Teaching Case

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.

Written informed patient consent for publication has been obtained.