Infectious myositis is an acute, subacute, or chronic infection of skeletal muscle. It was considered a tropical disease once but now seen in temperate climates as well, particularly with the emergence of HIV infection.
Viruses, bacteria (including mycobacteria), fungi, and parasites can cause infectious myositis.
Infectious myositis has a male predominance and is typically seen in young adults. Single or multiple muscle groups in the limbs can be involved. Trichinosis commonly involves orbital muscles.
Causes of Infectious Myositis
Known pathogens include the following:
- HIV-1 (one of the most common causes of myositis)
- group B coxsackievirus (epidemic myalgia)
- S aureus (most common, 70%)
- Streptococcus group
- Salmonella enteritidis
- Klebsiella pneumonia
- Clostridium freundii
- gram-negative organisms
- Escherichia coli
- Pseudomonas aeruginos
- Neisseria species
- Yersinia species
- Morganella morganii
- Citrobacter species
- Echinococcus granulosus
- T solium, T cruzi
- Cryptococcus neoforman
- Candida species
- Histoplasma capsulate
- Coccidioides species
- Aspergillus species
- Pneumocystis jiroveci
- Microsporidia Fusarium species
- Actinomyces species;
- Sporothrix schenckii
- Blastomyces dermatitidis
Mycobacterium avium-intracellulare complex
Common Types of Infectious Myositis
Viruses implicated include HIV-1, human T lymphotropic virus 1 (HTLV-1), influenza, coxsackieviruses, and echoviruses. The viral polymyositis is most likely autoimmune in origin. Influenza myositis could be due to direct viral invasion or autoimmune response.
In children, this condition occurs after an episode of fever, malaise, and rhinorrhea followed by severe pain, especially in the calves that coincide with subsidence of fever. The muscle pain worse with movement, especially with walking. It is also known as benign acute myositis and is a self-limiting condition that generally lasts 1-7 days.
In adults, there would be fever myalgias and generalized weakness. Muscle swelling may occur in some patients.
Complications include myocarditis and respiratory dysfunction.
Group A virus can cause an acute, diffuse inflammatory myopathy that may progress to rhabdomyolysis and myoglobinuria. Renal failure may be a complication leading to renal failure.
Group B virus infection causes severe pain in the muscles between the ribs or in the diaphragm [pleurodynia, also called Bornholm disease or epidemic myalgia.
It 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 and is worse on movements, breathing, or coughing.
Muscle tenderness and swelling may be noted in some patients.
Pyomyositis or Bacterial Myositis
Trauma, viral infection, and malnutrition have been implicated as the causes. Nutritional deficiency and associated parasitic infection in tropical climates may contribute. In the temperate climates, pyomyositis is seen most commonly in patients with diabetes, HIV infection, and malignancy.
The affected muscles are painful, swollen, tender, and indurated. Quadriceps muscle is the most common muscle involved followed by the psoas muscle, and then the upper extremities.
This may be difficult to distinguish clinically from inflammatory myopathy.
Findings may be subtle in immunocompromised persons.
The most common form is group A streptococci necrotizing myositis/streptococcal myonecrosis which is usually seen in the men and young adults. It is seen in the immunocompromised patients and is characterized by flu-like symptoms which evolve to severe local muscle pain, swelling, and fever. It may be rapidly progressive and can be fatal due to toxic shock syndrome, multiorgan failure.
The disease is transmitted by the bites of ticks of the Ixodes genus that carry the spirochete. Lyme disease may cause myositis by the invasion of muscle by the spirochete Borrelia burgdorferi or by autoimmune mechanisms.
It is characterized by pain and weakness of the proximal muscle groups in the vicinity of skin lesions. Local tenderness, swelling, and wasting of muscles may be seen.
Late ocular involvement may cause orbital myositis.
It is characterized by myalgia, periorbital swelling, and fever. Depending on the site of involvement, the features may be diplopia, dysarthria, dysphagia and. Mild involvement of limb muscles may be noted.
Myocarditis may occur.
It is an infection by the larval stage of the intestinal tapeworm Tenia solium. It is transferred from pigs to human by 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.
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.
During the acute stage of disease, patients may have fever and muscle tenderness.
- Most often seen in severely immunosuppressed patients
- Clinical presentation is similar to pyomyositis
- Often present with lower extremity pain and swelling
- May involve the paraspinal musculature
- Clinically presents with fever, rash, and muscle tenderness
It is caused by Toxoplasma gondii and usually is seen in immunocompromised individuals with disseminated toxoplasmosis.
