Facial paralysis is caused by dysfunction of the facial nerve which supplies the muscles to the face. This can occur with problems of the facial nerve itself or the brainstem. In the majority of cases, an underlying cause is not identified (idiopathic facial paralysis). Facial paralysis has been linked with low thyroid level. In some cases, facial paralysis is seen concurrently with problems in the vestibular system (system of balance). If the problem is inside the brain, possible causes include cancer, infection, inflammation, and stroke. There are usually additional neurologic signs in animals with brain disease (i.e. difficulty walking, change in level of alertness, abnormalities with other cranial nerves).
The most notable symptom is loss of the normal blink reflex. It can also cause drooping of the facial muscles on one side and drooling. There can also be decreased tear production in the eyeball, resulting in dry eye. A neurological exam will help determine if the problem is inside or outside the brain. If the problem is thought to be inside the brain, an MRI +/- spinal fluid analysis is recommended. If the problem is thought to be outside of the brain, ruling out hypothyroidism is recommended. Treatment for facial paralysis depends on the underlying cause. Animals with idiopathic facial paralysis typically improve over a period of weeks but can be incomplete. Prognosis for the other diseases that cause facial paralysis is variable.