A 7-year-old male patient presented to the emergency department with a history of sore throat and pain and swelling affecting the left leg, with no history of trauma. On physical examination the patient was feverish (39ºC), had swelling of the affected leg and the tonsils, which showed exudates on them.
Frontal and lateral view radiographies of the left leg were normal (Fig. 1).
Ultrasound revealed enlargement and heterogeneity of the solear muscle contacting the peroneal diaphysis. There were no obvious collections suggesting haematomas or abscesses (Fig. 2).
MRI performed two days later showed high signal intensity on T2WI involving the posterior tibial and flexor hallucis longus muscles and the deeper fibres of the solear muscle (Fig. 3). After gadolinium injection, a poorly defined central area with low signal intensity surrounded by a hyperenhancing rim was depicted, suggesting the presence of an associated abscess. There was also medullary bone oedema, but no cortical disruption or periosteal reaction were seen (Fig. 4).
MRI after 3 months of antibiotherapy revealed disappearance of the abscess and marked decrease of the oedema involving the referred muscles and the peroneal bone marrow (Fig. 5).
Punction of the abcess wasn't performed because the clinical condition was typical and there was good response to antibiotherapy.
Pyomyositis is a primary bacterial infection of skeletal muscles. It used to be considered a tropical disease but now it can be found in temperate climates, mainly because of the emergence of HIV infection. Risk factors for pyomyositis include rhabdomyolisis, muscle trauma, overlying cellulitis, infected insect bites, injection of illicit drugs, diabetes mellitus and bacteraemia from other sources (such as bacterial tonsillitis, which, in our case, was the probable source). Staphylococcus aureus is the most common pathogen in both tropical and temperate climates, and is responsible for 90% of the infections [1].
Primary pyomyositis can involve any muscle group in the body. Large muscles of the lower extremities are commonly affected, with the quadriceps muscle followed by the gluteal and iliopsoas muscles being the most common sites of infection [1].
Pyomyositis has three distinct stages, which represent a gradual progression from diffuse inflammation to focal abscess formation and to a septic state. Stage 1 involves the insidious onset of diffuse pain that progresses to erythema, swelling, and oedema of the affected muscle over a 1-2-week course (invasive stage). Stage 2 involves progressive induration, pain, and enlargement of the mass over a 2-3 week period (purulent stage), and stage 3 involves intensifying pain, suppuration, and muscle involvement with possible extension into an adjacent bone or joint eventually progressing to septicaemia, shock, and death [2].
Since delay in accurate diagnosis is frequent and clinical deterioration can be precipitous, early imaging is essential to detect, localize, and define the disease extent.
Depending on the stage of the disease, ultrasound can initially show a localized area of muscle oedema and later, in the course of the disease, an intramuscular fluid collection corresponding to a formed abscess [1].
MRI is the most useful imaging technique for the diagnosis of pyomyositis, as it clearly demonstrates diffuse muscle inflammation, with high signal intensity of the affected muscle(s) on T2FS and an hyperintense rim on T1 weighted images, and any subsequent abscess formation, which shows high signal intensity on T2, low signal intensity on T1 and peripheral contrast enhancement after gadolinium administration. There may also be diffuse muscle enlargement [3].
The choice of treatment for pyomyositis depends on the stage at presentation. During the early stage of the infection, the diffuse inflammatory changes can be effectively treated with antibiotics alone. However, if an abscess has formed, appropriate drainage before the initiation of antibiotic therapy is required and can be guided by ultrasound [2].
Pyomyositis
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
1. X-ray Findings: In the anteroposterior and lateral views of the left lower leg, overall bone continuity is intact, with no obvious signs of fracture. The bony trabecular structure is clear, showing no apparent bone destruction or periosteal reaction. Soft tissue swelling is evident, and increased density areas suggest potential soft tissue inflammation or edema, though it is difficult to precisely assess its extent.
2. MRI Findings: Patchy or irregular T2 hyperintense signals are observed in the left lower leg muscle tissues, appearing hypointense on T1. After contrast administration, ring-shaped peripheral enhancement is noted, suggesting abscess or liquefactive lesions within the muscle or intermuscular compartments. The corresponding muscle groups (e.g., gastrocnemius, tibialis anterior, or soleus) show uneven signal intensity, local disruption of muscle fiber structure, and moderate edema or inflammatory changes in the surrounding soft tissues.
3. Ultrasound Findings: Examination of the most painful site of the left lower leg reveals obvious thickening of the muscle layer, disorganized muscle fiber echogenicity, and local regions of anechoic or hypo-echoic areas with relatively unclear boundaries. These findings indicate fluid content (possibly pus).
Based on the child’s high fever, swelling and pain of the left lower limb, history of tonsillitis, and MRI findings revealing intramuscular fluid collections compatible with abscess formation, the most likely diagnosis is: Left Lower Limb Pyomyositis.
Staphylococcus aureus is the most common pathogen, especially given the concurrent tonsillitis as a possible primary infectious focus and hematogenous spread to the muscle.
1. Treatment Strategy:
– Antibiotic Therapy: Once Staphylococcus aureus infection is confirmed or strongly suspected, antibiotics effective against Gram-positive organisms, including methicillin-resistant S. aureus (MRSA), may be chosen (e.g., oxacillin, vancomycin, or combination therapy), with subsequent adjustment based on culture sensitivity results.
– Abscess Drainage: If a significant abscess is present, drainage (percutaneous under ultrasound or MRI guidance, or surgical) can relieve local pressure and facilitate recovery.
– Supportive Care: Includes fluid replacement, temperature control, and symptomatic management. Closely monitor the patient’s overall condition and regularly check laboratory parameters (such as complete blood count, C-reactive protein, etc.).
2. Rehabilitation and Exercise Prescription:
– Acute Phase (Abscess Stage): Emphasize rest and immobilization to prevent exacerbation of pain or spread of infection. Gentle active movement of the ankle and foot can be encouraged if tolerated, to reduce the risk of deep vein thrombosis in the lower limb.
– Early Recovery Phase (After Inflammation Control): Gradually restore joint range of motion and muscle strength training, following the FITT-VP principle:
• Frequency: 3–4 times per week of light exercises;
• Intensity: Start at a low intensity within the child’s tolerance level;
• Time: 10–15 minutes per session, avoiding marked fatigue;
• Type: Range-of-motion exercises, isometric contractions (e.g., gentle leg lifts);
• Progression: Increase the activity range and exercise duration as the condition improves and muscle strength returns;
• Volume: Adjust the daily or alternate-day exercise regimen as tolerated, ensuring adequate rest.
– Later Phase (Near Full Recovery): Under the guidance of the physician or rehabilitation therapist, incorporate muscle strengthening and coordination training (e.g., light weight-bearing ambulation, small-scale jumps). Carefully monitor for pain or swelling in the affected limb and increase intensity in a stepwise manner.
– Important Note: Since the patient is still in a growth phase, avoid excessive training or improper weight-bearing that may affect bone health and development. If low-grade fever, worsening local pain, or abnormal laboratory findings recur, promptly reassess to rule out relapse or worsening infection.
This report is based on the available medical data and imaging for reference only and cannot substitute an in-person consultation or the final judgment of a qualified physician. Specific diagnosis, treatment, and rehabilitation plans must be tailored to the patient’s individual condition upon comprehensive evaluation by specialists.
Pyomyositis
