Introduction

Pyomyositis is a rare but potentially fatal skeletal muscle infection that results in the formation of a muscle abscess. This condition mainly affects children living in areas with tropical climates, such as Asia and tropical Africa. 

In this article, we will explain what pyomyositis is, the causes and risk factors, and the available treatment options. 

What is pyomyositis?

Pyomyositis is a skeletal muscle infection often caused by the bacterium Staphylococcus aureus (S. aureus). It usually results in the formation of an abscess (inflammation and swelling with pus) in the muscles around the pelvis and lower limbs. The thigh muscles, particularly the quad muscles are most affected (Figure 1).^1,2,3 If diagnosed early, most cases can be treated without permanent damage to the affected muscles. However, if left untreated pyomyositis infections can lead to severe health issues and is potentially fatal, requiring hospital admission in approximately 1-4% of patients.¹

Figure 1: A magnetic resonance imaging (MRI) scan image of the quadriceps femoris thigh muscle in a 1-year-old child with pyomyositis. The arrow points to the muscle abscess (inflammation), which appears in bright white. (Image Source)

Incidence and prevalence 

Pyomyositis is a rare condition that primarily affects children living in regions of tropical climates characterised by high temperature and humidity. Regions with high incidences of pyomyositis include:¹

• Asia
• Tropical Africa
• Oceania
• The Caribbean Islands

This explains why pyomyositis is widely referred to as “tropical pyomyositis’’ (TP), “myositis tropicals’’, “tropical skeletal muscle abscess’’ and “tropical myositis’’.² 

People assigned male at birth (AMAB) with a median age of 10 years old in tropical areas are reported to have a higher susceptibility to pyomyositis than people assigned female at birth (AFAB).²

Despite its rarity especially in the United Kingdom (UK), Australia, and the United States of America (USA), the incidence of pyomyositis in Western countries is gradually increasing due to an increase in bacterial strains called “community-acquired methicillin-resistant S. aureus’’ (CA-MRSA). CA-MRSA refers to a specific strain of S. aureus that is resistant to an antibiotic called methicillin.¹ 

CA-MRSA secretes a harmful toxin called Panton-Valentine leukocidin (PVL) which has been shown to lead to more severe complications of pyomyositis in children with CA-MRSA compared to those who are methicillin-sensitive.¹

Causes 

S. aureus is the main bacterium responsible for pyomyositis in children, contributing to over 90% and 75% of pyomyositis cases in tropical and temperate regions worldwide, respectively.

Other less common bacterial pathogens that may cause pyomyositis include:² 

• Group A streptococcus (mainly in adults, however, and accounts for only 1-5% of pyomyositis cases)
• Group B, C, and G streptococcus
• Pneumococcus
• Haemophilus spp.
• Gram-negative bacilli

In rare cases, the following infectious agents may also lead to pyomyositis:¹ 

• Viruses e.g., influenza virus and coxsackievirus B. Particularly in school-aged children
• Fungi e.g., Candida albicans
• Parasites e.g., Trichinella spiralis. Particularly in immunocompromised patients 

Potential risk factors 

To date, the exact pathogenesis of pyomyositis remains unclear. However, certain risk factors are reported to increase your susceptibility to S. aureus and pyomyositis infection. These risk factors include:¹

Muscle trauma

The role of muscle trauma/injury in the development of pyomyositis was first theorised in the early 1900s. Scientists found that rabbits injected with S. aureus only developed abscesses in skeletal muscles that were exposed to trauma.  

These findings were further confirmed in later studies which showed a higher incidence of pyomyositis in patients, including children and soldiers who experienced muscle trauma due to strenuous physical activity. As the severity of physical activity, particularly vigorous aerobic exercise increased, so did the incidence of pyomyositis. Bicycle accidents appear to be another major source of muscle trauma that contributes to pyomyositis in children. 

Muscle trauma may also potentially explain why pyomyositis is more common in tropical areas, particularly during the July to October seasons. This is because, due to high temperatures, children are likely to play or work outside with lightweight clothes, thus making their muscles more prone to injury and S. aureus bacterial infections. 

For these reasons, muscle injury as a risk factor for pyomyositis in children appears plausible. However, this begs the question: how does S. aureus enter injured skeletal muscle and form an abscess? 

S. aureus requires high temperatures to grow and thrive as well as certain elements such as iron. Therefore, scientists speculate that upon muscle trauma, skin lesions develop in certain cases and the iron that is usually bound to myoglobin in healthy muscles becomes released. This allows S. aureus to move from the injured muscle into the blood (bacteraemia), which leads to subsequent abscess formation.  

Immune dysfunction

Some research studies suggest an association between S. aureus bacteraemia and the following states of immune dysfunction:¹

• Malnutrition
• HIV infection 
• Diabetes 
• Steroid use 
• Leukaemia
• Lymphoma 

However, this theory has received a lot of controversy and debate as children in tropical regions have been demonstrated to develop pyomyositis irrespective of their immune health. In contrast, children in temperate regions mainly develop pyomyositis if they are immunocompromised or have a severe health condition. 

