Keratoconjunctivitis Sicca (KCS), also known as dry eye, is a common issue many dogs face. Multiple conditions can result in KCS, including immune-mediated disease, endocrine disease, infectious disease, and neurogenic causes. Our team at Veterinary Vision Center wants to give you more information about how neurogenic KCS can affect dogs, and how this condition is addressed.
A dog’s tear film: Important to their eye health
Tears come in two forms (i.e., basal and reflex) to maintain a dog’s eye health and help them see clearly. Basal tears constantly provide a shield between a dog’s eye and the world, and lubricate, nourish, and protect the cornea. Reflex tears are produced in response to irritants, such as smoke or foreign bodies, and may also contain antibodies to fight infection.
The tear film, which contains enzymes, lipids, metabolites, and electrolytes, has three layers.
- Inner mucus layer — This layer ensures the tear remains on the eye.
- Watery middle layer — This thickest layer functions to hydrate the eye, repel bacteria, and protect the cornea.
- Outer oily layer — This layer smooths the tear surface and prevents the other layers from evaporating.
Neurogenic dry eye in dogs: Etiology
Neurogenic KCS occurs when the cranial nerve that provides parasympathetic innervation to the lacrimal gland is damaged by trauma, inflammation, cancer, or chronic ear infections. Since this nerve also supplies innervation to the nasal mucosa glands, dogs affected by neurogenic KCS will also have a condition called xeromycteria (i.e., dry nose) on the same side as the affected eye. Most KCS forms target both eyes, but neurogenic KCS usually affects only one eye.
Neurogenic dry eye in dogs: Signs
Early in the disease process in dogs affected by KCS, signs indicate eye pain and irritation.
- Mucoid ocular discharge — This is the most common sign observed in canine KCS. The decreased tear production results in a compensatory increase in mucin production. The thick mucoid discharge often sticks to the cornea and surrounding tissues as ropy strands.
- Blepharospasm — Squinting and frequent blinking indicates pain. As the eyelids move over the dry ocular surface, frictional irritation causes significant discomfort.
- Dull corneal appearance— A deficit in the tear film causes the cornea to lose its bright, clear appearance, and become lusterless.
- Reddened conjunctiva — Inflammation results in reddening of the soft tissues surrounding the eye.
- Blood vessel formation on the cornea — The tear film provides metabolic support to the cornea. When the film is deficient, the cornea attempts to replace this support by producing superficial blood vessels to bring in nutrients.
- Corneal ulceration — The increased friction makes dry eyes more susceptible to corneal ulceration.
As the disease progresses, the squinting response decreases, and the discharge becomes mucopurulent as inflammatory cells invade the corneal surface. Corneal scarring (i.e., hyperpigmentation) develops, and a dark film appears to be covering the cornea. If scarring is substantial, the dog’s vision may be reduced.
Neurogenic dry eye in dogs: Diagnosis
Indications in addition to reduced tear production confirm a dog is suffering from KCS. The Schirmer tear test measures the tear film production in one minute. A special wicking strip is placed under the dog’s eyelid. A normal measurement is greater than 15 mm wetting per minute. Ten to 14 mm wetting per minute indicates early KCS. Five to 10 mm wetting per minute indicates moderate KCS, and less than five mm wetting per minute indicates severe KCS. Dogs affected by KCS will also need their cornea stained, to check for corneal ulcers. The eye pressure will also be measured, to ensure the eye is not glaucomatous.
Neurogenic dry eye in dogs: Treatment
KCS treatment goals include returning natural tear production, lubricating the cornea, and restoring or maintaining the cornea’s visibility. Eye washes to remove mucoid discharge and artificial tears are helpful to keep the cornea hydrated until normal tear production returns. Artificial tears should be viscous (i.e., thick), to provide lubrication. Topical anti-inflammatories and antibiotics may be needed, especially if the cornea is ulcerated.
KCS is typically treated using medications that stimulate tear formation by modulating the immune system. However, neurogenic KCS does not respond to these drugs, although they can help improve comfort in some dogs. Neurogenic KCS is treated using lubricants and cholinergic medications that mimic the neurotransmitter acetylcholine. Cholinergic formulations can be administered topically or orally, although topical administration can cause irritation. These medications also can cause side effects, including excess salivation, vomiting, diarrhea, weakness, and decreased appetite. In cases caused by ear infections, the infection should be treated appropriately, and may help resolve the KCS.
In severe cases, surgery may be beneficial for long-term comfort. Parotid duct transpositioning surgically relocates the parotid duct, which carries saliva from the parotid salivary gland to the mouth, to a pocket below the eye. The eye is kept lubricated by saliva as opposed to tears.
Neurogenic KCS is a painful problem for many dogs and can lead to significant issues, including corneal ulcers and vision loss. If you are concerned your dog may be affected by this condition, do not hesitate to contact our team at Veterinary Vision Center to schedule an appointment.