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.
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:
- muscle trauma
- overlying cellulitis
- infected insect bites
- injection of illicit drugs
- diabetes mellitus
Presentation of infective myositis is with pain localized 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.
Although many pathogens, including viruses, bacteria (including mycobacteria), fungi, and parasites can cause myositis, the most common infectious agent is the bacterium, Staphylococcal aureus, accounting for over 75% of cases .
Findings include enlargement of the muscle, with or without abscess formation. Ultrasound may be employed to aid percutaneous drainage of a collection.
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 is the modality of choice to accurately assessing the extent of involvement. Muscle edema, characterized by high T2 signal is typically present. Abscesses appear, as elsewhere, as fluid collection (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, then it should usually be drained either surgically or percutaneously.
Potential complications of untreated infectious myositis include:
- compartment syndrome
- spread of infection to adjacent structures
- systemic spread
- septic shock
- distant abscess formation
For MRI appearances consider