Dogs and cats presented to the veterinarian with a history of "red eyes" often represent a diagnostic challenge. In many cases the condition is treated as conjunctivitis, however, a thorough examination, including both general examination as well as special eye examination is necessary to establish a correct diagnosis.
The conjunctiva covers the inside of the upper and lower eyelids, the third eyelid and parts of the globe, stopping about 1mm behind the limbus, the junction between the cornea and sclera. As conjunctivitis is inflammation or infection of the conjunctiva, hyperemic blood vessels stretching into the cornea could therefore not originate from the conjunctiva. It is important to be able to distinguish between conjunctival (superficial) and episcleral (deep) hyperemia. Episcleral blood vessels are straight and may continue into the cornea (circumcorneal injection) in inflammatory conditions. Thus, deep blood vessels indicate a problem in the episclera or, more common, in the eye itself. Small, straight episcleral blood vessels are seen in keratitis, uveitis, and also in early stages of glaucoma with only moderate increase in intraocular pressure. Large, tortuous blood vessels are seen in manifest glaucoma. A well-demarcated area of increased vascularization on the globe may be seen in episcleritis, an immune mediated reaction of the episclera. Note that some dogs, especially of miniature breeds, may have one or two large veins on the dorsal globe without any associated ocular disease.
Questions to ask are: What is conjunctivitis? Conjunctivitis is inflammation /infection of the conjunctiva. What can be confused with conjunctivitis? Without thorough examination: Almost all eye diseases. Do all animals with "red eyes" have conjunctivitis? No. There are multiple causes of red eyes in dogs and cats, including conjunctivitis. We should strive to establish the correct diagnosis for the animal to get optimal treatment and to prevent secondary blindness. Although conjunctivitis occurs frequently in dogs, primary infectious conjunctivitis is not a very common disease, but is caused by predisposing factors in most cases. This is in contrast to the cat, where primary infection, most often caused by Feline herpesvirus or Calicivirus, is a common cause.
Work-up of an animal presenting with red eye(s) includes
General examination. Many systemic diseases may present with ocular changes, and ocular signs may be the first noticed in some diseases
Vision testing in normal and dim light if the history includes blindness or reduced vision
Size and symmetry of the eyes should be checked
The amount, type and color of discharge should be noted
Measuring the tear production with Schirmer Tear Test is essential and should always be part of an eye examination.
Normal values in dogs
15-25 mm: Normal tear production
10-15 mm: Borderline, should be rechecked
< 10 mm: Keratoconjunctivitis sicca
Normal values in cats: 12-22 mm
The rest of the examination should be performed with a strong, focal light source under darkened conditions. Enlargement is strongly recommended.
The conjunctiva including the third eyelid and the cornea are examined and any irregularities noted. Fluorescein staining may be indicated but superfluous stain may obscure visualization of deeper structures. It may therefore be wise to include fluorescein staining later in the examination. The anterior chamber is examined by directing the light into the eyes from different directions. The aqueous should be clear. Haziness or cells in the aqueous are noted, as well as deposits on the ventral surface of the inside of the cornea, especially in cats, as this indicates uveitis. In cases of lens subluxation a little "cloud" of vitreous may be seen leaking into the anterior chamber between the iris and lens. The iris should be regular; both pupils of the same size and iris color the same in both eyes. There may be exceptions, as animals occasionally may have one blue and one brown eye. A darkened iris indicates a chronic inflammation; an edematous iris is seen in acute uveitis. Neovascularisation on the iris surface may be seen in uveitis, especially in cats. An undulating movement of the iris is seen if the lens is subluxated or luxated posteriorly, the iris loosing its support on the lens surface.
If an intraocular disease is suspected, intraocular pressure (IOP) should be measured. A lowered IOP indicates uveitis, because aqueous production is lowered in inflammation of the ciliary body, and drainage is increased through the uveoscleral outflow. However, clinical findings should always be evaluated, as some normal dogs may have a lower IOP. Elevated IOP indicates glaucoma. Rise in intraocular pressure occurs because aqueous drainage is compromised. If a combination of uveitis and glaucoma is present, IOP varies according to the predominating signs at time of examination. If the intraocular pressure has caused the eye to become enlarged (buphthalmic), the eye will not shrink noticeably even if the pressure is reduced at a later stage. To examine deeper structures dilatation of the pupil with tropicamide may be necessary. Note that the pupil should not be dilated if a lens luxation is suspected, or if the IOP is higher than normal. Normal findings include a transparent lens with normal tapetal reflex. Evaluation of the fundus requires some training, but one should be able to diagnose hemorrhages, detachments and degenerations.
