Why Is My Dog's Mouth Drooping on One Side?

By Adrienne Farricelli

If you notice your dog's mouth drooping on one side, suspect weakness or paralysis of the facial nerve. Accompanying symptoms denoting problems with the facial nerve of the mouth include messy eating and drooling. While in many cases the underlying cause of the facial nerve impairment remains unknown, it's important to seek out veterinary assistance to rule out certain medical conditions.

A Lesson in Anatomy

A dog's facial nerve, also known as cranial nerve VII, is responsible for controlling the muscles of your dog's face. This nerve branches off into several localized nerves meant to serve the dog's ears, nose, eyelids and lip muscles. A lesion of the auriculopalpebral branch of the facial nerve affects the eyelids and ear; whereas, a lesion of the palpebral branch of the facial nerve affects the eyelids. A drooping mouth is the result of a lesion of the buccal branch of the facial nerve, which affects the dog's lips and nostrils.

History of Trauma

One main cause of damage to the nerve controlling the dog's facial muscles is traumatic injury. Rough handling, a car accident or a complication from surgery are some examples of possible causes of traumatic injuries that could affect the buccal nerve branch of the facial nerve. In a dog with a history of trauma, an electromyography can be helpful in determining the extent of the injury; however, changes may not be noticeable until five to seven days following injury. Treatment may include massage, heat therapy, electroacupuncture and laser therapy.

Low Thyroid Function

When the dog's thyroid glands located in the neck produce low amounts of hormones, dogs are diagnosed with hypothyroidism. Hypothyroidism commonly affects medium to large middle-aged dogs. Golden retrievers, Doberman pinchers and Irish setters are particularly predisposed. It's estimated that facial nerve paralysis affects up to 70 percent of dogs showing clinical signs of hypothyroidism and nerve dysfunction. Along with a droopy mouth, affected dogs may exhibit the following symptoms:

  • Hair loss
  • A dull coat
  • Excessive shedding
  • Weight gain
  • Reduced activity
  • Sensitivity to cold
  • Ear infections

When a droopy mouth is triggered by low thyroid levels, thyroid replacement therapy may resolve facial paralysis completely.

Middle Ear Infection

Infection of the dog's middle and inner ear is a common culprit for facial nerve problems. Left untreated, a middle ear infection may progress and involve the dog's facial nerve, which runs right next to the dog's ear. Dogs may develop visible drooping affecting one side of the face and mouth along with the inability to blink. When the infection progresses to the inner ear, dogs may also lose their ability to effectively balance themselves and maintain equilibrium. Diagnosis can be obtained through an MRI or CT scan, and treatment involves a course of antibiotics.

If your dog shows signs of ear problems, see your vet to prevent an outer ear infection from potentially progressing to the middle and inner ear. Facial nerve paralysis stemming from an ear infection can be permanent.

Idiopathic Facial Paralysis

The term idiopathic is a medical term often used to refer to a condition with an unknown cause. Through a process of exclusion, after ruling out infections, low thyroid levels, injury or trauma, the vet may diagnose a dog with a droopy mouth of unknown origin as having "idiopathic facial paralysis." Unfortunately, since no cause can be found, no treatment can be initiated.

In the case of idiopathic facial paralysis, the droopy mouth can be either temporary or permanent. While a permanent droopy mouth may sound like bad news, facial nerve paralysis doesn't usually affect a dog's overall quality of life.