Facial paralysis is quite common in dogs, particularly in middle to old-age. The term is simply the description of drooping of muscles in the face, which is caused, not by damage to the muscles themselves, but to the nerves supplying them.
A dog with facial paralysis has a dropped appearance to the face on one side (a bit like a person that has suffered a stroke). One ear is held lower than the other and the lip hangs down on the affected side. Affected dogs are unable to blink and have a tendency to dribble saliva and drop food from the side of the mouth. Occasionally, the same signs may be present on both sides of the face which can make the changes difficult to recognise.
Sudden development of facial paralysis is a common presentation of stroke in humans. However, in most dogs, facial paralysis is not associated with disease of the brain, but rather with direct damage to the nerve that controls the muscles in the face (facial nerve). This nerve comes from the back of the brain to control the muscles of facial expression (ears, lips, eyelids and nostrils).
The most common cause of facial nerve paralysis in dogs is idiopathic facial nerve paralysis. The term idiopathic means that there is no known cause. Therefore in idiopathic disease all the tests results will be normal. A similar condition is recognised in human medicine as Bells Palsy.
The second most common cause of facial nerve paralysis is a deep-seated infection of the ear (otitis media/interna). In this case other signs such as Horners syndrome (decreased pupil size and third eyelid coming across the eye) and/or vestibular syndrome (head tilted to one side, and balance loss) are frequently seen in addition to facial paralysis.
Rarely, facial paralysis can be associated with disease affecting multiple nerves (polyneuropathy) or brain disease (tumour, infection or inflammation). In these cases, other neurological signs are usually observed in addition to facial paralysis.
Although idiopathic facial nerve paralysis is the most common form of this disease, investigations may be required to rule-out any other causes. Your vet will first want to perform a thorough ear examination (if necessary under sedation or anaesthesia using a scope) to detect signs of ear infection (such as a ruptured eardrum or an inflamed ear canal).
If this examination is normal then the deeper parts of the ear and brain may be examined using specialised imaging techniques. The deeper part of the ear (middle and inner ear) can be seen on X-Rays of the skull and the back of the brain can be examined using CT or MRI scans. In some cases it may be necessary to take a sample of fluid from around the brain to check for signs of inflammation.
Idiopathic facial nerve paralysis is usually left untreated because, by definition, no cause has been identified. Facial nerve paralysis secondary to an ear infection requires at least 4 to 6 weeks antibiotic treatment. If your pet has an ear infection that fails to get bet with medical treatment your vet may advise an operation to drain the ear.