Masticatory myositis in dogs is an inflammatory condition that affects the muscles of the jaw and those that are responsible for chewing (mastication). The cause of masticatory myositis is unknown, but it is thought to be caused by an autoimmune disease. Dogs with masticatory myositis may experience pain and swelling in the jaw muscles, and they may have difficulty opening their mouth. There is no cure for masticatory myositis, but treatment can help relieve symptoms. This article will review what masticatory myositis is, the symptoms, and treatment options.
What are the muscles involved in masticatory myositis in dogs?
Masticatory muscle myositis is inflammation of the muscles of mastication (chewing), namely the temporalis, masseter, and pterygoid muscles. The muscles work together to provide function in opening and closing the jaw. The most common inflammatory myopathy affecting dogs is masticatory myositis.
What causes masticatory myositis in dogs?
Masticatory myositis in dogs is a result of an autoimmune targeting process in which autoantibodies (IgG) are produced and target the myofibers of the muscles of mastication. The underlying trigger of this immune response is unknown. The autoimmune response is characterized by immune cell infiltration causing myofiber destruction which clinically manifests as muscle swelling and pain.
What are the clinical symptoms of masticatory myositis in dogs?
Any age or breed can be affected by masticatory myositis. Most affected dogs however are young adults, around 3-5 years of age at the time of disease onset. Early in the course of the disease, symptoms reflect inflammation of the masticatory muscles and can include:
- Reluctancy and difficulty opening the jaw
- Muscle swelling and pain around the facial area
- Enlarged lymph nodes
- Lethargy
- Fever
- Drooling
As the disease progresses and moves into the chronic phase, muscle fibrosis and atrophy occur resulting in symmetrical atrophy of the facial muscles and weight loss. Weight loss is usually attributed to a lack of caloric intake given the difficulty in apprehending food.
Approximately half of all dogs diagnosed with masticatory myositis have ocular involvement. This could include sunken eyes due to the atrophy of the temporalis muscle, elevated third eyelid, ocular discharge, and redness.
How do veterinarians diagnose masticatory myositis in dogs?
Your veterinarian will start by obtaining a detailed history of the symptoms present. A detailed physical examination will also be performed to aid in the diagnosis of masticatory myositis. Certain tests may be recommended to help make a diagnosis, which can include:
Immunological assays - This is a very accurate blood test. The presence of specific circulating autoantibodies involved can be considered diagnostic for masticatory myositis.
Muscle biopsy - this helps diagnose masticatory myositis and will show inflammatory cells and signs of muscle fiber damage. A muscle biopsy will only be performed if autoantibodies are not detected.
Electromyography (EMG) - This is a test that measures the electrical activity of muscles. Abnormal electrical activity can be seen in masticatory myositis as well as other muscle diseases.
Advanced imaging - Radiographs and CT scans may help rule out other causes of facial swelling and pain, such as tumors.
How is masticatory myositis treated in dogs?
There is no cure for masticatory myositis and the goal of treatment is to relieve clinical signs. Early in the course of the disease, masticatory myositis in dogs is very responsive to corticosteroids such as prednisone or dexamethasone. The dog will become comfortable quickly with a complete return of jaw function within 4 weeks. The dose of corticosteroids is tapered to a lower dose for long-term use.
Analgesics (pain medication) may also be required in the initial phase and encouraging jaw movements by playing with balls and chew toys may help promote the return of masticatory function.
For dogs experiencing difficulty eating and drinking, a slurried diet or gruel may be required. Most dogs do not have difficulty drinking water, however, it is worth noting that some dogs require the use of syringes to administer water.
What is the prognosis for dogs diagnosed with masticatory muscle myositis?
The earlier masticatory muscle myositis is recognized and treated, the better the prognosis. The return of normal jaw function is dependent on early detection and treatment. Research studies found that 91% of dogs had complete recovery of masticatory function and that 50% of patients were able to discontinue prednisolone.
In this article, we reviewed masticatory myositis, which is an uncommon inflammatory disease that affects the masticatory muscles (the muscles used for chewing) in dogs. The cause of masticatory myositis is unknown, but it is thought to be caused by an autoimmune disease. Dogs with masticatory myositis may experience pain and swelling in the jaw muscles, and they may have difficulty opening their mouth. There is no cure for masticatory myositis, but treatment can help relieve symptoms. If you have any questions about masticatory myositis, please contact your veterinarian.