Overview

Pyomyositis (PY) is a primary infection of the skeletal muscles that causes muscle fibre inflammation, pus production, and in later stages, necrosis (death of cells). It has long been believed that pyomyositis is a tropical condition. However, pyomyositis is increasingly detected in temperate climates, and its epidemiology may differ from that of tropical regions. The management of pyomyositis at the suppurative stage should include antibiotics and definitive incision and drainage.

Introduction

In skeletal muscle, pyomyositis is an acute bacterial infection that typically progresses from an invasive stage of diffuse induration (thickening or hardening of the skin due to inflammation) to a suppurative stage with drainable pus. 

In the United States and Europe, pyomyositis is a very uncommon condition that is frequently reported in several tropical nations. Unless the projected progression is known, it may not be detected right away. Scriba, a Japanese surgeon, provided the first thorough explanation of pyomyositis in 1885.¹ Pyomyositis has been reported in children from tropical regions of the South Pacific and Africa.² The phrase "bacterial pyomyositis" describes a profound, subacute soft tissue bacterial infection.

Although it was first only identified in tropical regions, the entity is becoming more common in temperate regions like the US. The existence of immunocompromising conditions like HIV/AIDS or liver illness is most likely the cause behind the condition.

The aetiology (cause) of the infection is thought to be transient bacteremia (presence of bacteria in the bloodstream) rather than continuous spread from surrounding tissues because there is typically no visible exterior lesion or portal of entry.²

Prevalence

Patients of any age can develop pyomyositis, however the condition peaks between the ages of two and five, when malnutrition may be a contributing factor, and again between the ages of thirty-five and forty. According to accounts from temperate regions, pyomyositis typically affects teenagers and young adults.

Nearly 60% of pyomyositis patients in nontropical regions have an underlying medical condition.¹ Patients with HIV infection, whether or not they had acquired immunodeficiency syndrome (AIDS), have been documented to develop pyomyositis on multiple occasions; S. aureus was the primary cause of this condition in most of these patients.⁴

Causes

Nowadays, some of the hospital admissions in some tropical countries are due to this acute infection of the skeletal muscle, which is typically caused by S. aureus.

Although infections of the calf, arm, scapula, chest wall, and neck have been reported, the quadriceps and gluteal muscles are the most often affected areas for abscess formation.

Bacteraemia (presence of bacteria in blood) is implicated in the aetiology of pyomyositis when a recently damaged muscle is present. Miyaki developed this connection in 1904. He discovered that Staphylococcus aureus accounts for 77% of cases, however recent publications have also identified streptococci, Salmonella, Escherichia coli, tuberculosis, and polymicrobial isolates as causes of infection.²

Clinical features

Muscle infections are typically not caused by primary infections of the surrounding skin, soft tissues, or bones; instead, pus builds up within the muscles first. The clinical manifestations of pyomyositis primarily include:

• Fever
• Soreness 
• Oedema (swelling and fluid build up)
• Localised muscular discomfort and stiffness

Though sometimes a feeling of heaviness may be the only initial symptom, pain is the most significant and early symptom. The pain can worsen and becomes unbearable, following which, the patient becomes pale, sweats and looks quite sick. This is followed by shock, tachycardia (fast heartbeat), hypotension (low blood pressure), and renal (kidney) failure. The patient may be nervous or apathetic, and they may sleep less but remain intellectually clear.⁴

Stages

The clinical differential diagnosis is initially broad and includes muscle strain, contusion, cellulitis, haematoma, seroma, septic joint, osteomyelitis, sarcoma, and diabetic myonecrosis due to these nonspecific symptoms and the possibility that the affected muscle is deep beneath the skin's surface, hiding visible signs.² Three overlapping stages have been identified in the clinical progression of pyomyositis.

Stage 1

The first stage is characterised by:

• Low-grade temperature
• Muscle soreness
• Nebbulous complaints (vague, unclear, hazy)

There are no signs of an abscess, but the afflicted muscle is indurated and irritated. In the early stages, the patient may just complain of a dull ache, and erythema (skin redness) may be seen at the skin's surface.²

Low-grade fever and cramping muscle pain are the hallmarks of stage I. The muscle fascia (connective tissue) and aponeurosis serve to isolate the inflammatory muscle from the subcutaneous tissue and superficial structures. When palpated, the afflicted muscle, if superficial, has a distinctive "woody" texture. Due to their ambiguous presentation, patients typically do not present at this stage, and when they do, they are frequently misdiagnosed.

