Eric C. Ledbetter, DVM, DACVO
1. General information
2. Essential anatomy and physiology
3. Clinical signs
4. Clinical evaluation
5. Conjunctivitis etiologies
a. Primary conjunctival diseases
b. Secondary manifestation of other ocular diseases
c. Secondary manifestation of systemic diseases
1) General Information
Conjunctivitis is a frequent condition for which small animals are presented for evaluation. The etiologies of conjunctivitis are numerous and include primary conjunctival diseases and conjunctivitis that develops secondary to other extraocular, intraocular, and systemic conditions. Clinical manifestations of conjunctivitis are frequently nonspecific and may be similar with diverse etiologies. Although the primary etiologies of conjunctivitis occur commonly, other severe ocular diseases and potentially life-threatening systemic conditions can initially present as conjunctivitis. A methodical clinical approach to conjunctivitis should be followed.
2) Essential Anatomy and Physiology
The conjunctiva is the mucous membrane covering the posterior aspect of the eyelids, palpebral (convex side facing the lids) and bulbar (concave side facing the globe) surfaces of the nictitans membrane, and anterior portion of the globe. The conjunctiva is composed of nonkeratinized, stratified epithelium and the underlying connective tissue ('the substantia propria'). Mucin-producing goblet cells are present in the conjunctival epithelium, and the substantia propria contains vessels, nerves, and lymphoid tissue. Rich vascular supply, loose arrangement of the substantia propria, and resident lymphoid tissue contribute to the conjunctiva's ability to rapidly, and often dramatically, respond to insults.
3) Clinical Signs
Conjunctivitis is associated with a combination of the following clinical signs: ocular discharge, chemosis, hyperemia, discomfort, pruritus, tissue proliferation, ulceration, and hemorrhage. Ocular discharge may be characterized by serous, mucoid, mucopurulent, or serosanguineous fluids. Epiphora (overflow of serous tears) results from excessive production or inadequate nasolacrimal drainage. Chemosis appears as conjunctival swelling and is conjunctival edema resulting from increased vascular permeability with fluid extravasation. Hyperemia is a red conjunctival discoloration ("red eye") and is a clinically observable manifestation of vasodilation and increased blood flow. Ocular discomfort and pruritus most commonly manifest as blepharospasm and periocular rubbing.
Tissue proliferation is divided into two distinct types: lymphatic and epithelial. Lymphatic proliferation is frequently termed follicular conjunctivitis and appears as small, round, semi-transparent elevated lesions representing lymphocytic aggregates. Occasional conjunctival lymphatic follicles on the posterior aspect of the nictitans membrane are normal; however, increased numbers of follicles or their presence in other anatomic locations is a pathologic change. Epithelial hyperplasia or keratinization results in variably sized, irregular, opaque, pink-to-red, elevated lesions. Both lymphatic and epithelial conjunctival tissue proliferation are indicative of chronic inflammation. Ulceration of conjunctival epithelium may develop with severe conjunctivitis. Hemorrhage may occur in the conjunctival epithelium or subconjunctival space and appears as variably shaped and sized, bright or dark, red regions.
4) Clinical Evaluation
The clinical approach to conjunctivitis is similar to any nonspecific ocular lesion that is associated with a variety of different potential etiologies. A thorough history, physical examination, and ocular examination are performed to identify the specific etiology or to narrow the differential diagnosis and exclude more serious causes of secondary conjunctivitis such as systemic or intraocular disease.
5) Conjunctivitis Etiologies
Etiologies of conjunctivitis include: primary conjunctival diseases, secondary manifestation of other ocular diseases, and secondary manifestation of systemic diseases. Primary etiologies of conjunctivitis are those where the disease process is limited to the conjunctiva and include allergic, frictional irritant, immune-mediated, infectious, and traumatic conditions.
A. Primary Conjunctival Diseases
Conjunctivitis associated with allergic conditions can be divided into three general types: atopic, drug reaction, and insect envenomation.
Atopic conjunctivitis is often accompanied by atopic dermatitis, and animals frequently present with mild and seasonal hyperemia, chemosis, epiphora, and ocular pruritus. Conjunctival follicle formation occurs with chronicity. Atopic conjunctivitis is generally a diagnosis of exclusion; however, concurrent atopic dermatitis and seasonality are suggestive.
