What is Glaucoma in Dogs & Cats?



Introduction


Fluid usually surrounds the structures inside the eye, e.g. the iris and lens. Normally, the movement of fluid through the eye is well controlled, but if the fluid doesn't circulate normally, fluid pressure will build up inside the eye.


What is glaucoma?

Glaucoma is a common term used to describe the increased pressure inside the eye. Glaucoma is a painful condition caused when pressure builds up within the eye, leading to the injury of the optic nerve that connects the eye to the brain. If left untreated, it can lead to irreversible blindness. Glaucoma may not be preventable in numerous cases, however, an early diagnosis and treatment may reduce the risk of permanent damage to the retina and blindness. In pets, glaucoma is more common in dogs than in cats and has many causes. There are two types of glaucoma depending on the cause: Primary glaucoma and Secondary glaucoma. Either of these can be open-angled glaucoma or closed-angle glaucoma.


What are the clinical signs of glaucoma?

Glaucoma can occur suddenly (acute) or develop over a long period of time (chronic). Depending on the cause, it can be unilateral or bilateral. Whether glaucoma occurs suddenly or over a longer period of time, the clinical signs are similar:

  1. Eye Pain (squinting, rubbing the eye or face against the floor or against furniture

  2. Cloudiness of the eye or "bluish" discolouration of the cornea

  3. Tearing

  4. Red eyes/Redness around the eye.

  5. Increased sensitivity to light

  6. Dilated pupils

  7. Unequally sized pupils

  8. Decreased appetite (due to pain)

  9. Partial or complete blindness

  10. "Head shyness" (reluctance to have the face or head touched/approached, due to pain and reduced vision)

  11. Visual deficits (might not be obvious to owner if contralateral eye retains vision).

  12. Raised intraocular pressure (eye pressure)


How does Glaucoma develop in dogs and cats?

The normal intraocular (eye) pressure is a balance between fluid production (aqueous humour) by the ciliary body processes by active secretion into the posterior chamber behind the iris, ultrafiltration and its outflow via the pupil into the anterior chamber. From the anterior chamber, the majority of aqueous humour drains via the iridocorneal angle (drainage angle). The main components of this angle are the pectinate ligament and ciliary cleft which contains the sieve-like trabecular meshwork. The majority of aqueous humour (~85%) filters through these structures and enters the angular aqueous plexus before entering the eye's blood vessels ('conventional' outflow). A minority of aqueous humour (~15%) bypasses the aqueous plexus and reaches the eye's blood vessels via the choroid and suprachoroidal space. Normal intraocular pressure in the dog is 10-25 mm Hg.

Drainage angle of the eye

In dogs, the only consistent risk factor for glaucoma is pathologically elevated intraocular pressure due to obstruction of the aqueous humour outflow. Aqueous humour obstruction can occur at any point from its release into the posterior chamber to its ultimate drainage into the angular aqueous plexus and include the following:

  1. Pupil block:

  • Iris bombe - adhesions between the posterior iris and anterior lens capsule for 360° in chronic uveitis.

  • Lens luxation and subluxation.

  • Vitreous prolapse (following lens removal or lens luxation).

2. Obstruction of the entrance to iridocorneal angle (drainage angle):

  • Abnormalities of the drainage angle structures in primary closed-angle glaucoma.

  • Obstruction by red blood cells (trauma), white blood cells and inflammatory debris (acute and chronic uveitis).

  • Anterior synechiae (chronic uveitis).

3. Obstruction within ciliary cleft:

  • Abnormalities of the drainage angle structures in primary closed-angle glaucoma.

  • Undefined anatomical and biochemical changes in primary open-angle glaucoma.

  • The collapse of the ciliary cleft in all forms of glaucoma.

  • Cancer extension.

  • Obstruction by red blood cells (trauma), white blood cells and inflammatory debris (acute and chronic uveitis).

Elevation of intraocular pressure causes interruption of blood supply to optic nerve head and neuronal fluid flow within optic nerve axons. An increase in some enzyme levels (glutamate levels) may cause excitotoxicity-mediated retinal ganglion cell death.

Primary glaucoma

Primary glaucoma is often an inherited genetic condition and has been identified in some dog breeds, such as Basset hounds, Chihuahuas, and Siberian huskies. It tends to affect both eyes but may not occur in both eyes at the same time.