Variable fever, local swelling with or without erythema, minor discomfort, and a little tenderness are all signs of a subacute start.

The area has a wooden consistency or is indurated. This period is frequently disregarded. Attention is diverted from infectious aetiology because the first swelling is firm, the pain is not noticeable, and/or it includes deep muscles that are difficult to evaluate at the bedside. If aspiration is tried, no pus is produced.⁴

Treatment

Antibiotics alone are typically sufficient to treat stage I illness, which is characterised by soreness over the affected location and no systemic symptoms in the early or invasive stage. Since the illness is deeply rooted, erythema is noticeably missing. This stage could end on its own, like fibromyalgia.³

The illness is subacute for the first two weeks, and symptoms are frequently disregarded. Local symptoms include erythema, swelling, little discomfort, and very little tenderness.

Stage 2

The second stage is characterised by:

• Fever
• Swelling
• Growing discomfort
• Formation of muscle abscess

This develops into a fluctuant suppurative phase, where pus is produced. Typically, this phase starts 10–21 days after the first signs appear. The symptoms include fever, oedema (swelling), and increased muscle pain. Additionally, the region surrounding the afflicted muscle may feel hot and erythematous (red). At this stage of pyomyositis, the majority of patients visit their doctor since both local and systemic infection symptoms are noticeable. When the muscle is aspirated, an abscess develops and pus is produced.

Along with noticeable muscle soreness and oedema (specific to the affected muscle), the patient will have a fever. Erythema is typically nonexistent, and the skin that covers it is warm and undamaged pus oozes out from the affected site.

The third stage is marked by local findings of erythema, exquisite tenderness, and fluctuance, as well as systemic signs of sepsis.⁴

At this point, a diagnosis is frequently made. Septic syndrome and high temperature chills are more noticeable general symptoms. Tenderness, swelling, fluctuation, myalgia (muscle pain), and inflammatory skin are examples of local abnormalities.

Stage 3

Pyomyositis advances to the third stage, which includes septicaemia and maybe septic shock, if the patient is not treated. The bacteria found in the majority of reported cases is Staphylococcus aureus, and the most often impacted muscles are those surrounding the pelvic region.

The advance stages of pyomyositis, including the third stage, is characterised by:

• Violaceous skin changes (skin changing to purple)
• Bullae (blisters)
• Skin necrosis

The initial invasive stage of the condition is characterised by muscle inflammation and swelling, followed by localised swelling, tenderness, and erythema.

High fever, septicaemia (serious infection in the bloodstream), and severe muscle degeneration are the hallmarks of the late phase, which usually affects individuals who are left untreated.¹

The characteristics of this stage include septicaemia and a toxic appearance. Acute renal failure, pericarditis, brain abscess, septic shock, and septic arthritis are additional infrequently described side effects of stage III S. aureus bacteraemia.²

Since the fascia covering the muscle abscesses keeps them contained, there may not be much localised erythema or warmth, and the process's severity may not be seen until the infection spreads to the subcutaneous tissues a few days to weeks later. The absence of regional lymphadenitis is a characteristic. Though they usually only affect one muscle region, additional muscle abscesses can arise following contiguous spread or be present at presentation.⁴

Serious systemic symptoms include fever and sepsis. Erythema, exquisite sensitivity, and noticeable fluctuance are seen upon local examination. If untreated, complications include metastatic abscesses arthritis, septic shock and renal failure

High dose penicillin G and clindamycin should be used in stage 3.

Treatment

The stage of presentation would determine the course of treatment. Cross-sectional imaging, like CT or MRI, can be used to identify the earliest phases with greater suspicion and recognition. As a result, necrotising infections and their clinical manifestations may overlap.
Antibiotics and surgical drainage of the abscess is frequently required for the treatment of stages II and III of the illness. 

After the first closure, suction drains should be inserted to allow the abscess cavity to drain, and any necrotic muscle tissue should also be surgically removed.²

Relapsing or recurrent pyomyositis is common in patients with advanced HIV infection; if the patient has a persistent fever following surgical or percutaneous needle drainage of a muscle abscess while on the right antibiotic medication, it may indicate the presence of multiple undrained suppurative foci.

Conclusion

Although the exact cause of pyomyositis is unknown, it seems likely to be caused by a temporary bacteremia that is accompanied by another muscular predisposition. A full recovery usually follows treatment with the right antibiotics and any required surgery.