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An Urban Experience
A. Strøm1 1Sweden
Ann R. Strøm, DVM, DACVO
Evidensia Malmö Djursjukhus, Malmö Cypressvägen 11, 213 63 Malmö, Sweden
Definition and classification
Glaucoma is one of the leading causes of blindness in small animals affecting 1-2% of our canine patients. It
is also a cause of ocular pain and is the most common reason for enucleation in dogs. Canine glaucoma is defined as “an elevation in intraocular pressure (IOP) beyond that which is compatible with healthy ocular tissues” and encompasses a diverse group of diseases associated with an increase in intraocular pressure resulting in retinal ganglion cell death. Canine glaucomas are classified on the basis of the possible cause (congenital, primary or secondary), the gonioscopic appearance of the drainage angle (open, narrow or closed as well as the degree of pectinate ligament dysplasia) and stage of the disease (acute, subacute, or chronic).
Congenital glaucoma develops early in life (<1 year of age) and is associated with severe abnormalities of the iridocorneal angle (ICA).
Secondary glaucoma results from obstruction of the normal flow of fluid out of the eye caused by other intraocular diseases. The signs include those usually associated with glaucoma (see below), along with associated signs of the underlying problem (i.e. aqueous flare, swollen iris, hypopyon, lens dislocation, iridal mass, etc.). Anything capable of obstructing the flow
of aqueous through the pupil or its exit through the trabecular meshwork can cause secondary glaucoma. Secondary glaucoma is the most frequently encountered type of glaucoma in veterinary medicine and occur
two to three times as frequently as primary glaucoma. Multiple eye conditions are known to cause secondary glaucoma including anterior uveitis, primary lens luxation, trauma to the globe, intraocular surgery, intraocular neoplasia, intraocular cysts, hyphema, cataracts and ocular melanosis. Depending on the cause of secondary glaucoma this condition may be treatable if the underlying condition can be treated and the intraocular pressure reduced in a timely manner.
Acute clinical symptoms include episcleral injection, corneal edema, mydriasis, varying degrees of pain,
and in the early stages there is often no obvious vision loss. Intraocular pressures can vary throughout the
day. In early stages of glaucoma dogs might have intermittent pressures spikes and therefore initial IOPs measured in the clinic may be normal or high normal. Chronic glaucoma symptoms include all the acute symptoms described above as well as varying degrees of buphthalmos, lens subluxation or luxation, a dark and cupped optic nerve head, and loss of vision.
Tonometry combined with clinical signs is essential
in diagnosing glaucoma. An IOP above 25 mmHg as measured by tonometry and clinical signs as described above is consistent with a diagnosis of glaucoma. Proper technique is required to obtain accurate tonometry reading as falsely elevated pressured can occur if there
is pressure on the neck or the globe itself or if the head is positioned below the heart. The most commonly
used tonometers utilized in veterinary medicine are the rebound tonometers and the applanation tonometers, both of which can obtain accurate and reliable IOP measurements in dogs and are relatively easy to use. While topical anesthesia is required for applanation tonometry it is not necessary for rebound tonometry.
Therapeutic targets for canine glaucomas include reduction of fluid production and improvement of fluid outflow.
  Primary glaucoma is inherited and several breeds
appear to be predisposed to developing the disease including the Cocker Spaniel, Basset Hound, Chow, Shar-Pei, Boston Terrier, Husky, Poodle, Akita, Malamute, Chihuahua and Beagle and several others. Primary glaucoma is not associated with any antecedent eye condition, but have been associated with dysgenesis of mesenchymal structures (pectinate ligament dysplasia) in the ICA and/or narrowing or closure of the ICA and/or the ciliary cleft (CC) in some breeds. Despite the suspicion
of an inherited defect clinical signs of primary glaucoma usually do not become evident until relatively late in life. Primary glaucoma is a bilateral disease although onset of disease varies between eyes (it takes on average 8 months from the first eye is diagnosed till the other eye is showing clinical signs).

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