Secondary glaucoma

Secondary glaucoma means that the pressure inside the eye was normal until another condition caused increased pressure in the eye. Conditions that can cause secondary glaucoma include trauma to the eye, inflammation involving the eye, or a tumour inside the eye. Glaucoma can also be classified as acute (occurring suddenly in response to trauma) or chronic (pressure builds up over a period of time).


What is the impact of Glaucoma on the eye?

Glaucoma can damage several important structures inside the eye, including the optic nerve, the retina (tissue at the back of the eye that is necessary for vision), and the cornea (the clear membrane on the front of the eye). Untreated glaucoma can also damage the lens of the eye, leading to cataracts. Injury to any of these structures, alone or in combination, can lead to permanent blindness.


Are there certain breeds/species that are more prone to Glaucoma?

The following are some of the dog breeds that are prone to glaucoma:-

  1. Basset Hound.

  2. Chihuahua

  3. Japanese Shiba Inu

  4. Flat-coated Retriever

  5. Siberian Husky.

  6. American Cocker Spaniel

  7. English Cocker Spaniel.

  8. Welsh Springer Spaniel

  9. English Springer Spaniel

  10. Border Collie

  11. Great Dane.

  12. Dandie Dinmont Terrier

  13. Hungarian Vizla

  14. Leonberger

  15. Welsh Terrier.

  16. Golden Retriever

How is glaucoma diagnosed?

The test used to measure the pressure inside the eye is called tonometry. The tonometer is applied to the surface of the eye so that the amount of pressure inside the eye can be estimated. Before tonometry is performed, a few drops of a solution shall be applied to the pet's eye to numb the surface of the eye (the cornea).

Tonometer

Depending on the situation, a full physical examination may be performed before tonometry. Physical examination findings, combined with the pet's medical history, provide valuable information about the possible cause of the problem and how long it has been going on. In some cases, a veterinary eye specialist (a veterinary ophthalmologist) may be recommended for tonometry to be performed and for treatment to begin.


How is glaucoma treated?

Treatment of glaucoma is aimed at controlling the flow of fluid through the eye and reducing the pressure inside the eye. Treatment may involve medication (drops placed into the eyes, pills given by mouth, or injections given in the hospital). In some cases, surgery is recommended.

  • Emergency treatment:

Travoprost

Primary closed-angle glaucoma requires emergency treatment to lower Intra Ocular Pressure quickly. Options include:

  1. Intravenous administration of hyperosmotic agents (e.g., 10% mannitol) reduces intraocular pressure via an osmotic effect on the vitreous and aqueous.

  2. Topical application of prostaglandin analogues (e.g., latanoprost, travoprost).

  3. If these emergency measures fail to reduce intraocular pressure to within normal limits then aqueocentesis (under sedation) may need to be considered.

  • Medical management:

Ocular antihypertensive medications

  1. Prostaglanding analogues (eg latanoprost 0.005%, travoprost 0.004%). Topical prostaglandin analogues are also used for longer-term treatment of glaucoma.

  2. Carbonic anhydrase inhibitors (eg dorzolamide 2% and brinzolamide 1%).

  3. Beta-blockers (eg timolol 2.5-5%). These drugs also increase aqueous outflow and are usually used 2-3 times daily.

  1. Analgesia:

  2. NSAIDs Analgesia

  3. Opioids Analgesia

  4. Anti-inflammatory medications:

  5. Indicated when intraocular inflammation is a component.

  6. Topical and systemic NSAIDs and corticosteroids may be used.


Surgical therapy

  • The main aim of surgery is to maintain or restore vision.

  • Surgery, if successful, may also eventually allow the frequency of necessary topical medications to be reduced.

  • Surgical treatments work by either reducing aqueous humour production or increasing its drainage from the eye.

  • Even following successful surgical treatment, relatively frequent topical medications are still usually required.

  • Cycloablation.

  • Gonioimplantation.

  • Enucleation Eye: enucleation and evisceration.


Monitoring

  • Monitoring of both eyes affected and the contralateral at risk eye is required.

  • Stable patients are usually examined at least every 3 months.

  • If there is a change in treatment then re-examination is performed after one week to ensure a satisfactory response to treatment.


Prevention

Most cases of glaucoma are not easily preventable. Primary glaucoma (the inherited form) eventually affects both eyes (although not always at the same time), so if one of your pet's eyes is affected, it is recommended to treat both eyes to help decrease the start of glaucoma in the normal eye.

