Masticatory muscle myositis

What it is

Masticatory muscle myositis (MMM) is a focal immune-mediated myopathy. The muscles of mastication contain a special fibre type, type 2M fibres, with a myosin isoform not found in limb muscles. In MMM the immune system produces autoantibodies against 2M fibre myosin, and lymphocytes and plasma cells infiltrate these muscles. Limb muscles are not affected, which distinguishes MMM from polymyositis.

Who it affects

Dogs of any breed, usually young adults. Large breeds such as the German shepherd, Labrador and golden retriever, Dobermann and Rottweiler are frequently affected, and the Cavalier King Charles spaniel is predisposed among small breeds.

Anatomy involved

The muscles of mastication are the temporalis, masseter, medial and lateral pterygoid muscles and the rostral belly of the digastricus, all innervated by the mandibular nerve, a branch of the trigeminal nerve. The temporalis fills the temporal fossa and inserts on the coronoid process of the mandible; the masseter runs from the zygomatic arch to the masseteric fossa and ventral border of the mandible. Swelling of the pterygoid muscles behind the eye pushes the eyeball forward, and their later atrophy lets it sink back into the orbit.

Clinical signs

• Acute phase: swelling of the temporalis and masseter, pain when opening the mouth or handling the head, trismus (inability to open the mouth), exophthalmos, fever and enlarged mandibular lymph nodes. This phase may be short or go unnoticed.
• Chronic phase: marked, symmetrical atrophy of the temporalis and masseter ("skull-like" head), enophthalmos and progressively restricted jaw opening from fibrosis. Many dogs are first presented at this stage.

Diagnosis

• Serum 2M antibody test: highly specific; it should be taken before immunosuppressive treatment, which can cause false negatives.
• Biopsy of the temporalis or masseter: inflammatory infiltrate targeting 2M fibres, necrosis and, in chronic cases, fibrosis.
• Creatine kinase may be raised in the acute phase. Electromyography shows abnormalities restricted to the masticatory muscles.
• Rule out trigeminal neuropathy (the dog cannot close the mouth, rather than cannot open it), temporomandibular joint disease, retrobulbar disease, polymyositis and infectious myositis.

Treatment

Immunosuppressive doses of corticosteroids, tapered slowly over at least 4 to 6 months to the lowest effective dose; other immunosuppressants such as azathioprine are added when needed. Forcing the jaw open under anaesthesia is not recommended. Soft food or a feeding tube supports nutrition while the dog cannot eat. Prognosis is good when treatment starts early; relapses are common if treatment is stopped too soon, and fibrosis that has already formed does not reverse.