Glaucoma

What is Glaucoma?

Put simply, glaucoma is an increase in the pressure inside the eye.  The big question is what is causing the increased pressure.  Before talking about this, let’s talk a little about the inside of the eye.  The eye can be divided into structures in front of the lens and behind the lens.  The globe of the eye behind the lens has a chamber that is filled in with a thicker material called vitreous humor.  In front of the lens the eye contains a thinner material called aqueous humor.  Eye pressures are the pressure inside the aqueous humor.  The aqueous is produced by the ciliary epithelium next to the len.  It then flows forward from the posterior chamber through the iris opening into the anterior chamber and then finally drains out through the trabecular meshwork into the aqueous plexus (and back into the blood stream).  The production of aqueous and its drainage is typically well regulated, but when the production exceeds drainage, pressures rise and glaucoma results.

 What are the clinical signs of glaucoma and how is it diagnosed?

When pets have glaucoma you can notice a number of different signs including redness of the eye, abnormal blood vessels on the eye, different sized pupils, squinting, pawing at the eye, and vision problems.  The diagnosis of glaucoma is fairly straightforward and involves measuring the pressures of the eye and documenting an increased eye pressure.  Most veterinarians have a tool called a Tonopen that accurately and quickly measures eye pressures.  The eye is first numbed; then the Tonopen is pressed against the eye multiple times and records the pressure.  Normal pressures for dogs and cats are between 10-25 mm Hg.  Individual pressures above this confirm glaucoma or a major difference (>8 mm Hg) between pressures in either eye may indicate a problem. 

What causes glaucoma?

The causes for glaucoma can be divided into two major categories: primary versus secondary.

1. Primary glaucoma typically means there is an inherent change (typically breed related) in the drainage angle of the eye that causes reduced drainage of aqueous.  A majority of these are caused by abnormal formation of the drainage angle of the eye that results in a narrower angle and poor drainage (closed-angle).  A smaller subset appears to have a normal drainage angle, but the drainage is still impaired (open-angle).  Breeds where primary glaucoma is common include Cocker Spaniels, Basset Hounds, Schnauzers, Beagles, Poodles, as well as many others.
2. Secondary glaucoma occurs when other eye problems occur that alter the drainage of aqueous.  This may occur if the drainage angle of the eye becomes blocked (due to inflammation, cancer, cyst) or the iris opening becomes blocked (lens luxation or chronic inflammation of the anterior eye chamber).  Long term treatment for this glaucoma is more about treating the primary cause while managing the secondary glaucoma.

How do we treat glaucoma?

The first question to ask when treating glaucoma is whether the eye is still visual, as this will determine what options are available for treatment.  Also, with secondary glaucoma, it is important to treat the underlying cause as this may ultimately fix the problem.

1. Visual Eyes: The first goal for visual eyes is to decrease the pressures as quickly as possible.  The longer the pressures stay elevated the better chance at long term damage to the retina and subsequently vision. 

1. Topical medications: Many topical medications are available that help decrease the pressure of the eyes.  With severe cases of glaucoma multiple medications are started that work in different ways.  In most cases of sudden, severe primary glaucoma a medication to open the drainage angle (latanoprost) along with medications to decrease aqueous production (dorzolamide) are used together.  An intravenous medication called mannitol may also be used that helps draw out some of the vitreous of the eye making more room for the aqueous.  Some eyes with glaucoma respond well to these medications and can be managed with just topical medications for a time.  However, a long term solution usually involves surgery.
2. Surgical options: These options are only available through a veterinary ophthalmologist and financial costs can be very high.  With primary glaucoma the recommendation will usually be a combination of two procedures.  The first is cyclophotocoagulation, which is a procedure that destroys part of the ciliary body (tissue responsible for aqueous production) with a laser.  This helps lower aqueous production.  In addition, some ophthalmologists will also recommend placing a shunt that drains the aqueous humor out into the conjunctiva.  Even with these procedures, topical medications may still be needed and complications can occur.

1. Non-visual Eyes: Once eye pressures have been high enough for long enough the retina will degenerate and the eye will become non-visual.  When this is the case, our goal is to make the pet comfortable (as glaucoma is painful).  If topical medications aren’t working, then a few procedures should be considered.

1. Enucleation: Removing the eye eliminates the problem of the high pressures and pain.  This procedure can be performed by most veterinarians; however, if a prosthesis is desired then an ophthalmologist may be needed.  Even without prosthesis, most owners are happy with the cosmetic appearance of an enucleation.

2. Intravitreal gentamicin injection: This procedure is only available for dogs and there cannot be inflammation present in the eye when performed.  Gentamicin is an antibiotic, but for this procedure is being used because it kills the ciliary body and decreases aqueous production.  It works in around 75% of dogs, but cosmetic changes (cataract, small eye, etc) may be undesirable.  It is typically used as an alternative to enucleation when cost is a factor.

What to do about the other eye?

In many cases primary glaucoma develops in one eye before the other; however, the other eye has the same changes that led to glaucoma in the bad eye.  Starting the other, non-glaucomatous eye on a topical medication can lengthen the time before that eye develops glaucoma.  The typical medications used are either dorzolamide (Trusopt) or timolol (Timoptic).