AUTHOR AND EDITOR INFORMATION

Section 1 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Multimedia
• References

INTRODUCTION

Section 2 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Multimedia
• References

Background

Infectious myositis is an acute, subacute, or chronic infection of skeletal muscle. Once considered a tropical disease, it is now seen in temperate climates as well, particularly with the emergence of HIV infection. In addition to HIV, other viruses, bacteria (including mycobacteria), fungi, and parasites can cause myositis. For a detailed discussion of HIV-associated myopathies, refer to eMedicine article HIV-1 Associated Myopathies.

Pathophysiology

Single or multiple muscle groups in the limbs can be involved, a notable exception being trichinosis, which commonly involves orbital muscles. In most instances, involvement of proximal muscles is predominant. Characteristic myopathic features and findings of polymyositis, including inflammatory infiltrates, may be seen.

Viruses: Viruses implicated in the pathogenesis of myositis include HIV-1, human T lymphotrophic virus 1 (HTLV-1), influenza, coxsackieviruses, and echoviruses. As in the non–HIV-infected population, HIV-associated polymyositis is most likely autoimmune in origin. Influenza myositis could be due to direct viral invasion or autoimmune response.

Pyomyositis: The pathogenesis is unclear, but trauma, viral infection, and malnutrition have been implicated. Although most cases of tropical pyomyositis occur in healthy individuals, other pathogenetic factors include nutritional deficiency and associated parasitic infection. In the temperate climates, pyomyositis is seen most commonly in patients with diabetes, HIV infection, and malignancy.

Lyme borreliosis: Musculoskeletal manifestations are noted frequently in Lyme borreliosis. The disease is transmitted by the bites of ticks of the Ixodes genus that carry the spirochete (see Image 1). The animal reservoirs are the white-footed mouse in the Eastern United States and the wood rat in California. Human infection results from the bite of infected ticks in the late spring and early summer. Lyme myositis may result from direct invasion of muscle by the spirochete Borrelia burgdorferi or by autoimmune mechanisms.

American trypanosomiasis: The causative organism is a protozoan, Trypanosoma cruzi. The insect vectors are reduviid bugs such as Rhodnius prolixus ("vinchuca"), Triatoma infestans, and Panstrongylus megistus. The insect defecates on the host's skin as it feeds, contaminating the bite wound with feces containing the parasites. T cruzi occurs in 2 forms in humans, the intracellular amastigote and the trypomastigote form in blood, which is ingested by the insects (see Image 2). The parasite reproduces asexually and migrates to the hindgut. In humans, the parasite loses its flagellum and transforms into the amastigote form, which may enter muscle and multiply, resulting in myositis.

Cysticercosis: Myositis also can occur in cysticercosis, which represents an infection by the larval stage of the intestinal tapeworm Taenia solium. Human infection results from ingestion of raw or incompletely cooked pork. Another mode of infection is by contamination of food and water by feces containing the eggs of the tapeworm. The larvae migrate throughout the body and may form fluid-filled cysts in a variety of tissues, including muscle.

Frequency

United States

• Pyomyositis: Approximately 330 cases have been reported in the US literature since 1971.
• Lyme disease: Endemic areas include the Northeast, mainly Connecticut, Massachusetts, Maryland, and New York; the North-Central region, mainly Wisconsin and Minnesota; and the West Coast, especially Northern California.

International

• In eastern Uganda, 400-900 cases of tropical myositis occur per year; it is rare in western Kenya.
• Cysticercosis is most prevalent in India, Eastern Europe, Central America, and Mexico.
• In endemic areas of Latin America, 8% of the population is seropositive for American trypanosomiasis.

Mortality/Morbidity

• A potentially life-threatening complication of pyomyositis is toxic shock syndrome.
• Rhabdomyolysis can complicate influenza and, rarely, coxsackievirus myositis.

Race

• In Hawaii, muscle abscesses were noted to be confined to the Polynesians.
• In the French Pacific islands, the disease is not seen in the French settlers.

Sex

Infectious myositis has a male predominance.

Age

Infectious myositis typically is seen in young adults.

