When the facial nerve does not work, dogs have an inability to blink, their lips droop, and there is decreased tear production on the affected side.
Twelve pairs of nerves, one on each side of the head, originate at the base of the brain and are responsible for certain neurological functions of the head and face. These paired nerves are called the cranial nerves and they are numbered 1 through 12. The 7th cranial nerve is the facial nerve, and it controls the muscles involved in facial expression, blinking and tear production. The cause of this condition is unknown. Although some cases of facial nerve paralysis have an identifiable origin, such as diseases of the ear, tumors and metabolic disorders, usually the cause of this disease is not well-defined.
Typically, a sudden weakness or paralysis occurs on one side of the face. If nerves on both sides of the head are affected, weakness is seen on both sides of the face. This weakness causes the ears and lips to droop. Animals may drop food or drool from the affected side of their mouth. Sensation, or feeling, of the face is normal. Because the facial nerve causes the eyelids to blink and controls the tear glands, affected animals may be unable to blink and may develop “dry eye” from a lack of tears on the affected side of the face. “Dry eye” may be associated with conjunctivitis, yellow-green discharge, and ulceration of the cornea. Vision remains normal.
Diagnosis is based on examination findings and the exclusion of other causes of facial nerve paralysis. Careful inspection of the ears is performed in affected animals. X-rays and CT scans and MRIs may be recommended in some animals. These tests are normal in animals with idiopathic facial paralysis. Dogs may also be tested for low thyroid function, which has been associated with facial nerve paralysis. Evaluation of tear production and other testing of the eyes may also be performed.
There is no specific treatment for idiopathic facial nerve paralysis. Artificial tears may be applied to the eyes on the affected side to prevent corneal ulceration. Initially, follow-up examinations may be done frequently to monitor for development of corneal ulcers. Notify your veterinarian if the eye on the affected side becomes red or squinty, if it has increased discharge, or if the cornea becomes cloudy, because these signs could indicate the presence of a corneal ulcer.
Prognosis for return of function of the facial nerve is very poor in most cases. Most affected animals do not regain function of the facial nerve. If function returns, it may take weeks for an improvement to be detected. Sometimes only partial recovery occurs. A full recovery does occasionally happen.
Dr. Karsten Fostvedt is a veterinarian at St. Francis Pet Clinic in Ketchum.