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:
- Alaskan Malamute
- Basset Hound
- Border Collie
- Boston Terrier
- Bouvier Des Flanders
- Cairn Terrier
- Cardigan Welsh Corgi
- Cocker Spaniel
- Dandie Dinmont Terrier
- English Springer Spaniel
- Giant Schnauzer
- Great Dane
- Manchester Terrier
- Miniature Pinscher
- Norfolk Terrier
- Norwegian Elkhound
- Norwich Terrier
- Pembroke Welsh Corgi
- Scottish Terrier
- Sealyham Terrier
- Shih Tzu
- Siberian Husky
- Smooth-Coated Fox Terrier
- Tibetan Terrier
- Welsh Springer Spaniel
- Welsh Terrier
- West Highland White Terrier
- 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.
Our Veterinary’s 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:
- Excessive tearing
- Green or yellow eye discharge,
- Reddened eye, or an eye that suddenly looks blue,
- Non responsive dilated pupil
- 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.
Our Veterinary’s 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:
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.
Glaucoma is a common disease in pets of Alta Loma, Rancho Cucamonga, Upland, Ontario, Claremont, Fontana and the Inland Empire, 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.