Introduction

Pyomyositis is a bacterial infection of the skeletal muscles that is typically caused by the bacterium Staphylococcus aureus. This condition can affect individuals of all ages, although it is more frequently observed in children and can occur in both sexes.¹

Depending on the origin, it can be classified as a primary or secondary infection. Primary pyomyositis arises from the spread of the infection through the bloodstream, whereas secondary pyomyositis occurs from a penetrating trauma.² 

This condition, typically found in tropical regions, is now being increasingly reported in temperate climates like the USA due to the rise of methicillin-resistant strain of Staphylococcus aureus (MRSA).³

The MRSA strain is one of the more invasive strains of S. aureus and is characterised by its high resistance to multiple antibiotics, making infections much harder to treat. 

An abscess, or the buildup of pus within the tissues, is commonly seen in this condition. Pyomyositis develops in three stages. The initial stage is characterised by acute fever and muscle cramps, often accompanied by muscle tenderness and oedema (buildup of watery fluid). The second stage, occurring a few weeks later, involves abscess formation and can cause severe pain, swelling, and fever. The final, and most severe stage is marked by systemic infection (sepsis), potentially leading to complications like organ failure or septic shock.⁴

The immune system plays a key role in battling pathogens such as S. aureus; therefore, an immunocompromised state (a weakened immune system) significantly increases an individual’s predisposition to pyomyositis. Conditions like diabetes, acquired immunodeficiency syndrome (AIDS), and cancer are known to compromise the immune system, making individuals more susceptible to such infections. Early detection and treatment of pyomyositis is key to effective rehabilitation. Here, key points about pyomyositis and its relation to immunocompromised individuals will be further discussed.

Pyomyositis and diabetes

Diabetes is the consequence of prolonged high blood sugar levels and is one of the most common causes of pyomyositis, particularly in non-AIDS patients. The disease manifests in different ways, the most common type being type 2 diabetes, which is also known as diabetes mellitus. Typical symptoms include fatigue, frequent urination, dry mouth, and vomiting.

In general, diabetes is associated with an increased susceptibility to more invasive colonisation by S.aureus^.1,2 Geerlings and colleagues revealed that children with diabetes presented slower and weaker immune responses.⁵ In addition, high sugar concentrations were reported in the urine of patients with diabetes, which often maximises the risk of various infections. 

Patients with diabetes typically report severe aches and pains in the lower limbs combined with high fever, fatigue, and dehydration. For instance, a 52-year-old male complained of prominent pain in the back and gluteal muscles.⁷ Blood tests reported high levels of inflammatory markers, with blood cultures testing positive for MRSA.⁴ A study on a 63-year-old man suffering from a swollen left thigh, hypertension, and diabetes also reported the presence of methicillin-sensitive S. aureus (MSSA), which later developed into the MRSA strain.⁸ In another study, a 26-year-old woman was initially diagnosed with primary pyomyositis following complaints of pain in her right thigh.⁹ She also had untreated diabetes, and her blood cultures tested positive for MSSA. Eventually, she also exhibited symptoms of systemic pyomyositis. 

Pyomyositis and HIV/AIDS

AIDS is a disease caused by the Human Immunodeficiency Virus (HIV) and is one of the most common sexually transmitted diseases. HIV progressively weakens the immune system, making individuals vulnerable to various infections. Though it is mainly transmitted through sexual contact, non-sexual transmission can also occur from an infected mother during childbirth and breastfeeding. 

HIV infections primarily target immune helper cells like the CD4⁺T cells and macrophages, significantly reducing their population and weakening cellular immune responses. This depletion, alongside the action of CD8⁺T killer cells, eventually results in the development of AIDS. Early detection is critical because, after integration into the genome, the virus can remain inactive without causing noticeable symptoms.

Pyomyositis has been known to complicate HIV prognosis. While more insights into the occurrence of this phenomenon are required, it is certain that S.aureus remains the causative agent that reduces the ability of immune cells to destroy bacteria.¹⁰ For instance, a recent study showed a strong association between HIV/AIDS and pyomyositis.¹ Patients with AIDS were five times more likely to develop the condition compared to those without.¹ Additionally, S.aureus bacteraemia levels (presence of bacteria in the blood) were 16.5 times higher in those infected with HIV.¹¹ However, this association is likely due to a general increase in susceptibility to infections in people with HIV, rather than a direct impact on skeletal muscle tissue.¹

Pyomyositis should, therefore, be considered in the differential diagnosis (a method of diagnosis that discerns one disease from another that presents with similar clinical symptoms) of individuals with HIV who present with fever, bacteraemia, and lower-body muscle pain. Approximately 80% of HIV-infected individuals also exhibit skin conditions such as psoriasis and folliculitis, which further increase the body’s predisposition to an S.aureus invasion.¹² As a result, it is important to note that the incidence of pyomyositis in many cases can be due to multiple factors. 

Pyomyositis and cancer

Cancer is a medical term used to describe diseases that are characterised by abnormal cell growth that can potentially spread to other regions of the human body (metastasis). It is one of the leading causes of mortality and can be a result of internal and external factors, including but not limited to diet and genetics. Cancers are often accompanied by symptoms such as abnormal lumps, headaches, and sudden weight loss. Symptoms can worsen over time, and late-stage cancers are a result of extreme metastasis, which is more difficult to control. Screening tests are crucial for early detection and can make recovery much easier. Treatment options include chemotherapy, surgery, and medication.

People with cancer or those undergoing chemotherapy have extremely fatigued immune systems and are therefore more likely to develop infections like pyomyositis.^13,14 However, so far, pyomyositis has been associated mostly with blood cancers.^13,14 In most cases, chemotherapy has been interrupted due to severe complications from pyomyositis, hence making early detection of the utmost importance.¹¹ For instance, a 78-year-old man undergoing chemotherapy for gastric cancer experienced sudden leg pain and high fever, leading to a diagnosis of pyomyositis after further testing. The condition was successfully treated with antibiotics, significantly improving his symptoms..

While the development of pyomyositis in cancers needs to be studied further, it is well-known that advanced pyomyositis can be fatal and complicate rehabilitative measures. Detailed computed tomography (CT) and magnetic resonance imaging (MRI) scans are essential tools for the effective diagnosis of a potential infection, and can also be used to map necessary therapeutic measures.

Summary

The natural immune responses of our bodies are set in place to fight off attacks to our health and well-being; however, when these responses are weakened, the immune system is compromised. An immunocompromised system is at a high risk for all kinds of infections, and pyomyositis is one such infection that has drastic effects on the functionality of the immune system. Conditions like diabetes, AIDS, and cancer occur along with pyomyositis, and while its onset has been documented in patients with such immunocompromised systems, its prognosis is often unpredictable.

Despite its involvement in some of the most common illnesses, pyomyositis has not been extensively studied. Multiple studies document the incidence of pyomyositis on a case-by-case basis; however, every study comes with its own limitations. The current avenues of treatment involves the surgical extraction of pus from an abscess and administration of antibiotics⁴; however, more awareness and diagnostic scans must be implemented for better prognosis and recovery.