Infectious myositis

Infectious myositis is an infection of skeletal muscle, and can be acute, subacute, or chronic. Pyomyositis refers specifically to a bacterial infection of skeletal muscle. 

Epidemiology

It is most often seen in young adults. Pyomyositis, or bacterial myositis, was once considered a tropical disease but is now seen in temperate climates, particularly with the emergence of HIV infection. Other risk factors for infectious myositis include:

• rhabdomyolysis

• muscle trauma

• overlying cellulitis

• infected insect bites

• injection of illicit drugs

• diabetes mellitus

Clinical presentation

Presentation of infective myositis is with pain localised to one or more muscles (although in most cases it is a single muscle), with variable degrees of systemic inflammatory manifestations, depending on the pathogen.  

Pathology

Although many pathogens, including viruses, bacteria (including mycobacteria), fungi, and parasites can cause myositis, the most common infectious agent is the bacterium Staphylococcus aureus, accounting for over 75% of cases ¹. 

Radiographic features

Ultrasound

Findings include enlargement of the muscle, withwhich adopts an ill-defined focally to diffusely hypoechoic appearance and may lose the characteristic "feathered" or without"starry" internal echotexture imparted by the perimyseal septae⁷. The investing fascia may demonstrate structural abnormalities such as a loss of its sharply demarcated margins, increased thickness, and may be visibly disrupted or adopt an increasing degree of outward convexity from the oedematous expansion of the muscle within ⁸. Colour or power Doppler may demonstrate tissue hyperaemia in the absence of necrosis or the presence of an abscess (e.g. pyomyositis) ⁶. The corresponding contralateral area should be examined routinely, when possible; this may elucidate subtle structural derangements.

In the appropriate scenario (e.g. puncture wound, IV drug use) one should carefully assess for a retained foreign body as a potentially infectious nidus. The presence of an abscess formationshould be also sought by recognition of a discrete collection, classically (but non-uniformly) heterogenous and less echo-dense compared to adjacent tissues, which moves independently from the surrounding structures during dynamic graded compression (i.e. sonographic "fluctuance"). The presence of irregular echogenic collections with "dirty" posterior shadowing (i.e. gas) should evoke concern for a necrotising soft tissue infection. Ultrasound may also be employed to aid percutaneous drainage of a collection. 

CT

CT shows enlargement and decreased attenuation of the affected muscle with effacement and stranding of surrounding fat planes.

If an abscess is present it will appear as an intramuscular fluid collection with peripheral rim enhancement, the presence of which is helpful in distinguishing focal low density regions from areas of necrosis, which will have little peripheral enhancement. 

MRI

MRI is the modality of choice to accurately assessing the extent of involvement. Muscle oedema, characterised by high T2 signal, is typically present. Abscesses appear, as elsewhere, as fluid collections (high T2, low T1) with peripheral contrast enhancement. There may also be diffuse muscle enlargement.

Treatment and prognosis

Depending on the infective agent systemic antibiotics are administered. If an abscess has formed, it is usually drained either surgically or percutaneously. 

Potential complications of untreated infectious myositis include:

• compartment syndrome

• spread of infection to adjacent structures

  • osteomyelitis

  • septic arthritis

• systemic spread 

  • septic shock

  • distant abscess formation

History and etymology

In 1847, Virchow reported inflammation of the skeletal muscle with suppuration and abscess formation. However, the first detailed description of pyomyositis is attributed to Scriba and Beitrang Zur in 1885, who described this condition as a pyogenic bacterial infection of skeletal muscle, endemic in the tropics, naming it tropical pyomyositis. In 1971, Levin reported the first case from a temperate region.

Differential diagnosis

For MRI appearances consider:

• differential diagnosis of skeletal muscle oedema on MRI

See also

• cellulitis

• necrotising fasciitis

• osteomyelitis

• septic arthritis