Introduction

None of us relishes the thought of billions of bacteria living on our skin, in our nose, guts or other parts of our bodies. We tend not to be comfortable with the notion of being the dwelling place for a variety of species of microorganisms. The fact is that bacterial cells outnumber human cells, which usually coexist in peace and harmony. 

However, this peace and harmony can be disrupted under certain conditions. These conditions include infections, injury, trauma, or underlying health conditions, which can lead to a hematogenous spread of bacteria. Simply put, the spread of bacteria via the blood to other parts of the body. When bacteria reach the muscles, they can cause a condition called pyomyositis. Read on to understand how the hematogenous spread of bacteria to the muscles causes pyomyositis. 

Understanding pyomyositis

Pyomyositis is an insidious condition characterised by bacterial infection of the skeletal muscles. It manifests mainly as single and multiple intramuscular abscesses.¹ Pyomyositis occurs mainly in individuals with a suppressed or poor immune system. These include people living with HIV/AIDS, type 2 diabetes, leukaemia, renal dysfunction, and autoimmune conditions.² Pyomyositis, also known as tropical pyomyositis or myositis tropicans, is related to insults to the skeletal muscles of the upper limbs, trunk, and spine. These insults can be high-intensity physical exercise, trauma, drug use, intramuscular injection, and underlying viral or parasitic infection.³ 

As the other name of pyomyositis implies, the condition is endemic in tropical areas and has been detected in Asia and Africa. ¹ For instance, the incidence is 1 in 1000 in Uganda, and 13% of recorded deaths in emergency units of hospitals in Nigeria are due to pyomyositis.⁴ However, epidemiology has changed since then. This is because pyomyositis is now detected at an alarmingly increasing rate in temperate areas as well, especially in patients with coexisting conditions.⁵ It commonly occurs in young adults, and different case studies showed a male predominance.⁵ 

Although pyomyositis is usually seen as a disease that occurs in teenagers and young adults, it can occur in other age groups, such as infants and young children.⁶ It has a higher prevalence in boys, but girls who engage in intense athletic activities have a higher risk of developing pyomyositis.⁶

Stages of pyomyositis

The clinical course of this condition progresses in 3 stages:

• Stage 1, or the invasive stage: The initial stage of the infection, characterised by acute muscle pain, localised swelling, and inflammation. This stage is not usually diagnosed in most patients and occurs between 1 to 3 weeks before progressing into stage 2.⁶ The symptoms at this stage are negligible and often overlooked. Other symptoms include fever and anorexia¹
• Stage 2 or suppurative stage: Diagnosis mostly occurs during the suppurative stage of the disease.⁶ This stage involves the formation of a muscle abscess, and patients usually have chronic pain, more swelling, tenderness and pain at the site of infection, and high fever. The affected area exudes pus with subsequent inflammation⁶
• Stage 3: Characterised by systemic manifestations with severe sepsis, fever, and erythema; In this stage, other complications can occur, which include multifocal abscess formation, arthritis, septic shock, and renal abnormalities⁷

How bacteria pass through the bloodstream to cause pyomyositis 

The primary causative organism in 95% and 70% of pyomyositis cases in tropical and non-tropical areas, respectively, is Staphylococcus aureus.⁶ However, to establish an infection, some type of muscle injury must occur. This is because an uninjured skeletal muscle is naturally protected and resistant to infection. This is because the protein found in muscle tissues, called myoglobin, firmly binds to iron, limiting its availability to bacteria.⁶ Iron is an essential compound for the growth and proliferation of organisms. 

However, when muscle injury occurs, the sequestered iron is released from the myoglobin of traumatised or injured muscles.⁶ This presents an essential nutrient for the bacteria to grow, cause abscess formation, and spread rapidly through the blood to the muscles. Additionally, there is a formation of hematomas around the site of injury.⁶ Consequently, this provides a favourable site for Staphylococci and other bacteria to attach to the injury site. This further impedes the host's immune response. 

Vigorous exercises, which can cause skin abrasions and bicycle accidents, are potential sources of muscle trauma.⁶ This could explain the predilection of pyomyositis to occur in tropical areas. Typically, skin infections and muscle trauma occur at a higher rate in these regions in some children who play outside with little clothing.⁶ Other immune-suppressive conditions associated with pyomyositis include HIV infection, steroid use, diabetes mellitus, leukaemia, and lymphoma. Furthermore, in temperate regions, tropical pyomyositis has also been diagnosed in immunocompromised individuals.⁸ However, cases of tropical pyomyositis during the rainy seasons of the year were reported in India and other tropical countries.⁸

Bacteria that cause pyomyositis

Apart from S. aureus, the 2nd most common group of bacteria that can cause pyomyositis is Group A Streptococci.⁹ Other causative organisms include; 

