Eye Tests for Pets: Fluorescein Staining, Schirmer Tear Test, and Tonometry

Fluorescein Staining

Fluorescein is a special type of stain that is administered to the cornea to determine the presence of corneal lesions. When applied to the eye, the orange stain turns fluorescent green and adheres to any part of the cornea where the epithelium (corneal surface) is missing.

When there is a break in the corneal epithelium, the underlying tissue layer composed of the stroma or corneal collagen is exposed. The fluorescein test also shows the extent of any corneal ulceration, specifically the ulcer’s size, depth, and type.

When might my vet use fluorescein stain on my pet’s eye?

In pets, fluorescein staining is a test that is used to assess the following situations:

• Red or painful eye
• Any irregularity in the cornea
• Trauma to the eye
• To determine if there is any problem with the lacrimal duct (tear duct) that can interfere in the passage of tears from the surface of the eye to the nasal cavity.

How Fluorescein Staining is Performed

Before the orange stain is administered, the eye surface is cleaned of any discharge that may be present. A drop of fluorescein stain is administered on the eye surface and the eyelids are then closed so the stain can spread over the entire surface of the eye.

To allow an exam of the eye and corneal surface, eyewash is administered to flush out the fluorescein. With the aid of a penlight or a specially filtered light, your vet will examine your pet’s eyes for any signs of the green stain. Where there is no damage to the cornea, no trace of the stain can be detected on the eye’s surface. But if there is corneal damage, the stain is not flushed out and remains to outline the damage.

If a problem in the lacrimal duct is suspected, each opening of your pet’s nostril will be examined for any sign of the fluorescent green stain.

Is fluorescein staining painful to pets?

Your pet won’t experience any pain or discomfort while undergoing the procedure. A local anesthetic may be applied if the animal is experiencing pain and won’t allow the vet to get too close to their face to examine and administer the stain. Fluorescein staining is very easy to perform and usually takes less than five minutes.

Schirmer Tear Test (STT)

The Schirmer Tear Test (STT) is used to determine the rate of tear production in pets. Tears are produced in the lacrimal gland and the gland of the third eyelid. Tears are important to keep the eyes healthy. If the glands do not produce enough tears, it can increase your pet’s risk of developing keratoconjunctivitis sicca (KCS) or dry eye.

The Schirmer Tear Test (STT) is the gold standard when diagnosing KCS.

Does my pet need a Schirmer Tear Test?

The STT is indicated in any patient with eye discharge or redness. The test should be performed BEFORE administering any topical drops, such as fluorescein or a local anesthetic, on your pet’s eye.

How is a Schirmer Tear Test performed?

The STT is performed using a strip of special filter paper that is placed inside the animal’s lower eyelid. Your vet places the tip of the paper inside your pet’s eye, closes the eyelid, and lets the strip stay there for exactly 1 minute. The test strip acts as a wick that absorbs the tears. The distance that is traveled by the tears within 1 minute can be distinctly seen on the paper strip.

The measurement is read immediately after the strip is removed since the tears that are absorbed will continue to spread along the strip. The result is compared with the normal value which is 15 millimeters or more in 1 minute.

Benefits of the Schirmer Tear Test

Inadequate tear production can have harmful effects on the eyes of pets. Chronic low production of tears can eventually cause persistent irritation of the eyes, inflammation, and pain. Without prompt veterinary attention, the cornea will be damaged, putting the patient at higher risks of developing more serious complications such as ulceration of the cornea, bacterial infections, and even blindness. Early diagnosis using the STT and appropriate treatment can help improve prognosis and response to treatment.

Tonometry (Eye Pressure Testing)

Tonometry is a procedure that is used for measuring the intraocular pressure (pressure within the eye) of pets. It is the test that is used to determine if your pet has glaucoma. A special instrument known as a tonometer is used for the procedure.

What does tonometry reveal?

Tonometry can diagnose glaucoma by measuring intraocular pressure. Fluid buildup within the eye causes pressure to be exerted by the eyeball against the outer layers.

The procedure is also useful for determining if there is low intraocular pressure (OIP) which is commonly associated with the presence of inflammation within the eye, known as anterior uveitis. Low OIP can also occur after eye surgery or as a result of dehydration.

How Tonometry is Performed in Pets

Two methods are generally used to measure IOP in pets - indentation tonometry and applanation tonometry.

Indentation Tonometry

The primary tool for performing indentation tonometry is called the Schiotz tonometer. The procedure involves the application of a certain weight to the corneal surface. The degree of the corneal indentation that is caused by the movement of the rod is inversely proportional to the intraocular pressure. This means that the higher the IOP, the harder the eye, and the lesser will be the indentation of the cornea.

Applanation Tonometry

This technique uses a mathematical calculation to determine the force that is required to cause an indentation in the cornea. This is performed using a tonometer, such as a Tono-Pen.

Is tonometry painful for dogs and cats?

No. The procedure is not painful for pets. Tonometry is a non-invasive procedure that can be used without sedation. A drop of local anesthetic is placed into each of your pet’s eyes before the test to numb the eyeball. This will allow your vet to examine your pet’s eye without causing undue stress to your pet, and also so your pet won’t feel the pressure exerted by the tonometer on the cornea.

Read more:

10 Facts About Your Dog’s Eyes

10 Facts About Cat Eyes

What causes itchy eyes in pets?

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