Note: I originally wrote this article for a National Samoyed publication, but the information applies to most breeds as well, so I hope it will be helpful here.

UNDERSTANDING GLAUCOMA IN THE SAMOYED
(and other breeds)
by Kerry Meydam

Since the Samoyed is one of those breeds considered to have a significant incidence of glaucoma, we should be familiar with what it is, how to recognize it, and what can be done for it. One of the primary problems in dealing with glaucoma in dogs is the difficulty in diagnosing the disease in the early stages, when the prognosis for retention of sight is most favorable. It is frequently misdiagnosed in the early stages and is most frequently identified only after irreversible changes in the eye have led to a total irreversible loss of vision, frequently accompanied by an eye that is painful and cosmetically unacceptable. If you know that this is a problem in our breed, and you can recognize what might be the early signs of glaucoma in your dog, you may be able to help your veterinarian by requesting that he measure IOP (intraocular pressure) on your dog, or you may have the benefit of a veterinary ophthalmologist near by who is available on an emergency basis, which glaucoma certainly is (an emergency).

In this article, I will try to describe the signs, testing, treatments available, and how your dog can be tested for an abnormality which predisposes him to glaucoma, which is not covered in your CERF exam, but does occur at a significant rate in our breed.

What is Glaucoma? Generally speaking, there are 2 classifications of glaucoma - Primary and Secondary.  Primary glaucoma occurs without previous trauma or ocular disease, and will eventually affect both eyes.  Secondary glaucoma is a consequence of some other disease, such as ocular inflammation, lens dislocation, intraocular tumors, or trauma.

 Primary glaucoma is known to occur in certain breeds at a significant rate and is thought to have a genetic basis - it may be familial or hereditary. At the present time there is no proof that it is inherited in the Samoyed, although with more study and more information compiled in the future, we may find that this is the case.

 There are 3 basic types of glaucoma:  Open angle glaucoma as seen in Beagles, Norwegian Elkhounds, Miniature poodles, and sometimes the Basset Hound; Narrow angle glaucoma as seen in American and English Cockers and the Basset Hound; Mesodermal Dysgenesis, or Goniodysgenesis, as seen in the Samoyed and Basset Hound.  (The Samoyed may also have narrow angle glaucoma, or a combination of both).

 Glaucoma is the increased pressure inside the eye due to a decrease in aqueous humor outflow. [See the diagram and glossary of terms] Aqueous humor is produced by the ciliary body cells and is secreted into the posterior chamber of the eye.  It flows over the anterior surface of the lens, through the pupil and into the anterior chamber. The fluid exits through the flow holes in the pectinate ligament.  The area of aqueous outflow, the iridocorneal angle, may be examined clinically with a corneal contact lens. Next, the aqueous humor is resorbed into the bloodstream. (See diagram)  A delicate balance between production and resorption maintains the normal intraocular pressure. The production of aqueous humor is not regulated by the intraocular pressure, however. Occlusion (blockage) of the primary outflow pathway, either at the pupil or iridocorneal angle, results in increased IOP (glaucoma).

 What is Goniodysgenesis:  In the embryo, the iridocorneal angle is a smooth, closed sheet of tissue.  Late in gestation and continuing in the early postnatal stage, the tissue undergoes progressive rarefaction (becomes less dense, more porous, thinner), and leaves a sheet of tissue now consisting of fine fibers (ligaments) with flow spaces between them. The pectinate ligament spans the area between the base of the iris and the sclera (white part of the eye). In some dogs, the pectinate ligament does not undergo enough rarefaction to form normal pectinate ligaments and flow spaces. This arrest in the development of the angle may occur as long as perhaps three weeks after birth. What results in some cases, is a broad sheet of tissue with few openings, which impedes the outflow of aqueous humor. Most animals have rather long slender strands making up the pectinate ligament. These strands occasionally branch and join adjacent strands, and some animals have rather stout ligaments that are often correspondingly short. This shortness may be associated with a webbing of iridal tissue between strands that bridge the width of the angle from 1/3 to 1/2  the width. Extreme examples of this may be associated with glaucoma. Goniodysgenesis signals a potential pre-glaucoma anatomy.

 Diagnosis:  The classical signs of glaucoma, which are often signs of advanced glaucoma, include:  episcleral congestion, or a "red eye", due to resistance to venous flow and veins become dilated;  megaloglobus, or enlargement of the eye, which is a sign of advanced and usually chronic glaucoma (although a young dog with less scleral rigidity may show enlarged globes earlier than older dogs);  the eyelid may not be able to completely close over the enlarged eye, sometimes resulting in corneal ulceration. Most veterinarians will recognize glaucoma at this stage, although by this time, vision is permanently lost. Dislocation of the lens may also occur at this time due to stretching of the fibers which hold it in place.