Clinical presentation

Signs and symptoms

The signs and symptoms of pyomyositis vary depending on the stage the disease has progressed to. Pyomyositis has three main stages.

Stage 1 (invasive stage) lasts for approximately 10 to 21 days and may cause symptoms, such as:

• Muscle pain and/or muscle cramps
• Low-grade fever 

Stage 2 (suppurative (production and accumulation of pus) stage) lasts for approximately 1-3 weeks. During this stage, the muscle abscess forms and is often unifocal (confined to a single location). Over 90% of patients, including children, are diagnosed with stage 2 pyomyositis.¹ 

In addition to the muscle abscess, other signs and symptoms at stage 2 include:

• Severe muscle pain
• Fever and chills
• Mobility difficulties e.g., difficulties walking if the leg is the main affected site 

Stage 3 is the final and most severe stage of pyomyositis where the S. aureus bacterium seeded is thought to move into the bloodstream (bacteraemia). This results in patients usually having more than one muscle abscess at this stage (multifocal).¹

Potential complications

If left untreated, stage 3 pyomyositis can lead to severe and life-threatening symptoms and complications including:

• High fever
• Septicaemia: this occurs when the S. aureus bacterium multiplies in numbers and spreads from the blood throughout the body 
• Septic shock: this occurs when the body goes into ‘shock mode’ and responds by dropping blood pressure levels dramatically (hypotension) after the immune system ‘overreacts’ to the bacterial infection (sepsis)
• Organ damage or failure: if left untreated, septic shock may result in brain damage and/or lung, heart, or kidney failure, and eventually, death

Diagnosis

A healthcare provider will diagnose a suspected patient with pyomyositis by conducting a series of tests in the following order.^1,3

1. Physical examination

The healthcare provider will look for signs of muscle swelling and symptoms, such as fever, muscle pain etc.

2. Blood tests

Since muscle infection and inflammation are major signs and symptoms of pyomyositis, the healthcare provider will expect a suspected patient’s blood test to show:¹

• Low full blood count (FBC): indicating anaemia
• Leukocytes with “left shift’’: indicating a high number of white blood cells (WBCs), specifically neutrophils in the blood 
• Elevated erythrocyte (red blood cells) sedimentation rate (ESR) and acute-phase reactants (APRs) (plasma proteins)  

3. Imaging studies

The healthcare provider will usually order a magnetic resonance imaging (MRI) test to take detailed images of the muscle abscess at the affected site

Management and treatment 

Current management and treatment options for pyomyositis include:¹

Antibiotic therapy

One antibiotic or more commonly, a combination of anti-S. aureus antibiotics are used to treat pyomyositis. Examples include:

• Vancomycin, teicoplanin, linezolid, cloxacillin and/or clindamycin (mainly prescribed for patients with CA-MRSA S. aureus isolates)

• Nafcillin or oxacillin (mainly for patients with CA-MSSA S. aureus isolates)

Antibiotics are usually discontinued once ESR and C-reactive protein (CRP) levels have returned to normal. 

Surgical drainage

Although antibiotics alone can be used to treat pyomyositis in the early stages, approximately 50% of cases require subsequent surgical drainage of the pus in the abscess.

Prevention strategies

Pyomyositis may be prevented in children by:^1,3

• Wearing clothes that, at a bare minimum offer thigh-coverage, especially in tropical areas during the July to October seasons

• Maintaining personal hygiene and immune health 

When to see a doctor?

See a doctor immediately if your child has:

• Unexplained fever
• Ongoing muscle pain 

This is especially important if your child is included in the high-risk group category of pyomyositis discussed above. 

Summary 

Pyomyositis is a rare but potentially fatal infection that leads to the formation of an abscess mainly in the quad muscles of children AMAB in tropical areas of Africa and Asia. It is therefore widely referred to as ‘’tropical pyomyositis’’ or ‘’tropical myositis’’. 

Over 90% of tropical pyomyositis cases are caused by the bacterium S. aureus. The exact pathogenesis of pyomyositis in children remains a mystery to date. However, many scientists believe that S. aureus moves inside skeletal muscles and forms a muscle abscess in children in the following order: 

1. Muscle injury due to vigorous aerobic exercise or accidents
2. Symptoms of low-grade fever and muscle pain
3. Development of an S. aureus skin lesion
4. Bacteraemia 
5. Unifocal abscess formation

Pyomyositis is currently treatable at these early stages using antibiotics and/or surgical drainage of the abscess. 

However, if left untreated, bacteraemia can lead to the following complications:

• Severe muscle pain and mobility problems 
• Septicaemia 
• Multifocal abscess formation
• Sepsis 
• Septic shock 
• Organ damage or failure e.g., brain damage or kidney failure
• Death 

Currently, pyomyositis in children is mainly diagnosed when a blood test shows low FBC, high ESR, APRs, and WBCs, and is confirmed when an MRI scan detects a muscle abscess. 

To prevent your child from developing pyomyositis, it is important to engage in good hygiene practices and seek immediate medical attention if the following symptoms arise:

• Fever
• Ongoing muscle pain