Updates on Canine Glaucoma

The definition of glaucoma is an elevated intraocular pressure. Normal intraocular pressures in dogs are 12-24mmHg. The pressure is best measured by tonometry and the most common tonometers are the Tonopen, TonoVet, and Schiotz tonometers. The Tonopen and TonoVet are more easily utilized but the Schiotz can also be accurate when the user is adequately trained in its use. Glaucoma can be either be primary or secondary in cause. Primary glaucoma is due to an abnormality in the drainage angle that prevents the release of aqueous humor. Secondary glaucoma is due to a different condition causing secondary blockage of a normal drainage angle. The most common cause of secondary glaucoma is uveitis with obstruction of the drainage angle from inflammatory cells or red blood cells or a preiridal fibrovascular membrane. Other causes include neoplasia, pupillary blockade from lens luxation, melanocytic deposition, or posterior synechia leading to pupillary blockade and iris bombe. Primary glaucoma is most common in purebred dogs. The most common breeds of dogs affected are Cocker Spaniel, Basset Hound, Chow, Shar-Pei, Boston Terrier, Husky, Poodle, Akita, Malamute, Chihuahua, Beagle, and Great Dane. The age of presentation is generally between 4 and 10 years. Acute clinical symptoms include episcleral injection, corneal edema, mydriasis, varying degrees of pain, and they are usually visual. Because IOPs can vary throughout the day, initial IOPs may be normal or high normal. Chronic symptoms include all of the acute symptoms but also buphthalmos, lens zonule tearing leading to lens subluxation or luxation, dark and cupped optic nerve head, and loss of vision.