Introduction

Pyomyositis is a rare bacterial infection that leads to the formation of pus-filled lesions in skeletal muscles. The most commonly affected muscles in the body are usually the thigh muscles. The primary cause of this disease is a strain of bacteria called Staphylococcus aureus, which often lives harmlessly on the skin or in the nose but is capable of causing infection when it enters the muscle tissue.¹  

Fever, muscle soreness and pain are common early signs of pyomyositis. As the illness worsens, the infected muscle can swell, potentially leading to pus-filled abscesses if left untreated. For better outcomes in patients, early detection of these symptoms is essential.² 

This disease typically affects residents in tropical areas, with pyomyositis accounting for 1.4% of all hospital admissions ³. However, although less frequent, pyomyositis has been reported in temperate regions, particularly in people with compromised immune systems. Most cases in these non-tropical regions are linked to serious immunodeficiency disorders, like HIV infection or cancer therapy. Although people of any age can contract the infection, children and young adults are most commonly affected ³. This article aims to explore key aspects of diagnosing pyomyositis, focusing on its clinical presentations, the role of various imaging techniques, and the laboratory tests necessary to confirm the infection.

How does pyomyositis develop? 

The exact cause of pyomyositis is not fully understood, but several key factors are thought to play a role. One idea for how this disease develops is that the bacteria spread through the bloodstream during brief periods of bacteraemia (the presence of bacteria in the bloodstream) and then infect the muscles. Normally, muscles are well-equipped to resist bacterial infections, possibly because myoglobin in the muscles binds to iron, which helps prevent bacteria from growing.⁴ 

Certain situations can increase the likelihood of developing pyomyositis. For example, muscle injury can make it more vulnerable to bacteria, allowing the infection to start. People with weakened immune systems, such as those with HIV, are also at a higher risk because their bodies have a harder time fighting off infections. In tropical areas, factors such as poor nutrition and parasitic infections can also increase the risk of developing pyomyositis. These factors make it easier for the bacteria to invade and cause an infection in the muscles.⁵

Clinical presentation of pyomyositis 

The clinical presentation of pyomyositis in patients differs depending on the stage of the disease.

The invasive phase 

The first stage of the disease is the invasive phase, which occurs two weeks after the infection starts. The symptoms of the invasive phase are usually nonspecific and include a fever and general feelings of discomfort and unease. However, the skin over the muscle usually appears normal at this stage.⁶

Purulent phase 

If there is no intervention during the invasive phase of the disease, the infection may then progress to the purulent phase, which is characterised by more intense symptoms. These symptoms include high fever, headache, fatigue, and other general signs of systemic infection. Local symptoms at the muscle may also intensify, with worsened muscle pain, muscle tension, and the development of pus-containing (purulent)  abscesses.. The skin over the affected area may become red and warm due to inflammation.⁷

Late phase

Pyomyositis can lead to severe complications, including more extensive abscess formation, joint inflammation (arthritis), kidney failure, and sepsis. Sepsis is a life-threatening condition where the body has an intense response to infection, leading to widespread inflammation all over the body. If sepsis is not treated quickly, it can progress to septic shock, which is a condition where blood pressure drops dangerously low, causing organs to stop working. Some other symptoms during the late phase include extreme pain in the affected muscle and a high fever.⁶

However, these symptoms can overlap with symptoms of other musculoskeletal infections. Therefore, accurate imaging strategies and clinical evaluation skills are essential to confirm a diagnosis of pyomyositis. 

Imaging techniques 

1. ​Ultrasound imaging: a valuable tool used in diagnosing pyomyositis. Point-of-care ultrasound (POCUS) allows healthcare providers to quickly identify signs of infection, including fluid collection within muscles. However, while ultrasound is effective in detecting abscesses, it may not be able to identify early-stage infections⁸
2. Magnetic Resonance Imaging (MRI): MRI plays a crucial role in diagnosing pyomyositis. It is a type of imaging that provides highly detailed images, helping to identify the extent and precise location of the muscle infection. MRI is particularly effective in detecting soft tissue infections and collections of fluid, which indicate abscess formation. This can appear on images as high signal intensity of T2-weighted images and as hyperintense sections on T1-weighted images⁹
3. ​Computed Tomography (CT) scans: CT scans are incredibly useful in diagnosing pyomyositis because they are very good at identifying muscle enlargement and also the reduction in the ability of the muscle to absorb or block energy. These are both factors that are indicative of infection. Also, administering contrast agents during CT scans can highlight abscesses. Although CT scans are helpful, they are usually not preferred over MRI scans as MRI scans have greater sensitivity and are also safer due to the lack of radiation exposure¹⁰

Laboratory tests 

Lab tests, combined with checking the patient's symptoms and using imaging tests, are crucial in diagnosing pyomyositis. However, laboratory findings are often nonspecific, therefore, integrating these results with clinical assessments and imaging studies is essential for an accurate diagnosis and effective management.​

• Blood tests: Routine blood tests often show an elevated white blood cell count in patients with pyomyositis, which indicates an ongoing infection (leukocytosis). More blood test indications for pyomyositis include elevated C-reactive protein, which indicates inflammation. Despite muscle involvement, certain enzyme levels, such as creatine kinase, remain unchanged¹¹
• Blood cultures are an important part of diagnosing pyomyositis, as they help doctors identify the specific bacteria causing the infection. Once the specific strain of bacteria is identified, doctors can prescribe the right antibiotic to treat the infection. In pyomyositis, blood cultures show the presence of bacteria in about 5% to 30% of cases. The most common type of bacteria found in these cases is Staphylococcus aureus¹²
• Microbiological Analysis of Pus: When abscesses are present, drawing pus for Gram staining and culture is vital for diagnosis. This analysis provides an accurate identification of the bacteria and can further help doctors choose the most suitable antibiotics for treatment. Both anaerobic and aerobic cultures, along with antimicrobial sensitivity testing, should be performed to ensure effective treatment¹²
• Other: Testing for specific pathogens may be requested based on clinical evaluations and suspicions; however, this is uncommon

FAQs

What happens if pyomyositis is left untreated?

If not treated promptly and effectively, pyomyositis can lead to severe complications such as widespread abscesses, sepsis, joint inflammation, and organ failure.

Is pyomyositis common?

Pyomyositis is a rare condition, but it is more common in tropical regions. It can also occur in people with weakened immune systems, such as those with HIV or undergoing cancer treatment.

Is pyomyositis contagious? 

No, pyomyositis itself is not contagious. The bacteria which often cause pyomyositis, Staphylococcus aureus, can spread through wounds but is generally not passed from person to person unless there is direct contact with the infected area.

Can pyomyositis be prevented? 

There is no guaranteed way to prevent pyomyositis; however, maintaining a healthy immune system and promptly treating any injuries can help reduce the risk. People with compromised immune systems should also take extra care to avoid cuts and other infections that could lead to pyomyositis.

How do doctors know if pyomyositis is getting worse? 

Doctors can monitor symptoms such as swelling, fever, and the presence of new abscesses. Imaging tests like MRI or ultrasound are used to track the progression of the infection, and lab tests can help to identify whether or not the bacteria is resistant to treatment. 

Summary 

Staphylococcus aureus is the common cause of pyomyositis, a rare bacterial infection that causes pus-filled abscesses in muscles, usually in the thighs. Individuals with compromised immune systems and those residing in tropical regions are primarily affected. Fever, swelling, and muscle pain are early symptoms and serious side effects like organ failure and sepsis may result from the infection if it is left untreated. Clinical assessment, imaging methods such as MRI, CT, and ultrasound, as well as laboratory tests like blood cultures and pus analysis, are all part of the diagnostic process.