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807.e2 Polymyositis, Immune-Mediated

Polymyositis, Immune-Mediated
VetBooks.ir Possible Complications

Serum biochemistry profile typically reveals
BASIC INFORMATION
elevations in muscle enzymes. Confirmation If megaesophagus is present, use caution when
Definition is obtained with muscle biopsy in conjunction initiating immunosuppressive therapy with
Immune-mediated polymyositis is a general- with negative infectious disease testing. Because prednisone to avoid worsening pneumonia.
ized inflammatory myopathy characterized by an infectious cause is identified in about 30%
mononuclear cellular infiltration in multiple of cases of generalized polymyositis, it is critical PROGNOSIS & OUTCOME
skeletal muscles of the body. to rule these out.
The prognosis for immune-mediated polymyo-
Epidemiology Differential Diagnosis sitis is good. It can be controlled in most dogs
SPECIES, AGE, SEX • Infectious polymyositis with medications, although many need lifelong
Any breed of dog can be affected, although • Polyarthritis therapy to prevent clinical relapses. Some dogs
large breeds tend to predominate. The disease • Polymyopathy, noninflammatory can be completely weaned off medications
typically affects adults, with a mean age of 5 • Myasthenia gravis after a minimum of 4-6 months of treatment.
years (range, 1-10 years). • Meningitis However, if the polymyositis is pre-neoplastic,
the long-term prognosis is grave.
GENETICS, BREED PREDISPOSITION Initial Database
Breed-specific syndromes have been reported • CBC: typically normal PEARLS & CONSIDERATIONS
for Vizslas and Newfoundlands. A preneoplastic • Serum biochemistry profile: elevated creatine
inflammatory myopathy has been described kinase (CK) and aspartate aminotransferase Comments
in boxers, which develop lymphoma within (AST) levels Do not try to wean prednisone too rapidly.
a year of diagnosis. Dermatomyositis, a rare Relapses can be more difficult to treat than the
condition that affects the skin, skeletal muscle, Advanced or Confirmatory Testing initial disease, and slow and gradual weaning
and vasculature, has been reported in collies • Infectious testing: Toxoplasma gondii, Neos- of no more than 0.5 mg/kg q 4 weeks is
and Shetland sheepdogs. pora caninum, Hepatozoon americanum, and recommended.
Borrelia burgdorferi
Clinical Presentation • Thoracic radiographs: evaluate for mega- Technician Tips
DISEASE FORMS/SUBTYPES esophagus and aspiration pneumonia Technicians can serve a crucial role in helping
Polymyositis is a generalized inflammatory • Electromyography: reveals abnormalities of owners cope with the adverse effects of gluco-
myopathy that affects skeletal muscle in the fibrillation potentials, positive sharp waves, corticoids or in suggesting when alternative
entire body, usually in a symmetrical fashion. and complex repetitive discharges immunosuppressive therapies may be indicated
Some affected animals present with primary • Muscle biopsy: gold-standard diagnostic if adverse effects are intolerable.
signs of dysfunction of laryngeal, pharyngeal, test, demonstrates multifocal mononuclear
and esophageal muscles. infiltrates Client Education
Discuss the potential side effects of prednisone,
HISTORY, CHIEF COMPLAINT TREATMENT and explain that other immunomodula-
Clinical signs can be acute or chronic. The tory drugs can be used to reduce or replace
chief complaint typically includes generalized Treatment Overview prednisone.
weakness, diffuse muscle loss, and a stiff, stilted The goal of treatment is immunosuppression to
gait. Dysphagia, regurgitation, and dysphonia achieve remission of clinical signs. Prednisone is SUGGESTED READING
also can be observed. the most commonly used therapy for immune- Evans J, et al: Canine inflammatory myopathies: a
mediated polymyositis; it is administered at clinicopathologic review of 200 cases. J Vet Intern
PHYSICAL EXAM FINDINGS immunosuppressive doses if the diagnosis is Med 18:679-691, 2004.
Generalized weakness and diffuse muscle confirmed. If there is suspicion of an infectious
atrophy are typically noted. Dogs display a cause of polymyositis, prednisone can be used ADDITIONAL SUGGESTED
stiff, stilted gait with kyphosis and low head in combination with an antimicrobial pending READING
carriage. Spinal reflexes can be diminished. the results of infectious disease testing. Podell M: Inflammatory myopathies. Vet Clin North
Dysphagia and dysphonia are also common. Am Small Anim Pract 32:147-167, 2002.
Fever, an inability to open the jaw, and myalgia Acute General Treatment
are observed infrequently. Prednisone therapy should be instituted at AUTHOR: Sheila Carrera-Justiz, DVM
1-2 mg/kg PO q12h. This dose should be EDITOR: Karen R. Muñana, DVM, MS, DACVIM
Etiology and Pathophysiology continued for a minimum of 2 weeks or until
This is an immune mediated disease character- a clinical response is apparent.
ized by a mononuclear inflammatory infiltration
in the skeletal muscles. Chronic Treatment
Prednisone dose should be gradually tapered
DIAGNOSIS over a period of 4-6 months. Lifelong therapy
may be required. A second immune-modulatory
Diagnostic Overview drug should be considered in dogs that experi-
Polymyositis should be suspected based on the ence unacceptable side effects on prednisone.
presence of weakness and diffuse muscle atrophy.

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