Hespeler Animal Hospital

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Image of Dog - Red Eye in dogs: Usually an emergency that may cause blindness!


Red Eye in dogs: Usually an emergency that may cause blindness!


Bloodshot eyes are common in dogs and cats. Red eyes are a sign of inflammation, but this is not a diagnosis, just a sign of one of several underlying diseases. Without a proper diagnosis, blindness or loss of the eye might result.  It’s important to contact a veterinarian IMMEDIATELY if your pet has a red eye.


Source of redness

  • Red eyes can be at the level of the conjuctiva (the conjunctiva is the lining of the eyelids).  The conjunctival blood vessels appear more noticeable, and is associated with diseases outside of the eyeball.
  • Red eyes can be due to bleeding into the eyeball or under the conjunctivae.
  • Episcleral injection causes redness because of congestion of the deep episcleral vessels, and is characterized by straight and immobile episcleral vessels, which run 90° to the junction of the white part of the eyeball and the clear part of the eyeball (referred to as the limbus).  The sclera is the connective tissue portion of the eyeball. Episcleral injection is an external sign of intraocular disease, such as anterior uveitis and glaucoma.
  • Corneal Neovascularization (blood vessels on the clear portion of the eye)


Diseases & Diagnostics

All red eyes must be evaluated for 3 key ocular diseases that may cause vision loss in an eye :

  1. Corneal ulceration
  2. Glaucoma
  3. Uveitis

A few basic diagnostic procedures can quickly assess whether these diseases are present; they should be performed in the following order for all patients with bloodshot eyes:

  1. Schirmer tear test (STT): Aids in diagnosis of conditions associated with decreased tear production, such as keratoconjunctivitis sicca (KCS), and should be performed before any medications are administered to the ocular surface.   The veterinarian will place a strip of special paper called a Tear Test Strip between the eyelid and the eyeball and will measure the amount of tears produced within one minute. 
  2. Fluorescein stain: Is critical for diagnosis of corneal ulceration.  This is a special stain that sticks to the water-soluble tissues inside the cornea.  A normal cornea does not have a water-soluble surface, so the stain should not stick to it. 1,2  
  3. Tonometry: Is critical for diagnosis of glaucoma and uveitis.  A tonometer is an instrument that measures the pressure inside the eyeball.  With glaucoma, the pressure is too high.  With inflammation of the eyeball (uveitis), the pressure is too low.  1,2  

Once an examination and these diagnostics are completed, the eye’s condition can be classified as:

  • Extraocular (conjunctival or corneal)
  • Intraocular (glaucoma or uveitis)
  • Ocular manifestation of systemic disease.  Some diseases of the organs or infection can manifest as eye disease.  For example, animals with a serious bacterial infection (blood poisoning) can have severely bloodshot eyes.


Corneal Ulcers

Corneal ulcers result in abnormal blood vessels on the cornea, and secondary inflammation inside the eyeball,  which appears as a “red eye.”

Corneal blood vessels are an indication of chronic disease and, generally, take 1 to 3 days to proliferate on the corneal surface. Uncomplicated corneal ulcers typically heal in 3 to 5 days; ulcers that do not heal in this time period must be closely evaluated for confounding factors. Underlying disease that can impede healing include:

  • KCS = “Dry Eye”  (low tear test measurements)
  • Diseases of the eyelids (entropion = rolled-in eyelids, distichia = eyelashes that face inwards vs outwards, ectopic cilia= hairs growing under the eyelid and rubbing the cornea)
  • Chronic corneal exposure (lagophthalmos = droopy eyelids, exophthalmos= an extra large eyeball).

They are often associated with other signs of corneal melanosis (or “pigmentation”). Corneal vascularization can occur with nonulcerative corneal disease as well.

Clinical Note:Blepharospasm (=squinting)  is seen with most forms of corneal disease but is a common and nonspecific sign of pain associated with many ocular diseases.


Diagnosis & Classification

Once an ulcer has been identified with positive fluorescein staining, further classification allows proper therapeutic interventions and prevents catastrophic complications related to lack of treatment.