Drug reaction conjunctivitis is a hypersensitivity reaction that may result in severe clinical signs. Concurrent blepharitis (inflammation of the eyelash follicles), often with dermal ulceration, is frequent. Conjunctivitis may develop at any time during drug use and with any medication. Diagnosis is by discontinuing all ophthalmic medications for one to two weeks and slowly reintroducing medications individually until the offending drug is identified.
Insect bites and stings may produce severe conjunctivitis. This form of conjunctivitis typically has a rapid onset and is characterized by severe bilateral chemosis with the conspicuous absence of other clinical signs of conjunctivitis. acutely.
Frictional Irritant Conjunctivitis
Endogenous and exogenous irritants may result in conjunctivitis. These irritants include conjunctival foreign bodies, dermoids, and abnormalities of the eyelids, cilia, and periocular facial hair. Diagnosis of frictional irritant conjunctivitis is made by identifying one or more of these conditions during ocular examination.
Diffuse episcleritis and nodular, granulomatous episcleritis are canine conditions that present with conjunctival hyperemia and thickening. Peripheral keratitis is common. Lesions may be unilateral or bilateral. Nodular, granulomatous episcleritis is associated with formation of one or more distinct, firm, smooth-surfaced masses. Diffuse episcleritis is clinically similar, but without nodule formation. Atypical pannus is an alternative presentation of canine pannus (aka chronic superficial keratitis) where the primary clinical lesions occur in the conjunctiva, as opposed to the cornea. Atypical pannus presents clinically as a bilateral hyperemic thickening of the nictitans conjunctiva associated with multifocal follicle formation and varying degrees of depigmentation and pigmentation.
Eosinophilic conjunctivitis is a condition of cats that clinically presents as unilateral or bilateral conjunctivitis, often with associated keratitis. Lipogranulomatous conjunctivitis is a feline disease associated with nonulcerative, white nodules in the palpebral conjunctiva, adjacent to the eyelid margin. Diagnosis of these conditions in dogs and cats is made by clinical appearance and conjunctival cytology or biopsy.
Conjunctivitis may result from bacterial, viral, and parasitic infection. Bacteria are not a known etiology of primary conjunctivitis in dogs; however, secondary bacterial conjunctivitis is common, exacerbates clinical lesions, and complicates the management of conjunctivitis associated with other etiologies in both cats and dogs. Chlamydophila felis and Mycoplasma infection are common causes of feline conjunctivitis.
Primary conjunctivitis may result from direct viral infection of the conjunctival epithelium, including canine herpesvirus-1, canine adenovirus-2, feline herpesvirus-1, and feline calicivirus. Conjunctivitis may also be a prominent feature of various other systemic viral infections (e.g., canine distemper virus, canine influenza), but these animals present with concurrent systemic illness.
Parasitic conjunctivitis may result from thelaziasis or onchocerciasis. Thelazia spp. are nematodes transmitted by flies that reside in the conjunctival fornix and nasolacrimal duct. Diagnosis is by identifying the parasites on the ocular surface. Onchocerca spp. are filaria that produce conjunctivitis associated with single or multiple bulbar conjunctival nodules. Diagnosis is by histopathologic demonstration of the parasites.
Blunt or penetrating trauma can produce conjunctivitis. Both forms may be self-inflicted in small animals with other painful or pruritic ocular diseases. Conjunctival trauma is often associated with dramatic initial chemosis, subconjunctival hemorrhage, and ulceration of conjunctival epithelium.
B. Secondary Manifestation of Other Ocular Diseases
Anatomical proximity, shared blood supply, and extensive vascular and lymphoid tissue may result in the conjunctiva being secondarily affected by other ocular diseases. Conjunctivitis is a frequent manifestation of a variety of serious extraocular and intraocular diseases, including blepharitis, ulcerative keratitis, keratoconjunctivitis sicca, uveitis, glaucoma, and orbital disease. Failure to exclude other ocular conditions as the etiology of conjunctivitis is avoided by performing a complete ocular examination. When conjunctivitis is a secondary manifestation, therapy is directed at the primary condition.
C. Secondary Manifestation of Systemic Diseases
Systemic diseases may first manifest in a detectable manner as conjunctivitis. The extensive vascular and lymphoid tissues within the conjunctiva also render it susceptible to generalized vascular and lymphatic diseases. Infectious, neoplastic, and vascular conditions are especially likely to present as conjunctivitis, with or without additional ocular abnormalities. In the majority of animals, historical and physical examination findings will suggest a systemic disease process.