Pyomyositis
Synonyms and keywords: Tropical pyomyositis; Myositis tropicans; Bungpagga; Lambo lambo; Tropical skeletal muscle abscess; Myositis purulenta tropica; Bacterial myositis; Suppurative myositis; Epidemic abscess

# Overview
Pyomyositis, also known as tropical pyomyositis or myositis tropicans is an acute primary deep bacterial infection of the skeletal muscles that results in a single or multiple pus-filled abscesses. This condition is more common in tropical areas but may also occur in the temperate zones. The majority of pyomyositis is caused by the bacterium Staphylococcus aureus. In tropical regions, the infection often follows minor trauma, while in temperate zones the infection typically occurs in people with immune deficiencies. The abscess within the muscle is drained surgically and antibiotics are administered to fully clear the infection. The infection can affect any skeletal muscle, but most commonly infects the large muscle groups such as the quadriceps or gluteal muscles.

# Historical Perspective
- Traquair credited Virchow for the earliest mention of pyomyositis, however it was first described by Scriba in 1885 as a disease endemic to the tropics, hence the term tropical myositis.
- Levin et al. reported the first case from a temperate region in 1971.

# Classification
- Pyomyositis may be classified according to the International Classification of Diseases-10 (ICD-10) into:
- M60.0	Infectious myositis, Tropical pyomyositis (optionally, B95-B97 to indicate the infectious agent; B95.6 for Staphylococcus aureus)
- Based on prognosis, pyomyositis is classified into three stages:
- Invasive stage
- Purulent or suppurative stage
- Late or septic stage

# Pathophysiology

## Pathogenesis
- Pyomyositis is an acute primary deep bacterial infection of the skeletal muscle.
- Pyomyositis may be associated with IgM antibody deficiency or with defective phagocytosis.
- The exact pathogenesis of pyomyositis is not fully understood.
- Under normal circumstances, the skeletal muscle tissue is intrinsically resistant to bacterial infections. It is thought that there is sequestration of iron by myoglobin, which is an essential nutritional requirement of proliferating bacteria. This results in slower growth of bacteria, allowing cell-mediated and humoral defences to enter infected zones and thereby preventing establishment of infection.
- Pyomyositis results from hematogenous invasion of bacteria to damaged skeletal muscle (20% with subsequent abscess formation).
- It does not include:
- Intermuscular abscess
- Abscesses extending into muscles from adjoining tissues such as bone and subcutaneous tissues
- Abscesses secondary to previous septicemia

## Common locations
Commonly involved muscles include:
- Quadriceps
- Glutei
- Pectoralis major
- Serratus anterior
- Biceps
- Iliopsoas
- Gastrocnemius
- Abdominal muscles
- Spinal muscles

## Microscopic histopathological analysis
On microscopic histopathological analysis, the following are characteristic findings of pyomyositis in sequence:
- Edematous separation of muscle fibers
- Coagulation necrosis
- Patchy myocytolysis and inflammatory infiltrate
- Loss of striations and granular degeneration of myofibers
- Appearance of lymphocytes, plasma cells and eosinophils when myofibrils degenerate

# Causes
Pyomyositis is an acute bacterial infection of skeletal muscle.
Bacteria
Most common cause of pyomyositis includes:
- Staphylococcus aureus (90% cases in tropical and 75% cases in temperate regions)
Less common causes of pyomyositis include:
- Group A streptococci (1%-5% of cases)
- Group B,C,G streptococci
- Streptococcus pneumoniae
- Escherichia coli
- Pseudomonas mallei
- Pseudomonas pseudomallei
- Pneumococcus
- Neisseria
- Haemophilus
- Aeromonas
- Serratia
- Yersinia
- Klebsiella
Rare causes of pyomyositis include:
- Salmonella
- Citrobacter
- Fusobacterium
- Anaerobes
- Mycobacterium

# Differentiating Pyomyositis from Other Diseases
- Pyomyositis must be differentiated from other diseases that cause muscle pain, fever, and/or leukocytosis such as:
- Osteomyelitis
- Pyrexia of unknown origin
- Muscle contusion
- Cellulitis
- Muscle hematoma
- Thrombophlebitis
- Appendicitis
- Septic arthritis
- Deep venous thrombosis
- Muscle rupture or muscle strain
- Osteosarcoma
- Rhabdomyosarcoma
- Trichinosis
- Leptospirosis
- Polymyositis
- Cysticercosis cellulosae
- Trypanosomiasis or toxoplasmosis
- Spontaneous gangrenous myositis
- It does not include:
- Intermuscular abscess
- Abscesses extending into muscles from adjoining tissues such as bone and subcutaneous tissues
- Abscesses secondary to previous septicemia

# Epidemiology and Demographics

## Tropical countries
- Majority (1,000-4,000 per 100,000 of all hospital asmissions) of pyomyositis cases are reported in some tropical countries such as Asia, tropical Africa, Oceania (islands of tropical pacific ocean), and the Caribbean islands.
- Between 1980-1989, the incidence of pyomyositis was approximately 2,000 per 100,000 of surgical admissions in Ecuador. 

## Temperate countries
- In temperate countries, such as the United States, pyomyositis was a rare condition (accounting for 1 in 3000 pediatric admissions), but has become more common since the appearance of the USA300 strain of MRSA, especially in immunocompromised patients.

## Age
- In tropical countries, pyomyositis most commonly affects children and young adults but may occur in any age group.
- In temperate countries, the majority of pyomyositis cases occur in adults and elderly patients.

## Gender
- In tropical countries, males are more commonly affected than females.
- In temperate countries, both males and females are equally affected.