It is prudent to refer all patients with deep stromal ulcers, descemetoceles, and ruptured eyes to an ophthalmologist for surgical evaluation in order to save the globe and vision. Surgical interventions include conjunctival flap, corneal graft, or corneal-conjunctival transposition.4,5



In a patient presenting with glaucoma, redness of the eye is due to episcleral injection (enlargement of the blood vessels on the white part of the eye, the sclera), with deep corneal vessels that form a 360° perilimbal pattern (the red vessels circle the white part of the eye) if the condition is chronic.

Clinical Note: Pain, corneal edema, and disturbance of vision may be present with glaucoma.



The only diagnostic sign of glaucoma is increased intraocular pressure (IOP).

  • IOP is measured with a special instrument called a tonometer.



Initial medical therapy for acute primary glaucoma is aimed at rapidly reducing IOP.    Combinations of medications to maintain lower IOP within the normal or acceptable range for dogs are often needed.

Clinical Note: Referral should be considered early in the disease process, especially if IOP fails to respond to medical management.

Glaucoma is ultimately a surgical disease. Medical therapy will typically become ineffective within a year. Surgical intervention may prolong vision but has a relatively poor long-term success rate.



Generally, primary glaucoma begins as a unilateral (one-sided) disease but, eventually, the other eye develops it as well. Treating the “second” eye with a prophylactic medication even if IOP is within the normal range is critical in order to delay onset of disease.


Anterior Uveitis

When anterior uveitis is present, redness of the eye is due to episcleral injection, with 360° deep corneal vascularization if the disease is chronic (Figure 4 and 11). Other causes of redness related to anterior uveitis include hyphema (blood in the anterior chamber) and rubeosis iridis (iris neovascularization).

  • Aqueous flare (cloudiness of the aqueous humor, the fluid in the front part of the eyeball)
  • Fibrin within the anterior chamber
  • Hypopyon (= pus in the eyeball)
  • Keratic precipitates
  • Synechiae = threads that extend from one part of the eyeball to another.

Nonspecific clinical signs include:

  • Blepharospasm (squinting)
  • Epiphora (excess tearing)
  • Miosis (small pupils)
  • Ocular discharge.

All patients with anterior uveitis should have the following performed:

  • Complete blood count and serum biochemical profile
  • Urinalysis
  • Titers for tick-borne diseases (for example, a 4DX test)
  • Chest (3 views) and abdominal radiographs (to check for cancer and organ disease)
  • Abdominal ultrasound (to check for cancer and organ disease).

Clinical Note: Ocular ultrasound can help determine extent of ocular disease if anterior segment changes are severe and the posterior segment cannot be visualized.


Systemic Disease

Uveitis, whether unilateral or bilateral, is most often due to a systemic cause. Systemic causes of uveitis include infectious, autoimmune, and neoplastic (cancer) disorders. Uveitis is considered non-systemic if there is evidence of:

  • Cataracts (lens-induced uveitis)
  • Corneal ulceration
  • Intraocular neoplasia (cancer in the eyeball, often seen as a mass in the eye)
  • Other external evidence of trauma.

As for any other inflammatory conditions, underlying cause of the red eye should be determined in order to institute proper therapy. However, this determination is often impossible even with thorough diagnostic investigation.



Treatment should be directed at addressing the primary cause and decreasing pain and inflammation.

Clinical Note: Referral to an ophthalmic specialist is appropriate for management of severe or resistant cases of uveitis.


Other causes of red eye

This article has focused on causes of red eye that are most likely to threaten vision. However, it is important to remember that there are other causes for the apparently “acute” red eye, including more insidious disorders that are perceived as acute by the owner. These include:

  • Inflammation of the eyelids, associated with lid lacerations or eyelid infections
  • Conjunctivitis (KCS/Dry Eye, allergic conjunctivitis) and prolapsed gland of the third eyelid (“cherry eye”)
  • Nonulcerative keratitis, such as chronic superficial keratitis (pannus), qualitative tear film abnormalities, and other keratopathies without ulceration
  • Orbital diseases (infection or cancer), which routinely present with conjunctival hyperemia (redness) and chemosis (swelling of the inner lining of the eyelids).


In Summary

Correct diagnosis and treatment of the red eye are important to prevent loss of vision, the eyeball itself, or, in some cases, loss of life. Each of the conditions listed above requires prompt and specific treatment in order to ensure a positive outcome.

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