Overview

Pyomyositis is a bacterial infection of the skeletal muscles, which causes the formation of pus-filled abscesses and inflammation in the skeletal muscle with ‘pyo-’ meaning pus, ‘myo-’ meaning muscle and ‘sitis’ indicating infection. It primarily affects large muscle groups such as the back, gluteal and quadriceps, and is more prevalent in tropical populations.^1,2

The main symptoms of pyomyositis are unexplained muscle pain, fever and/or swelling.^3,4

Magnetic resonance imaging (MRI) has emerged as the primary imaging modality to diagnose pyomyositis due to its ability to detect early signs of inflammation, identify abscesses in the muscles, and distinguish between similar conditions with overlapping symptoms.^5,6,7 MRI also plays a key role in treatment planning, such as deciding between antibiotics, aspiration, or surgery. Earlier diagnoses using imaging can also vastly change the treatments chosen and their effect on the patient.

This article will walk through how MRI is used to both diagnose and treat pyomyositis, delineating the challenges and existing methods used to manage pyomyositis.

What is pyomyositis?

Pyomyositis is a bacterial infection which can cause abscess formation (‘pockets of pus’) in the muscles. It is endemic to tropical regions but has an increasing presence in temperate regions.^2,8

Common symptoms include:

• Swelling and/or tenderness in the affected muscle
• Fever and chills
• Reduced range of motion in the affected limb
• Localised muscle pain
• General fatigue

The most common pathogen which causes pyomyositis is Staphylococcus aureus, although Streptococcus and other bacteria may also be responsible.^4,6 The entry of the pathogen into the muscle usually occurs through trauma, surgery, and even minor injuries that compromise muscle integrity.⁴ 

People with diabetes, cancer, HIV, or recent viral infections are at a higher risk, including any other conditions that impair the individual’s immune function.^5,6

Early diagnosis may lead to better outcomes, as there is a higher likelihood that the infection can be treated using non-surgical medical interventions such as antibiotic therapy, avoiding the potential risks associated with image-guided procedures or even surgery.^1,6

Why is MRI critical for diagnosing pyomyositis?

MRI is the gold standard for diagnosing pyomyositis due to its ability to distinguish between different types of soft tissue in detail.^5,6,9 MRI scans use strong magnetic fields and radio waves to detect structural changes in a person’s anatomy by using the properties of water in people’s bodies, allowing different tissues to be identified by differing water content. Inflammation and abscesses contain more fluid relative to healthy muscle tissue, allowing them to be visually identified against muscle tissue.^6,7,10

Different scanning sequences can be used to highlight different features of the infection and inflammation in the muscle. Typically, radiologists combine multiple sequences to gain a complete overview of the infection or inflammation.

MRI sequences used in the diagnosis and management of pyomyositis:^6,7,9,10

Common misdiagnoses and MRI clues for differentiation

Due to vague symptoms, pyomyositis is often misdiagnosed with other conditions, and the list below highlights a select few of the most common misdiagnoses that can be differentiated using MRI.

Pyomyositis is usually not the first diagnosis, especially in temperate regions where it is infrequent. It can present as cellulitis, muscle strain, or even sciatica if specific areas are affected.^4,8,10 This is why pyomyositis is often undiagnosed until more severe symptoms appear.^5,9 

Common misdiagnoses

How MRI helps in treatment planning and management

MRI has a key role in not just diagnosis, but also managing the condition.

MRI scans can provide insight and feedback for doctors, as patients are monitored before, during and after treatment, to gain an insight into whether an infection is responding to antibiotics, or whether the aspiration or surgery was effective in draining an abscess and assessing the likelihood of recurrence.

How MRI supports treatment

• Localising the abscess: MRI can be used to pinpoint the location, size, depth and shape of the abscess, and this information can be used to guide treatment on whether aspirations can be used or surgical intervention is a necessity^6,7
• Tailoring antibiotic selection: MRI helps differentiate whether the infection is diffuse or localised, which guides what antibiotics are selected and treatment duration^1,4,5
• Monitoring recovery

Follow-up MRI scans can be used to show:

• Changes in inflammation after treatment^1,7,9
• Shrinking of the abscess 
• Whether the infection has recurred or spread

FAQs

What is an abscess?

An abscess is a pocket of pus, which forms due to the body's immune response to infection.

Is MRI always necessary for pyomyositis?

MRI is not always required, but it is the gold standard for imaging infectious diseases generally, and for the diagnosis of pyomyositis.

Can you treat pyomyositis without surgery?

Treatment selection is guided by how early the progression of the infection is diagnosed. The earlier, the higher the probability, and less-invasive interventions such as antibiotics or aspiration can be used.

What are the long-term outcomes after recovery?

Most patients make a full recovery in 6-8 weeks, whereas some may experience long-term occasional muscle fatigue.

Can pyomyositis recur?

Yes, especially in people with weakened immune systems. To prevent this, underlying conditions have to be managed carefully, and any minor infections or trauma must be dealt with promptly. 

Summary

• Pyomyositis, an infection of the skeletal muscles, can progress from mild muscle inflammation to dangerous intramuscular abscess formation and infection, so earlier diagnosis is critical for better health outcomes for people
• MRI is the most effective tool to detect these changes early and guide safe treatment
• Unlike CT or ultrasound, MRI has higher sensitivity to more subtle differences in soft tissue, hence it can identify muscle infections and inflammation at an earlier stage
• MRI can be used to determine whether patients can be treated with antibiotics alone, or need image-guided aspirations or even surgery in more severe cases