Clinical features are similar to those of polymyositis, with manifestations of fever and muscle weakness.
The clinical features are given in the individual description. The patient should be enquired for
- Risk factors for Staphylococcus aureus pyomyositis
- Strenuous activity
- Muscle trauma
- Skin infections
- Infected insect bites
- Drugs via injection
- Connective tissue disorders
- Overseas travel
- Consumption of poorly cooked meats
- Tick bites
- inflammatory Demyelinating Polyradiculoneuropathy
- Femoral Mononeuropathy
- Paraneoplastic Autonomic Neuropathy
- Metabolic Myopathies
- Neuromuscular and Myopathic Complications of HIV
Following lab tests are done
Leucocytosis is seen in pyomyositis. Trichinosis and other parasitic infections may show eosinophilia. ESR and CRP is increased across all the types.
- Muscle enzyme levels are increased.
- IgE levels are increased in parasitic infections.
- Lyme antibody titer may be done using ELISA in suspected cases. Similarly, Toxoplasma antibodies titer would be raised in myositis caused by Toxoplasma.
- Viral serology for suspected viral cases.
- Muscle biopsy in cryptococcal myositis may reveal intracytoplasmic organisms
- Fungal stain and culture may be done to identify a fungal cause
Aspiration, Stain, and Pyomyositis
Needle aspiration is useful for the detection of purulent material and for microbiologic identification of the causative organism.
Histopathological findings differ according to the cause of myositis. Inflammation and/or necrosis is seen across all the etiologies.
Characteristic features of Different Infectious Myositis
- Interstitial infiltrates composed mainly of eosinophils
- Trichinella species larvae or encapsulated cysts may be visible
- Focal calcification
- Widespread necrosis
- Pleomorphic inflammatory response
Muscle necrosis and gram-positive bacteria in chains are noted.
Little or no tissue reaction.
Ruptured cyst – acute inflammatory response with a pleomorphic exudate
- Atrophic fibers
- and an infiltrate consisting of lymphocytes, plasma cells, and macrophages.
- Borrelia burgdorferi can be detected
- Muscle fiber necrosis without inflammatory
- Viral particles can be identified in muscle fibers under electron microscopy.
- Inflammatory response consisting of mononuclear and polymorphonuclear leukocytes.
- Muscle fiber regeneration may be seen
Characteristic fungal appearances may be seen
MRI is the image of choice in myositis. MRI is able to differentiate pyomyositis or tuberculous abscess from osteomyelitis. It also differentiates early muscle inflammation from abscess formation.
Gadolinium-enhanced MRI is used to identify tuberculosis.
MRI is able to identify cysticercosis with orbital involvement
In trichinosis MRI, may show a clear cyst with a scolex [also can be seen on CT and ultrasound]
MRI can distinguish between tumors and hematomas from an abscess.
Calcifications are best visualized on CT scans. CT scanning may show hypertrophy of involved muscle groups and effacement of the fat planes. Contrast enhancement may indicate abscess formation.
The gallium scan is useful for localization in the early stages of illness.
Features are similar to idiopathic polymyositis.
- Short-duration motor unit potentials
- Low-amplitude polyphasic motor unit potentials
EMG may reveal fibrillation potentials.
Changes of denervation may be seen in the involved muscles.
Treatment of Infectious Myositis
Exact treatment would depend on the diagnosis. Rest and medication are mainstays of the treatment. The drugs given in a particular case would depend on the diagnosis and causative agents.
- Treatment comprises bed rest, intravenous fluids, and symptomatic management with antipyretics and analgesics.
- Antiviral agents such as amantadine could be considered in adults.
- HIV- corticosteroids
Thiabendazole is effective if administered within 24 hours of infection but less effective in established infection.
- Antibiotics depending on the causative organisms
- Abscess drainage if required
An antifungal agent such as amphotericin B.
Complications are varied and depend on the type of myositis
For example, rhabdomyolysis may be sen in viral type.
Pyomyositis can be complicated by sepsis and toxic shock syndrome.
Gas gangrene may occur in clostridial pyomyositis.
Myocarditis and/or diaphragmatic involvement in trichinosis.
Prevention of Myositis
- Prevent trichinosis and cysticercosis by adequately processing pork.
- Protection from insects by using repellents, insect killers or bed nets.
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