Glaucoma

an elevation in pressure in the eye out of its normal range

What is it?
The eye is a fluid filled structure.  The fluid, aqueous, is produced by the ciliary body located behind the iris.  Fluid flows around the lens, through the pupil and into the front of the eye (anterior chamber).  At the periphery of the iris, where the iris attaches near the edge of the cornea is the drainage angle where aqueous leaves the eye.  Glaucoma occurs when the ability of aqueous to leave the eye through the drainage angle is less than the amount of aqueous being produced by the ciliary body.  Glaucoma is an elevation in pressure in the eye out of its normal range of 15 to 25 mm Hg.  As pressure is raised above normal levels for even a short period of time the retina can become damaged and result in blindness.

How is it diagnosed?
Glaucoma can only be properly diagnosed with a tonometer.  A tonometer measures the pressure within the eye.  Normal pressure will range from about 15 mm Hg up to about 25 mmHg.  A second instrument that may be used is a gonioprism.  The gonioprism is a special contact lens that allows the ophthalmologist to look at the drainage angle.  Examining the drainage angle can help determine the type of glaucoma affecting the animal.

What causes it?
Glaucoma is generally divided into two types, primary and secondary.  Primary glaucoma is considered an inherited disease and is commonly seen in Basset Hounds, Cocker Spaniels, Beagles, Poodles, and Chow Chows.  It is primary because the defect is isolated to ocular tissues and from a routine eye exam there is no apparent cause.  Primary glaucoma is generally associated with a narrowing of the drainage angle.  Secondary glaucoma is due to changes that affect the eye such as a change in lens position, inflammation (uveitis), and trauma.  The type of glaucoma determines the course of treatment chosen.

How is it treated?
Primary glaucoma is most often diagnosed when one eye is enlarged (buphthalmic) and blind and the other eye still normal.  It is not unusual to have one eye blinded since the patient cannot talk and early signs of glaucoma such as redness and discharge can be associated with many eye diseases.  When one eye remains visual, treatment is designed to prevent future pressure elevations and preserve vision.  The selection of treatment depends on the breed, preexisting damage, and economics.  The enlarged or buphthalmic eye is addressed with a salvage procedure.  Generally a prosthesis is recommended to preserve the eye cosmetically and remove a source of irritation.  Removal of the eye will also eliminate the irritation but is not as cosmetically acceptable.  Careful monitoring of the remaining eye by the owner and ophthalmologist is critical to preserving vision long-term.  In some cases, an eye affected with primary glaucoma can be saved and vision preserved.  Surgery is generally the best method of saving sight in these eyes.  Laser surgery and cryosurgery may be used to reduce the amount of aqueous produced.  A valve may be implanted to provide an alternative pathway for aqueous to leave the eye when the drainage angle is narrowed.

What is the prognosis?
Secondary glaucoma can often be treated successfully.  Removing the cause of the pressure elevation will generally result in reduction of pressure to normal levels and restoration of vision.

The prognosis depends on the character of the eye upon presentation, the type of glaucoma, and the initial response to therapy.  The best way to think of glaucoma is like cancer with keeping an eye visual either through prophylactic treatment or primary treatment of an eye like maintaining a cancer patient in remission.  A close adherence to treatment protocols and observation of the patient by the owner is critical to long-term success.