Facial Nerve Paresis (Paralysis) in Dogs
Dr. Erika de Papp
Idiopathic (unknown cause) facial nerve paralysis is the most common form of the disease in dogs. A large study of dogs with clinical signs consistent with facial nerve paralysis demonstrated an unidentifiable cause of disease in 75 percent of the cases. In idiopathic cases the signs are usually unilateral and there are no other abnormal physical exam or neurologic findings. Otitis media/interna. Another neurologic manifestation of middle ear disease may be Horner's syndrome. This is an abnormality of the sympathetic nervous system (highly specialized part of the nervous system), which results in a small pupil, raised third eyelid and drooping of the upper eyelid, as well as a sunken appearance to the eye.
In dogs with concurrent otitis media/interna, there may be a history of chronic ear infections or physical evidence of disease associated with the external ear. However, they may not show obvious evidence of ear disease on physical exam, as inflammation confined to the middle and/or inner ear cannot be seen externally.
Evidence of middle and inner ear disease may be demonstrated by additional neurologic deficits seen as abnormalities of the vestibular system (part of the neurologic system that controls balance). These signs might include a head tilt, nystagmus, strabismus (abnormal eye position), and general lack of coordination. Occurrence of facial nerve paralysis and vestibular abnormalities is strongly suggestive of concurrent ear disease, or brain disease affecting the 7th and 8th cranial nerves at their points of origin in the brainstem.
The impact of facial nerve paralysis on your pet depends on whether or not there is an identifiable underlying cause of disease and what other signs are present. Facial nerve paralysis is a physical exam finding and should not be confused with any other disease process, although there are a number of associated causes that must be ruled out, including:
Trauma. Traumatic accidents may cause direct compression of the facial nerve via local tissue swelling or fractures of the skull. In these cases, painful responses to physical examination would likely be noted.
Surgery. Surgical procedures that are often done to treat chronic (long term) ear disease or to remove ear tumors can result in direct damage to the facial nerve resulting in nerve paralysis. These procedures include ear canal ablation (removal of the external ear canal) and bulla osteotomy (opening of the bony portion of the middle ear).
Neuromuscular diseases. Diseases in this category affect transmission between the nerves and their associated muscles. Animals with neuromuscular abnormalities will most often demonstrate other signs in addition to facial nerve paralysis such as generalized weakness, as a result of multiple nerve involvement. Examples of neuromuscular diseases include myasthenia gravis and botulism (toxin). Facial nerve paralysis in these cases is often bilateral.
Cancer. Tumors involving the middle and inner ear may cause direct damage to the facial nerve.
Polyneuropathies. These are neurologic conditions involving multiple nerves in the body, and may include involvement of the facial nerve. Polyneuropathies may be caused by endocrine diseases such as hypothyroidism, cancerous conditions such as insulinoma (insulin producing pancreatic tumor) and immune mediated processes (diseases in which the body's immune system attacks its own components, in this case nerve cells). Similar to neuromusclular disease, polyneuropathies generally cause other signs in addition to facial nerve damage, and the facial paralysis may be bilateral.
Inflammatory diseases of the CNS. Inflammation in the CNS may affect the facial nerve where it originates in the brainstem. If facial paralysis results secondary to a CNS disease, the animal would also likely demonstrate ataxia, behavior changes, possible seizures, and other cranial nerve deficits.