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