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tvpjournal.com Today's Veterinary Practice July/August 2014 16 K eratoconjunctivitis sicca (KCS) is a relatively common condition in dogs. Although KCS can be diagnosed readily with a thorough ophthalmic examination, the diagnosis is often overlooked. KCS is an inflammatory condition of the cornea and conjunctiva, secondary to a deficiency of the precorneal tear film (PTF). KCS is cat- egorized by tear film defi- ciency: • Quantitative KCS is a decrease in the aqueous component of the tear film as measured with the Schirmer tear test (STT); it is recognized more commonly in veterinary medicine. • Qualitative KCS is a decrease in the lipid or mucin components of the tear film and diag- nosed by document- ing decreased tear film breakup time (TBUT). PATHOPHYSIOLOGY Tear film deficiencies lead to: • Chronic inflammation of the ocular surface sec- ondary to increased sur- face friction • Secondary infection • Dehydration and malnu- trition of the corneal and conjunctival epithelium. This latter combination makes ulcerations more prone to infection, possibly resulting in keratomala- cia and perforation. Diagnosis & Treatment of Keratoconjunctivitis Sicca in Dogs Lori J. Best, DVM; Diane V.H. Hendrix, DVM, Diplomate ACVO; and Daniel A. Ward, DVM, PhD, Diplomate ACVO University of Tennessee Peer revieweD THe LAcrImAL SYSTem & TeAr FILm Normal PTF is estimated to be anywhere from 3 to 45 microns thick in humans and, in most species, is composed of aqueous, lipid, and mucin layers, which were once thought to be present in a laminar arrangement (Table 1). 1,2 More recent evidence suggests that PTF may resemble a muco-aqueous pool covered in a very thin lipid layer rather than a trilaminar structure. 3 Lacrimal secretion is stimulated via sensory input from the cornea, periocular structures, and globe. The ophthalmic and maxillary divisions of the trigeminal nerve serve as the afferent part of the reflex arc; then motor input travels to the lacrimal glands via the parasympathetic division of the facial nerve as the efferent arc. Tears are then secreted following contraction of lacrimal acinar myoepithelium. TabLe 1. Structure of Precorneal Tear Film AreA OF PrODUcTION FUNcTION TYPe OF DeFIcIeNcY DIAGNOSTIc TeST LIPID Meibomian glands • Limits evaporation • binds tear film to cornea • Provides surface tension to prevent tear film overflow Qualitative Decrease in TbUT AQUeOUS Orbital and nictitans lacrimal glands • Provides corneal nutrition, surface lubrication, and smooth surface for optical clarity • Removes waste material and bacteria Quantitative Decrease in STT value mUcIN Conjunctival goblet cells • enhances spread of tear film Qualitative Decrease in TbUT