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What is Glaucoma and How Does it Develop?

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What is Glaucoma and How Does it Develop

Glaucoma is a group of neurodegenerative diseases with the most consistent clinical finding being increased intraocular pressure (IOP).  Increased IOP is painful and causes damage to the retina and optic nerve, which can rapidly lead to irreversible blindness.  Glaucoma is a leading cause of blindness in animals and people.

Glaucoma develops due to a disturbance in the flow of fluid within and out of the eye.  Aqueous humor is the fluid inside the front chamber of the eye, which is produced by another part of the eye called the ciliary body.  In dogs and cats, it is predominantly drained out of the eye by the iridiocorneal angle (ICA) at the base of the iris (colored portion inside the eye).  When this angle, or drain, is obstructed for any reason, aqueous humor builds up inside the eye and results in increased IOP.  The concept is similar to a faucet that is always turned on, and a drain that is always open.  When the drain becomes plugged and the faucet continues to run, pressure builds up inside the eye.

There are two classifications of glaucoma; primary and secondary.  Primary glaucoma is inherited, so affected animals are born with an abnormality of their ICA.  Predisposed breeds include the Cocker Spaniel, Basset Hound, Beagle, Norwegian Elkhound, Great Dane, Siberian Husky, Jack Russell Terrier, Samoyed, Chow Chow and Chinese Shar Pei.  Primary glaucoma is a bilateral condition, but both eyes are not typically affected at the same time.  Once one eye is affected, the other eye has an increased risk for development of glaucoma, however, with preventative medical therapy, the time interval of which the other eye is affected may be extended.  Secondary glaucoma is acquired, and affected animals have a secondary disease or injury causing increased IOP.  It is typically unilateral, and causes of secondary glaucoma include but are not limited to an anterior lens luxation (movement of the entire lens into the front of the eye), advanced cataracts, intraocular masses, and trauma to the eye.

How is Glaucoma Diagnosed

Diagnosis of glaucoma requires complete ophthalmic examination of the eye and appearance and shape of the optic nerve.  A specialized instrument called a tonometer is useful in the treatment and monitoring of intraocular pressure elevations associated with the disease process.  Clinical signs of glaucoma can be vague and nonspecific but include redness, cloudiness, tearing, loss of vision, and an enlarged eye.  Glaucoma is a painful disease and systemic signs of lethargy and inappetence can be seen in patients with increased intraocular pressure.

In some cases, the drainage structure of the eye can be examined using a special lens (goniolens), which can help classify the type of glaucoma.  Understanding whether a dog has primary or secondary glaucoma is helpful, as this information can help provide a more accurate prognosis and direct whether treating the unaffected eye would be beneficial.

How is Glaucoma Treated

There is no cure for glaucoma, and it can be difficult to control.  Both medical and surgical treatment options are available.  Several different topical medications with different mechanisms of action can be used to control glaucoma.  If these medications fail, surgical options are available.  Surgical options include the use of a laser to damage cells of the ciliary body which produces aqueous humor, and surgical placement of implants that drain excess aqueous humor from the eye.

Due to the aggressive and progressive nature of the disease, some animals lose vision despite treatment.  There are still options for blind and painful eyes, including eye removal with or without the placement of an intraocular prosthesis.

What is the Prognosis for Glaucoma

Prognosis is dependent on early detection and response to treatment.  Immediate evaluation of a patient with glaucoma by a board certified veterinary ophthalmologist is key in providing the best possible outcome.  With early detection and aggressive therapy, patients can remain comfortable and maintain vision for extended periods of time.  It is also important to understand that a diagnosis of glaucoma requires lifelong treatment and monitoring by a veterinary ophthalmologist.

By DJ Haeussler, Jr., BS, MS, DVM, DAVCO

Christina Korb, DVM