of glycerol (50% solution, 0.5-1gm/pound orally) may lower intraocular pressure. Glycerol often causes vomiting.

I have not treated enough cases of glaucoma in cats to describe treatment. Dogs should also receive oral carbonic anhydrase inhibitors to reduce intraocular pressure. These agents reduce the production of fluid within the eye. Acetazolamide (Diamox) is given at 2-4mg/pound body weight two or three times a day. Side effects of this drug commonly include increased thirst, depression, confusion, poor appetite, vomiting and diarrhea. They occur in almost half the patients. Neptazane (1-2mg/lb two or three times a day) or Daranide (1-2mg/lb two or three times a day) have much fewer side effects. However I was not able to find them at pharmacies the last time I prescribed it. The two drugs still have numerous entries on the internet. A new human pharmaceutical, topical dorzolamide (Trusopt) shows promise in dogs. All dogs should receive a potassium supplement while on these medications.

I have found pilocarpine to be too irritating to my patients. When it is prescribed, a 2% solution is used three or four times a day. Instead, I use another eye drop preparation, 0.5% timolol maleate (Timoptic) twice a day.

Some specialists administer the adrenergic agent, Dipivefrin hydrochloride (Propine) drops to help reduce intraocular pressure. Others have experimented with latanoprost (Xalatan) as an alternative to timolol.

Surgery

Eventually, all drugs I have mentioned will no longer reduce intra-ocular pressure or side effects become intolerable. When this time comes, three types of surgery are available to treat glaucoma. This surgery should be performed by an experienced veterinary ophthalmologist.

The first destroys part of the organ that produces fluid in the eye (the ciliary body). The second creates an alternative escape rout for the fluid. Destruction of the ciliary body is called cyclo-destruction. It requires cryosurgery (cold) or a surgical laser. To the best of my knowledge it rarely restores vision.

Several surgical techniques open a new passage for fluid to leave the eye or implant a small tube to shunt the fluid to below the sclera. These procedures have a high incidence of complications.

I feel that the best long-term solution for glaucoma is the removal the contents of one or both eyes, retaining only the non-functional globe or the injection of gentamycin into the affected eye to destroy the fluid-secreting apparatus (ciliary body). I have performed this surgery in zoo animals but not in dogs or cats.

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