Inside the eye, a clear fluid called aqueous humor circulates in the front chamber between the cornea and the pupil. The aqueous humor leaves the eye through a small sieve-like structure between the base of the cornea and front of the iris. Normal pressure is a dynamic function of the balance between aqueous production and drainage. When the drainage sieve does not work properly or becomes clogged, or when the flow of aqueous through the pupil becomes obstructed, intraocular pressure can increase to an unsafe level. This is particularly true if aqueous production continues at a normal level. Glaucoma is never caused by over production of aqueous humor. It is always a drainage problem.

Glaucoma occurs far more frequently in dogs (0.675%) than in cats (0.197%). Primary, hereditary, breed-related glaucoma is most commonly seen in purebred dogs. Cats usually have secondary glaucoma that is associated with chronic inflammation of the iris (uveitis) or with intraocular tumors. Primary glaucoma in cats is very rare but Siamese and Burmese cats may be predisposed. Over forty different breeds of dogs are predisposed to glaucoma, with common ones being the cocker spaniel, beagle, basset, Akita, chow chow, Samoyed, Bouvier de Flandres, Shih Tzu and Chinese Shar Pei.

Glaucoma can be a very painful disease for pets – more so than for people – as the intraocular pressure can become rapidly elevated to levels much higher than typically occur in people. In people, the pain feels like a constant bad headache. Normal intraocular pressure in people is 12-22 mmHg. The normal pressure for dogs and cats is 10-25 mmHg. With glaucoma in cats and dogs, the pressure may go up to 30 mmHg and higher. Values above 50 mmHg rapidly cause blindness, are painful and may cause the eye to stretch and enlarge.

An affected eye may look normal to a pet owner when the glaucoma is mild. Early signs can include a bloodshot eye, cloudy cornea, dilated pupil, and squinting/holding the eye closed. Over time, the eye size can increase and it may bulge.

Unfortunately in cases of severe glaucoma, the eye is often permanently blind by the time of diagnosis. Pets can act normal with vision in just one eye, which is one reason glaucoma is often diagnosed late in the disease’s course.

Diagnosis and Treatment

Glaucoma can be detected by measurement of eye pressure using an instrument called a tonometer. Tonometry can be done by your veterinarian during the eye exam. Pressures are measured in both eyes and then compared.

If glaucoma is found, then numerous medications are available to lower the pressure. All glaucoma medications used in dogs and cats are medications that have been manufactured for human glaucoma. The treatment of glaucoma requires faithful, consistent application of the medications. These are not medications that can be skipped or forgotten without risk of the eye pressure rapidly deteriorating.

Some cases of primary glaucoma are also treated with surgery. A major emphasis in secondary glaucoma is to determine the underlying cause and to treat it, as well as the elevated pressure. If primary glaucoma is diagnosed in one eye, it is common for the other eye to be started on preventative medication because both eyes are prone to the disease.

Removal of the eye (enucleation) is sometimes recommended. Examples include a blind eye in which the glaucoma does not respond to medications; any eye containing a tumor; when the eye is blind and the owner cannot afford glaucoma medications; and when infection is the cause of the glaucoma and it does not respond well to appropriate therapy.

Prognosis

In most cases, glaucoma is not curable and once vision is lost it will not return. With primary glaucoma, vision is often lost over time, even with appropriate treatment. For secondary glaucoma, prognosis depends on the underlying disease and its response to treatment.

The iris, which is the colored part of the eye, surrounds the pupil and regulates how much light passes through it. Behind the iris and pupil is the lens, a transparent structure that also focuses light onto the retina. The retina converts the light into nerve impulses, which travel to the brain through the optic nerve. The brain interprets these impulses into an image. Abnormalities in these and other structures of the eye may lead to blindness.

Some of the more common causes of blindness in dogs and cats include the following.

• Uveitis: Uveitis is a painful condition in which the uvea becomes inflamed. The uvea is a blood vessel-rich tissue consisting of the iris, the ciliary body (which produces fluid inside the eye) and the choroid (which nourishes the retina). Causes of uveitis include infections such as feline infectious peritonitis (FIP) and tick-borne ehrlichiosis, tumors, immune-mediated conditions, eye trauma, toxins and eye irritants.
• Cataracts: A cataract is a cloudiness in the lens. The lens is supposed to be transparent to allow light to pass through, but cataracts impair this function. Cataracts may affect only a small part of the lens initially, but progress to affect more of the lens over time. The most common cause of cataracts in cats is uveitis, while genetics and diabetes are the two most common causes in dogs. Other causes include eye trauma, toxins, nutritional deficiencies, radiation, electric shock and age-related degeneration.
  
• Glaucoma: Glaucoma is increased pressure in the eye. In a healthy eye, the volume of fluid that goes in and out of the eye is balanced to maintain a normal eye pressure. In glaucoma, a problem draining that fluid causes the eye pressure to rise, a condition that can quickly and painfully cause loss of sight. Primary glaucoma results from genetics while secondary glaucoma results from conditions such as uveitis, eye tumors and anterior lens luxation (see section on lens luxation). 
• Retinal detachment: The retina is a 10-layered structure that converts light signals into nerve impulses that are sent to the brain for interpretation into an image. When specific layers of the retina separate or detach from one another, the retina can no longer carry out this function properly, resulting in impaired vision. Causes of retinal detachment include genetics, trauma, tumors, infections, immune-mediated conditions, uveitis, eye surgery and high blood pressure. 

• Progressive retinal atrophy (PRA): PRA is an inherited disease that occurs in dogs and more rarely in cats. In pets with PRA, the retina degenerates over time, leading to blindness. In most cases, the pet is initially blind only in low-light conditions, but will eventually become blind in all conditions. Cataracts may also accompany PRA in some dogs.
  
• Sudden acquired retinal degeneration syndrome (SARDS): This syndrome is a disease in dogs in which the retina rapidly and irreversibly deteriorates, leading to blindness within days to months. The cause of SARDS is unknown.
  
• Optic neuritis: Optic neuritis is inflammation of the optic nerve, which carries impulses from the eye to the brain to be interpreted as images. Dogs and cats affected by this condition generally end up with partial or complete blindness. Causes of optic neuritis include such infections as distemper in dogs and cryptococcosis, a systemic fungal infection, in cats. Inflammation, immune-mediated conditions, toxins such as rodenticides, eye trauma and tumors are other potential causes.
  
• Lens luxation: The lens is normally held in position by fine ligaments. If these ligaments fail, then the lens shifts – or luxates – from its normal position. A forward shift can block circulation of the eye’s fluid and quickly lead to painful glaucoma. A backward shift is not as immediately harmful, but secondary glaucoma and retinal detachment can occur. Terrier breeds are predisposed to primary, inherited luxations. Lens luxation may also occur due to cataracts, glaucoma, trauma, tumors, and uveitis.
  
• Corneal diseases such as keratoconjunctivitis sicca, pigmentary keratitis, and pannus will completely scar the cornea if untreated.

If you notice any signs of blindness or changes in your pet’s eyes, take your pet to the veterinarian. Oftentimes, older dogs and cats develop an age-related cloudiness in their eyes that is usually harmless to vision. However, it can look similar to a cataract, so it is still worth getting checked by a veterinarian to make certain it’s harmless.