Introduction

Pyomyositis is a bacterial infection of the skeletal muscles of the body that is commonly accompanied by the formation of pus or abscess.¹ Although it was first described by Scriba in 1885, the condition had been in existence before then, with Virchow being credited for its initial mention.² It was initially thought pyomyositis was highly prevalent in the tropics, hence its initial name, tropical pyomyositis. However, more recent studies have shown that it also occurs in temperate regions, particularly among individuals with immunosuppression, diabetes, and malignancy.³ 

Pyomyositis has a variety of names, including tropical myositis, tropical skeletal muscle abscess, myositis tropicans, myositis, temperate myositis, infectious myositis, and spontaneous bacterial myositis. It mostly affects large, bulky muscles such as the gluteus muscles, gastrocnemius, pectoral, and biceps muscles, with a predilection for the lower limbs and pelvis.¹ Other smaller muscles have also been reported to be affected, such as the piriformis muscle.⁴ 

Pyomyositis is a rare condition which, in early stages, can present similarly to other diagnoses, making it an illness that can be easily missed. 

When detected and treated early, it completely resolves. However, life-threatening and devastating consequences occur when there is a delay in its treatment. 

As such, a high index of suspicion, early detection and timely treatment often improve the prognosis of the condition/ outcome from its occurrence.

Etiology

Pyomyositis is caused by bacterial microorganisms. The most common cause is Staphylococcus aureus, accounting for 90% and 75% of cases in the tropics and temperate regions, respectively.²  Streptococcus organisms account for the second most common organisms responsible. 

Other microorganisms responsible include gram-negative bacteria, such as Neisseria and Pseudomonas organisms, anaerobes, and mycobacteria, such as Mycobacterium tuberculosis.^1,2 

The exact pathogenesis of this condition is not known. This is likely due to the inherent resistance of skeletal muscle to infection, attributed to its rich vascular supply. ^1,5 

The possible risk factors that may predispose the muscle to this infection include: traumatic muscle injury, malnutrition, immunosuppression, low level of immunoglobulins, diabetes, intravenous drug use, and tropical climates.^1,6,7,8 Other risk factors includes strenuous exercises that may lead to muscle injury, gynaecological surgery including vaginal delivery, forceps delivery, and unsafe abortions. ^1,4 

Clinical features 

Pyomyositis occurs worldwide, and affects males slightly more often than females, with a male-to-female ratio of 1.5:1. It can affect individuals of all age groups and primarily involves the skeletal muscles. 

This condition may be unifocal, affecting only a single site, or multifocal, affecting multiple skeletal muscles. This occurs in 12-40% cases. ^1,8,9

The disease progresses through the following stages:

Invasive stage (Early)^1,7

This is the early stage occurring between one to two weeks of the onset of the disease. It is characterised by pain, cramps and swelling at the site of a muscle. This is accompanied by worsening pain and fever. 

Redness may not be pronounced as the infection is deep-seated. Occasionally, the skin over the area may be shiny. 

When touched, the affected area is painful and has a ‘woody hard’ consistency. No pus is formed at this stage; as such, when a needle is put in the muscle, there will be no aspirate found. 

This stage has the best prognosis when treated early, as antibiotic treatment along with supportive care is sufficient in its management. The disease can be resolved with no sequelae.

Suppurative stage (Mid)

This occurs during the 2nd and 3rd week of the onset of the illness. It is characterised by the presence of pus. The temperature of the patient is high, suggesting the presence of fever.

The patient complains of excruciating pain at the affected site and has a general feeling of being unwell. When a needle is inserted into the swelling for aspiration, pus is present.

The prognosis of this stage is also good when the diagnosis is made. Incision and drainage is the treatment option for the pus present, along with antibiotic treatment and supportive care.

Late stage (Septic/Disseminated)

 This occurs after 3 weeks, when the diagnosis is not detected and the disease progresses without treatment. This is seen as the spread of infection into the bloodstream, causing multiple collections of pus in various sites, sepsis, septic shock, multiple organ failure and sometimes death.

The prognosis of this stage is poor due to prolonged treatment and subsequent recovery. 

Diagnosis 

Pyomyositis can be diagnosed by obtaining the following 

• History 
• Physical examination 
• Blood tests - Full blood count, serum electrolytes to assess kidney function
• Pus aspirate microscopy, culture and sensitivity
• Blood culture 
• Ultrasound scan of the muscle
• Computed tomography (CT) scan 
• Magnetic Resonance Imaging (MRI) scan 
• Gadolinium contrast scan
• Diffusion-weighted imaging scan 
• Muscle biopsy and histological examination- This investigation shows the classical diagnostic feature of pyomyositis, which is ‘ the presence of myositis without an abscess’

Pus aspirates and blood cultures are often negative in cases of pyomyositis in both tropical and temperate regions. However, detection rates tend to be higher in temperate regions, likely due to more advanced laboratory processing techniques.

