Primary sternal osteomyelitis caused by Actinomyces israelii.

To the Editor: Primary sternal osteomyelitis (PSO) is a rare entity. Diagnosis is usually difficult due to the nonspecific clinical picture and late radiological findings. We describe a case of PSO due to Actinomyces israelii and how ultrasonography (US). CT, and MRI helped to make the diagnosis and guide treatment.

A previously healthy 50-year-old woman without history of trauma presented with painful presternal swelling and fever. A 5-cm warm, fluctuating mass overlying the sternal manubrium was palpated. There was leukocytosis with neutrophilia. Blood cultures were sterile. A chest x-ray was normal. On US, thickening of the insertion of the right pectoralis major muscle on the manubrium, an adjacent heterogenous hypoechoic collection, and irregularities of the contour of the sternum were detected. An US-guided aspiration yielded purulent material. A contrast-enhanced CT demonstrated hypodense soft-tissue swelling, with enhancement surrounding the sternum and right chondrosternal joint. She was unresponsive to cloxacillin and surgical debridement. A MRI showed an extensive involvement of the sternal manubrium with decreased signal intensity on T1-weighted images (T1-W1) and increased signal on T2-W1 of the bone marrow, progression of the cortical erosions involving the anterior and posterior aspects of the sternum, and the 2nd-4th chondrosternal joints, with associated pleural thickening. Right pectoralis major muscle and peristernal soft tissues appeared thickened and hypointense on T1-W1 (Fig. 1). Culture of the bony material yielded A israelii.

She was treated with IV penicillin G and extensive surgical debridement, guided by MRI findings, leading to quick disappearance of the fever and improvement of the presternal swelling. Dental caries and periodontal disease of the first and second molar teeth bilaterally were identified. These teeth were removed, and anaerobic bacteria, but not A israelii, were isolated. On the sixth week of treatment she was discharged afebrile on oral antibiotherapy.

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Most cases of sternal osteomyelitis are complications of sternotomy or thoracic trauma, and less frequently are secondary to spread from contiguous foci. (1) PSO is caused by hematogenous dissemination, usually in the setting of IV drug abuse or immunosuppression. Most cases of PSO are caused by Staphylococcus aureus, Gram negative bacteria, Candida, and Aspergillus sp. (2) Actinomyces sp are Gram positive anaerobic bacteria found in the normal oral flora which are involved in oral-cervicofacial infections. They have seldom been reported as the cause of osteomyelitis of the facial bones. (3) Periodontal disease, damage to mucosal barriers, aspiration, and immunosuppression are risk factors. A breach in the oral mucosa leads to its introduction into the soft tissues, from where it may spread to the facial bones in a contiguous fashion. In our patient the presumptive portal of entry for A israelii was oral and an ulterior hematogenous seeding probably caused the sternal infection. The ongoing penicillin treatment, together with the difficulties inherent to the culture of Actinomyces may have prevented its isolation from the teeth.

Diagnosis of sternal osteomyelitis is straightforward in cases secondary to sternotomy or trauma and bone destruction on x-rays. However, PSO is often difficult to diagnose due to the nonspecific symptoms and late radiological changes. Plain film findings, which lag 1 to 2 weeks, include increased density, displacement of soft tissues, and bone loss. (1,2) Technetium-99 scintigraphy is positive earlier and has a good sensitivity, but lacks specificity. (4) Despite the limitations of US in the assessment of bone, in our case, the irregularities of the sternal contour and the inflammatory changes of the muscles raised suspicion of osteomyelitis and prompted further evaluation. CT may show a peristernal soft-tissue mass, periostitis, and bone destruction. But occasionally, it may be difficult to distinguish inflammatory soft-tissue infection with reactive periostitis from true PSO. In such cases, MRI provides better visualization of soft-tissue planes and inflammatory changes in the bone, an excellent resolution, and multiplanar capacity that allow precise delineation of the extent of soft tissue and osseous involvement, which are invaluable in the pre-surgical assessment. (4) In our patient, MRI showed a more extensive osseous and soft-tissue involvement, pleural thickening, excluded mediastinum involvement, and enabled surgeons to tailor surgical therapy and achieve a complete debridement.

A prompt diagnosis of PSO is essential for appropriate treatment and to avoid extension to the mediastinum. US may have a role in the assessment of sternal abnormalities, and provides an accessible means of obtaining tissue samples. Both CT and MRI are extremely useful in the diagnosis, but we favor MRI as the method of choice for assessing its extension and for surgical planning.

I. Pinilla, MD, PhD

C. Martin-Hervas, MD, PhD

E. Gil-Garay, MD, PhD

Departments of Radiology and Orthopedic Surgery

Hospital Universitario La Paz

Madrid, Spain

References

1. Franquet T, Gimenez A, Alegret X, et al. Imaging findings of sternal abnormalities. Eur-Radiol 1997;7:492-497.

2. Lin JC, Miller SR, Gazzaniga AB. Primary sternal osteomyelitis. Ann Thorac Surg 1996; 61:225-227.

3. Yenson A, deFries HO, Deeb ZE. Actinomycotic osteomyelitis of the facial bones and mandible. Otolaryngol Head Neck Surg 1983; 91:173-176.

4. Moylett E, Chung T, Baker C. Magnetic resonance imaging in a child with primary sternal osteomyelitis. Pediatr Infect Dis J 2001;20:547-550.