eMedicine Specialties > Neurology > Neurological Infections

Infectious Myositis: Differential Diagnoses & Workup

Author: Mohammed J Zafar, MD, FAAN, Associate Clinical Professor of Medicine, Kalamazoo Center for Medical Studies, Michigan State University; NeuroImager, Premier Medical Care, PC, Premier Radiology, Kalamazoo Neurologic Institute
Contributor Information and Disclosures

Updated: Aug 12, 2009

Differential Diagnoses

Other Problems to Be Considered

Cellulitis
Deep vein thrombosis
Hematoma
Osteomyelitis
Septic arthritis
Wasting syndrome
Idiopathic polymyositis

Workup

Laboratory Studies

• Pyomyositis
  • Leukocytosis
  • Elevated erythrocyte sedimentation rate
  • Serum creatine kinase (CK) and aldolase usually normal
  • Blood culture results generally negative.
• Trichinosis: Eosinophilia is suggestive of the diagnosis.
• Serology
  • Viral titers
  • Toxoplasmal antibodies - Found more frequently in patients with polymyositis and dermatomyositis than in the general population
  • Lyme antibody titer - If enzyme-linked immunosorbent assay (ELISA) screening test result is positive, confirmed with a western blot test
• Cysticercosis - Electrocardiogram
• HTLV-1 infection - Serological tests to detect HTLV-1 antibodies in serum
• Trypanosomiasis
  • Serological tests - ELISA (highly sensitive) or direct agglutination test.
  • Electrocardiogram
• Influenza myositis
  • Elevated CK, mostly consisting of the MM (muscle) isoenzyme, as high as 500 times normal
  • Urine myoglobin - May be positive
  • Liver enzymes (aspartate aminotransferase [AST] and lactate dehydrogenase [LDH]) - May be elevated
  • Electrocardiogram
• Coxsackievirus myositis
  • Rising coxsackievirus antibody titers
  • Isolation methods for group B coxsackievirus - Include inoculation of cell cultures

Imaging Studies

• Pyomyositis
  • MRI is the imaging modality of choice for the diagnosis of pyomyositis. MRI is helpful in differentiating pyomyositis from osteomyelitis. It is especially useful in differentiating early muscle inflammation from abscess formation. MRI is also the best imaging modality for evaluation of pelvic infections.⁶
  • CT scanning may show hypertrophy of involved muscle groups and effacement of the fat planes. Contrast enhancement may indicate abscess formation. CT is also useful for distinguishing tumors and hematomas from abscess.
  • Ultrasound or MRI also may be used to localize involved muscle.
  • Gallium scan is useful for localization in the early stages of illness.
• Tuberculous pyomyositis
  • MRI: Findings are consistent with an abscess (ie, low signal intensity on T1-weighted images and high signal intensity on T2-weighted images of affected muscles).
  • Following gadolinium infusion, peripheral rim enhancement is noted. This is indicated by subtle hyperintensity on T1-weighted images and hypointensity on T2-weighted images.
• Cysticercosis with orbital involvement: Multiplanar imaging with MRI may be obtained to identify a cyst with a mural nodule. With gadolinium, a nodule shows intense enhancement.

Other Tests

Trichinosis: Skin test with trichinellar antigen is unreliable.

Procedures

• HIV polymyositis: Electromyography (EMG) findings are similar to those of idiopathic polymyositis.
  • Short-duration motor unit potentials
  • Low-amplitude polyphasic motor unit potentials
  • Fibrillations
• Trichinosis: EMG may reveal fibrillation potentials.
• Lyme myositis: Changes of denervation may be seen in the involved muscles.
• Pyomyositis: Needle aspiration is useful for the detection of purulent material and for microbiologic identification of the causative organism.

Histologic Findings

Trichinosis

Muscle biopsy is required to confirm diagnosis. Findings in the acute stage of larval invasion of the muscles include segmental necrosis and interstitial infiltrates composed mainly of eosinophils. The Trichinella species larvae sometimes can be seen in the muscle biopsy. However, encapsulated cysts (without larvae), granulomas, and focal calcification are more likely to be encountered.

Pyomyositis

Widespread necrosis of muscle fibers, perimysium, and blood vessels is noted. Pleomorphic inflammatory response consisting of both neutrophils and lymphocytes is noted.

Cysticercosis

The viable larvae produce little or no tissue reaction. However, rupture or death of a cysticercus (mature larva) evokes an acute inflammatory response with a pleomorphic exudate composed of neutrophils and eosinophils. Over time, fibrous tissue encapsulates the cysts. A chronic granulomatous response may surround the cysts.

Lyme myositis

Muscle biopsy shows atrophic fibers and an infiltrate consisting of lymphocytes, plasma cells, and macrophages. The spirochete of borreliosis can be detected in muscle fibers by the modified Dieterle silver stain method (see Media file 4).

Sections of infected tissues may reveal clusters of amastigotes in muscle cells surrounded by acute or chronic inflammation.

Influenza myositis

Muscle fiber necrosis without inflammatory change is observed. Influenza viral particles have been identified in muscle fibers under electron microscopy. Muscle fiber regeneration is seen in some, with an inflammatory response consisting of mononuclear and polymorphonuclear leukocytes.

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