Masticatory Myositis in Dogs and Cats

Masticatory myositis is an immune-mediated, inflammatory myopathy in which specific autoantibodies are directed against type II muscle fibers in the muscles of mastication (temporalis, masseter, and pterygoid muscles) of affected animals. The masticatory muscles have distinct myosin and myofiber types (type II M), which are embryologically distinct from appendicular myofibers and are believed to be antigenically distinct as well. This may explain the targeted immune-mediated attack against the masticatory muscles.

Many dog breeds (mostly large breeds) are affected by masticatory myositis, and most dogs with this condition are young adults.

Clinical signs in acute cases in dogs include the following:

• swollen masticatory muscles

• difficulty opening the jaw

• exophthalmos (occasionally)

• fever (occasionally)

Clinical signs in chronic cases in dogs include the following:

• anorexia

• weight loss

• difficulty opening the jaw

• muscular atrophy (see chronic masticatory myositis image)

Chronic masticatory myositis, muscular atrophy, dog

Image

Courtesy of Shinichi Kanazono, DVM, DACVIM (Neurology).

Masticatory myositis has rarely been reported in cats (1, 2). The clinical signs in cats were similar to those typically observed in dogs, and the results of a canine ELISA were positive for type II M antibodies.

Diagnosis of masticatory myositis is by testing for type II M antibodies, usually in a serum sample. This antibody test is highly sensitive and specific.

Serum CK activity may be elevated in patients with this condition, and electromyographic evaluation reveals abnormal spontaneous electrical activity in affected muscles.

CT scan or MRI results may show contrast enhancement, size changes, and focal changes in attenuation of masticatory muscles.

A biopsy sample of the temporalis muscle may show myofiber necrosis and phagocytosis, and can be helpful in determining the prognosis for recovery. A biopsy demonstrating substantial fibrosis may indicate that improvement in the jaw's range of motion and muscular atrophy is unlikely.

Treatment of masticatory myositis is with steroid therapy(eg, prednisone at 2 mg/kg, PO, every 24 hours, tapered over weeks to months [3, 4]). Other immunosuppressive agents may help lessen the adverse effects of steroid treatment.

Manual jaw opening under general anesthesia is not recommended because of the risk of iatrogenic fracture.

If a patient struggles to open its jaw because of muscular stiffness or fibrosis, nutritional support may be necessary. Soft food, in the form of a gruel, is likely the best option for patients with a small range of jaw motion. Alternatively, IV nutritional support can be used in acute but severe cases. In more chronic cases, a feeding tube can ensure adequate nutritional support.

In general, physical therapy is not needed for these cases. Treatment of the underlying inflammation will restore normal or near-normal jaw movement in acute cases. In chronic cases, if the muscle has been mostly replaced with fibrous tissue, physical therapy offers no benefit.

Relapses of masticatory myositis are common; however, the long-term prognosis for dogs and cats with this condition is favorable.

For other inflammatory myopathies in dogs and cats, see the Overview for this chapter.