The animal has been repeatedly treated for conjunctivitis, but is loosing vision-what is wrong?
Animals do not loose vision because of conjunctivitis, so the diagnosis is most probably incorrect. One exception is herpes virus infection in cats, in which conjunctival swelling and concurrent ulceration on the conjunctival and corneal surfaces may lead to secondary synechia. Diseases that may be confused with conjunctivitis include keratitis, episcleritis, uveitis and glaucoma.
Keratitis. Measuring tear production is essential to rule out keratoconjunctivitis sicca (KCS). Lowered tear production causes chronic conjunctivitis and keratitis with vascularization and pigmentation of the cornea. A typical mucoid discharge is usually present. Causes of KCS include congenital diseases, trauma, intoxication by drugs, neoplasia and autoimmune diseases. The aim of treatment is threefold: To increase tear production, to substitute the lacking tears and to prevent secondary bacterial infection. Keratitis may also be caused by other factors than KCS. In dogs, primary chronic keratitis is often immune mediated. In cats, however, feline herpes virus infection is a frequent cause. Secondary keratitis may be caused by predisposing factors, including entropion, trichiasis (hairs rubbing on the cornea) and foreign bodies.
Episcleritis. An immune mediated episcleritis may occur in the dog, clinical signs varying according to type and severity of the disease: Uni- or bilateral episcleral hyperemia, focal or generalized. Nodule(s) may form in the episclera, and in severe cases the cornea may also be affected. Corneal changes present as dystrophic area(s) adjacent to or parallel to the limbus. The disease is treated as an immune mediated disease with topical and/or systemic anti-inflammatory agents. Occasionally there is a concurrent inflammation of the sclera, scleritis, which may lead to thinning of the sclera.
Uveitis. As described, differentiation between superficial (conjunctival) and deep (episcleral) blood vessel hyperemia is essential to establish a correct diagnosis, and a common mistake in general practice is to overlook uveitis. Signs of acute uveitis are most obvious in inflammation of the anterior uvea. Diagnosis of posterior uveitis requires pupil dilatation and funduscopic examination. Signs of uveitis include (All signs may not be present in all cases of acute uveitis): Pain, photophobia, epiphora, deep blood vessel hyperemia, circumcorneal vessels, miosis (small pupil), iris changes (swollen iris, neovascularisation on iris surface, later darkened iris), corneal edema, increased cell and/or fibrin content of the aqueous humor, lowered intraocular pressure, reduced vision or blindness.
Uveitis is often associated with systemic diseases, and a generalized work-up should include: General examination, urine and blood analysis (hematology, biochemistry). In addition one may consider specific tests, including test for leishmaniasis, fungi, ehrlichiosis, borreliosis, and in the cat FIP, FIV, FeLV and toxoplasmosis, imaging (X-ray, ultrasound, other) and lymph node biopsy if malignant lymphoma is suspected. An important cause of uveitis that is often overlooked in general practice is chronic low-grade uveitis secondary to cataract.
The aim of treatment is to treat the uveitis, to treat the underlying condition and to reduce pain. Medications include systemic and/or topical steroids, NSAIDS, topical mydriatic and antibiotics if indicated. Note that systemic steroids and non-steroidals must not be given together. Length of treatment must be decided through frequent rechecks and measuring of intraocular pressure.
Complications in uveitis include posterior synechia between lens and iris, iris bombé, secondary glaucoma corneal edema, cataract, retinal detachment, iris atrophy, retinal atrophy and phthisis bulbi.
Glaucoma indicates a condition where the IOP is abnormally high. Glaucoma may be primary and secondary, where primary glaucoma is caused by abnormal conditions in the drainage angle and secondary glaucoma by other ocular diseases, most often uveitis. Glaucoma may also occur secondary to lens luxation, especially luxation to the anterior chamber. Glaucoma is treated either medically or surgically. The aim of secondary glaucoma is to treat both the primary condition as well as the secondary rise in intraocular pressure.
In this lecture the most common causes of "red eyes" are described. Without proper examination most of the conditions may easily be confused with conjunctivitis. But by thorough work-up one should be able to differentiate conjunctivitis from other ocular disorders, thus ensuring correct treatment and a better prognosis for animals presented with an eye problem.
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