MAGNETIC RESONANCE IMAGING IN A CHILD WITH PRIMARY STERNAL OSTEOMYELITIS

Abstract

Primary sternal osteomyelitis (PSO) is rare. In a 9-year-old boy with Staphylococcus aureus bacteremia, magnetic resonance imaging helped to promptly establish the diagnosis of PSO. Eleven previously reported cases are reviewed, emphasizing surgical drainage and antimicrobial therapy for optimal outcome. Magnetic resonance imaging may be superior to other imaging studies for the diagnosis of PSO. Primary sternal osteomyelitis (PSO) is a rare clinical entity and typically occurs in adult patients who use illicit intravenous drugs. 1, 2 Within the pediatric population sternal osteomyelitis may be encountered as a complication of open cardiac surgery; recovery is slow and requires extensive surgical intervention. 3 PSO affecting children is infrequent. In the antibiotic era 10 cases of PSO in children 18 years of age or younger have been reported in the English language literature. 4–8 These documented cases include 5 children with sickle cell disease (SCD) whose primary sternal osteomyelitis was caused by Salmonella sp. 8 PSO is characterized by an insidious onset of clinical symptoms with nonspecific findings on physical examination. Early diagnosis and appropriate treatment are of paramount importance if a satisfactory outcome is to be achieved. Effective management includes antibiotic therapy as an adjunct to surgical intervention. 2 Magnetic resonance imaging (MRI) has become an important tool for the diagnosis of osteomyelitis and is considered more sensitive and equal to or more specific than three phase 99mTc-methylene diphosphonate (MDP) bone scan. 9–11 MRI allows superior visualization of soft tissue planes and differentiation of cellulitis from osteomyelitis; it also delineates the extent of tissue injury and abscess formation preoperatively so that specific surgical techniques can be used to optimize tissue healing after sternal infection. 2 Because of the paucity of pediatric cases of PSO, optimal therapy remains uncertain. We report a child with PSO successfully treated with antibiotic therapy and surgical drainage and highlight the usefulness of MRI in providing a prompt and accurate diagnosis. A review of reported pediatric PSO cases is included. Case report. A previously healthy 9-year-old African American boy was admitted to our hospital with a 6-day history of chest pain associated with progressive swelling over the midsternal area. The pain was described as being acute in onset, pleuritic in nature, without radiation and relieved by mild analgesia. There was no history of trauma, cough, night sweats, weight loss or fever before admission. The patient appeared to be in moderate distress secondary to pain. Examination revealed a 5- by 4-cm swelling overlying the midsternal area which was warm, pink, nonfluctuant and acutely tender to palpation. The lung fields were clear to auscultation, and no adenopathy was noted. The remainder of the physical examination was unremarkable. The patient remained afebrile until initiation of antimicrobial therapy on Hospital Day 2 when maximum temperature of 103°F was recorded. Laboratory studies revealed a total white blood cell count of 12 200/mm 3 with 73% polymorphonuclear leukocytes, 9% band forms and 10% lymphocytes. The erythrocyte sedimentation rate was 91 mm/h. Routine chemistries were normal. A blood culture drawn before initiation of antibiotic therapy grew methicillin-susceptible Staphylococcus aureus. Radiographic imaging included a lateral chest radiograph that showed thickened soft tissues overlying the sternum (Fig. 1) and a 99mTc-MDP bone scan detected increased tracer uptake in the sternum. MRI of the sternum revealed findings compatible with osteomyelitis of the first and second sternal segments, with a fluid collection within the intervening joint space (Fig. 2). Nafcillin (200 mg/kg/day) and gentamicin (3 mg/kg/dose) treatment was initiated, and the patient was taken to the operating room where the sternal abscess was incised and drained and necrotic bone was curetted. Six milliliters of purulent material obtained from within the second sternal segment grew methicillin-susceptible Staphylococcus aureus. Clinical improvement was noted by Postoperative Day 2, but the patient remained febrile until Postoperative Day 5. A repeat blood culture, drawn after initiation of antibiotic therapy, was sterile. The patient received 7 days of nafcillin and gentamicin therapy and then completed a further 6 weeks of intravenous clindamycin therapy (40 mg/kg/day) at home. The patient made an uneventful recovery and by Postoperative Week 3 the erythrocyte sedimentation rate was 5 mm/h.Fig. 1: Lateral radiograph of the chest indicating a prominence of the soft tissues (arrows).Fig. 2: T2-weighted image of the sternum (sagittal plane) showing subtle increased signal intensity involving the first and second sternal segments. The abnormal increased signal intensity surrounding the joint space between the first and second segments (arrow) represents infected fluid proven at surgery. Elevation of the posterior sternal periosteum is depicted in this image (arrow).Discussion. Our patient brings to 11 the total reported cases of PSO affecting children 18 years of age or younger during the postantibiotic era and highlights the usefulness of MRI in diagnosing this rare clinical condition. His course is comparable with that of the limited number of cases reported in that there was an insidious onset of nonspecific symptoms associated with gradual swelling over the sternal area. At presentation the presence of localized swelling and pain over the sternal area raised concern regarding an underlying infectious process, but noninfectious and malignant processes could not be excluded. The MRI aided in distinguishing between infection limited to the soft tissues or underlying bone as well as between an infectious or malignant process. Children with PSO caused by Salmonella sp. were included in this review because before the report by Narchi, 8 few cases of Salmonella sp. PSO had been reported and SCD was not identified as a risk factor. Apart from underlying SCD no other factors (e.g. intravenous drug abuse, immunodeficiency, acne fulminans, palmoplantar pustulosis or vascular access infection) were identified in these patients. As with other sites of involvement in acute hematogenous osteomyelitis, primary infection of the sternum appears to have a male preponderance with a male:female ratio of 2.3:1. