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.