# Risk Factors
The common risk factors in the development of pyomyositis include:
- Blunt trauma (20%-50% have history of trauma to the affected muscles)
- Vigorous exercise
- Intravenous drug abuse
- Intramuscular injections
- Nutritional deficiencies
- Immunosupression
- HIV
- Immunosupressive drugs
- Organ transplantation
- Liver cirrhosis
- Renal insufficiency
- Corticosteroid therapy
- Diabetes mellitus
- Viral infections
- Parasitic infections

# Screening
According to the U.S. Preventive Service Task Force (USPSTF), there is insufficient evidence to recommend routine screening for pyomyositis.

# Natural History, Complications, and Prognosis

## Natural History
If left untreated, pyomyositis is a potentially life threatening condition (mortality rate of 0.5-2%) which may progress to develop bacteremia, septicemia, metastatic abscess, acute renal failure, septic shock, and death.

## Complications
Common complications of pyomyositis include:
Cardiac complications
- Pancarditis
- Staphylococcus aureus endocarditis
- Micro abscesses in myocardium
- Diffuse acute myocarditis
- Arrhythmias
- Valve infection
Other complications
- Metastatic abscesses (microabscesses in liver, spleen, kidney, lung, and other organs)
- Anterior uveitis
- Local wasting and deformity
- Interference with normal development of adjacent bones in children.
- Septicemia
- Uremia
- Confusion and coma

## Prognosis
- Based on prognosis, pyomyositis is classified into three stages:
- Invasive stage
- Purulent or suppurative stage
- Late or septic stage
- Depending on the clinical stage of pyomyositis at the time of diagnosis, the prognosis may vary. However, the prognosis is generally good when diagnosed and treated properly.

# Diagnosis
Early diagnosis of pyomyositis is critical for good prognosis but is often missed due to following:
- Unfamiliarity with the disease
- Atypical presentations
- A wide range of differential diagnosis
- Lack of early specific signs

## History
A detailed and thorough history from the patient is necessary. Specific areas of focus when obtaining a history from the patient include:
- Trauma
- Intravenous drug abuse
- Intramuscular injection
- Vigorous exercise
- Immunosupression
- HIV
- Diabetes mellitus
- Corticosteroid therapy
- Chemotherapy
- Organ transplantation
- Malnutrition
- Other infections
- Viral
- Parasitic

## Symptoms
The symptoms of pyomyositis are divided into three stages

### Atypical symptoms
The atypical symptoms of pyomyositis include:
- Toxic shock syndrome
- Pyrexia of unknown origin (when invasive stage is prolonged)
- Acute abdomen
- Spinal cord compression
- Compartment syndrome
- Cervicobrachial neuralgia (when localized to neck muscles)

## Physical Examination
Common physical examination findings of pyomyositis include:

### Images

## Laboratory Findings
Laboratory findings consistent with pyomyositis include:
- Needle aspiration (gold standard for diagnosis)
- Negative for pus in invasive stage
- Positive for pus in suppurative and septic stage
- Aspirated pus is sent for culture (aerobic and anaerobic culture)
- Biopsy (gold standard for diagnosis)
- Biopsy of the abscess wall and muscle taken at the time of abscess drainage should be cultured
- Blood culture and sensitivity helps to identify the following:
- Septicemic process in blood
- Antibiotics of choice
- Hematological tests:
- Complete blood count
- Leukocytosis
- Neutrophilia
- Eosinophilia
- Anemia
- Raised erythrocyte sedimentation rate
- Creatine phosphokinase and aldolase (markers of muscle damage are usually normal)
- Raised C-reactive protein
- Other laboratory tests to identify risk factors

## Imaging Findings
The following non invasive imaging studies can be used in the diagnosis of pyomyositis:

### Ultrasonography
- Ultrasonography is the initial imaging study of choice.
- On ultrasonography, pyomyositis is characterized by the following features:
- Muscle swelling
- Heterogenous hypoechoic areas
- Hypoechoic areas
- Hyperechoic areas

### CT scan
- On CT scan, pyomyositis is characterized by the following features:
- The affected muscle shows areas of low attenuation with loss of muscle planes
- Surrounding rim of contrast enhancement
- It is also useful in differentiating tumours, hematoma and thrombophlebitis from abscess.

### MRI
- On MRI, pyomyositis is characterized by the following features:
- The affected muscle may appear swollen, with loss of architectural definition
- Heterogenous areas of low intensity appear on T1-weighted images
- In the early stage, the only finding may be edema (area of high signal intensity on fluid-sensitive sequences)

### Plain radiography
- Plain radiography is used to rule out bony lesions.

## Other Diagnostic Studies

### X ray Chest
- All patients should have initial radiography of chest and also repeated after an interval to detect complications of pyomyositis(like lung and cardiac complications).

### Echocardiography
- Echocardiogram is used to diagnose cardiac complications of pyomyositis.

### ECG
- ECG is used to diagnose cardiac complications of pyomyositis.

# Treatment

## Resuscitation
The following conditions require immediate resuscitation:
- Anemia: Blood transfusion
- Malnutrition: Nutritional support and rehabilitation
- Shock

## Surgery
Surgery is the mainstay treatment for pyomyositis.
- Abscess drainage includes:
- Incision and drainage
- Percutaneous drainage (ultrasound or CT guided)

## Medical Therapy

### Analgesia
Appropriate analgesics are given when the patient presents with pain.

### Antimicrobial therapy
- Antimicrobial therapy is recommended for patients with pyomyocitis:
- Preferred regimen (1): Nafcillin
- Preferred regimen (2): Oxacillin 2 g IV q4h
- Preferred regimen (3): Cefazolin 2 g IV q8h (if MSSA)
- Alternate regimen: Vancomycin 1 g IV q12h (if MRSA)

## Prevention

### Primary prevention
There are no primary preventive measures available for pyomyositis.

### Secondary prevention
There are no secondary preventive measures available for pyomyositis.