“Red eye” is one of the most common ophthalmic presentations in clinical veterinary practice. Although relatively “minor” conditions are responsible for the majority of “red eyes”, this presentation may signal more severe, vision-threatening or even life-threatening disorders. A logical, step-wise approach to the diagnosis of red eye is vital to ensure that serious ocular and systemic disorders are identified and treated promptly and appropriately.
Causes of Red Eye
These may be considered from an anatomic standpoint, in a logical progression, from external / adnexal diseases, to anterior segment and intraocular disorders (Table 1).
Table 1. Common causes of red eye in dogs.
Abscess/ cellulitis Proptosis Neoplasia
Anterior segment & adnexal disease
Blepharitis Conjunctivitis Third eyelid protrusion Episcleritis / scleritis Conjunctival or scleral hemorrhage
Keratitis – ulcerative Keratitis – non-ulcerative
Uveitis Glaucoma Hemorrhage Neoplasia
Clinical Approach to Differential Diagnosis of Red Eye
The importance of obtaining a thorough clinical history is emphasized. A careful clinical examination to localize the source of the red appearance; identify key features of disease and to exclude or diagnose serious intraocular disorders can be achieved in general clinical practice, within a limited time frame and using only simple diagnostic equipment.
Key features to evaluate: (See Table 2)
Onset: Acute vs chronic?
Pain: Signs include squinting, lacrimation, rubbing
Globe size & position: Enlarged vs exophthalmos; reduced in size vs enophthalmos?
Pupil size: Are pupil sizes equal, large, small? (evaluate in dim and bright light)
Pupillary light reflexes: Both direct and consensual
Vision: Menace, dazzle, obstacles, tracking
Episcleral congestion vs conjunctival hyperemia: Congested conjunctival blood vessels are less worrisome than episcleral vessels. Conjunctival vessels remain mobile within the bulbar conjunctiva, extend into the fornix and tend to be more tortuous, bright red and branching vs the relatively straight and immobile, darker episcleral vessels
Ocular discharge: May indicate infection if purulent Discharge adherent to the ocular surface is suggestive of dry eye (KCS).
Corneal vascularization: Deep, straight circum-limbal vessels form a “brush border”, appearing at the limbus and indicate deep corneal disease, and /or intraocular disease e.g., uveitis and glaucoma. These should be differentiated from branching, superficial vessels that originate in conjunctiva and cross over the limbus.
Corneal edema: Blue, hazy, “steamy” appearance that may be focal or diffuse?
Aqueous flare: Use small, focal, bright light beam to detect protein in anterior chamber.
Lens abnormalities: Distant direct ophthalmoscopy can detect cataract, lens luxation / subluxation.
Posterior segment changes: Attempt to evaluate for hemorrhage, chorioretinitis, retinal detachment
Additional basic supplies include: Schirmer tear test strips, fluorescein stain, Topical anesthetic, swabs for bacteriology, supplies for cytology (microscope slides and a blade for obtaining scrapes, stain (e.g., Diff-Quik). Additional instrumentation that may or may not be available include Tonometer, Ultrasonography, Goniolens.
Table 2. Differentiating common causes of red eye.
++ ( if chronic)
Pupil size & PLR
Miotic / PLR
Dilated (absent PLR)
Normal or miotic (if reflex uveitis)
Vision – affected eye
-/+ (if deep keratitis/ulcer)
Variable / “sticky” if KCS
Superficial / deep (depends on disease)
Synechiae, cataract or sublux ?
Luxation or subluxation? Cataract?
– (unless penetrating wound)
Chorioretinitis/ retinal detachment/ hemorrhage
Optic disc cupping
Retinal detachment if trauma
(or if 2° glaucoma)
(may be normal if chronic / 2° to uveitis)
-/ (if reflex uveitis)
+/- normal (if visible)
Opposite eye – pectinate ligament dysplasia?
Normal (if visible)