CLINICAL

Section 3 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Multimedia
• References

History

• Key historical points should be confirmed.
• • Risk factors for Staphylococcus aureus pyomyositis - Strenuous activity, muscle trauma, skin infections, infected insect bites, illicit drug injections, and diabetes
  • Overseas travel
  • Consumption of poorly cooked meats (especially pork products in the case of trichinosis or cysticercosis)
  • Tick bites
• Pyomyositis
• • Fever and malaise
  • Psoas abscess - Subtle symptoms such as fever and flank and hip pain; may manifest as pyrexia of unknown origin
• Trichinosis
• • Cardinal features - Myalgia, periorbital swelling, and fever (see Image 3)
  • Depending on site of involvement - Diplopia, dysarthria, dysphagia, dyspnea
• Lyme myositis
• • Pain and weakness of the proximal muscle groups
  • Symptoms in the vicinity of skin lesions or in limb muscles
• Cysticercosis with myositis - Fever, myalgias
• Trypanosomiasis with myositis
• • Acute stage
  • • May be asymptomatic or characterized by fever
    • Myositis occurring in the early stage of infection - Symptoms such as muscle weakness and myalgias mimicking those of polymyositis
  • Chronic stage - Myalgias
• Toxoplasma myositis - Fever, myalgias, and muscle weakness
• Influenza myositis - Childhood and adult forms recognized
• • Childhood form
  • • Fever, malaise, and rhinorrhea followed 1-7 days later by severe pain, especially in the calves
    • Muscle pain worse with movement, especially with walking
    • Symptoms of myositis - Generally last 1-7 days
  • Adult form
  • • Fever, myalgias, generalized weakness
    • Muscle swelling in some patients
• Acute coxsackievirus myositis
• • Group A virus infection - Myalgias, weakness
  • Group B virus infection - Causes epidemic pleurodynia (Bornholm disease or epidemic myalgia), which is considered a form of myositis
  • • This is an acute, febrile disorder with abrupt onset of pain in the abdomen or lower thoracic region.
    • Pain can be referred to the back and shoulders.
    • Pain is worse with movement, breathing, or coughing.

Physical

• Pyomyositis
• • Muscles are painful, swollen, tender, and indurated.
  • Quadriceps muscle is involved most commonly.
  • The second most common location is the psoas muscle, followed by the upper extremities.
  • Depending on the site of involvement, it may mimic appendicitis (psoas muscle), septic arthritis of the hip (iliacus muscle), or epidural abscess (piriformis muscle).
  • Findings may be subtle in immunocompromised persons requiring a high index of suspicion for diagnosis.
• Trichinosis
• • Involvement of orbital muscles can result in diplopia and strabismus.
  • Dysarthria or dysphagia can result when tongue and pharyngeal muscles are affected.
  • Limb muscles usually are mildly involved.
  • Other manifestations include myocarditis and dyspnea from diaphragmatic involvement.
• Lyme myositis
• • Weakness and atrophy of the proximal muscle groups can occur, accompanied by local swelling and tenderness.
  • Muscle weakness may be a major presenting feature of this disease.
  • Rarely, late ocular involvement, including orbital myositis, may occur.
• Cysticercosis with myositis
• • The most common sites of involvement are the skeletal and cardiac muscle, brain, and eyes.
  • When skeletal muscles are involved, palpable cysticerci (mature larvae) appear in subcutaneous tissues.
  • A notable feature of this type of myositis is muscle pseudohypertrophy, which may be seen in the tongue or calf muscles.
  • During the acute stage of disease, patients may have fever and muscle tenderness.
• Trypanosomiasis with myositis
• • The acute stage of the disease may be characterized by fever, lymphadenopathy, and hepatosplenomegaly.
  • At the site of the insect bite, local inflammation (involving subcutaneous tissues and muscle) results in a swelling known as a chagoma.
  • Contamination of the eyes produces unilateral periocular and palpebral edema with conjunctivitis and preauricular lymphadenopathy (Romaña).
  • Extraocular involvement is rare. It may present with features of subacute orbital myositis and may mimic an orbital tumor.
  • During the acute parasitemic stage, intense infection of the myocardium may occur, producing severe myocarditis and disturbances of cardiac conduction.
  • Clinical manifestations in the early stage of myositis include muscle weakness, tenderness, and erythema mimicking those of polymyositis and dermatomyositis.
  • Skeletal muscle may be involved in the chronic stage as well and can last for decades.
• Toxoplasma myositis
• • Muscle invasion by Toxoplasma gondii usually is seen in immunocompromised individuals with disseminated toxoplasmosis.
  • The clinical features are similar to those of polymyositis, with manifestations of fever and muscle weakness.
  • Polymyositis is a prominent feature even in the congenital form of toxoplasmosis.
• Influenza myositis
• • Muscle weakness, tenderness, and swelling
    • More severe in adults
    • Proximal muscles are affected predominantly.
    • In children, involvement of the gastrocnemius-soleus muscles causes calf pain and difficulties with walking( toe-walking, wide-based gait)
    • Complications include myocarditis and respiratory dysfunction.
• Acute coxsackievirus myositis
• • Group A virus
    • These viruses can cause an acute, diffuse inflammatory myopathy.
    • This may progress to rhabdomyolysis and myoglobinuria, leading to renal failure.
  • Group B virus infection (epidemic myalgia)
    • Muscle tenderness and swelling may be noted in some patients.
    • Relapses can occur 2 weeks to a few months after the initial presentation.

