Also known as 'hard eye', glaucoma is an increase in the intraocular pressure (IOP), leading to damage to the optic nerve. This nerve connects the eye to the brain and any damage can result in partial or total 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. Intraocular fluid is constantly being made by the ciliary body and drained away at the angle where the iris and cornea meet, exiting via a series of drainage canals (known as the 'trabecular meshwork') where it is eventually 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, however, drainage slows down due to a partial or complete blockage, impeding outflow, resulting a build up of fluid in the eye which increases intraocular pressure. The cause may be primary or secondary and 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 is divided into two categories. 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.
What are the causes of glaucoma in cats?
Uveitis (inflammation of the uvea) is one of the most common causes of glaucoma in cats. Inflammation can cause scar tissue to form in the fine drainage meshwork. Other causes include trauma which causes bleeding in the eye, diabetes, infection, lens luxation (displacement of the lens), advanced cataracts and eye tumours.
What are the symptoms of glaucoma?
Glaucoma is a painful condition, however, your cat may only display subtle signs initially. Common symptoms can include:
- Enlarged eye
- Dilated pupil
- Cloudiness of the cornea
- Vision loss
How is glaucoma diagnosed?
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 firstly 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:
- 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 - An ophthalmoscope is used to look at the back of the eye, where the optic nerve is located.
- Ultrasound - To look for tumours, lens displacement, trauma.
- Baseline tests such as biochemical profile, serum chemistry and urinalysis may be run to evaluate the overall health of your cat and check for diabetes.
- If uveitis is suspected to be the cause, further testing may include FIV and FeLV tests, fluorescein test to check for corneal ulcers, and checking the blood pressure.
How is glaucoma treated?
Treatment is aimed at reducing pressure within the eye, relieving discomfort and addressing the underlying cause, if one is found. This may include:
- Oral or topical drugs to bring down intraocular pressure. These drugs work by reducing production of aqueous humour.
- If inflammation if the cause, steroids may be prescribed.
- Medications to relieve pain.
- Cryosurgery involves freezing a portion of the ciliary body, which also serves to reduce production of aqueous humour.
- If blindness has occurred, removal of the eye may be recommended. This is known as enucleation.