Differential diagnosis 

Pyomyositis can mimic several other conditions and may be misdiagnosed as:

• Deep vein thrombosis
• Muscle contusion
• Muscle hematoma
• Cellulitis
• Muscle or tendon rupture
• Septic arthritis of the hip
• Osteomyelitis
• Osteosarcoma
• Polymyositis
• Spigelian hernia
• Prolapsed intervertebral disc

An iliopsoas abscess may present similarly to acute appendicitis. In some cases, pyomyositis of the piriformis muscle has been misdiagnosed as acute pyelonephritis due to symptoms such as bilateral flank pain and fever.¹⁰ 

Timely treatment and its benefits

The treatment of pyomyositis depends on the stage at which the diagnosis is made. In the early (invasive) stage, antibiotic therapy alone may be sufficient to resolve the infection. However, in the suppurative stage, when abscess formation has occurred, surgical or image-guided drainage in combination with antibiotics is typically required. 

Antibiotics use 

• In the early stage, the use of antibiotics is the main treatment option. Initially, empirical antibiotics are used.  Cloxacillin is the first-line treatment. Once culture and sensitivity results are available, therapy is adjusted accordingly
• In patients with penicillin allergy, cefazolin, a first-generation cephalosporin, is a suitable alternative. Antibiotics are generally administered intravenously and continued for an extended duration, often until clinical signs of infection have resolved¹¹
• In cases caused by the methicillin-resistant Staphylococcus aureus, vancomycin is the preferred agent
• Despite early treatment, some cases may still progress to abscess formation, requiring drainage in addition to antibiotics

Surgical drainage 

This is the treatment of choice when there is a collection of pus. Drainage options include image-guided drainage using either ultrasound or CT scan image guidance. 

Open incision and drainage may be performed when image-guided percutaneous drainage is not available or feasible. 

Supportive care 

This includes the use of pain medications, bed rest, adequate hydration with intravenous fluid, and dialysis for patients with impaired kidney function.

Treatment of comorbidities or concurrent illnesses and underlying risk factors for the disease, such as immunosuppression, diabetes, and intravenous drug use, when instituted, also improves patient outcomes. 

Recovery timeline 

Most patients recover within weeks with early treatment. Those who present later in the disease course may require several months for full recovery. In the majority of cases, treatment results in complete resolution of the infection. 

However, a subset of patients may experience relapse or recurrence. A treatment relapse is defined as a repeat episode occurring within six months of the initial infection, while recurrent disease refers to a new episode occurring more than six months after the original illness.¹¹

Recovery outcomes

Timely treatment 

The prognosis of pyomyositis is good with timely treatment. Although only a few percentage of patients are diagnosed in the early stage, once treatment is initiated promptly, long-term muscle damage is avoided.

Delayed treatment 

Delayed treatment increases the morbidity of the illness, leading to the onset of complications, muscle damage, systemic infection, and eventually prolonging the duration of hospital stay. Mortality from pyomyositis, especially in this group, is 0.5-2%. It ranges from <0.5% in the early stage of disease to 15% of patients who present late for care.^5,12

Chronic or recurrent pyomyositis 

This is uncommon, occurring in < 30% of cases.  It is common in immunosuppressed individuals. Proposed risk factors for recurrence are the presence of infection, which triggers an overactive immune response.¹²

Factors affecting prognosis

1. Presence of other illnesses

The presence of other illnesses, such as diabetes, acquired immunodeficiency syndrome (AIDS), or other immune conditions, especially when poorly controlled, can slow the onset or progression of the disease, alter the course of the disease and delay recovery.  

2. Stage of disease 
3. The prognosis is better if diagnosis and treatment are commenced at an early stage. Treatment adherence

Adhering to treatment protocol, including taking the full course of antibiotics and treating comorbidities, also improves outcomes. The prognosis is poorer for those who adhere poorly to treatment. 

Long-term outlook

Most patients recover fully with little or no muscle loss after treatment. 

Muscle strength can be improved by physical therapy to restore strength in patients with severe infections.  

Summary 

• Pyomyositis is a bacterial infection of the skeletal muscle accompanied by the formation of pus. It occurs worldwide, affecting all age groups
• The clinical features mimic those of other conditions
• Prognosis is excellent with timely early diagnosis and treatment. With delayed intervention, the prognosis is poor and, in a few cases, may lead to death
• Awareness of symptoms, early diagnosis and early intervention are key to preventing complications and promoting full recovery