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!

What is the neuroanatomic lesion localization for the following case?
Signalment: 4 year old FS DSH
History: Acute onset inability to blink one eye. No history of trauma. She is an indoor only cat.

To answer this question, of course you must start with a cranial nerve exam. At its most basic level, the cranial nerve examination is a process of elimination. Let's start with the blink reflex. If you touch the medial and lateral canthus, what cranial nerves are you testing? (CN 5 and CN 7) How do you know which nerve is affected? To do this, we try to isolate each cranial nerve in the reflex to see which one misbehaves. How can you isolate these two nerves from each other to see which is the affected nerve? Lets's try a corneal reflex! When done correctly, the cotton swab touches the cornea and the eye retracts into the socket. Doing this tests CN 5 (sensory) and 6 (motor). Voila! If the cat does not blink when you touch the medial or lateral canthus, but DOES retract the globe when you do corneal reflex which nerve is affected? Think you know... scroll (or read!) to the bottom to see the answer.

But wait! That is only part of the question. We have now localized which cranial nerve is affected but we don't know if this is a central or peripheral nerve lesion localization, right? To look at the brainstem we focus on the nerve pathways running towards and from the forebrain and determine if they're affected. The nerve pathways that are easiest to test are proprioception and motor/tone. Watching the animal walk you may be able to detect a toe drag or delayed placement but ultimately we have to test proprioception through paw replacement testing or my personal favoriate for cats: tactile placing. Similarly, when watching the patient walk you may see a hemiparesis (weakness on one half of the body). Often this is more obvious in the pelvic limb but both ipsilateral limbs can be affected. The last piece to the puzzle is an evaluation of level of mentation. If the animal is obtunded, stuperous or in a coma, we have an effect to the brainstem RAS. If you have 1 or more of these signs, the animal has a brainstem disease. If we DON'T have delayed proprioception, evidence of hemiparesis or a change in mentation, we are more likely to be dealing with a peripheral neuropathy.
Remember:
1) cranial nerve deficit + delayed paw replacement/tactile placing, weakness or decreased mentation = brainstem.
2) cranial nerve deficit without the above = peripheral

Answer: Cranial nerve 7 is affected. (5 is normal in corneal reflex therefore it is not the problem in the blink reflex either.)

Thanks for reading and have a great week! Do you have a case that is puzzling you? Please reach out - I'd love to help. Did you know I also do onsite or virtual private CE for hospitals? Reach out for more details, if you're interested.