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Masticatory Myositis 637

Masticatory Myositis Client Education
Sheet
VetBooks.ir Diseases and Disorders
HISTORY, CHIEF COMPLAINT
BASIC INFORMATION
• Dysphagia/jaw pain • Tetanus
• Disease of the temporomandibular joint or
Definition • Ptyalism facial bones
Masticatory myositis (MM) is a focal inflam- • Abnormal facial appearance ○ Trauma, neoplasia, osteopathy, foreign
matory myopathy of autoimmune origin that • Weight loss body
targets the unique muscle fibers (type 2M) of • Decreased activity/lethargy • Idiopathic inflammatory polymyopathy in
the muscles of mastication. Hungarian Vizsla
PHYSICAL EXAM FINDINGS • Phenobarbital-responsive hypersalivation
Synonyms Acute stage: Chronic disease: other causes of masticatory
Masticatory muscle myositis (MMM), mastica- • Fever, lymphadenopathy (mandibular, muscle atrophy
tory myopathy, historically: eosinophilic myositis prescapular) • Loss of trigeminal nerve function
(acute stage), atrophic myositis (chronic stage) • Trismus ○ Peripheral nerve sheath tumor, inflamma-
• Pain on palpation and attempted forced tion, or other neoplasia
Epidemiology opening of the jaw
SPECIES, AGE, SEX • Swelling of the muscles of mastication Initial Database
• Canine, all ages, either sex • Exophthalmos ± blindness ± decreased • Biochemistry profile: mild elevations in
• Feline rarely reported pupillary light responses (PLRs) muscle enzymes creatine kinase (CK),
Chronic stage: alanine aminotransferase (ALT), aspartate
GENETICS, BREED PREDISPOSITION • Atrophy of the muscles of mastication aminotransferase (AST) in the acute phase
• Large-breed, young adult (2-3 year old) to • Trismus (less than elevations associated with general-
middle-aged (4-9 year old) dogs • Enophthalmos ized polymyositis); hyperglobulinemia
• Overrepresented breeds: German shepherd, • Rarely, inability to close the mouth • CBC: leukocytosis with neutrophilia or
Labrador retriever, Doberman pinscher, and eosinophilia
golden retriever Etiology and Pathophysiology • Urinalysis: ± proteinuria
• Rottweilers are more prone to peripheral • The muscles of mastication include the • Muscle or lymph node aspirates can reveal lym-
eosinophilia and eosinophilic muscle temporalis, masseter, pterygoid (medial and phoplasmacytic or eosinophilic inflammation
infiltrates due to their propensity for lateral), and rostral digastricus innervated • Thorough head and jaw evaluation (sedation
hypereosinophilic syndrome by the mandibular branch of the trigeminal may be required)
• Cavalier King Charles spaniels: possible nerve that also innervates the tensor tympani ○ Measure the extent of jaw opening.
genetic predisposition; can develop signs and tensor veli palatine muscles. • Skull radiographs under sedation to rule out
of disease at a much younger age (12 weeks) • They have a unique embryologic origin trauma, neoplasia, or foreign body
(brachial arch vs. paraxial mesoderm),
ASSOCIATED DISORDERS myofiber types (2M vs. 2A), and distinct Advanced or Confirmatory Testing
MM has been documented to occur concur- myosin isoform compared with limb muscles. • Serum 2M antibody: high specificity (100%)
rently with myasthenia gravis, and masticatory • Autoantibodies are produced against mastica- and sensitivity (85%-90%)
muscle involvement can occur as an overlap tory myosin-binding protein-C; may involve ○ Negative < 1:100, borderline 1:100,
syndrome with generalized polymyositis. molecular mimicry with bacterial antigens; positive > 1:100
results in necrosis and phagocytosis with ○ Previous glucocorticoid therapy or the
Clinical Presentation fibrotic infiltrate of the muscles chronic form of the disease can result in
DISEASE FORMS/SUBTYPES false-negative
Signs occur bilaterally in the muscles of mas- DIAGNOSIS • Electromyography (EMG)
tication but are not always equal in severity ○ Fibrillation potentials; positive sharp
or time of onset from one side to the other. Diagnostic Overview waves; bizarre, high-frequency discharges
Dogs can often be weeks into the acute stage Diagnosis is often based on clinical presenta- in the acute phase
of disease before owners present the dog for tion and a positive serum 2M antibody titer. ○ Decreased insertional activity in the
diagnosis. Signs can wax and wane during this However, results of this test can vary according chronic phase
time. Patients can present in the chronic stage to the stage of disease (acute or chronic). It is ○ Limb muscles should also be tested to
without owner recognition of the acute phase. important to consider a multimodal approach differentiate generalized polymyositis
Acute stage: to the diagnosis of this disease, including serum ○ Muscle enzyme levels should be tested
• Trismus: inability to open the jaw (even testing and biopsy exam. prior to EMG to prevent false enzyme
under anesthesia) elevations
• Myalgia: pain on palpation of temporalis Differential Diagnosis • Muscle biopsy
and masseter muscles Acute disease: other causes of exophthalmos ○ Commonly biopsied muscles include the
• Swelling of the muscles of mastication or pain on jaw opening: temporalis or masseter.
• Ocular • Extraocular myositis (EOM) ■ Care must be taken not to biopsy the
○ Exophthalmos from swelling of the • Retrobulbar abscess thin frontalis muscle located superficial
pterygoid muscles; can result in blindness • Generalized polymyositis to the temporalis muscle.
from stretching of the optic nerve if severe, ○ Initially can present with only masticatory ■ Important not to biopsy areas where
and also exposure conjunctivitis muscle involvement an EMG was performed
Chronic stage: • Infectious myopathy ○ Necrosis and phagocytosis of muscle fibers
• Atrophy of the muscles of mastication ○ Toxoplasma gondii, Neospora caninum, Bor- ○ Perivascular infiltration of mononuclear
• Enophthalmos (differs from acute stage) relia burgdorferi, Ehrlichia canis, Rickettsia cells
• Permanent trismus rickettsii, Hepatozoon spp ○ Perimysial and endomysial fibrosis

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