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
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.
Lagophthalmos is incomplete closure. Watch dogs in your exam room to see if they blink and ask owners to 'sneak up' on pets
when sleeping to see if eyes are completely closed. Dogs such as Shih Tzus, Pugs, Boston Terriers with shallow orbits, prominent
eyes with a lot of white scleral show, even with normal STT and reflexes often have incomplete coverage of their central corneas.
The interpalpebral fissure epithelium may roughly pick up stain and often have an immediate tear film break up time (TFBUT)-normal
is 20 seconds. We will discuss this later but essentially many of these dogs warrant attention to preventing central dryness.
COMPLETION of the blink response is critical.
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.