Vet360 Vet360 Vol 05 Issue 03 - Page 15

OPTHALMOLOGY • • • • - - Cavalier King Charles Spaniel - - Chihuahua - - English bulldog - - English Cocker Spaniel - - English springer spaniel - - Lhasa apso - - Miniature schnauzer - - Pekingese - - Pug - - Samoyed - - Shih Tzu - - West highland white terrier - - Yorkshire terrier Idiopathic Debilitated and dehydrated animals Trauma Drug-induced – this is important to takeinto account when testing STT - - Sulphonamides - - Atropine - - General anaesthesia - - Topical anaesthesia - - Etodolac The latter may occur due to increased aqueous tear production in an attempt to compensate for a loss of mucin. Mucins are required to hydrate, lubricate and protect the ocular surface. Additionally they provide an “anchor” for the aqueous portion of the tear film to the hydrophobic ocular epithelium. A reduced Tear Film Break Up Time (TBUT) test is used to assess for a qualitative defect in tear production. • Apply 1 drop of fluorescein stain to the eye, holding the eyelids open. • Under cobalt-blue illumination, examine the cornea. Note how many seconds it takes for dark spots to appear as the tear film “breaks up” the fluorescein layer. • A normal TBUT is approximately 20 seconds in dogs and 17 seconds in cats. Animals with quantitative deficiencies often have a TBUT of < 5 seconds, which indicates an unstable tear film. Alternatively, a conjunctival biopsy and histopathology can be done to quantify the goblet cell density. Lipid deficiency QUANTITATIVE DISORDERS Aqueous Deficiency Most cases will fit into this category. Deficiency of the aqueous portion of the tear film is more commonly seen in dogs than cats or horses, and leads to the typical KCS we see. The aqueous portion of the tear film is produced by the lacrimal gland and the gland of the third eyelid. Here, true quantitive KCS needs to be differentiated from animals in which corneal drying is due to increased tear film evaporation. Increased evaporation occurs when the ability to blink is decreased due to facial nerve disease, lagopthalmos (incomp