Causes

HIV infection is one of most important causes of viral myositis. Opportunistic infections can cause myositis in immunosuppressed patients. Known pathogens include the following:

• Viral - HIV-1, HTLV-1, cytomegalovirus, group B coxsackievirus (epidemic myalgia), influenza
• Bacterial - S aureus (most common, 70%); Streptococcus viridans; Streptococcus pyogenes; Streptococcus pneumoniae; Salmonella enteritidis; Klebsiella pneumoniae; Clostridium freundii; Bartonella; gram-negative organisms including Escherichia coli and Pseudomonas aeruginosa, Neisseria, Yersinia, Morganella morganii, and Citrobacter species
• Spirochetal - B burgdorferi
• Mycobacterial - Mycobacterium avium-intracellulare complex
• Parasitic - T gondii, Trichinella spiralis, Echinococcus granulosus, T solium, T cruzi, microsporidia
• Fungal - Cryptococcus, actinomyces

DIFFERENTIALS

Section 4 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Multimedia
• References

Other Problems to be Considered

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

WORKUP

Section 5 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Multimedia
• References

Lab 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
• • 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. MRI is helpful in differentiating pyomyositis from osteomyelitis. MRI is also the imaging modality of choice for evaluating pelvic infections.
  • 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 Image 4).

Trypanosomiasis

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.

TREATMENT

Section 6 of 10  

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• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Multimedia
• References

Medical Care

All medical care should be provided in conjunction with an infectious disease specialist and the primary care physician.

• HIV polymyositis: Corticosteroids remain the mainstay of treatment of polymyositis.
• Trichinosis
  • Thiabendazole is effective if administered within 24 hours of infection. It has minimal effect in established infection.
  • Optimal dosage has not been established.
  • It can be combined with prednisone 40-60 mg/day in patients with severe pain and weakness.
• Trypanosomiasis
  • Benznidazole is a trypanocidal drug that is quite effective in the acute phase of the illness.
  • It reduces cardiac complications and parasitemia and has been found to be beneficial in the early chronic phase.
  • Successful treatment is evinced by serological tests remaining negative for at least 1 year after conclusion of treatment.
• Viral myositis
  • Treatment comprises bed rest, intravenous fluids, and symptomatic management with antipyretics and analgesics.
  • Antiviral agents such as amantadine could be considered in adults.
• Tuberculous and toxoplasmal myositis, cysticercosis: Please refer to the following eMedicine articles: HIV-1 Associated Myopathies, Neurocysticercosis, and Neuroimaging in Neurocysticercosis.
• Pyomyositis
  • Promptly administer systemic antibiotics. This could eliminate the need for surgical drainage in selected cases.
  • The choice of antibiotic is determined by identification of the causative organism.
• • Antibiotics initially are given intravenously until clinical improvement is noted, followed by oral antibiotics for a total course of 3 weeks (eg, cefazolin or ceftriaxone IV followed by cephalexin PO).

Surgical Care

• Pyomyositis: During the suppurative phase, abscess aspiration under ultrasonic or CT guidance may be required. Surgical drainage is especially necessary for large abscesses.

Consultations

• Neurologist
• Infectious disease specialist

MEDICATION

Section 7 of 10  

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• Introduction
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• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Multimedia
• References

Treat the underlying cause of infectious myositis. Use appropriate antibiotics for pyomyositis. Prednisone may be effective to treat HIV-1–associated polymyositis.

Drug Category: Corticosteroids

These agents decrease inflammatory reactions by reversing increased capillary permeability and suppressing PMN activity.

Drug Category: Anthelmintic

Parasite biochemical pathways are sufficiently different from those of the human host to allow selective interference by chemotherapeutic agents in relatively small doses.

Drug Category: Antibiotics

Empiric antimicrobial therapy must be comprehensive and should cover all likely pathogens in the context of the clinical setting.

