Glaucoma is an eye disease marked by an elevation of the normal pressure within the eye, called intraocular pressure. This elevated pressure leads to pain and a loss of vision. In fact, glaucoma is the leading cause of blindness in animals.

To better understand the mechanisms of glaucoma, it helps to understand the normal flow of fluids that maintain intraocular pressure. The intraocular fluid, also known as aqueous humor, is produced behind the iris in the ciliary body.

This fluid flows through the pupil and drains from the eye at the iridocorneal angle, a sieve-like network located between the iris and cornea. Aqueous humor is produced and passed from the eye at approximately the same rate, resulting in a stable pressure within a range of 15 to 25 mmHg (millimeters of mercury).

Glaucoma is a result of a blockage in the outflow of aqueous humor and a subsequent build-up of pressure within the eye. High pressures compress the optic nerve, leading to blindness.

Generally speaking, there are two categories of glaucoma:

1. Primary glaucoma occurs without any previous ocular disease. It is known to occur within certain purebred dogs and thought to be an inherited trait. There are several types of primary glaucoma, including the following:
   v Open-angle glaucoma, as seen in beagles and Norwegian elkhounds
   v Narrow-angle glaucoma, as seen in American and English cocker spaniels
   v Goniodysgenesis (abnormal drainage angle formation), as seen in basset hounds, cocker         spaniels, samoyeds, and chow-chows

2. Secondary glaucoma is a consequence of some other primary ocular disorder that interferes with the natural flow of ocular fluid. Disorders that commonly cause secondary glaucoma include ocular inflammation, lens dislocation, and ocular trauma.

Glaucoma results in blindness because the elevated pressure causes retinal degeneration and blocks the visual impulse through the optic nerve. Once the retina and optic nerve have been permanently damaged, vision cannot be restored.

With early aggressive and appropriate medical and surgical therapy, some vision can be retained. Unfortunately, with extreme elevations of pressure, the eye is permanently blinded and, without treatment, painful. When a patient’s glaucoma has reached this point, our goal in treating the patient is to maintain a cosmetic, pain-free eye.

Diagnosis
The diagnosis of glaucoma is based on history, clinical signs, tonometry, and gonioscopy.

Clinical signs of glaucoma include a red eye with a fixed and dilated pupil. In later stages, the eye might become enlarged.

Tonometry is the measurement of intraocular pressure. A variety of techniques for estimating intraocular pressure are available, including Schiotz tonometry and applanation tonometry.

Gonioscopy is a technique used to evaluate the iridocorneal angle. This technique involves placing a dome-shaped contact lens (goniolens) on the corneal surface. This lens permits the direct visualization of the drainage angle. Gonioscopy occasionally requires sedation but, in calm and cooperative patients, can be performed with only topical anesthetics. Gonioscopy is essential in evaluating the eye for its predisposition to glaucoma or for the risk of future glaucoma attacks.

Treament
After we diagnose glaucoma, we generally treat the patient with aggressive medical therapy. This therapy might require a brief period of hospitalization. During periods of hospitalization, we might give osmotic diuretics orally or intravenously to quickly reduce intraocular pressure.

Once we have controlled the pressure, we prescribe maintenance levels of medications for long-term therapy. We commonly use medications that work to improve the outflow of aqueous humor, such as pilocarpine, demercarium bromide, and latanoprost (xalatan). We also use medications that work to suppress the production of aqueous humor, including carbonic anhydrase inhibitors and beta-blockers.

In some instances, controlling the pressure with medications alone is impossible. A variety of surgical techniques have been developed to help control elevated pressures within the eye when medication cannot do so.

Cyclocryothermy is a freezing procedure that was developed several years ago to decrease the secretion of aqueous humor. With a small probe, this freezing process destroys sites within the ciliary body, which produce the aqueous humor. Several sites are frozen depending on the degree of pressure elevation.

Laser cyclophotocoagulation is a similar procedure that requires the use of a diode laser. This laser can offer a more precise destruction of sites within the ciliary body and causes less inflammation.

Filtering procedures are surgical procedures aimed at promoting an alternate outflow tract for the aqueous humor. For example, in one such procedure, we surgically remove a section of the sclera (the white part of the eye) along with a portion of the iris. To prevent closure of the wound, we implant a sponge soaked with an anti-scarring drug underneath the conjunctiva (the mucous membrane that lines the inner surface of the eyelid). The result is a small bleb (or blister) that allows for an alternate outflow of fluid. In a similar procedure, we implant a filtering device or shunt between the anterior chamber and conjunctiva to create an alternative outflow that prevents the build-up of intraocular fluid. Unfortunately, these procedures provide only temporary relief because most of the shunts become clogged by inflammatory debris.

Another technique lowers intraocular pressure through the injection of an antibiotic compound into the posterior segment of the eye. This antibiotic at high concentration has a toxic effect on the ciliary body; the result is a reduction or even cessation of aqueous humor production. We use this technique only in an eye that is already blind because the antibiotic is also toxic to the retina. The drawback to this technique is that we cannot always achieve a cosmetic effect.

The implantation of an intraocular prosthesis is a salvage procedure we can use to permanently relieve elevated intraocular pressure and associated pain. The result of this procedure is a cosmetic, pain-free eye. In this procedure, the intraocular contents are removed and replaced with a silicone prosthetic. The extraocular tissues remain in place, and the eye can move in concert with the other eye.

Another practical option for patients with a non-visual eye plagued with unrelenting and painful glaucoma is to remove the eye. This surgery offers patients a quick path to comfort.

Prognosis
The prognosis for a patient with glaucoma depends not only upon appropriate medical therapy and regular, consistent examinations, but also (and most important) on an early diagnosis. Many animals are presented too late for us to restore vision in the first affected eye. In these cases, we focus our attention on retaining vision in the good eye and comfort in the glaucomatous eye.

If you have any questions or suspect Glaucoma, please do not hesitate to call us at Eye Care for Animals.