Diagnosis
The area of the fluctuance in the right thigh was incised and pus was drained and sent for culture. Due to the severity of the diseases empirical therapy with intravenous piperacillin-tazobactam 4-0.5 g every 6 hours and vancomycin 1 g every 12 hours was commenced. Culture of the pus grew Pseudomonas aeruginosa susceptible to the commenced beta lactam-lactamase inhibitor. Blood and urine cultures were sterile.

A skin-muscle biopsy of the right thigh and the left arm was performed. A moderate degree of striated muscle degeneration was disclosed with the presence of infiltration of inflammatory cells consistent with myositis.

Differential diagnosis
Serology for Leptospira interrogans, Bartonella henselae, Borellia burgdorferi, and Legionella pneumophila was negative. An autoimmune screen including anti-ds DNA and anti-Jo1 was negative.

The patient improved gradually and was discharged after three weeks of hospitalization. All abnormal laboratory indices returned to normal. At follow-up (8 months after her discharge) she is doing extremely well.

Teaching points
- Pyomyositis is an infection of the skeletal muscles that usually afflicts immunocompromised patients; nevertheless immunocompetent patients are not spared. It is usually due to Staphylococcus aureus in about 90% of cases.1 Other pathogens that may cause pyomyositis are various coagulase-negative Staphylococcus species, various streptococcal species, Gram-negative bacteria, as well as anaerobic bacteria and fungi. The disease may affect practically all skeletal muscles of the body, including muscles in the extremities and the trunk.2

- Pyomyositis due to P. aeruginosa is rare.3 The skin overlying the involved muscle is not invariably afflicted. Additionally, isolated P. aeruginosa infection of intact skin may take several forms: the green nail syndrome, the webbed space syndrome, cutaneous folliculitis, and ecthyma gangrenosum.4 As far as we know large diameter erythematous patches overlying several areas of inflammatory myositis as was the case in our patient has not been described. Though antibiotic treatment may suffice in the occasional patient, incision and drainage is a mainstay in management.3

References
1. Crum NF. Bacterial pyomyositis in the United States. Am J Med. 2004;117:420-8.
2. Gomez-Reino JJ, Aznar JJ, Pablos JL, Diaz-Gonzalez F, Laffon A. Nontropical pyomyositis in adults. Seminars in Arthritis and Rheumatism 1994 ;23:396-405.
3. Korten V, Gurbuz O, Firatli T, Bayik M, Akoglu T. Subcutaneous nodules caused by Pseudomonas aeruginosa: healing without incision and drainage. J Chemother. 1992;4(4):225-7.
4. Agger WA, Mardan A. Pseudomonas aeruginosa infections of intact skin. Clin Infect Dis. 1995;20(2):302-8.

Acknowledgments
1. The case was prepared for our website by Drs. Peter Rafailidis and Anastasios Kapaskelis.
2. The case report was submitted for consideration for publication.