 Early signs of glaucoma:  The early signs of glaucoma, which may not be recognized as such, may consist of mild conjunctival inflammation, which can mimic conjunctivitis; increased tearing and pain; and photophobia (sensitivity to light which results in some "squinting" of the eye). Visual impairment is rarely observed at this stage, even when both eyes are involved. The cornea begins to become cloudy as well. The general practitioner is not always aware of the early signs or of the predisposed breeds, and that is where we can be of help, by requesting that he/she measure IOP (intraocular pressure) as a part of the examination of the eye. If IOP is elevated above 50 mm Hg, corneal edema (cloudiness), inflammation, and a dilated fixed pupil are characteristic. Normal IOP ranges from 15 to 25 or 30 mm Hg (20 or below is preferred).  If IOP is 30 to 50 mm Hg, the pupil may be moderately dilated and have a sluggish reaction to light.

The diagram below shows the anatomy of the eye concentrating on the drainage angle and its relationship to the cornea and lens, and the direction of aqueous outflow from the ciliary body to the aqueous vein.

GLOSSARY OF TERMS: 

Aqueous humor: The watery fluid present in the anterior chamber of the eye.  
Choroid: That part of the uvea that nourishes the retina with blood.  
Ciliary body:
The part of the uvea that produces the aqueous humor.  
Iridocorneal angle:
A sieve‑like network present at the junction between the iris and the cornea, responsible for drainage of intraocular fluid.  
Conjunctiva: The tissue lining the eyelids which is continuous with the eyeball.  
Glaucoma: An elevation of pressure within the eye incompatible with normal vision.

Goniodysgenesis: Examination of the iridocorneal angle and pectinate ligaments with a special optical instrument.  
Pectinate ligament:
Spans the area between the base of the iris and the sclera (white part of the eye) and form flow spaces.  
Uvea: A blood vessel rich layer consisting of the iris, ciliary body, and choroid.  
Aqueous vein:
Outflow vein where aqueous humor leaves the eye and is resorbed into the blood stream.  
Cyclocryotherapy:
Using very cold temperatures to kill some of the cells producing intraocular fluids, resulting in a decrease in intraocular pressure (pressure inside the eye).


Some veterinarians will have a Schiotz tonometer to measure IOP (all veterinarians should have one in their office).  It is a simple and quick procedure in most cases, requiring only a topical anesthetic on the cornea. The instrument consists of a footplate that approximates the radius of curvature of the human cornea (which makes it not quite as accurate on the canine eye, but will still give a good reading), a plunger (which only sits gently on the cornea to take the measurement), a holding bracket, and a recording scale.  Don't be afraid to ask your vet to take a pressure reading if your dog has any of the above symptoms (insist if you have to). You may save his vision.

 Most ophthalmologists will have an "applanation" tonometer or TonoPen.  Following topical anesthesia (eye drops), a small probe is applied to the center of the cornea with a gentle tapping motion, so the probe contacts the cornea for only a fraction of a second. The contact probe converts IOP into an electrical signal which causes a recording to be made on paper, which is then read to determine IOP.  This method will give the most accurate reading.  The other method, digital palpation is at best crude and unreliable. Instrumental tonometry should always be used.

 Treatment of glaucoma: There is not really an absolute "cure" for primary glaucoma. And it is extremely important to keep in mind that primary glaucoma is a bilateral disease.  When an animal has one eye affected, you must be aware that a similar episode is likely to occur in the other eye (usually anywhere from 6 months to 2 years later).  Preventive medical management may be called for in the "good" eye as well.

 There are several different forms of medical management, including the use of several different types of eye drops which have different modes of action, either to decrease the production of aqueous humor within the eye, or to increase the outflow, or both. I won't go into all the different types;  frequently several are used together. Topical and systemic ocular hypotensive medications usually are not effective in controlling IOP in the dog over extended periods of time, and many times surgical management of the glaucoma is required. A variety of surgical procedures have been developed to aid in the control of IOP or for cosmetic reasons in a non-visual eye. Cyclocryosurgery and Laser Cyclophotocoagulation  are  relatively non-invasive techniques used on eyes to try to save vision. This involves freezing or destruction of the aqueous-producing ciliary body tissue in several locations, which reduces the IOP. An Anterior Chamber Shunt is a surgical procedure where a small valve-like device is implanted just under the surface of the white of the eye. This device has a small tube which enters the eye through a tiny incision and this tube provides an alternate drainage pathway for the aqueous fluid to leave the eye. While some ophthalmologists report frustration with this technique since the little tube may become blocked with fibrin, or the functioning of the valve may be compromised by scarring, other ophthalmologists report considerable success with the procedure. In certain situations, a laser procedure AND the implantation of a glaucoma valve is indicated.

