Feline Glaucoma

Due to my severe vision impairment, I spent several years attending a School for the Blind growing up.  There, glaucoma was not an uncommon condition (though this is not the condition that causes my vision problems).  However, I didn't know that cats can also suffer from glaucoma!

What is Glaucoma?

As in human glaucoma, feline glaucoma is a condition in which the watery fluid (aqueous humor) in the front part of the eye right behind the lens cannot drain properly.  Aqueous humor is produced by cells in the eye and provides nutrients to tissues inside the eye and helps the eye maintain its shape.  In a healthy eye, aqueous humor drains through the eye's drainage angle.  When the eye's drainage angle is partially or completely blocked, aqueous humor builds up in the eye, causing high intraocular pressure.  

The accumulation of aqueous humor puts pressure on the optic nerve, a nerve that runs from the back of the eyes to the brain.  Pressure on the optic nerve causes nerve damage, which prevents a cat from seeing normally.  Without treatment, partial or total blindness can occur.  Unfortunately because the signs and symptoms of glaucoma are often very subtle, it can be a difficult condition to diagnose early.

Glaucoma can affect one or both eyes, depending on what type of glaucoma a cat has.  

Primary glaucoma is an inherited condition and is rare in cats.  Thomas Kern, DVM, associate professor of ophthalmology at Cornell University's College of Veterinary Medicine notes that Siamese and Burmese cats are among the breeds predisposed to primary glaucoma.  In primary glaucoma, both eyes will be affected with the condition.

Secondary glaucoma is more common in cats.  It is not inherited, and one or both eyes may be affected.  The most common cause of secondary glaucoma is uveitis, which is severe eye inflammation that produces proteins and other debris that block an eye's normal drainage ducts.  As a result, the aqueous humor builds up and causes pressure within the eye.  

Uveitis has several causes, which include feline Immunodeficiency Virus (FIV), Feline Leukemia Virus (FeLV.), Feline Infectious Paritonitis (FIP), and Toxoplasmosis.  Secondary glaucoma tends to occur in middle-aged or older cats.

Causes of Feline Glaucoma

Though uveitis is the most common cause of secondary glaucoma in cats, several other things can also cause the condition.

Anterior Dislocation of the Lens: This occurs when the eye's lens falls forward and blocks the eye's drainage ducts, trapping aqueous humor behind the dislocated lens.

Damage to the Lens: When the eye's lens is damaged, it can leak proteins into the eye, which can cause inflammation.  The inflammation can result in swelling, which can block the eye's drainage angle.  Trauma is the most common cause of lens damage.

Intraocular Bleeding: If bleeding occurs within the eye, a blood clot can form and block the eye's drainage angle, trapping aqueous humor in the eye.

Tumors: Tumors in the eye can also lead to the blockage of the drainage angle.

Symptoms

The signs of glaucoma in cats are often subtle and progress slowly over time.  Unfortunately, this means that oftentimes a cat has little to no vision in one eye before glaucoma is diagnosed.  Some of the most common signs of glaucoma include:

• Pain: You may notice that your cat's eye is only partially open or that she squints a lot.  She may also rub the affected eye.  Your kitty may turn away from you if you touch her, especially near the side of her face.
  
  As many of you know, cats are masters at hiding pain.  However, other signs that your feline companion is hurting may include decreased activity levels, decreased appetite, irritability, and less desire to play or interact with you.
• Swelling and Bulging of the Eyeball: The sclera (white part of the eye) looks bloodshot and swollen.
• Watery Discharge: You may notice a clear, watery discharge coming from your kitty's eye.  Your companion may also appear unresponsive and depressed.
• Cornea Discoloration: The cornea (the clear part of the eye) may become cloudy or appear bluish in color.
• Dilated Pupil.
• Blindness.

As your cat ages, you may want to get her eye pressure measured at her bi-annual veterinary exams, especially if she is a breed at risk for developing glaucoma.  

VCA Animal Hospitals notes that acute glaucoma is an emergency.  If you notice any of these symptoms in your cat, please get her to a veterinarian right away.  An emergency referral to a veterinary ophthalmologist may be required.  

Diagnosis

If your veterinarian suspects that your cat has glaucoma, she will refer you to a veterinary ophthalmologist who will conduct a thorough eye exam and use a tonometer to measure your cat's intraocular pressure.  The tonometer rests on your kitty's cornea to measure intraocular pressure, and the test is quick and painless.  Diagnosis is based on the documentation of increased intraocular pressure and consistent clinical signs.  Other tests may also be conducted to determine the cause of the glaucoma.  For instance, x-rays and ultrasounds can be utilized to detect eye abnormalities and tumors.  Blood tests can be done to check for feline leukemia, feline immunodeficiency virus, and to check kidney, thyroid, liver, and pancreas functioning.  It is essential to determine the cause of secondary glaucoma and treat it whenever possible.  

A cat's normal intraocular pressure varies with time of day, age, and gender.  For instance, intraocular pressure goes down with age, and it is not uncommon for senior cats to exhibit low intraocular pressure.  Intraocular pressure is highest at night.  Due to the fluctuations in intraocular pressure in cats, it can be helpful to check both eyes for discrepancies in pressure readings.  

Treatment

The goal of treatment is to reduce intraocular pressure in the eye.  

• Carbonic Anhydrase Inhibitors: Topical carbonic anhydrase inhibitors, such as Azopt (brinzolamide) and Trusopt (dorzolamide) are the most commonly prescribed medications for glaucoma in cats.  These medications reduce aqueous humor production.  Though research suggests that dorzolamide is more effective, clinical experience shows it may be more irritating to the eye.  Systemic side effects to topical carbonic anhydrase inhibitors are rare but may include gastrointestinal upset.
• Timolol: Timolol is a non-selective beta blocker, which is also commonly used in the treatment of glaucoma in cats.  It works to reduce aqueous humor production.  Caution should be used in giving Timolol to cats with asthma or heart disease because Timolol can cause bronchospasms and bradycardia.  
• Cholinergics: Cholinergics reduce intraocular pressure by increasing aqueous humor drainage.  However, because cholinergics can destabilize the blood-ocular barrier, they are less commonly utilized in glaucoma treatment.  Demecarium and pilocarpine are two medications in this class of drugs.
• Adrenergic Agnosists: The non-specific adrenergic agnost, epinepherine, has been associated with reducing the production of aqueous humor as well as increasing its outflow.  
• Surgery: Surgical removal of the eye is used as a last resort.  In these cases, the eye is blind, and surgical removal of the eye is done to relieve pain and discomfort
• Other Medications: Depending on what the cause of the glaucoma is, other medications may also be prescribed.  

Though glaucoma cannot be prevented, regular intraocular testing allows for early detection and treatment, which can lead to better treatment outcomes.  

Sources:

Cornell University College of Veterinary Medicine: Feline Glaucoma.

VCA Animal Hospitals: Glaucoma in Cats.

Healthy Pets: Recognize the Early Warning Signs of Glaucoma in Pets.

Pet Health Network: Glaucoma in Cats.

National Institute of Health: National Library of Medicine: Feline Glaucoma - A Comprehensive Review.