Even though glaucoma may not be preventable in many cases, early diagnosis and treatment may reduce the risk of permanent damage and blindness. Some of the early signs of glaucoma may be detected during regular wellness examinations. Monitoring your pet at home for any signs of discomfort or changes in behaviour can also increase the chance of identifying problems like glaucoma.


Control

  • Dogs susceptible to abnormalities that may lead to glaucoma should be examined first before breeding.

  • Dogs of breeds at risk of primary open-angle glaucoma e.g. Border Collies should receive DNA tests before breeding.

  • Affected dogs should be removed from the breeding population.

Prophylaxis

  • Prophylactic treatment of the fellow eye delays the onset of primary glaucoma (Miller et al, 2000).

  • Most ophthalmologists advise topical carbonic anhydrase inhibitors or beta-blockers (or both).

  • Topical prostaglandin analogues are also sometimes used but might not be the best choice as they cause shallowing of the anterior chamber and narrowing of the iridocorneal angle (Tsai et al, 2013).


References

Ahonen, S. J. (2014). A novel missense mutation in ADAMTS10 in Norwegian Elkhound primary glaucoma. . PloS one, 9(11).


Bras, I. D., Robbin, T. E., Wyman, M., & Rogers, A. L. (2005). Diode endoscopic cyclophotocoagulation in canine and feline glaucoma. Veterinary Ophthalmology, 8(6), 449.


Brooks, D. E., Garcia, G. A., Dreyer, E. B., Zurakowski, D., & Franco-Bourland, R. E. (1997). Vitreous body glutamate concentration in dogs with glaucoma. . American journal of veterinary research, 58(8), 864-867.


BSAVA. (2014). BSAVA Manual of Ophthalmology (3rd ed.). (D. Gould, & G. McLellan, Eds.)

Cook, C. S. (1997). Surgery for glaucoma. Veterinary Clinics of North America: Small Animal Practice, 27(5), 1109-1129.


Dog Well Net. (2021). Retrieved from International Partnership for Dogs (IPFD): https://dogwellnet.com/

Gelatt, K., Gilger, B., & Kern, T. (2013). Veterinary Ophthalmology (5th ed.). (G. B. Gelatt K, Ed.)


Kuchtey, J., Kunkel, J., Esson, D., Sapienza, J. S., Ward, D. A., Plummer, C. E., & Kuchtey, R. W. (2013). Screening ADAMTS10 in dog populations supports Gly661Arg as the glaucoma-causing variant in beagles. Investigative ophthalmology & visual science, 54(3), 1881-1886.


Kuchtey, J., Olson, L. M., Rinkoski, T., MacKay, E. O., Iverson, T. M., Gelatt, K. N., & Kuchtey, R. W. (2011 ). Mapping of the disease locus and identification of ADAMTS10 as a candidate gene in a canine model of primary open-angle glaucoma. PLoS Genet, 7(2).


Miller, P. E., Schmidt, G. M., Vainisi, S. J., Swanson, J. F., & Herrmann, M. K. (2000). The efficacy of topical prophylactic antiglaucoma therapy in primary closed-angle glaucoma in dogs: a multicenter clinical trial. Journal of the American Animal Hospital Association, 36(5), 431-438.


Nasisse, M. P., Davidson, M. G., English, R. V., Jamieson, V., Harling, D. E., & Tate, L. P. (1990). Treatment of glaucoma by use of transscleral neodymium: yttrium aluminium garnet laser cyclocoagulation in dogs. Journal of the American Veterinary Medical Association, 197(3), 350-354.


Pearl, R., Gould, D., & Spiess, B. (2015). Progression of pectinate ligament dysplasia over time in two populations of Flat‐Coated Retrievers. Veterinary Ophthalmology, 18(1), 6-12.


Samuelson, D. A., Gum, G. G., & Gelatt, K. N. (1989). Ultrastructural changes in the aqueous outflow apparatus of beagles with inherited glaucoma. Investigative Ophthalmology & visual science, 30(3), 550-561.


Sightsavers. (2021, April 24). What is Glaucoma? Retrieved from Sightsavers: https://www.sightsavers.org/


The Kennel Club. (2021). Worldwide DNA Tests. Retrieved from The Kennel Club: https://www.thekennelclub.org.uk


Tsai, S., Almazan, A., Lee, S. S., Li, H., Conforti, P., Burke, J., . . . Robinson, M. R. (2013). The effect of topical latanoprost on anterior segment anatomic relationships in normal dogs. Veterinary Ophthalmology, 16(5), 370-376.



53 views0 comments

Recent Posts

See All