Section: Information In-depth
Glaucoma is an elevation of the pressure within the eye that is incompatible with normal function of the eye. It is a disorder of the outflow of fluid (aqueous humor) from the eye and not a disease of overproduction of fluid within the eye. Sudden, high elevations of pressure within the eye can cause devastating and irreparable damage to the retina (which acts like the film a the camera) and the optic nerve (which sends information from the eye to the brain).
The causes of glaucoma are both primary (spontaneous, probably inherited) and secondary (arise in association with other diseases within the eye).
Primary Glaucoma
The fluid outflow pathway in the eye is commonly known as the drainage angle. For proper outflow, the angle needs to be open and functioning properly. The drainage area looks and acts almost like the sieve in a kitchen drain. Rarely in cats, this drainage angle can be abnormal. Either the access to the sieve is narrow or the drain behind the sieve does not work properly. Even though the fluid passes through the sieve it does not drain into the veins around the eye well and backs up, causing the pressure in the eye to increase.
Secondary Glaucoma
Inflammation inside of the eye that gives rise to glaucoma has many different causes. In the cat, chronic anterior uveitis (an inflammation of the iris and surrounding tissues) is caused by feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), feline infectious peritonitis virus (FIP), and toxoplasmosis. In some cases of chronic inflammation, the cause is never determined. One or both eyes may be affected by the inflammation, and one or both eyes may develop glaucoma.
Tumors within the eye generally occur in older pets. Only one eye is usually affected and the progression to glaucoma may be relatively slow.
Trauma can happen to any outdoor cat, at any age (hit by car, hit in the head, falling, attacked by another animal).
Lens luxations (dislocations) usually only occur in the cat in association with chronic anterior uveitis. The chronic inflammation causes the small fibers that hold the lens in position to break down. When enough fibers are broken, the lens can become dislocated. Glaucoma is most likely when the lens falls into the front chamber of the eye and becomes trapped in front of the pupil.
What to Watch For
Many diseases can cause a red, squinty, teary and cloudy eye, but glaucoma should always be considered as a cause of these signs. Frequently, when the pressure is high, the cat is quite painful and holds the eye closed or keeps the third eyelid (a pink/red membrane) up over the eye.
If the pressure inside the eye remains elevated for some time, the eyeball may actually stretch and enlarge. As the eye enlarges or becomes buphthalmic, the eyelids may no longer be able to protect the surface of the eye, and the eye may be traumatized easier.
Assessing vision in the cat can be difficult. Your pet should blink and try to close the eye when a very bright light is shown into the eye. Without creating airflow or touching any whiskers, see if your cat blinks when you wave your hand in front of the eye (known as a "menace test" because your pet is responding to a menacing gesture). You can also try to throw objects like cotton balls (anything that won't make a sound when it lands) in front of your pet to see if he follows the object. Remember that a cat with good vision in one eye will act normally, so it may not be possible to tell if vision is altered in the glaucomatous eye.
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