Diagnosing Eye Disease

Photos courtesy of Long Beach Animal Hospital  unless otherwise indicated.


Graphic by Cranach.

Inherited eye disease, eye infections and eye trauma are all problems we see in dogs and cats on a regular basis. Eye problems are often uncomfortable or painful, and some can progress to permanent damage or loss of vision, so properly diagnosing eye problems is essential.

If you notice anything unusual about your pet’s eyes, a trip to the vet as soon as possible is the best plan. Your veterinarian will most likely recommend one or more of the following diagnostic, in order to figure out what’s going on:

Ophthalmic Exam

Using a light source or an ophthalmoscope, the surface of the cornea and surrounding structures as well as the pupils’ response to light are examined. The pupils should be symmetrical (identical in size and shape to each other) and should constrict in response to direct light. For instance, a light shone directly into the left eye should result in constriction of the left pupil. This is called a direct pupillary light response (PLR). But at the same time, the right pupil should also constrict in response to the action of the left eye. This is the indirect or consensual pupillary light response. The other eye (the one not having the light shone into it) also constricts at the same time. This is because of the connection of the optic nerves before they enter the brain.

Eye light reflex1

A vet shines a light into a cat’s eye during an ophthalmic exam.

The surface of the cornea and the surrounding structures are examined. If the lining of the eyelids, the third eyelid in the medial corner of the eye (closest to the nose) or the surface of the eyeball is swollen, irritated and reddened, often with a moist discharge, this is conjunctivitis. This inflammation of the membranes surrounding the eye can come from irritation, infection or trauma. Antibiotic drops or ointment are often used for treatment. Note: It’s very important to avoid the use of steroids in the eye until it’s been determined that there are no scratches or ulcers in the cornea itself. Steroids can make this worse.

The surface of the cornea is examined. It should be completely clear and transparent to allow light to pass through the pupil (lens) to the back of the eye. If the surface of the cornea looks hazy or bluish, this could be corneal edema, a collection of fluid in the cornea itself in response to infection or injury.

The aqueous humor is examined. This is the clear gel-like liquid filling the front of the eye between the iris and the cornea, also known as the anterior chamber. This should be completely transparent. If this portion of the eye appears hazy, it’s known as uveitis. This occurs when inflammatory cells accumulate in this normally clear liquid. Sometimes known as an aqueous flare, uveitis can arise either from infection or inflammation of the cornea or the eye itself, or it can be a reflection of systemic disease such as toxoplasmosis, which we covered in a previous article. This is why your veterinarian may recommend blood tests in a cat with uveitis. He or she may be testing for parasitic, bacterial or viral infections.

Fundic Exam

The fundus refers to the structures in the large part of the eye, behind the iris and pupil. Dilating the pupil allows visualization of the back of the eye, where abnormalities can help diagnose disease. A drop of atropine may be used to dilate the pupil, and an ophthalmoscope is used to examine the back of the eye, including the retina, the blood vessels and the optic disc. This is where the optic nerve exits the back of the eye, connecting it to the brain. Swelling, discoloration or other abnormalities of the optic disc can be seen with inflammatory and viral diseases, and retinal detachment appears as a sheer curtain billowing in the back of the eye.


An ophthalmoscope is used to examine the back of an animal’s eye.

Fluorescein Stain

An animal with blepharospasm, or squinting, would be showing signs of pain. This could be coming from a scratch or an ulcer on the surface of the cornea, but that can be very hard to see with the naked eye. A fluorescein stain is used to evaluate the surface of the cornea, usually used in conjunction with a topical anesthetic, proparacaine, since the eye is often painful. The stain shows up as a green patch on any defect in the cornea’s surface. If there is no stain uptake, it’s an indication that the surface of the cornea is intact.

Florcein testNormal eye with no ulcer

This is a normal eye with no ulcer. You can see the vivid green fluorescein dye around the margins of the eye because it is being illuminated with an ultraviolet light.

A fluorescein stain is often recommended before using any steroids in the eye. Antibiotic drops or ointment may be used for treating infections such as conjunctivitis, but sometimes, steroids will help with chronic inflammatory diseases. However, using steroids when there’s a corneal abrasion or ulcer can lead to a worsening of the condition, slowing down healing and causing complications. It’s always best to seek medical attention first before using any medications in your pet’s eyes.

Schirmer Tear Test

This test is done with a small strip of absorbent paper, which evaluates the quantity of tears an eye produces in a short period of time. Poor tear production results in a constantly irritated cornea, which can lead to chronic changes over time. This condition is known as keratoconjunctivitis sicca, shortened to KCS or “dry eye,” and is treated with daily eye ointment.

tear test

This little dog is undergoing a tear test to determine whether he has dry eye.


The Schiotz tonometer is used to test the pressure inside the eyeball. An anesthetic eye drop is first used to temporarily desensitize the cornea, and the tonometer is gently rested against the surface to test pressure. Normal intraocular pressure (IOP) is between 10 and 20 mm Hg, which is a unit of measurement for pressure. Increased pressure can indicate a serious and painful condition called glaucoma, which may lead to blindness and loss of an eye.

tear test

Administering a test for glaucoma.

These tools are essential in evaluating the eye, in looking for changes which could indicate a serious problem with the eye or even systemic disease. Any change in your pet’s eyes merits a visit to your veterinarian, who will most likely recommend several of these diagnostic tools. For more information, visit the Long Beach Animal Hospital website and search for eye disease.


LBAH often examines wildlife. Here, a veterinarian uses specialized equipment to diagnose a sea lion’s ocular issues.

Share this: