Section: Information In-depth
It is important to differentiate whether the engorged blood vessels that cause an eye to appear "red" are superficial or deep. Engorgement of superficial vessels indicates surface irritation, while reddening of deep blood vessels represents inflammation of more important ocular structures. Deep inflammation is more frequently associated with vision-threatening disease.
Red eye due to conjunctivitis is relatively common and does not require emergency treatment. However, when the redness reflects inflammation of deeper structures, particularly if the eye is painful, then your veterinarian should be consulted promptly.
Causes
conjunctivitis is one of the more common causes of red eye and, in cats is typically due to infectious agents such as Herpesvirus or Chlamydia psittaci.
Conjunctivitis is associated with engorgement (filling up of fluid) of the smaller, superficial blood vessels of the conjunctiva and will appear different to your veterinarian than engorgement of the deeper (episcleral) blood vessels. Determining the cause of the conjunctivitis and beginning treatment will usually provide significant reduction of inflammation and redness. This is not a vision-threatening disease.
Blepharitis is inflammation of the eyelids. This can be part of a more widespread dermatitis (skin inflammation) or can be localized to the eyelids only. Multiple causes are possible including allergy, parasitic, fungal or bacterial infections and immune-mediated inflammation. Blepharitis causes red eye by inflammation of the adjacent conjunctiva. Concurrent treatment of the conjunctival and skin inflammation is necessary.
Keratitis refers to inflammation of the cornea and is a frequent cause of reddening of the surrounding blood vessels. With more serious or chronic keratitis, blood vessels may begin to encroach (intrude or invade) on the cornea from the surrounding sclera or conjunctiva. In general, keratitis is a more serious diagnosis than conjunctivitis. You should be suspicious of corneal involvement if your pet is showing signs of discomfort such as squinting. Your veterinarian will look for other signs of corneal involvement such as ulceration or opacity of the cornea.
Scleritis is a rare diagnosis. This form of red eye involves the deep blood vessels and usually reflects an altered immune response to the animal's own sclera. Diagnosis and treatment of this syndrome can be quite challenging and your local veterinarian may recommend referral to a veterinary ophthalmologist.
Uveitis refers to inflammation of the inner vascular layer of the eye including the colored iris and related structures. This is a serious and potentially vision-threatening disease. Uveitis may even indicate that your pet has inflammation of structures within the body other than the eye, especially if both eyes are involved. In this case your veterinarian may recommend some blood tests or radiographs (X-rays) to assess your pet's health further. Uveitis produces engorgement of the deep blood vessels and is frequently painful. Your animal may appear generally depressed. More serious signs, such as these, warrant an urgent visit to your veterinarian and potentially referral to a veterinary ophthalmologist.
Glaucoma refers to an increase in intraocular pressure (pressure within the eye). This diagnosis is made after tonometry (measurement of intraocular pressure) is performed on your pet. This is a painful disease that will cause engorgement of the deep blood vessels around the eye. If intraocular pressure is excessive or remains elevated for long periods, vision is lost. Ultimately, if untreated, glaucoma causes the eye to stretch and enlarge. This disease can be challenging to treat. Frequently, your veterinarian will recommend referral to a veterinary ophthalmologist for management of glaucoma.
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