FOLLOW-UP

Section 8 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Multimedia
• References

Further Inpatient Care

• Pyomyositis: Hospitalize for systemic antibiotics.

Deterrence/Prevention

• Prevent trichinosis and cysticercosis by adequately processing pork.
• When traveling in endemic areas of Latin America, apply insect repellents such as N-diethyl-meta-toluamide (DEET) to avoid American trypanosomiasis. Pyrethrin insecticides also may be used to kill insect vectors. Using bed nets to keep away insects is advisable.

Complications

• Influenza and, rarely, coxsackievirus myositis can be complicated by rhabdomyolysis.
• Pyomyositis: Life-threatening complications include sepsis and toxic shock syndrome.
• Trichinosis
• • Heavy infestation may be fatal because of myocarditis and/or diaphragmatic involvement.
  • Myocarditis can lead to cerebral embolism.

Prognosis

• Pyomyositis: Prompt administration of antibiotics can result in complete resolution.
• Trichinosis: Most patients with myopathic involvement recover after several weeks. Infection may be fatal if severe and involving other organs systems (eg, cardiac, pulmonary, CNS).

Patient Education

• Travelers visiting area of endemic trichinosis should be educated on the hazards of eating raw or undercooked pork.
• Educate traveling diabetic patients concerning the need for prompt treatment of cutaneous infections and infected insect bites and to avoid strenuous activity.
• The Myositis Association of America serves as a resource for patients and the medical community.
• For excellent patient education resources, visit eMedicine's Bites and Stings Center. Also, see eMedicine's patient education article Ticks.

MULTIMEDIA

Section 9 of 10  

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• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Multimedia
• References

Media file 1:  Ixodes scapularis (dammini), tick vector for Lyme disease. Courtesy of Centers for Disease Control and Prevention.
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Media file 2:  Trypanosoma cruzi in blood smear. Courtesy of Centers for Disease Control and Prevention.
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Media file 3:  A patient with trichinosis and ocular involvement. Courtesy of Centers for Disease Control and Prevention and Dr. Thomas F. Sellers, Jr.
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Media file 4:  Histopathology: Borrelia burgdorferi spirochetes in Lyme disease, with Dieterle silver stain. Courtesy of Centers for Disease Control and Prevention and Dr Edwin P. Ewing, Jr.
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Media type:  Photo

REFERENCES

Section 10 of 10

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
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• Follow-up
• Multimedia
• References

• Belman AL, Preston T, Milazzo M. Human immunodeficiency virus and acquired immunodeficiency syndrome. In: Goetz, Pappert, eds. Textbook of Clinical Neurology;1999:898-900. [Medline].
• Crum NF. Bacterial pyomyositis in the United States. Am J Med. Sep 15 2004;117(6):420-8. [Medline].
• Hays AP, Gamboa ET. Acute viral myositis. In: Engel, Franzini-Armstrong, eds. 2nd ed. Myology: Basic and Clinical;1994:1399-409.
• Heffner RH Jr, Schochet SS Jr. Skeletal muscle. In: Anderson's Pathology. 10th ed. Vol 2. 1996:2666-7.
• Kim JY, Park YH, Choi KH, et al. MRI of tuberculous pyomyositis. J Comput Assist Tomogr. May-Jun 1999;23(3):454-7. [Medline].
• O''Neill DS, Baquis G, Moral L. Infectious myositis. A tropical disease steals out of its zone. Postgrad Med. Aug 1996;100(2):193-4, 199-200. [Medline].
• Patel SR, Olenginski TP, Perruquet JL, Harrington TM. Pyomyositis: clinical features and predisposing conditions. J Rheumatol. Sep 1997;24(9):1734-8. [Medline].
• Reimers CD, de Koning J, Neubert U, et al. Borrelia burgdorferi myositis: report of eight patients. J Neurol. May 1993;240(5):278-83. [Medline].
• Simpson DM, Citak KA, Godfrey E, et al. Myopathies associated with human immunodeficiency virus and zidovudine: can their effects be distinguished?. Neurology. May 1993;43(5):971-6. [Medline].
• Trusen A, Beissert M, Schultz G, et al. Ultrasound and MRI features of pyomyositis in children. Eur Radiol. May 2003;13(5):1050-5. [Medline].
• Tulio AM et al. American trypanosomiasis. In: Strickland TG, ed. 7th ed. Hunter's Tropical Medicine;1991:628-37.

Article Last Updated: Jan 16, 2007