The Facial Nerve
The facial nerve is a large motor nerve, originating from the medulla, that causes the muscles of the face to contract. These are often called the "muscles of facial expression". What happens when this important nerve malfunctions?
What does paralysis look like?
The facial nerve innervates the muscles of facial expression in dogs and cats as well as innervation to the lacrimal eye glands. Clinical signs are typically unilateral and, result in an inability to move the eyelids (inability to blink), inability to move the lips (dogs may accidentally chew on their lips and look "droopy" ), lack of ear movement (especially noticeable in cats), and a dry, red eye with possible ocular ulceration. They look bored because they cannot make a facial expression.
What causes facial nerve paralysis?
Facial nerve paralysis can be caused by peripheral or central dysfunction. Diseases affecting the peripheral nerve might include hypothyroidism, neoplasia, otitis media, polyps, rarely neuritis and everybody's favorite - idiopathic. Idiopathic facial nerve paralysis is actually the most common cause of peripheral facial nerve paralysis in dogs. Central facial nerve paralysis occurs when something dysrupts the nerve function at the level of the brainstem. In additional to facial nerve paralysis, dogs will show ipsilateral paw replacement deficits, changes in mentation, and occasionally hemiparesis with central dysruption. The causes of brainstem facial nerve paralysis are often neoplasia, meningoencephalitis or vascular.
The Diagnostic Challenge
After localizing the lesion, the next step is to make the diagnosis. Animals with central disease are best diagnosed with MRI +/- spinal tap. Animals with peripheral disease would benefit from a T4, neospora IFA, toxoplasma IgG/IgM (and other geographically specific infectious disease testing), and an MRI. Idiopathic facial nerve paralysis is diagnosed when all causes have been ruled out.
Treatment for facial nerve paralysis is clearly based on the diagnosis. All animals with facial nerve paralysis benefit from eye lube at night (they cannot blink or close their eyelids) and eye drops during the day until the blinking returns.
I hope this TidBit was useful. This summer I'm going to spend some time looking at the cranial nerves in TidBit fashion so buckle your seatbelts! Have a great week and I look forward to working with you soon!