|What is glaucoma Causes Symptoms Diagnosis Treatment|
At a glance:
About: 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.
Causes: There are a number of causes of glaucoma including trauma, diabetes, infection, displacement of the lens, advanced cataracts and tumours of the eye.
Symptoms: Pain, squinting, redness, dilated pupil in the affected eye, one eye appearing larger than the other, cloudiness of the cornea and vision loss.
Diagnosis: Baseline tests to evaluate the overall health of your cat as well as a thorough physical examination. Tonometry to measure the pressure of the eye, ophthalmoscopy to look at the optic nerve, ultrasound and FIV and FeLV tests.
Treatment: Medications to bring down the intraocular pressure, steroids to reduce inflammation and analgesics to relieve pain.
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 (trabecular meshwork) and is reabsorbed into the bloodstream. This fluid in/fluid out should occur at the same rate, to keep pressure within the eye stable.
Glaucoma occurs when fluid continues to be produced, but drainage slows down due to a partial or complete blockage which impedes outflow. A build-up of fluid and increase in intraocular pressure develop as a result.
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, and occurs most often in Siamese, Burmese and Persians. 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 (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 (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 is the normal eye pressure in cats?
10 to 25 mm Hg
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 include:
- Trauma, which can cause bleeding in the eye
- Lens luxation (displacement of the lens)
- Cataract surgery
- 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:
- One eye may look larger than the other as the pressure increases
- 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. 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: Examination of the angle of the anterior chamber of the eye with a gonioscope.
- Ophthalmoscopy: To examine the optic nerve at the back of the eye.
- Ultrasound: To look for tumours, lens displacement, trauma.
- Fluroscein: A yellow stain is applied to the eye to check for corneal ulcers.
- FIV and FeLV tests: To evaluate for these viral infections
Glaucoma is incurable and the goal of treatment is to reduce pressure within the eye, relieve discomfort and address the underlying cause, this may include:
- Osmotic agents: These systemic medications can reduce IOP rapidly and works by increasing blood plasma osmality, once plasma osmality is greater than that of the intraocular fluid, water moves from the eye to the plasma reducing IOP. Mannitol, glycerin and isosorbide are the most common osmotic agents.
- Carbonic anhydrase inhibitors: CAI’s reduce the formation of bicarbonate ions, which is necessary for aqueous humour production. Medications include topical and oral medications. Topical: dorzolamide and brinzolamide or oral: methazolamide and acetazolamide.
- Prostaglandin analogues: These drugs increase intraocular fluid from the eye. Medications include prostaglandin analogs such as latanoprost, travaprost, unoprostone and bimatoprost.
- Beta-blockers: These medications reduce aqueous humour production although the exact mode is not understood. The most used medication is Timolol, but other options include evobunolol, betaxolol, metipranolol and carteolol.
- Steroids: To reduce inflammation.
- Cryosurgery: To freeze a portion of the ciliary body which reduces the production of aqueous humour.