In this VETgirl online veterinary continuing education blog, Dr. Shelby Reinstein, DVM, DACVO reviews keratoconjunctivitis sicca (KCS or “dry eye”) in dogs. There are many forms of dry eye or KCS in dogs, so read on to learn it. Come back next week to read her blog on how to treat KCS in dogs!
Abnormalities in the quantity or quality of the tears will result in ocular surface pathology. The conjunctival and corneal epithelial cells suffer from hypoxia and desiccation, with further injury occurring as frictional irritation ensues. Debris that would normally be flushed from the ocular surface (dead cells, mucus, particulates) accumulates, and bacteria more easily colonize the compromised surface. These consequences of tear film disease culminate in inflammation of the ocular surface, leading to cell damage and progressive disease.
Mechanisms of Tear Deficiency
• Quantitative Tear Deficiency: A decreased secretion of tears may be the result of glandular dysfunction, or from interruption of the nerve supply to the glands. Insufficient aqueous tear production results in the clinical syndrome of keratoconjunctivitis sicca (KCS).
• Qualitative Tear Deficiency: Deficiencies in either the lipid or mucin tear layers can also result in symptoms of ocular surface inflammation.
• Distributional Tear Deficiency: Disorders that result in poor tear film coverage, such as lagophthalmos, facial nerve palsy, or other eyelid disorders will result in surface inflammation and a dry eye type syndrome.
Quantitative Tear Deficiency
KCS is the most common form of tear deficiency in dogs. Compromise to the glandular secretion can be caused by a variety of diseases affecting the lacrimal gland, 3rd eyelid gland, or both. Infectious diseases, such as canine distemper virus and blepharoconjunctivitis, can induce a lacrimal adenitis with subsequent KCS. Certain breeds, such as Pugs and Yorkshire Terriers, are predisposed to lacrimal aplasia leading to clinical KCS at a very young age. Certain drugs are known to reduce tear secretion (atropine, general anesthesia), while others may be toxic to the lacrimal gland (sulfasalazine, trimethoprim sulfa, etodolac). Trauma to the eye or the orbit as well as radiation therapy may damage the lacrimal gland and lead to KCS. Certain metabolic conditions are associated with decreases in tear production, including diabetes mellitus, hyperadrenocorticism, and hypothyroidism. However, the majority of KCS cases in dogs are the result of idiopathic, lymphoplasmacytic inflammation with secondary glandular atrophy. Certain breeds have a much greater incidence as compared to the general population suggesting a genetic component. English Bulldogs, Cavalier King Charles Spaniels, West Highland White Terriers, Shih Tzus, and Pugs are some of the most commonly affected breeds. Loss of the parasympathetic innervation to the lacrimal gland is known as neurogenic KCS, and can occur with chronic otitis, peripheral neuropathies, idiopathic disease, or primary neurologic diseases.
Qualitative Tear Deficiency
Diseases that affect the eyelid margins can result in abnormal secretions by the meibomian glands and subsequent instability to the tear film. The lack of a functional oily surface layer leads to premature evaporation of the tears and surface irritation. Inflammation of the eyelid margin, known as marginal blepharitis or meibomianitis, causes swelling of the eyelid margins and abnormal lipid secretion. Certain autoimmune conditions that affect the eyelids, such as lupus erythematosus and bullous pemphigoid, may also result in abnormal meibomian gland secretions. Chronic conjunctivitis can lead to a deficiency in the mucin production by the conjunctival goblet cells and also lead to an unstable tear film.
Distributional Tear Deficiency
Poor eyelid conformation or function will result in abnormal tear coverage over the ocular surface. Lagophthalmos (incomplete blinking) caused by buphthalmos, exophthalmos, or facial nerve paralysis will result is axial (central) corneal disease secondary to poor tear distribution.
Come back next week to learn HOW to treat dry eye in dogs!