Surreal image of shoulder pain transforming into muscle fibers, representing pyomyositis.

Uncommon Culprit, Common Symptoms: When Shoulder Pain Signals More Than Just Arthritis

Mira Elwood in Health & Wellness December 2025 • 4 min read.

"Discover how pyomyositis can mimic septic arthritis and why a high index of suspicion is crucial for early diagnosis and treatment."

Pyomyositis, while relatively uncommon, is often considered a tropical disease, with higher prevalence rates in regions like Africa and the South Pacific. However, it's increasingly being recognized in Western countries. Pyomyositis requires a high degree of suspicion due to its potential for misdiagnosis.

Pyomyositis is essentially a deep-seated bacterial infection affecting skeletal muscles, most frequently in the lower limbs, such as the quadriceps, hamstrings, gluteal muscles, and iliopsoas. While the exact cause is often unclear, several risk factors have been identified, including immunosuppression, intense physical exertion, and direct trauma to the muscle.

The challenge with pyomyositis is that it can easily be overlooked, either because it isn't considered in the initial differential diagnosis, or because patients present with nonspecific symptoms that mimic other conditions, such as septic arthritis. This can delay appropriate treatment and potentially lead to complications.

Case Study: A Diagnostic Puzzle

Surreal image of shoulder pain transforming into muscle fibers, representing pyomyositis.

Consider the case of Mrs. ET, a 54-year-old woman who presented with a two-week history of progressive, dull aching pain in her right shoulder. Her symptoms had worsened in the days leading up to her admission, accompanied by fever, rigors, and significant pain with even minimal shoulder joint movement. Her medical history was unremarkable, except for hypertension. There were no indications of diabetes, immunosuppression, or any recent trauma.

Upon examination, Mrs. ET had a body temperature of 37.9° Celsius, but otherwise displayed normal physiological parameters. While there was no obvious erythema, there was slight swelling and localized temperature elevation around the right shoulder joint. The tenderness was diffuse, spanning from the scapula to the sternoclavicular joint. Her range of shoulder movement was reduced in all directions, with specific tenderness noted during arm adduction against resistance.

Initial laboratory tests revealed elevated inflammatory markers:

White cell count (WCC): 9.62 × 109/L
Neutrophils: 8.73 × 109/L
C-reactive protein (CRP): 52 mg/L

Radiographs of the right shoulder were unremarkable.

Given her symptoms, septic arthritis of the shoulder was suspected. However, two attempts at shoulder aspiration (via anterior and posterior approaches) yielded no fluid. Blood cultures later identified Staphylococcus aureus, sensitive to flucloxacillin. Subsequently, Mrs. ET was started on intravenous antibiotics after consulting with the infectious diseases (ID) team. An echocardiogram was also performed to rule out infective endocarditis, and the results were negative.

The Takeaway: Think Beyond the Obvious

Mrs. ET's case underscores the importance of maintaining a broad differential diagnosis, even when initial presentations suggest common conditions. Pyomyositis, though rare, should be considered in patients presenting with shoulder pain and signs of sepsis, even in the absence of typical risk factors. Early diagnosis, coupled with appropriate antibiotic therapy and, if necessary, surgical intervention, can lead to favorable outcomes and prevent serious complications.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.3390/diseases6040100, Alternate LINK

Title: An Unusual Presentation Of Pectoralis Major Pyomyositis Presenting As Septic Arthritis Of The Shoulder: A Case Report And Review Of The Literature

Subject: General Medicine

Journal: Diseases

Publisher: MDPI AG

Authors: Ahmed Saad, Shafiq Shahban, Tarek Elgamal

Published: 2018-11-10

Everything You Need To Know

What exactly is pyomyositis, and why is it important to recognize?

Pyomyositis is a deep-seated bacterial infection that primarily affects skeletal muscles. While it's more commonly found in tropical regions, it can occur elsewhere. It is often caused by bacteria such as Staphylococcus aureus. Risk factors for pyomyositis include immunosuppression, intense physical exertion, and trauma to the muscle. It is significant because it can be mistaken for other conditions, such as septic arthritis, potentially delaying appropriate treatment.

What is septic arthritis, and why is it so important to diagnose it quickly?

Septic arthritis is an infection within a joint. It's significant because it can cause rapid joint damage if not treated promptly. The symptoms, such as pain, swelling, and reduced range of motion, can overlap with other conditions, making accurate and timely diagnosis crucial. Septic arthritis can be caused by bacterial or fungal pathogens. Diagnosis often involves joint aspiration to analyze the fluid for signs of infection.

What do elevated inflammatory markers like white cell count and C-reactive protein tell doctors, and why are they significant?

Elevated inflammatory markers, such as white cell count (WCC) and C-reactive protein (CRP), are indicators of inflammation in the body. Elevated WCC indicates the body is fighting an infection or dealing with inflammation. Elevated CRP signifies an acute inflammatory response. These markers are significant because they help clinicians assess the presence and extent of inflammation or infection, guiding further diagnostic and treatment decisions. However, they are not specific and can be elevated in various conditions.

What is shoulder aspiration, and why is it done when joint infection is suspected?

Shoulder aspiration is a diagnostic procedure where a needle is inserted into the shoulder joint to extract fluid for analysis. It's typically performed when septic arthritis is suspected. The fluid is then examined for signs of infection, such as elevated white blood cell count or the presence of bacteria. It is significant because it helps confirm or rule out an infection within the joint and identify the causative organism, guiding appropriate antibiotic treatment. A negative aspiration result doesn't always rule out infection, as seen with pyomyositis affecting surrounding tissue.

What is Staphylococcus aureus, and why is its identification important in treating infections?

Staphylococcus aureus is a common type of bacteria that can cause a variety of infections, ranging from skin infections to more serious conditions like septic arthritis and pyomyositis. Its significance lies in its ability to cause invasive infections if it enters the bloodstream. Identifying Staphylococcus aureus through blood cultures is crucial for guiding antibiotic therapy, as different strains may exhibit varying levels of resistance to antibiotics. Treatment typically involves intravenous antibiotics, such as flucloxacillin, to eradicate the infection.