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Letters to the Editor

Pyomyositis of the Iliopsoas and Piriformis Muscles Caused by Panton-valentine Leukocidin-positive Staphylococcus aureus

Polat, Meltem MD; Sabir, Nuran MD

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The Pediatric Infectious Disease Journal: September 2021 - Volume 40 - Issue 9 - p e358-e359
doi: 10.1097/INF.0000000000003217
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To the Editors:

We read with great interest the report by Pedoto et al regarding primary pyomyositis due to methicillin-resistant Staphylococcus aureus (MRSA) in 2 children.1 Pyomyositis is a purulent infection of skeletal muscles, mostly caused by S. aureus and commonly affecting children in tropical regions.2 However, as stated in this report,1 the incidence of pyomyositis has recently raised significantly among healthy children living in temperate regions, which is thought to be associated with the emergence of Panton-Valentine leukocidin (PVL) toxin-producing community-acquired MRSA strains.2,3 PVL is a staphylococcal virulence factor associated with severe pneumonia, skin and soft tissue and musculoskeletal infections.3 Herein, we describe a case of pyomyositis of the left iliopsoas and piriformis muscles caused by PVL-positive MRSA, which resolved after the addition of clindamycin to treatment.

A previously healthy 15-year-old boy presented with a 4-day history of fever, left hip pain and inability to weight bear on that side. There was no history of recent trauma or infection. On examination, the patient had fever up to 39.4 °C, and toxic-looking. The movements of the left hip joint were restricted and painful. Laboratory tests showed elevated C-reactive protein (29 mg/dL) and erythrocyte sedimentation rate (48 mm/h), with normal leukocyte count (5400/mm3) and creatine phosphokinase level (154 U/L). After blood cultures were obtained, treatment with empirical intravenous ceftriaxone and vancomycin was started for presumed septic arthritis of the left hip. Contrast-enhanced magnetic resonance imaging (MRI) revealed the myositis and myonecrosis of the left iliopsoas-priformis muscles, with no findings compatible with septic arthritis (Fig.1). On day 3, 2 sets of blood cultures grew clindamycin-susceptible MRSA, which was subsequently typed as PVL-positive. Due to persistent fever, severe hip pain and elevated CRP level (320 mg/dL), intravenous clindamycin was added to treatment on day 6. Echocardiographic examination showed no evidence of infective endocarditis. Repeat MRI demonstrated the similar findings with the previous one. He became afebrile within 24 hours after the addition of clindamycin and his hip pain gradually resolved over the next days. On day 14, the patient was able to walk without support, and acute phase reactants were back to normal values in 20 days. He completed 3 weeks of parenteral antibiotic therapy and was discharged without sequelae.

Contrast-enhanced magnetic resonance imaging demonstrates diffuse edematous signal changes and marked contrast enhancement in the left iliopsoas-priformis muscles (arrow).

Pyomyositis involving the muscles around the hip joint can mimic septic arthritis. The disease has been divided into three clinical stages: invasive, suppurative (abscess formation), and late (septicemic). The management depends on the stage of the disease. In the early invasive stage, it can be treated with antibiotics alone. However, most of the patients (>90%) present at the suppurative stage, and therefore require computed tomography or ultrasound-guided percutaneous drainage of the abscesses in combination with antibiotic therapy. Surgical drainage may be required in the setting of extensive muscle involvement with necrosis or deep-seated infection.4

Empirical antimicrobial therapy for pyomyositis will depend on local resistance patterns and antibiotic efficacy. As the frequency of community-acquired MRSA has increased in recent years, empirical treatment should cover these strains. It is important to suspect PVL-positive MRSA disease if infection is associated with a necrotizing process and fails to respond to initial empirical treatment.2,3 The addition of clindamycin to the treatment regimen may be helpful in patients with necrotizing pyomyositis due to PVL-positive MRSA, as it has previously been shown to inhibit the production of the PVL toxin.5

Clindamycin has been approved by the US Food and Drug Administration for the treatment of staphylococcal infections. Although not specifically approved for treatment of MRSA infection, it has been successfully used for the treatment of invasive susceptible CA-MRSA infections in children. Clindamycin penetrates well into the bone and abscess cavities. However, because of its bacteriostatic nature, it should not be used if there is concern for endovascular infections, such as infective endocarditis or septic thrombophlebitis.6 The other concerns regarding the clindamycin use are the increasing rate of clindamycin resistance among CA-MRSA isolates and the development of resistance during the treatment with clindamycin.7 Future studies are needed to examine whether clindamycin administration could improve the outcome of severe infections caused by PVL-positive MRSA strains.


1. Pedoto D, Diana A, Pennacchio ML, et al. Primary pyomyositis in children is no more a rare condition: presentation of 2 clinical cases [published online ahead of print March 2, 2021]. Pediatr Infect Dis J. doi: 10.1097/INF.0000000000003113.
2. Pannaraj PS, Hulten KG, Gonzalez BE, et al. Infective pyomyositis and myositis in children in the era of community-acquired, methicillin-resistant Staphylococcus aureus infection. Clin Infect Dis. 2006;43:953–960.
3. Young CB, Earle SG, Soeng S, et al. Panton–Valentine leucocidin is the key determinant of Staphylococcus aureus pyomyositis in a bacterial GWAS. Elife. 2019;8:e42486.
4. Elzohairy MM. Primary pyomyositis in children. Orthop Traumatol Surg Res. 2018;104:397–403.
5. Dumitrescu O, Boisset S, Badiou C, et al. Effect of antibiotics on Staphylococcus aureus producing Panton-Valentine leukocidin. Antimicrob Agents Chemother. 2007;51:1515–1519.
6. Liu C, Bayer A, Cosgrove SE, et al.; Infectious Diseases Society of America. Clinical practice guidelines by the Infectious Diseases Society of America for the treatment of methicillin-resistant Staphylococcus aureus infections in adults and children. Clin Infect Dis. 2011;52:e18–e55.
7. Khamash DF, Voskertchian A, Tamma PD, et al. Increasing clindamycin and trimethoprim-sulfamethoxazole resistance in pediatric Staphylococcus aureus infections. J Pediatric Infect Dis Soc. 2019;8:351–353.
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