What is pyomyositis?

Pyomyositis is an infection of the skeletal muscles caused by bacterial pathogens. This infection results in the formation of an abscess in the localised area.¹ Staphylococcus aureus is the bacterium that causes most cases, accounting for 90% of cases in the tropics and 75% in temperate areas.² While the exact pathogenesis is still not fully understood, this infection is thought to occur by the spread of pathogens via the bloodstream (hematogenous seeding).³ Without proper diagnosis and treatment, Pyomyositis complications can include sepsis.⁵ Previously known as or more commonly referred to as Myositis tropicans; it got this name due to its prevalence in tropical climates.² However, due to increased prevalence in temperate climates, it is also known as non-tropical pyomyositis, ‘infectious pyomyositis’ or ‘primary pyomyositis’.³ The disease can affect individuals of all ages, but is especially prevalent in immunocompromised patients.⁴

The pathogenesis of pyomyositis 

The Pyomyositis pathogenesis is still not understood to this day,¹² however, there are currently several associated risk factors.

• Blunt trauma¹⁴
• Muscular injuries 
• Rigorous exercise¹³
• Immunocompromisation
• Malnutrition³

Pyomyositis usually affects a single muscle site, most notably the thigh, but multiple muscle groups can be affected simultaneously.¹⁵ Other muscle groups that are typically affected include the pectorals, biceps, and glutei.¹⁶

The three stages in which pyomyositis can be clinically categorised are the 

1. Invasive Stage¹²
2. The Suppurative Stage¹²
3. The Late Stage¹²

The invasive stage 

This stage involves localised swelling and sometimes a fever. This stage can vary anywhere from 1 to 3 weeks. As the initial infection is still quite deep-seated at this stage, it can be difficult to diagnose and often overlooked,¹² thus leading to disease progression. There is often no redness or skin colour change (erythema) of the skin, either at this stage.¹²

The suppurative stage

Most pyomyositis diagnoses are made at this second stage, due to high spiking fevers and paired with extreme pain or tenderness in the affected areas. At this stage, although erythema of the affected area may not occur, a diagnosis can be made based on the presence of an abscess using imaging studies such as ultrasound, MR imaging, and CT scans.¹² This is the stage at which most treatment plans begin. 

The late stage

The late stage of Pyomyositis usually occurs when pyomyositis is left untreated, leading to multiple abscesses and even more fatal complications such as septic shock and, in extreme cases, multiorgan failure system.¹²

Reviews of multiple studies have shown that pyomyositis is usually misdiagnosed as another condition due to the similar clinical presentations it shares with many other conditions, eg, appendicitis, deep vein thrombosis, and osteomyelitis, just to name a few^17,12 A result of these misdiagnoses led to pyomyositis complications, which could have been avoided.¹⁸

Pyomyositis in tropical regions

We know that in its early years, due to a higher prevalence of cases in tropical climates, the name ‘Tropical pyomyositis’ ⁶ was coined. At one point, pyomyositis cases accounted for up to 4% of hospitalisations across the African continent. It was observed that most early tropical cases presented in healthy patients following physical exertion.¹³ While pyomyositis affects all age groups, it is known to have a higher prevalence in young children.²  A link has been found between the prevalence of pyomyositis cases and immunocompromised individuals, such as cases in Brazil, found patients with toxocariasis later developed pyomyositis.²⁰ A similar trend has also been observed in Uganda with HIV infected patients and pyomyositis.²¹ 

Pyomyositis in temperate regions

The united states

In temperate regions, pyomyositis was once considered an extremely rare disease,³ with only 98 patients being diagnosed with the disease between 1971 and 1992 in the entirety of North America.² A lot has changed in recent decades, with a staggering 246 cases from 1981 to 2002 in the United States alone.⁷ That number continues to rise with an even more recent study finding an increase in pyomyositis hospitalisations between 2002 to 2014^9, with S. aureus as the cause for half of them in a US-based study across three  institutions.⁸

Increasing pyomyositis cases in temperate locations such as the US have been linked to the rise of immunocompromised patients, such as HIV- Positive patients⁷ and patients with liver disease.¹⁰ Another contributing factor to the higher number of pyomyositis cases in these climates is due to better healthcare access for these patients, leading to more immunocompromised patients infected with pathogens difficult to culture and thus identify and treat (atypical infections).¹¹

Treatment and outcomes

Diagnosis

Aspiration & muscle biopsies

To diagnose pyomyositis, withdrawal of pus (pus aspiration) from the affected area is a commonly used method for cultures; however, in the early stages of the disease, pus is not always present, thus presenting a challenge for early diagnosis. Another issue with this diagnostic method is the fact that in up to 30% of cases, no organism may be isolated.¹⁹ However, muscle biopsies are a much more reliable diagnostic method.¹⁹ 

Imaging

While CT, ultrasounds, and MR imaging can all be used, MR imaging is the most specific as it can image the swelling and fluid-filled abscess.¹²

Treatment

In the initial Invasive stage, antibiotics alone will suffice, and the patient monitored once the diagnosis has been confirmed. Once pyomyositis has progressed to the Suppurative stage, any abscess formed should be drained with ultrasound guidance.¹² 

Summary

Pyomyositis is a disease that can manifest in both tropical and temperate climates. Its exact pathogenesis is still not fully understood, and more research needs to be done on it. As pyomyositis presents with a myriad of symptoms common to many other conditions, it can be quite difficult to diagnose. However, once caught early and treated with adequate antibiotics, most patients can expect to make a full recovery. If left undiagnosed and untreated, the consequences can be fatal.