The precorneal tear film is a substantial structure both in its size and functional importance. Yet it's difficult to directly evaluate. Various studies have come up with imaging techniques to measure the thickness of the tear film and although one study measured it at a whopping 40um, essentially equaling the size of the epithelium, most still feel it sits somewhere around 10um.

It's easy to forget the magnitude of the tear film functions such as to nourish with O2, glucose, electrolytes; cleanse; lubricate; maintain corneal clarity; protect: immunoglobulins, enzymes, antibacterial, growth factors, and fibronectin. Alteration in any one or all of the above may lead to discomfort, infection, poor wound healing, visual abnormalities, and adjacent tissue dysfunction of the conjunctiva and eyelids.

Normal tear film distribution relies on 1. normal anatomy, 2. normal tear film quality, and 3. normal tear film quantity. The blink reflex relies on normal trigeminal nerve sensory interpretation to touching of the eyelid and the facial nerve motor ability to close the eyelids. Careful evaluation of all patients for the ability to blink is a rapid and critical test. Do not get in front of the eye as your hand will be visualized and you will be checking the menace response or vision. If pets have sensation but lack the ability to blink due to facial nerve dysfunction, they may actively pull the eyeball back with retrobulbar muscles CN VI, raise their third eyelids, and/or pull away when you tap them. However, lack of sensory innervation to cornea (V) decreases corneal health by limiting growth factors, nutrients, and immunoglobulins leading to a neurotrophic keratitis despite a normal STT and ability to blink.

Small eyelid masses are often underestimated in their potential for irritation. Impaired sebum escape from underlying meibomian glands eventually leads to impaction, chalazion formation, and if rupture, severe, diffuse granulomatous blepharitis. Even small eyelid papillomas on older dogs that remain stable for years may cause significant eyelid margin trauma in these cases. Subtle problems with tear film distribution locally may lead to direct epithelial damage. Client education about possible problems is essential. Large marginal eyelid masses can be significantly interfering factors in normal tear film distribution. In addition, they may be malignant, destroy unique eyelid margin anatomy, and should be excised. Irritation to the cornea and conjunctiva may lead to pruritis and adjacent blepharitis.

Prolapsed nictitans glands should be repositioned for many reasons not the least of which is data clearly indicating an association with the development of KCS in the future particularly in the Bulldog, Cocker Spaniel, Cavalier, and Shih Tzu. The large, malpositioned gland itself, however, is a mass lesion that significantly affects normal tear film distribution, quality of its constituents, pH, bacteria, and mucous balance. The inflamed glandular tissue will eventually be replaced by nonfunctioning fibrous tissue and often significant bending of the associated cartilage. Surgical correction should be done early.

Trichiasis in the feline may be initiated by chronic feline herpes virus (FHV), trauma, and more rarely than in the dog, anatomy. Unresolved trichiasis often leads to a unique necrotic, brown corneal sequestrum. The cycle of keratitis, pain, enophthalmia, trichiasis, more pain, entropion, more blepharospasm, and erosion can be difficult to control in the cat without surgical intervention as the eyelids are so tight fitting.

Medial entropion is often something we see in the brachycephalic cats and yet the contribution of trichiasis to tear film abnormalities and epithelial damage is variable. Some cats with hairs touching the corneal tear film do sustain epithelial damage. It is not uncommon, however, for some cases to need medication and even surgical intervention in the long run. It is important to evaluate these pets with their eyes in different positions to determine if the hairs may have greater contact with the eye looking down, nasally, or laterally. In contrast, Pugs with medical trichiasis, often have a dramatic amount of pigmentation. As most of these pigmentary keratitis cases starting medially, it is assumed that the medial trichiasis with chronic epithelial irritation and abnormal tear film distribution initiate or significantly contribute to the pigmentation. Correction of the trichiasis intuitively makes sense to improve comfort and prevent continual damage. However, once the pigmentation process begins, even correction of the entropion may not lead to resolution of the altered pigment accumulation. This is important to convey to owners. I do find some improvement of pigmentation density as well as area with surgical repair, use of lubricants, tacrolimus or cyclosporine, and if vascularization is present, mild steroidal anti-inflammatories such as once daily hydrocortisone ointment with caution.

Macropalpebral fissure or elongation of the eyelids contributes to a combination of ectropion/entropion and lateral canthal instability. The determination as to whether or not to recommend surgery revolves around the degree of discomfort, damage, bilateral or unilateral, the age of the dog, and lastly, the desire to maintain cosmetics. If dogs are older than 4 or 5 and have lived comfortably with the eyelids and only recently are suffering from clinical signs often initiated by a trauma, a temporary tarsorrhaphy may alleviate the entropion. It is important to warn the owners that once sutures are removed, entropion may 'come back'. Keep the sutures in place as long as is needed to assure comfort with significant resolution of the corneal inflammation. Properly placed sutures (4-0 to 5-0 Prolene) may remain in place for weeks. Many young dogs with severely elongated eyelids and a combination of entropion/ectropion will need permanent surgical intervention. These can be tricky and I often use a combination of techniques. Although some of my colleagues recommend waiting until the large breed dogs with these problems such as St. Bernards are mature, I find that they do very well with correction at an early age and rarely if ever require second surgeries. Most owners are eager to resolve the problem. Some breeders, however, very much want to maintain the Diamond Eye look. Communication of expectations in these cases is critical. If owners are not willing to perform surgery, lubricating ointments, gels, or long contact time liquids are essential in maintaining a somewhat improved tear film integrity.

Tear Film Disorders also include the inability of the tears to flow properly out the nasolacrimal duct system. Epiphora may be due to 1. an increase in production such that it overwhelms the outflow system or 2. a decrease/blockage in outflow. Careful evaluation of the eyelid margins for the correct anatomic position, the presence of cilia emanating from the meibomian glands or conjunctiva as ectopic cilia, and the meibomian glands themselves may elucidate reasons for overproduction. Pain may also result from foreign bodies or inflammation. Diminished tear film stability may lead to a sensation of dryness/discomfort and therefore, lead to overproduction. In addition, if the trilaminar tear film constituents are not present, the tears may not bind to the cornea and just pool ventrally. The decrease in outflow may be due to a blocked duct due to a foreign body, mass effect, tooth root abscess. Or the tears may take the path of least resistance due to entropion, malposition of the eyelid margin, caruncle trichiasis, and eyelid notching. This list is not inclusive as once I found a worm as the culprit.

The Jones Test reveals abnormal passage down the nasolacrimal duct system. One drop is put in each eye at the same time, the dogs head is pointed to the floor, and the nares periodically examined. Most large breed dogs will have evidence at the distal nares in 15-30 seconds but time travel may be variable. The difference between the two will often be significant highlighting the affected side to determine if there is a blockage. Cats and brachycephalics may never flow distally if stain flows into the nasopharynx and the test may be nondiagnostic. If flushing is desired, some large dogs will allow it awake after multiple doses of topical anesthetic and a wait of at least 10-15 minutes. If a nasolacrimal cannula is not available, a 20-25g IV catheter sleeve will often work. Nonsterile water may be used in a 3cc syringe. Use magnification! Most small dogs and cats need to be sedated to accomplish a proper flush. While flushing from the lower duct, hold off the upper duct to force fluid down the nose and then alternate.