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Canine : Pathophysiology * University of Missouri and Diagnosis DACVO, MS, DVM, of Elizabeth Guiliano, Courtesy

❯❯ Shelby L. Reinstein, Abstract: Canine glaucoma encompasses a diverse group of diseases associated with an increase DVM, MS in that results in death and eventual blindness. Dogs may The University of Pennsylvania have congenital, primary, or secondary glaucoma. The diagnosis is made by recognizing clini- ❯❯ Amy J. Rankin, DVM, cal signs, interpreting ophthalmic examination findings, and determining intraocular pressure. MS, DACVO Identifying the underlying cause of the glaucoma is essential to providing appropriate treatment ❯❯ Rachel Allbaugh, DVM, recommendations. MS, DACVO Kansas State University queous humor is produced by the with an acute onset of buphthalmia and and flows through the corneal edema. The disease may be uni- pupil into the anterior chamber of lateral or bilateral and may be associated A 2 the . In dogs, most aqueous humor with other ocular anomalies. exits the eye through the iridocorneal Primary glaucoma is considered a heri- angle; approximately 10% to 15% exits the table condition in some breeds2 (Box 1). At a Glance eye through uveoscleral outflow.1 Normal The disease is progressive and may result Canine Glaucoma intraocular pressure (IOP; range: 10 to 25 from changes in the iridocorneal angle or Page 450 mm Hg2–4) is maintained by an equilib- from abnormal metabolism of the trabe- Diagnosis rium between aqueous humor production cular cells within the outflow pathway.2 Page 451 and drainage. Canine glaucoma is usu- Primary glaucoma is further classified as Top Five Breeds of Dogs ally due to a disturbance in the conven- open-, narrow-, or closed-angle, based on With a High Prevalence of tional outflow pathway that results in an the appearance of the drainage angle.2,5 It Primary Glaucoma increased IOP. is always a bilateral condition. Page 451 Secondary glaucoma results when another Comparison of Available Canine Glaucoma condition disrupts aqueous humor outflow. Tonometry Methods Traditionally, glaucoma has been defined Several disease conditions can lead to sec- Page 452 as an elevated IOP beyond that which ondary glaucoma, including cataract, lens permits normal visual function.2 Recent luxation, hyphema, intraocular neoplasia, research has indicated that canine glau- retinal detachment, and uveitis.2 In some coma is a common end point of several of these conditions, the release of vasoac- ophthalmic diseases. Canine glaucoma tive factors may lead to the formation of a can be classified as congenital, primary, preiridal fibrovascular membrane and sub- or secondary. sequent secondary glaucoma.2 Although Congenital glaucoma is rare in dogs. It secondary glaucoma is not considered *A companion article, “Canine is caused by abnormalities in the aqueous heritable, some of the inciting causes do Glaucoma: Medical and Surgical Treatment Options,” begins on humor outflow pathways. Puppies gener- have a genetic basis (e.g., cataract, lens lux- page 454. ally present young (3 to 6 months of age) ation). Dogs of breeds that are predisposed

450 Compendium: Continuing Education for Veterinarians® | October 2009 | CompendiumVet.com FREE Canine Glaucoma: Pathophysiology and Diagnosis CE to these conditions that have developed a high Box 1 IOP in one eye should have the contralateral eye routinely monitored for the development Top Five Breeds of Dogs of disease.6,7 Additionally, if cataract surgery is With a High Prevalence of performed, postoperative glaucoma is a poten- Primary Glaucomaa tial complication that can be vision threaten- ing.8,9 If secondary glaucoma can be diagnosed American cocker spaniel (5.52%) early and managed appropriately, vision may  (5.44%) be preserved. Chow chow (4.70%) Shar-pei (4.40%) Diagnosis Boston terrier (2.88%) The history, clinical presentation, and ophthal- mic examination findings assist in diagnosing aGelatt KN, MacKay EO. Prevalence of the breed-related in pure-bred dogs in North America. Vet Ophthalmol glaucoma. It is crucial for the veterinarian to 2004;7(2):97-111. determine the stage of the glaucoma (acute or chronic) to provide appropriate treatment. may regain some visual function in days to Clinical Signs weeks if aggressive medical or surgical man- Glaucoma can be a painful condition. Signs of agement is pursued early in the course of the ocular pain include blepharospasm, epiphora, IOP elevation. and an elevated third eyelid. Episcleral con- Determination of IOP in both involves QuickNotes gestion or corneal edema may also be present, proper restraint (avoiding neck pressure) and and owners may describe the eye as reddish correct use of equipment to obtain accurate If secondary or blue.2,10 Dogs may also present without measurements.4 A recent study13 demonstrated glaucoma can be vision, lacking menace responses, pupillary that body position can affect IOP readings in diagnosed early light reflexes (PLRs), and dazzle reflexes.11 dogs without glaucoma; therefore, it is impor- and managed Unfortunately, most subtle or transient tant to keep the dog’s body position consis- appropriately, increases in IOP lack overt clinical signs in tent during IOP measurement. There are three vision may be the acute phases, and most dogs present methods of measuring IOP: indentation, appla- preserved. with chronic glaucoma. IOP, corneal edema, nation, and rebound tonometry. Indentation and visual status may be similar in acute tonometers, such as the Schiotz, indent the and chronic glaucoma. Globe size and fun- corneal surface and provide a measurement dic examination help determine the duration that can be converted for use in dogs by using of the disease. Signs of acute glaucoma may the accompanying human conversion table.14 include a normal-sized globe with corneal Applanation tonometers, such as the Tono- edema, mydriasis, and a relatively normal Pen VET (Reichert), measure IOP by flattening . Dogs with chronic glaucoma generally the corneal surface and are commonly used present with buphthalmia, blindness, corneal in general practice. Rebound tonometers (e.g., edema, and fixed, dilated pupils in one or TonoVet, Icare) measure IOP by projecting a both eyes. Fundic examination may reveal ret- small probe at the corneal surface and analyz- inal degeneration and optic disc cupping.2,12 ing the characteristics of its rebound. Rebound tonometers have been shown to be as accurate Ophthalmic Examination and easy to use as applanation tonometers.3,15 The first objective of the ophthalmic exami- Because IOP measurements obtained using nation is to determine the visual status and different instruments vary,15 it is recommended potential of each eye. Menace responses and that the same instrument be consistently used direct and consensual PLRs should be assessed. when monitoring a patient. Table 1 compares If these responses are absent, dazzle reflexes the three most commonly used tonometers. should be assessed by shining a bright light Canine glaucoma does not usually present into each eye in turn and monitoring for a symmetrically, and because primary glaucoma blink response. A recent study11 demonstrated is always a bilateral disease, it is critical to thor- that dogs with acute glaucoma and absent oughly evaluate and routinely monitor the con- menace responses, PLRs, and dazzle reflexes tralateral eye. Depending on the breed of dog,

