Introduction

Pyomyositis is a bacterial infection that leads to inflammation and swelling of the muscles that surround the bones. It usually affects the thigh muscles and presents with non-specific symptoms, such as fever, pain, swelling in the affected area, and cramps.

Once a tropical condition, pyomyositis is increasingly diagnosed in temperate climate regions and primarily affects immunocompromised individuals.¹

Early stages of pyomyositis can be treated with antibiotics, while the advanced stage disease can cause septic shock and organ failure, making it a life-threatening condition if improperly treated or misdiagnosed. Considering pyomyositis is a rare disease with generalised symptoms, it is commonly mistaken for other conditions with similar presentations, such as cellulitis, deep vein thrombosis (DVT), and necrotising fasciitis.^2,3,4 In cases where a patient presents with symptoms which can be attributed to several conditions, healthcare professionals can rule out possible diagnoses by taking a closer look at the clinical presentation and diagnostic results, until they find the right cause of the illness, a process called differential diagnosis.

This article explores the similarities and differences between the four conditions, with a focus on their clinical presentations and factors that can differentiate pyomyositis and achieve accurate and early diagnosis.

Pyomyositis

Pyomyositis is a bacterial infection of the muscle that surrounds our skeleton, called the skeletal muscle. This infection is usually caused by Staphylococcus aureus (including MRSA), a bacterium that is commonly found inside the nasal cavity and on our skin.¹ Such bacteria are often a part of our normal flora and do not pose a threat to our health because, our immune system and organs (in this case, our muscles) keep them in check.¹

The way the bacteria responsible for pyomyositis find its way to the skeletal muscle is usually through minor injury or muscle trauma, that compromises those protective mechanisms. Individuals with suppressed immune systems, such as people affected by cancer, HIV/AIDS, or people undergoing steroid treatments, may also be at risk of low protection against such bacteria, as their immune system is weakened and therefore unable to protect and defend the body against bacterial threats. 

Additionally, people with diabetes mellitus may be more prone to uncontrollable infections that can lead to pyomyositis, due to the condition’s ability to compromise skin integrity, immune defense, and blood circulation.⁵

Why pyomyositis is no longer just a tropical disease

Pyomyositis is considered a tropical disease, as it is traditionally found in humid parts of the world, where puncture wounds from insect bites are more common, and skin infections are more difficult to heal. However, an increasing number of diagnoses can be seen in temperate regions due to growing risk factors (immune system conditions, immunosuppressive therapy) and better diagnostics.¹

Pathophysiology and common stages of pyomyositis

Pyomyositis symptoms can be split into three stages, depending on how the infection has progressed:⁶

• Invasive stage: it is characterised by low-grade fever, mild swelling, or pain in the affected muscle. Symptoms at this stage may be disregarded as non-specific or associated with common symptoms of mild skin inflammation following injury. Antibiotics are usually effective in stopping the spread of the infection at this stage, and are typically received orally
• Suppurative stage (10-21 days later): it is characterised by fever, chills, and topical symptoms like pain, tenderness, feeling a firm lump under the skin, and mobility issues. At this stage, the majority of cases receive a diagnosis, as the pyomyositis-characteristic abscess has most likely formed within the muscle. Oral antibiotics are no longer a treatment option, but surgical drainage/aspiration paired with IV antibiotics can be an effective approach to control the spread and eliminate the infection
• Septicemic stage (after 3 weeks if untreated): it is characterised by extreme pain, very high fever, chances of bacteraemia, septic shock, and organ failure. Individuals with late-stage pyomyositis require hospitalisation, possibly in intensive care, where broad-spectrum IV antibiotics that tackle subsequent manifestations and abscess drainage are required

Diagnostic tools

Diagnosis of pyomyositis incorporates blood work, physical exams, and imaging techniques, depending on the progression and severity of the condition. Blood tests checking for infection markers, such as elevated white blood cell (WBC) count (immune system cells fighting infections) and c-reactive protein (CRP) levels (a marker for inflammation), can point towards an active infection in the body. The most effective method to diagnose pyomyositis is through magnetic resonance imaging (MRI), as muscle inflammation and abscess presentation can be easily identified.⁷

Conditions that clinically mimic pyomyositis

Cellulitis

Cellulitis is also an infection caused by a bacteria, usually Streptococcus pyogenes (S. pyogenes)  as well as Staphylococcus aureus (S. aureus). Cellulitis results in a superficial infection of the skin, specifically of the subcutaneous (under the skin) tissue. This type of infection usually occurs in the lower leg area and is caused by tear in the skin that allows the bacteria to enter the body. Cellulitis is commonly seen in patients who have undergone surgery or are in hospital care, while lymphoedema (a chronic condition causing tissue swelling and restricted infection site-immunity) and other risk factors similar to pyomyositis can predispose individuals to cellulitis.⁸

Similarities with pyomyositis

Pyomyositis and cellulitis have quite similar clinical presentations, which often leads to misdiagnosis of pyomyositis, since cellulitis occurs more frequently. Both conditions present with fever, pain, and swelling on the lower extremity with warm and red-looking skin (erythema). Diagnostic blood tests may show elevated markers of inflammation in both conditions, as they are caused by similar or the same bacteria.²

