A 12-year-old girl presented with a non-healing, tender swelling of the left upper leg with warm skin moulting despite outpatient antibiotic therapy (Figure 1). The enlargement started five weeks before the presentation while staying in Togo. The patient was in general good health without a remarkable medical history or history of trauma.
Ultrasound revealed an extensive abscess collection superficially to the musculus rectus femoris. Focal interruption in the anterior wall of the m. rectus femoris, with debris bulging from intramuscular into the adjacent anterior collection (Figure 2a). Furthermore, a second, smaller collection was more proximal in the m. rectus femoris and surrounding soft tissue inflammation (Figure 2b).
MRI showed a large, heterogeneous, T1-hyperintense collection located sub-/interfascial on the anterior side of the fascia lata with thick septae and central abscessation. Extension into the m. rectus femoris and m. vastus lateralis through an interruption of the deepest fascia lata sheet, with only mild surrounding muscle oedema (Figures 3a and 3b). No bone contact with the femur nor bone oedema.
After intravenous contrast administration with gadoteric acid, the sub-/interfascial collection showed extensive wall enhancement, with multiple multilocular micro-abscesses subfascial and intramuscular (Figures 3c and 3d). The dynamic bolus sequence showed a slow first pass phase and a stable second pass without wash-out.
Background
Pyomyositis is a bacterial infection of skeletal muscle tissue, endemic in the tropics [1,2]. It may develop from direct inoculation, extension from adjacent infection, or haematogenous spread [2–4]. Mycolicibacterium fortuitum (M. fortuitum) is a non-tuberculous mycobacterium frequently isolated from (processed) water and soil [5,6]. M. fortuitum is a rare human pathogen outside the tropics, primarily affecting immunocompromised adults [1,4,7]. Investigations of our patient’s immune status were negative.
Clinical Perspective
Skin and soft tissue infections caused by M. fortuitum may develop with a variable presentation, including cellulitis, ulcers, and abscesses, often taking 4–6 weeks to develop [5,6]. The nonspecific clinical presentation and the low incidence often result in delayed diagnosis [1]. Pyomyositis develops deep in the muscles inaccessible to direct physical examination; therefore, the diagnosis is often based on imaging [1,4,8]. MR-imaging with gadolinium contrast is the method of choice because of its usefulness in detecting abscesses, adjacent osteomyelitis, or septic arthritis, altering treatment [1,2,8].
Imaging Perspective
In the early stage of the disease, pyomyositis will present as muscular enlargement with oedema and high signal intensity on STIR- and T2-weighted sequences [2,9]. With inflammatory progression, MRI will reveal one or more intramuscular abscesses [2]. These can be detected as a collection with mass effect and high signal on fluid-sensitive sequences, identified through a hyperintense rim on T1-weighted images, representing blood products and the margin of drainable pus [1,10]. After Gadolinium administration, the abscess enhances peripheral [1,2,4,9,10]. The purulent material in the abscess cavity has a variable signal on T1-weighted imaging depending on the proteinaceous content and will not enhance [2] but show diffusion restriction on DWI, often surrounded by increased diffusion due to expanded extracellular space [8]. The adjacent soft tissues may show subcutaneous oedema and unorganised phlegmonous collections [2,9].
Outcome
When muscle abscesses are present, the standard of care is surgical intervention and prolonged antibiotic therapy, using imaging follow-up to evaluate for resolution [8]. Antibiotic treatment alone is frequently correlated with recurrences, chronic drainage, and sinus tract formation [10]. Our patient underwent drainage of the abscess. M. fortuitum was isolated from the sample culture, and treatment with antibiotics was implemented. The patient showed a good response with complete clinical remission.
Teaching Points
This case highlights the importance of considering mycobacteria as the causative agent in pyomyositis, particularly when they have a tendency for chronicity [11], characterised here by only mild surrounding inflammation and clinical signs compared to the extent of the abscesses. MRI is the preferred imaging modality for diagnosis and to pinpoint the exact location and extent of muscle abscesses. MRI should generally rule out bone (osteomyelitis) and joint involvement (septic arthritis) as complications/differentials in infection [1,2,8]. Furthermore, it is non-specific for the underlying pathogen of the infection.
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Pyomyositis caused by Mycolicibacterium fortuitum
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According to the provided ultrasound and MRI images, local swelling of the left thigh muscle tissue is observed. Ultrasound shows a relatively well-defined hypoechoic area within the muscle layer, suggesting the presence of pus or a liquefied lesion. On MRI, T2-weighted and STIR sequences demonstrate high signal intensity in the affected region, indicating pronounced exudation or liquefaction; on T1 sequences, the lesion appears with uneven or iso- to low-intensity signals. Post-contrast imaging shows ring-enhancement, consistent with a typical intramuscular abscess. Surrounding soft tissues also exhibit varying degrees of edema signal, suggesting possible inflammatory spread or infiltration.
No obvious signs of bone destruction are noted on the images, making it difficult to determine whether the bone is involved. Further clinical correlation and additional imaging sequences are needed to rule out osteomyelitis or joint pathology.
Taking into account the patient’s age, symptoms (non-healing, elevated temperature, pain, and local skin changes), as well as the intramuscular abscess findings on ultrasound and MRI, and pathogen culture results, the final diagnosis supports: Myositis caused by nontuberculous mycobacteria (Mycolicibacterium fortuitum purulent myositis).
If there is still uncertainty or if the lesion is complex, follow-up imaging during treatment or further pathological or genetic testing can be obtained to confirm the pathogen definitively.
Once the acute inflammation is effectively controlled and the abscess drainage or surgery is stable, rehabilitation exercises can be gradually introduced. Adhere to the FITT-VP principle (Frequency, Intensity, Time, Type, Volume, Progression):
The entire rehabilitation process should be tailored according to the patient’s recovery status, wound healing progress, and immune condition. If significant pain or discomfort occurs during exercise, training should be paused, and evaluation should be conducted in consultation with the attending physician or rehabilitation therapist to ensure safety.
Disclaimer:
This report is based on existing imaging data and clinical information for a preliminary analysis only and is for reference purposes. It does not replace an in-person consultation or professional medical advice. If you have any questions or changes in your condition, please seek medical attention promptly.
Pyomyositis caused by Mycolicibacterium fortuitum
