Glaucoma is a group of disorders characterized by elevated pressure within the eye and is a common cause of blindness. Cells inside the eye produce a clear fluid that maintains the shape of the eye and nourishes the tissues inside the eye. This fluid is constantly being produced, circulates around the lens, and exits the eye through the iridocorneal angle. Intraocular pressure elevation occurs as a result of poor fluid outflow. The prognosis for vision in an eye with glaucoma depends on the degree and duration of elevated pressure. Irreversible damage to the retina and optic nerve may occur within a day or two if the pressure is markedly elevated.

Most cases of glaucoma in the cat are secondary to other diseases, and many patients are blind when they are first examined. All cats with glaucoma should be examined by an ophthalmologist to identify the primary disease so appropriate treatment can be initiated.  Many cats have glaucoma secondary to having the fluid in the front of the eye directed to the chamber in the back of the eye (aqueous misdirection). Congenital glaucoma has also been reported in cats and is likely related to a defect in fluid drainage. 

Increased intraocular pressure is painful. Dogs, cats, and humans have normal intraocular pressures between 10 and 20 mmHg. Feline glaucoma patients may have pressures over 45 mmHg. In people the pain is in the form of a constant headache or migraine.  In cats, this discomfort can result in decreased activity, less desire to play, irritability, or decreased appetite, and is often not apparent to the owner.

Clinical signs of glaucoma in cats may be subtle and include increased eye size, cloudy appearance of the eye, redness of the conjunctiva and sclera and increase in pupil size.  Other conditions may produce similar signs, so the diagnosis is confirmed by measuring the cat’s intraocular pressure.

Topical medications are used to treat glaucoma, but the response to treatment must be adjusted to each individual. Timolol maleate 0.5 % is a mild glaucoma medication that is well tolerated by cats and can reduce intraocular pressure by 3-10 mmHg. It is a beta blocker and should not be given to a cat with asthma.  Brinzolamide topical 1% or dorzolamide topical 2% are frequently used to treat glaucoma in cats. These are topical carbonic anhydrase inhibitors that reduce the production of fluid in the eye.  They are the most effective topical glaucoma medications for cats.  More side effects are associated with dorzolamide (Trusopt®) than with brinzolaminde (Azopt ®). 

Surgical options for cats are limited and a veterinary ophthalmologist should be consulted. When vision has been lost and pain is present, removal of the eye is the best option. Pain-free, blind cats use their other senses and compensate very well.