Key differences

Several key differences can help to distinguish one condition from the other, as misdiagnosis due to similar symptoms or non-specific testing can lead to severe health complications. Some factors that differ between the two conditions include:

• Location: cellulitis is more superficial, while pyomyositis affects deep muscle layers
• Symptom intensity: cellulitis causes warm-to-touch skin with red patches that don’t have defined edges, while pyomyositis causes muscle stiffness and may affect movement
• Imaging findings: only pyomyositis shows abscess formation in the muscle through MRI
• Response to antibiotics: cellulitis is often treated faster

Deep vein thrombosis (DVT)

Deep vein thrombosis (DVT) is a condition that causes swelling of the lower limbs due to the formation of blood clots (thrombus) in deep veins. Blood clots can form in sedentary individuals due to a lack of movement or be caused by medication that promotes blood thinning, such as anticoagulant and antiplatelet drugs. Correct diagnosis and treatment of DVT is crucial, as clots can enter the bloodstream and get stuck in the lungs, causing pulmonary embolism, a severe and possibly life-threatening condition.⁹

Similarities with pyomyositis

Similarities between pyomyositis and DVT include swelling of the lower limbs, warmth, and pain in the affected area, as well as erythema.

Key differences

Differences that can help in the differential diagnosis process include:

• Different symptoms: DVT typically doesn’t present with fever or muscle tenderness
• Risk factors: a person with restricted mobility or thrombophilia, a condition that makes you prone to blood clot formation, is more likely to have DVT
• Imaging: doppler ultrasound detects blood clots in the veins in DVT, while pyomyositis only shows abscess formation in the muscle through MRI¹⁰
• Blood tests: DVT cases show minimal blood test abnormalities, while pyomyositis shows clear inflammation

Necrotising fasciitis

Necrotising (tissue death) fasciitis (subcutaneous inflammation) refers to a rapidly spreading bacterial infection that affects the fascia (layer of tissue under the skin).¹¹ It can be divided into type 1 (polymicrobial) and type 2 (monomicrobial), depending on how many bacterias are causing the infection, while S.aureus and group A S.pyogenes are usually responsible for type 2 necrotising fasciitis. This infection occurs from cuts on the skin (injury, surgery) and thrives in individuals with impaired immune response, like people with obesity, diabetes, cancer, pregnant individuals, and people with vascular problems.¹²

Similarities with pyomyositis

Pyomyositis and necrotising fasciitis present similar symptoms, which include fever, pain, and swelling at the injury site with erythema.

Key differences

The key differences between the two conditions that aid in their differentiation include:

• Disease progression: necrotising fasciitis spreads rapidly and shows severe symptoms across the body (e.g. pain, nausea, diarrhoea, necrosis, and sepsis)
• Pain level: necrotising fasciitis triggers intense nerve pain, not limited to the injury site
• Presentation: necrotising fasciitis causes blisters and crepitus (popping sensation under the skin, caused by gas produced by bacteria)
• Imaging: fascial thickening, gas in tissues,while pyomyositis only shows abscess formation in the muscle through MRI
• Labs: LRINEC score (Laboratory Risk Indicator for Necrotising Fasciitis) is useful for correct diagnosis ¹³
• Surgical emergency for necrotising fasciitis

Comparative table 

Definitive clinical features of pyomyositis

After reviewing each condition, let’s have a look at the definitive signs that an inflammation of the lower extremities with possibly misleading presentations is actually pyomyositis:

• Abscess formation in skeletal muscle, detected via MRI
• Muscle involvement:  pain is not superficial and stems from deep within the muscle, leading to muscle tenderness and swelling, and ultimately restrictions in movement
• Blood tests show high inflammatory marker levels (WBC and CRP are often raised), paired with pain limited to the injury site, which shows localised infection
• MRI imaging is the golden standard for diagnosis, allowing the identification of intramuscular (within the muscle) abscess, a presentation unique to pyomyositis

Summary

Differential diagnosis is a systematic approach that can help eliminate conditions that mimic each other, through similar clinical presentations and diagnostic test results, in order to identify the correct condition promptly. This process can be challenging, especially when a rare disease with generalised symptoms is at play, as in the case of pyomyositis.

Pyomyositis is caused by bacteria that reside deep within the skeletal muscles after a minor injury, usually on the legs, and its symptoms may mimic more common conditions like cellulitis and DVT, as well as the rare but alarming necrotising fasciitis. While these conditions present a similar clinical profile, several key factors differentiate them and can help in the correct diagnosis. While necrotising fasciitis is the most severe and rapid of all the conditions mentioned in this article, progression of pyomyositis, cellulitis, and DVT can lead to serious health complications that require specific treatment plans to control and eradicate. Maintaining a high index of suspicion and not overlooking rare diseases is crucial in diagnosis, as an incorrect or delayed treatment plan can lead to serious, life-threatening outcomes.