Glaucoma in Cats – Causes, Symptoms & Treatment

What is glaucoma   Causes  Symptoms   Diagnosis   Treatment

Glaucoma in cats

What is glaucoma?

Also known as hard eye, glaucoma is an increase in the intraocular pressure (IOP), leading to damage to the optic nerve, which connects the eye to the brain. Any damage can cause partial or full blindness.

The eye contains a transparent jelly-like fluid known as intraocular fluid or aqueous humour, which maintains the shape of the eye and nourishes the tissues within the eye.

The ciliary body is constantly making intraocular fluid. It drains away at the angle where the iris and cornea meet, exiting via a series of drainage canals (known as the trabecular meshwork) and is reabsorbed into the bloodstream. This fluid in/fluid out should occur at the same rate, keeping pressure within the eye stable.

Glaucoma occurs when fluid continues to be produced, but drainage slows down due to a partial or complete blockage. This impedes outflow, resulting a build-up of fluid in the eye and increased intraocular pressure.  There are two types of glaucoma in cats, primary or secondary. It can affect one eye (unilateral) or both eyes (bilateral).

Primary glaucoma is rare in cats. It is caused by a congenital eye abnormality. Secondary glaucoma is when another eye disease is present. Whichever the type, the result is improper drainage and never an overproduction of the aqueous humour.

Glaucoma comes in two forms, open-angle and closed-angle.

Open-angle (also known as wide-angle) glaucoma is the most common type and is caused by a slow and progressive partially clogging of the trabecular meshwork and slowing down drainage of the aqueous humour.

Closed-angle (also known as angle closure) glaucoma occurs when the iris bulges forward completely clogging access to the trabecular network. This is by far the most serious type of glaucoma as pressure can rise very quickly within the eye.


The most common cause of glaucoma is uveitis (inflammation of the uvea), which leads to the formation of scar tissue in the fine drainage meshwork.

Other causes of glaucoma include:

  • Trauma, which can cause bleeding into the eye.
  • Diabetes
  • Infection
  • Lens luxation (displacement of the lens)
  • Advanced cataracts
  • Eye tumours


Glaucoma is a painful condition, however, your cat may only display subtle signs initially and it can be difficult to detect. Common symptoms can include:

  • Pain
  • Squinting
  • Redness
  • One eye may look larger than the other
  • Dilated pupil
  • Cloudiness of the cornea
  • Vision loss


Your veterinarian will perform a complete physical examination of your cat and obtain a medical history from you. He will need to perform some diagnostic tests first to determine if the cause is primary or secondary and if secondary (as it almost always is), to look for an underlying cause. Unfortunately, a large number of cats are already blind in the eye at the time of diagnosis.

Tests he may perform include:

  • Baseline tests – To evaluate the overall health of the cat and check for diabetes with a biochemical profile, complete blood count, and urinalysis.
  • Tonometry – Measuring the pressure within the eye (which is usually between 10-20 mm/Hg). Eye drops are applied to the eye(s) to numb them, then a tonometer is used to gently press the cat’s eye to determine how much force is required to flatten the cornea. In cats with glaucoma, the eye is harder, requiring more force.
  • Gonioscopy –  An optical instrument known as a ‘gonioscope’ is used to examine the angle of the anterior chamber of the eye.
  • Ophthalmoscopy – To examine the optic nerve at the back of the eye.
  • Ultrasound – To look for tumours, lens displacement, trauma.
  • FIV and FeLV tests, and fluorescein test to check for corneal ulcers, FIV and FeLV tests and blood pressure checks if uveitis is the cause.


Treatment to reduce pressure within the eye, relieve discomfort and address the underlying cause, this may include:

  • Oral or topical drugs to bring down intraocular pressure. These drugs work by reducing production of aqueous humor.
  • Steroids to treat inflammation.
  • Medications to relieve pain.
  • Cryosurgery which involves freezing a portion of the ciliary body, which also serves to reduce production of aqueous humor.
  • Removal of the eye, known as enucleation, if blindness has occurred.