 A non-visual eye may be injected with an antibiotic compound that will reduce pressure by destroying the ciliary body, but this compound is also toxic to the retina and is only used for irreversibly blind and painful eyes.  Sometimes the eye must be removed (enucleation), or an intraocular prosthesis may be placed inside the eviscerated globe to provide a cosmetic, pain free eye usually with minimal complications.

 The Prognosis is largely dependent upon how early the glaucoma is detected.  Usually the dog is presented to the veterinarian long after any restoration of vision is possible. At this point the goal becomes a pain free and cosmetic affected eye and preventing glaucoma in the "good" eye, which will probably become affected from 6 months to 2 years after the first eye. Glaucoma requires long term management including proper medical and/or surgical therapy and regular and consistent re-evaluations.

 Does your dog have a normal Iridocorneal Angle? How do you know? The GONIOSCOPY. It is recommended that Samoyed owners have their dogs tested for goniodysgenesis and/or narrow angles. This test is NOT part of the CERF exam, but can easily be done in conjunction with it, if requested. (note: I would also recommend that owners of other breeds who are at higher risk for glaucoma do the same - Basset Hounds, Cocker Spaniels, Beagles, etc.)

 Gonioscopy is a technique used to evaluate the iridocorneal angle, which will determine the etiology of a glaucoma or predisposition for glaucoma. Both eyes should always be done  The test takes about 15 minutes and involves placing a dome-shaped contact lens (goniolens) on the cornea. This lens permits the ophthalmologist to directly view the drainage angle and determine if the dog has goniodysgenesis, a narrow angle, etc. Either will signal a predisposition for glaucoma.  Medical therapy may then be initiated to prevent or reduce risk of future problems, and you may want to re-evaluate your breeding plans for that dog if the angle is not normal.

 The gonioscopic examination can be performed with the use of only a topical anesthetic, and can be done in conjunction with your CERF exam at a regular club eye clinic if requested in advance and there is sufficient time for scheduling. If you request a gonioscopy and the club holding the eye clinic doesn't know anything about it, ask them to contact the ophthalmologist who will be doing the clinic for information re:  price, scheduling, etc. Remember that the gonioscopic exam must be done prior to the drops to dilate the pupils for the CERF exam being administered to your dog.

 The gonioscopy does NOT have to be repeated yearly like the CERF exam. Samoyeds may be done as early as 6 months of age, if the eye is large enough for the goniolens. Or it may be done at any age after that.

 Conclusion:  Glaucoma remains a leading cause of blindness in animals. It is a treatable disease that may be held in check with medicine, surgery or both. Most important, it requires careful and prompt professional attention and client understanding. Your veterinary ophthalmologist teamed with your regular veterinarian can recommend appropriate therapies suitable for your dog and for your situation. I highly recommend you have all your dogs checked with a gonioscopic examination as well as your regular CERF exam. Don't be afraid to ask your veterinarian or veterinary ophthalmologist as many questions as you can think of.

Just a few more interesting facts about glaucoma:

* Glaucoma occurs more frequently in northern regions and episodes of glaucoma occur more frequently during winter months.

* The left eye is the first to be affected in approximately two thirds of the cases.

* The disease is nearly three times more frequent in bitches than in dogs.

* The incidence of goniodysgenesis appears to be greater in Bassets, Samoyeds, Siberians, Chows, American & English Cockers and Poodles than in other breeds. Open angle glaucoma is seen in Beagles and Norwegian Elkhounds. Narrow angle glaucoma is most frequently seen in American & English Cockers.

* Glaucoma can be precipitated by immune-mediated eye diseases, such as uveitis.

* Average age for glaucoma in the Samoyed is between 2 and 5 years of age.

* The second eye is usually affected from 5 months to two years after the first eye.

I would like to thank Melanie, Sue, and Mike (see references) for all their help and patience in answering all my many questions.    
References:
  1. Dr. Michael Zigler, B.Sc., DVM, Cert. V. Ophthalmology, Bronte Road Animal Hospital, Oakville, ON
  2. Dr. Melanie Williams, DVM, Dip. ACVO; Dr. Sue West, DVM, Dip. ACVO, Toronto North Veterinary Referral Services, Richmond Hill, ON
  3. Dan W. Lorimer, DVM, Eye Clinic for Animals, Tucson, Arizona.
  4. Charles L. Martin, DVM, MSc.  "Gonioscopy"
  5. Robert L. Peiffer, Jr., DVM - “Diagnostic and Therapeutic Challenges in Small Animal Ophthalmology I: Glaucoma” and “Determination of Intraocular Pressure (IOP) in Small Animal Practice”.
  6. Malcolm E. Miller, Bs, DVM, MS, Ph.D.:"Miller's Anatomy of the Dog", W.B. Saunders Co., Philadelphia, PA

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copyright February 2000 Kerry Meydam