Infectious myositis is a rare diagnosis with potential for severe local and systemic complications [1]. Infectious myositis is most commonly the result of hematogenous infection and typically develops in areas compromised by trauma, foreign body, ischemia or surgery [2]. Conditions known to increase the risk of infectious myositis include HIV infection, malignancy, diabetes mellitus and systemic sclerosis [2].

Peak incidence is between 20–40 years. Several cases have been reported of children affected in equatorial climates, whilst infectious myositis remains rare in temperate climates [3] [4] [5] [6][7][8]. We present a case of infectious myositis of the iliacus muscle in a child, diagnosed clinically and radiologically, treated successfully with empirical antibiotic therapy. Interest in this case lies in the rarity of the condition, diagnostic difficulty and radiological findings.

A 13-year-old boy of South Asian descent presented with a 24-hour history of pyrexia, malaise and reluctance to bear weight on his right lower limb. A working diagnosis of hip joint sepsis was made. On examination the patient appeared systemically unwell, had exquisite tenderness to palpation over the right anterior superior iliac spine (ASIS), mildly limited range of movement of his right hip joint with hip extension most affected and walked with an antalgic gait. Several eczematous skin lesions over his upper and lower limbs, with a localized area of erythema and slight ooze on the dorsal aspect of his right foot were also noted. Abdominal examination was unremarkable and deep palpation of his hip joint did not illicit tenderness. On admission he was noted to be pyrexial with a temperature of 38.1 °C. Initial blood results revealed a white cell count (WCC) of 10.8, C-reactive protein (CRP) of 61, and erythrocyte sedimentation rate (ESR) of 33. The patient was admitted for observation. Serial blood cultures were taken at times of pyrexia and the patient was initially treated symptomatically whilst awaiting an MRI scan of pelvis. Skin swabs were taken from the area of erythema affecting his right foot. MRI findings of signal hyperintensity in the right iliacus on T2 weighted sequences, (Figure 1), and hypointensity on T1 weighted sequences paired with the biochemical changes seen on serial blood tests led to a diagnosis of infectious myositis of the iliacus muscle. Accordingly, empirical IV antibiotic therapy was initiated. Blood culture results did not identify a causative pathogen. However, skin swabs taken from the right foot skin lesion revealed growth of Staphylococcus aureus and Acinetobacter lwoffii. The patient displayed prompt resolution of symptoms following initiation of parenteral antibiotic therapy guided by sensitivities (Clindamycin and Flucloxacillin) and was discharged home on oral antibiotics for 4 weeks. A serial MRI scan was performed six weeks later and revealed complete resolution of radiological inflammatory changes.

This case represents a rare clinical diagnosis, with only one such case previously described in British Orthopedic literature [3]. Infectious myositis of pelvic musculature is known to present with striking similarities to septic arthritis of the hip joint and indeed often an initial working diagnosis of septic arthritis is erroneously made [2] [9][10] [11] [12][13]. Subtle features on clinical examination can guide the clinician towards the correct diagnosis. Pelvic pyomyositis may present with pain and limited range of motion of the hip, with symptoms most pronounced in a particular plane of motion, corresponding to stretch of the affected muscle(s), unlike the globally reduced range of motion seen in septic arthritis [2] [10][11][14][15] [16][17]. In this case, the patient complained of more significant pain on passive extension of the hip joint, representing the stretched iliopsoas muscle complex. It is vital for the clinician to be aware that the complexity of pelvic anatomy, including the close association of the iliacus to the femoral nerve, lateral cutaneous nerve of the thigh, intra-abdominal viscera, sacroiliac joint and hip joint, can give rise to a wide range of presenting symptoms [3] [16][17][18][19][20]. Given the inconclusive symptom profile in our patient early MRI scan proved to be essential in determining a definitive diagnosis. Early diagnosis of infectious myositis is essential in order to prevent, or limit, the development of potentially life threatening complications [1] [12][21]. MRI scan is the imaging modality of choice for identifying pelvic soft tissue infections and has demonstrated high sensitivity for identifying early inflammatory changes [1] [3][12][8]. MRI scan has the additional benefit of excluding other hip or pelvic pathology [3]. The combination of signal hyperintensity in the iliacus muscle on the T2 weighted images and signal hypointensity on T1 weighted images, with otherwise normal anatomical appearances enabled a diagnosis of infectious myositis of the iliacus to be made. It is noteworthy that in our patient there was no evidence of abscess formation which has often been encountered and necessitates aspiration or debridement [2] [4] [5] [6][7] [8] [21] [22]. Infectious myositis is most commonly bacterial in origin and causative pathogens include Staphylococcus aureus, Group A streptococci, Gram negative aerobic and facultative bacilli. Staphylococcus aureus is the causative pathogen in 90% of cases [1][2] [3]. Fungal and viral etiologies are also seen, more commonly, in immunocompromised patients [21]. Infection can be a complication of local trauma, foreign body, illicit drug use injection, surgery. Causative pathogens may be indigenous local flora. However, hematogenous infection is most common. Often contiguous sites of infection are identified such as skin infections, infected ulcers or wounds. In our patient, the identification of Acinetobacter lwoffii and Staphylococcus aureus from a superficial area of eczema suggests a possible source of infection. Acinetobacter lwoffii is a normal commensal of skin, oropharynx and perineum.

Acinetobacter lwoffii bacteremia has been reported, though this has most often been in immunocompromised patients. [23]. Serial blood cultures failed to identify a blood-borne organism in our patient. The absence of a clear collection meant that aspiration was not possible and we were unable to definitively identify a causative organism. Therefore, we are only able to postulate that the infected skin lesion acted as a possible source of infection which resulted in hematogenous spread to the iliacus muscle. The prompt resolution of both skin lesion and symptoms from the infectious myositis following initiation of parenteral antibiotics lend support to this theory.