• Streptococcus pyogenes⁹
• Escherichia coli
• Streptococcus dysgalactiae
• Proteus mirabilis
• Klebsiella oxytoca
• Klebsiella pneumoniae¹⁰
• Yersinia enterocolitica
• Salmonella species
• Aeromonas hydrophila
• Haemophilus influenzae 
• Citrobacter freundii 

Although most cases of pyomyositis are caused by S. aureus, some cases of pyomyositis result from polymicrobial infections.⁹ This is applicable in pyomyositis not caused by the commonly-associated pathogens: Staphylococci, Streptococci, and enteric Gram-negative organisms. Other organisms that can cause pyomyositis include Actinomyces spp, Mycobacteria (M avium complex, M. tuberculosis, and M. haemophilum), Nocardia spp, fungi, and viruses. ⁹ 

Predisposing factors associated with pyomyositis 

Pyomyositis is a condition that predominantly affects the muscles of the lower limbs (i.e., the thighs), although the muscles of the upper limbs, trunk, and spine can also be involved.¹¹ Usually, a single muscle is affected, but multiple muscle involvement occurs in 12-40% of cases. The muscles of the forearm, abdominal wall, pelvic muscles, and extraocular muscles can be affected.¹¹

However, the predisposing factors that can lead to the onset of pyomyositis include: 

• Muscle trauma or injury
• Viral and parasitic infections
• Some kind of immunodeficiency or immunosuppressive condition like HIV
• High-intensity exercise
• Malnutrition 
• Bacteremia 

Treatment and management of pyomyositis

Early diagnosis of this disease makes treatment more effective with antibiotic therapy, depending on the causative organism. Initial antibiotic therapy should include a broad-spectrum agent that covers major Gram-positive and Gram-negative organisms.¹¹ The coverage of gram-negative organisms in the treatment regimen is paramount in the treatment of immunocompromised individuals.¹¹ 

Therefore, the treatment and proper management of pyomyositis are usually related to the stage of disease at clinical presentation in the hospital. Early initiation of intravenous antibiotics can prevent the formation of abscesses and avoid surgical drainage.¹² In stage 1 or the invasive stage, before the formation of an abscess, diffuse inflammation can be treated solely with antibiotics.¹² Considering the most culpable bacteria, cloxacillin is a common choice. Also, a combination of cloxacillin and aminoglycoside achieves a synergic effect for immunocompromised patients.¹¹ The duration of therapy can vary from 2 to 6 weeks, depending on clinical severity and the patient’s response to treatment. Patients with an underlying disease that suppresses their immunity may require a longer period of antibiotic treatment, either orally or intravenously.¹¹

Can pyomyositis be prevented? 

There is no specific way to prevent pyomyositis. This is because the exact cause can be difficult to identify, particularly in people with a combination of risk factors. However, there are some strategies to prevent bacterial infections that can trigger pyomyositis.¹² These include: 

• Prompt treatment of injuries to prevent infection with bacteria
• Management of underlying health conditions such as HIV and diabetes, and strict compliance with prescriptions from your doctor
• General hygiene practices and avoid sharing needles with people 
• If you have a history of staph infections, consider testing your nasal passages to eliminate the bacteria that might be residing there
• Individuals at risk should be monitored, especially if they have underlying health conditions 

Summary

• Pyomyositis is an acute intramuscular infection that occurs when bacteria spread to the skeletal muscles
• The hallmark sign of pyomyositis is the formation of abscesses in the affected muscles 
• People with a weak or suppressed immunity, chronic conditions, muscle injury, and those who engage in vigorous exercise are at increased risk of developing pyomyositis
• The hematogenous spread of bacteria to cause this condition occurs after a muscle injury or trauma
• The clinical presentation of pyomyositis occurs in 3 stages: stage 1, stage 2, and stage 3, with associated symptoms
• The symptoms depend on the muscle affected, but they generally include muscle pain and tenderness, fever, malaise, redness and swelling, and mobility issues
• Early diagnosis is essential for prompt treatment, which includes the use of antibiotics and, in extreme cases, surgical incision and drainage
• Prompt treatment of wounds and muscle injury, with the incorporation of good hygiene practice, is one technique to prevent this potentially life-threatening condition

FAQs

How is pyomyositis diagnosed?

This condition is usually diagnosed through clinical evaluation, carrying out some blood tests, and imaging studies such as MRI or ultrasound. 

What are the long-term complications of pyomyositis?

Untreated or delayed diagnosis of pyomyositis can lead to serious complications such as septic shock, arthritis, osteomyelitis, metastatic abscesses, and even death.  

Is pyomyositis the same as myositis? 

No, these are different conditions. Myositis is an infection of the skeletal muscle, which can be acute, subacute, or chronic, depending on the causative organism and severity. On the other hand, pyomyositis is a chronic condition characterised by the hematogenous spread of bacteria via the blood to the muscles and has a hallmark symptom of muscle abscess.