Understanding Glaucoma

Written on October 18, 2011 by Staff Veterinarian

 Glaucoma is a disease most commonly diagnosed by veterinarians as an increase in the normal pressure within the eye which can lead to pain and loss of vision.  Diagnosing the disease requires recognition of the early clinical signs, pressure evaluation, and ophthalmic examination.  Unfortunately glaucoma is frequently misdiagnosed because the early clinical sign are similar to those of other eye diseases, and many owners/companions do not realize how quickly the blinding disease occurs.  Too often it is identified after irreversible changes occur in one or both eyes.

The normal physiology of the eye calls for an equal rate of fluid production and fluid drainage from the eye.  Cells of the ciliary body (located behind the iris) are responsible for production of the intraocular fluid (aqueous humor) which delivers a clear nutrient rich fluid to the clear structures of the eye.  This fluid circulates from behind the iris into the anterior chamber.  To keep the eye at normal volume and pressure, the aqueous then leaves via an intraocular drain known as the iridocorneal angle.

If drainage is not occurring properly, then pressure rises because the delivery of fluid continues.  In animal species this scenario of a blockage in the outflow of aqueous humor and a subsequent build-up of pressure within the eye is the most common factor leading to vision loss.  Glaucoma related blindness occurs because the elevated pressure causes retinal nerve degeneration and blocks the visual impulse through the optic nerve.  Because permanent loss of vision can occur very quickly (within 24 hours), recognition of clinical signs is essential to having treatment options for the prevention of vision loss.



Primary glaucoma is seen in certain breeds and is often thought to be inherited.  The following are types of primary glaucoma:

  • Narrow angle glaucoma:  common in Cocker Spaniels
  • Goniodygenesis:  common in Basset Hounds and Chow Chows
  • Open angle glaucoma:  Not Common.  Found in some Beagles and Norwegian Elkhounds

Secondary glaucoma is a result of some other ocular disorder that interferes with the drainage of ocular fluid.  Some examples include:

  • Inflammation
  • Trauma or hemorrhage (bleeding of the eye)
  • Lens luxation
  • Tumors


Tonometry is the measure of intraocular pressure (IOP).  A variety of instruments are used to measure pressure in animals.  Most commonly applanation tonometers (TonoPens), rebound tonometers (TonoVet), or indentation tonometers (Schiotz) indirectly measure the pressure within the eye in mmHg.  Pressures less than 24mmHg are normal, however as animals age pressure decreases.  Therefore normal puppies may have readings of 24mmHg and a 10 year old may have 10mmHg.  Interpretation of the pressure is again just one factor is establishing a diagnosis of glaucoma.


Gonioscopy is a technique used to evaluate the iridocorneal angle. A dome-shaped contact lens (goniolens) is placed on the corneal surface to permit direct visualization of the drainage angle. Gonioscopy is helptful in evaluating the eye for its predisposition to glaucoma or for the risk of future glaucoma attacks.

Retinal and optic nerve head examination with direct or indirect ophthalmoscopy.  These are means of directly looking at each of these structures to understand the presence, extent/severity of damage.All of these methods are necessary for establishing a prognosis and treatment options.



One of the most important things for you, the owner, is to be able to recognize the earliest signs. Early signs may include:

  • The white of the eye becomes very red.
  • Squinting and tearing are common.
  • Corneal haziness (due to edema) is commonly noted.
  • Non reactive or unequal size of the pupil.
  • Pain.
  • They may not want their head touched or yelp if touched.
  • Vision loss (temporary if early)

Chronic signs of glaucoma

  • Change in size of the eye (stretched or enlarged eye).
  • Lens subluxation
  • Blood vessels within the cornea.
  • Dilated pupil not responsive to light
  • Severe retinal degeneration and optic nerve head cupping
  • Variable amounts of discomfort.  Many animals have adapted to the pain of the condition.


Glaucoma is an ophthalmic emergency and must be treated immediately and aggressively. As mentioned above, if the pressure remains elevated, permanent vision loss occurs.

Several options are available depending on whether the patient still has vision, the classification of glaucoma, concomitant systemic disease of the patient, and financial considerations.

Medical treatment strategies for primary glaucoma involves applying various eye drops to the eye.  The topical medications either improve the outflow of fluid from the eye or slow the production of fluid. Which of the medications is selected will depend of a variety of factors and is the decision of your veterinarian or veterinary ophthalmologist.

These medications need to be given very regularly (2-3 times daily in some cases) without fail if there is to be any hope to preserving vision in the affected eye.  The other “good” eye should also be monitored very regularly and treated to delay onset of the disease.

In dogs, medications can be administered and delay the return of elevated pressures.  Unfortunately the pressures will elevate again which obviates the need for other forms of long term management.

In animals where medications are insufficient to control the intraocular pressure surgery is performed to place a shunt (gonioimplant) in the eye to drain excess fluid into the frontal sinus.  Other implants are being researched for their success in treating canine and feline glaucoma.

Alternate surgical options include intra- or extra- ocular laser delivery ablation of the ciliary body cells.  This slows the production of fluid into the eye.

All surgical options entail risk.  The success rates of surgery for glaucoma are not as high as those for other routine intraocular surgeries such as cataract surgery.  There are many factors leading to these lowered success rates including timing to referral, classification of the glaucoma, patient and owner compliance with post operative care, and coincident ophthalmic disease.  Determining if a patient is a surgical candidate and the risks of the surgery are reviewed during the appointment.

If a patient’s eye is too advanced for vision sparing surgeries and has a non-visual eye with painful glaucoma, then other surgical options exist.


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