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table 1 Comparison of Available Tonometry Methods3,10,15,a Criteria Indentation Tonometry Applanation Tonometry Rebound Tonometry

Instrument Schiotz tonometer Tono-Pen VET TonoVet

Average costb ~$300 $2500+ $2500+ Corneal contact areac 3 mm 3 mm 1 mm Clinical considerations Requires appropriate patient Minimal patient positioning and Minimal patient positioning and positioning and restraint restraint restraint Readings must be converted to Digital display of pressures and Digital display of pressures and mm Hg using a chart accuracy accuracy Requires topical anesthetic Requires topical anesthetic Does not require topical anesthetic

aLeiva M, Naranjo C, Pena MT. Comparison of the rebound tonometer (ICare) to the applanation tonometer (Tonopen XL) in normotensive dogs. Vet Ophthalmol 2006;9(1):17-21. bPrices vary by supplier. c The amount of corneal exposure required may exceed this area.

primary glaucoma usually manifests in mid- 8 months) than eyes treated with prophy- dle or old age2; therefore, routine monitoring lactic medication (median: approximately 31 of at-risk puppies is not useful. When a dog months). Sharing this timeline with owners presents with unilateral glaucoma, gonios- of dogs with primary closed-angle glaucoma copy (iridocorneal angle examination) can be helps keep the progressive nature of the dis- QuickNotes performed by a veterinary ophthalmologist ease in perspective and provides motivation to to determine if the drainage angle is abnor- maintain compliance with the recommended It is important to mal. This information helps differentiate pri- prophylactic medications. keep the dog’s mary and secondary glaucoma. Prophylactic body position medical therapy in the contralateral eye of an Conclusion consistent during affected dog can significantly prolong visual Canine glaucoma may be congenital, primary, IOP measurement. status.16 In a multicenter clinical trial,16 topi- or secondary in origin. It is crucial to deter- cal 0.5% betaxolol twice daily or topical 0.25% mine the underlying cause of the glaucoma, demecarium bromide once daily and a topical the chronicity of the condition, and the visual corticosteroid once daily significantly delayed potential of the affected eye because these or prevented the onset of glaucoma in the sec- factors decide the appropriate treatment rec- ond eye. Untreated control dogs developed ommendations and affect prognosis. glaucoma in the second eye earlier (median: REFERENCES ON page 466

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1. Canine glaucoma usually results from 2. Normal canine IOP is ______mm Hg. a. an increase in the amount of aqueous humor a. 5 to 10 c. 15 to 30 produced by the ciliary body. b. 10 to 25 d. 20 to 30 b. a disturbance in the aqueous humor outflow pathway. 3. Which statement is true with regard to c. external pressure on the globe (tumor, cyst, congenital canine glaucoma? abscess). a. It is a common condition in certain breeds d. a and b of dogs.

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b. It is usually a result of an early traumatic injury to the globe. c. It becomes clinically evident in the first 2 to 3 years of life. d. It may be unilateral or bilateral and may be associated with other ocular anomalies.