This case of infectious myositis of the iliacus muscle in a child represents a rarely seen clinical entity. One such case has previously been reported in British orthopedic literature. This case has demonstrated the diagnostic difficulty in differentiating septic arthritis of the hip from infectious myositis of pelvic musculature. Early MRI investigation is essential to enable early diagnosis and commencement of parenteral antibiotic therapy. Whilst this represents a very rare clinical entity, both orthopedic and pediatric clinicians should consider it as a differential for any unwell child presenting with fever, joint or muscle pain.

1. Brook I. Microbiology and management of myositis. Int Orthop 2004 Oct;28(5):257–60.   [CrossRef]   [Pubmed]    
2. Roca B, Torres V. Pyomyositis of the iliacus muscle complicated with septic sacroiliitis. QJM 2008 Dec;101(12):983–4.   [CrossRef]   [Pubmed]    
3. Peckett WR, Butler-Manuel A, Apthorp LA. Pyomyositis of the iliacus muscle in a child. J Bone Joint Surg Br 2001 Jan;83(1):103–5.   [CrossRef]   [Pubmed]    
4. Chen W-S, Wan Y-L. Iliacus pyomyositis mimicking septic arthritis of the hip joint. Arch Orthop Trauma Surg 1996;115(3-4):233–5.   [CrossRef]   [Pubmed]    
5. Hall RL, Callaghan JJ, Moloney E, Martinez S, Harrelson JM. Pyomyositis in a temperate climate. Presentation, diagnosis, and treatment. J Bone Joint Surg Am 1990 Sep;72(8):1240–4.   [Pubmed]    
6. Renwick SE, Ritterbusch JF. Pyomyositis in children. J Pediatr Orthop 1993 Nov-Dec;13(6):769–72.   [CrossRef]   [Pubmed]    
7. Andrew JG, Czyz WM. Pyomyositis presenting as septic arthritis. A report of 2 cases. Acta Orthop Scand 1988 Oct;59(5):587–8.   [CrossRef]   [Pubmed]    
8. De Boeck H, Noppen L, Desprechins B. Pyomyositis of the adductor muscles mimicking an infection of the hip. Diagnosis by magnetic resonance imaging: a case report. J Bone Joint Surg Am 1994 May;76(5):747–50.   [Pubmed]    
9. Chiedozi LC. Pyomyositis. Review of 205 cases in 112 patients. Am J Surg 1979 Feb;137(2):255–9.   [CrossRef]   [Pubmed]    
10. Birkbeck D, Watson JT. Obturator internus pyomyositis. A case report. Clin Orthop Relat Res 1995 Jul;(316):221–6.   [Pubmed]    
11. Freedman KB, Hahn GV, Fitzgerald RH Jr. Unusual case of septic arthritis of the hip: spread from adjacent adductor pyomyositis. J Arthroplasty 1999 Oct;14(7):886–91.   [CrossRef]   [Pubmed]    
12. Bertrand SL, Lincoln ED, Prohaska MG. Primary pyomyositis of the pelvis in children: a retrospective review of 8 cases. Orthopedics 2011 Dec 6;34(12):e832–40.   [CrossRef]   [Pubmed]    
13. Harbarth SJ, Lew DP. Pyomyositis as a nontropical disease. Curr Clin Top Infect Dis 1997;17:37–50.   [Pubmed]    
14. Chacha PB. Muscle abscesses in children. Clin Orthop Relat Res 1970 May-Jun;70:174–80.   [CrossRef]   [Pubmed]    
15. Rogers DW. Case of pyomyositis occurring in London. Br Med J 1973 Sep 29;3(5882):679.   [CrossRef]   [Pubmed]    
16. Watts RA, Hoffbrand BI, Paton DF, Davis JC. Pyomyositis associated with human immunodeficiency virus infection. Br Med J (Clin Res Ed) 1987 Jun 13;294(6586):1524–5.   [CrossRef]   [Pubmed]    
17. O'Reilly M, Cook JV, O'Reilly PM, Wood BP. Radiological case of the month. Psoas pyomyositis. Arch Pediatr Adolesc Med 1996 Dec;150(12):1305–6.   [CrossRef]   [Pubmed]    
18. Yuh WT, Schreiber AE, Montgomery WJ, Ehara S. Magnetic resonance imaging of pyomyositis. Skeletal Radiol 1988;17(3):190–3.   [CrossRef]   [Pubmed]    
19. Pai VS, Yee E, Lawson D, Curtis G. Iliacus pyomyositis with involvement of lateral cutaneous nerve of the thigh. N Z Med J 1994 Sep 28;107(986 Pt 1):378–9.   [Pubmed]    
20. Kinahan AM, Douglas MJ. Piriformis pyomyositis mimicking epidural abscess in a parturient. Can J Anaesth 1995 Mar;42(3):240–5.   [CrossRef]   [Pubmed]    
21. Crum-Cianflone NF. CInfection and musculoskeletal conditions: Infectious myositis. Best Pract Res Clin Rheumatol 2006 Dec;20(6):1083–97.   [Pubmed]    
22. Taksande A, Vilhekar K, Gupta S. Primary pyomyositis in a child. Int J Infect Dis 2009 Jul;13(4):e149–51.   [CrossRef]   [Pubmed]    
23. Ku SC, Hsueh PR, Yang PC, Luh KT. Clinical and microbiological characteristics of bacteremia caused by Acinetobacter lwoffii. Eur J Clin Microbiol Infect Dis 2000 Jul;19(7):501–5.   [CrossRef]   [Pubmed]