4. Which statement is true with regard to primary canine glaucoma? a. It is a heritable condition in many breeds. b. It is the result of a malformation of the drainage angle. c. It is always a bilateral disease process. d. all of the above

5. Secondary canine glaucoma can be caused by a. lens luxation, uveitis, or intraocular neoplasia. b. conjunctivitis and blepharitis. c. cataracts and cataract surgery. d. a and c

6. A 6-year-old spayed bassett hound pre­sents with buphthal- mia, corneal edema, and a dilated pupil with no PLR. A cupped optic disc and retinal degeneration are visible on fundic examination. This dog has ______glaucoma. a. acute c. congenital b. chronic d. none of the above

7. If a dog with elevated IOP lacks a menace response and direct and consensual PLRs, a. the prognosis for vision is grave. b. the dog must have chronic glaucoma. c. dazzle reflexes should be assessed. d. magnetic resonance imaging should be performed to attempt to identify intra­cranial disease.

8. Accurate determination of IOP requires a. proper restraint and avoidance of excessive neck pressure. b. sedation or light anesthesia. c. a dark environment. d. that no topical medications be applied to the eye.

9. Which statement regarding tonometers is true? a. The reading obtained with the Schiotz tonometer must be converted to mm Hg using the supplied conversion chart. b. Applanation tonometry and rebound tonometry require a topical anesthetic. c. The newest tonometer, the TonoVet, is the only style of tonometer that provides a digital display of the IOP. d. Rebound tonometry has been shown to be more accurate than applanation tonometry.

10. In a case of apparently unilateral glaucoma, evaluation of the contralateral eye is crucial because a. primary glaucoma is always a bilateral disease, even though it may present asymmetrically. b. may help to identify an eye that is at risk for developing glaucoma. c. prophylactic medical therapy can significantly prolong visual status in an eye at risk for developing glaucoma. d. all of the above

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References 1. Abrams KL. Medical and surgical management of the glaucoma 9. Biros DJ, Gelatt KN, Brooks DE, et al. Development of glauco- patient. Clin Tech Small Anim Pract 2001;16(1):71-76. ma after cataract surgery in dogs: 220 cases (1987-1998). JAVMA 2. Gelatt KN, Brooks DE, Kallberg ME. The canine glaucomas. 2000;216(11):1780-1786. In: Gelatt KN, ed. Veterinary Ophthalmology. 4th ed. Ames, Iowa: 10. Gelatt KN. The canine glaucomas. In: Essentials of Veterinary Blackwell Publishing; 2007:753-811. Ophthalmology. Ames, Iowa: Blackwell Publishing; 2005:165-196. 3. Knollinger AM, La Croix NC, Barrett PM, Miller PE. Evaluation 11. Grozdanic SD, Matic M, Betts DM, et al. Recovery of canine of a rebound tonometer for measuring intraocular pressure in dogs retina and function after acute elevation of intraocular and horses. JAVMA 2005;227(2):244-248. pressure: implications for canine glaucoma treatment. Vet Ophthal- 4. Pauli AM, Bentley E, Diehl KA, Miller PE. Effects of the applica- mol 2007;10(suppl 1):101-107. tion of neck pressure by a collar or harness on intraocular pressure 12. Martin CL. Evaluation of patients with decreased vision or blind- in dogs. JAAHA 2006;42(3):207-211. ness. Clin Tech Small Anim Pract 2001;16(1):62-70. 5. Reilly CM, Morris R, Dubielzig RR. Canine goniodysgene- 13. Broadwater JJ, Schorling JJ, Herring IP, Elvinger F. Effect of sis-related glaucoma: a morphologic review of 100 cases look- body position on intraocular pressure in dogs without glaucoma. ing at inflammation and pigment dispersion. Vet Ophthalmol Am J Vet Res 2008;69(4):527-530. 2005;8(4):253-258. 14. Miller PE, Pickett JP. Comparison of the human and canine 6. Gelatt KN, MacKay EO. Secondary glaucomas in the dog in Schiotz tonometry conversion tables in clinically normal dogs. North America. Vet Ophthalmol 2004;7(4):245-259. JAVMA 1992;201(7):1021-1025. 7. Johnsen DA, Maggs DJ, Kass PH. Evaluation of risk factors for 15. Gorig C, Coenen RT, Stades FC, et al. Comparison of the use of development of secondary glaucoma in dogs: 156 cases (1999- new handheld tonometers and established applanation tonometers 2004). JAVMA 2006;229(8):1270-1274. in dogs. Am J Vet Res 2006;67(1):134-144. 8. Lannek EB, Miller PE. Development of glaucoma after pha- 16. Miller PE, Schmidt GM, Vainisi SJ, et al. The efficacy of topical coemulsification for removal of cataracts in dogs: 22 cases (1987- prophylactic antiglaucoma therapy in primary closed angle glauco- 1997). JAVMA 2001;218(1):70-76. ma in dogs: a multicenter clinical trial. JAAHA 2000;36(5):431-438.

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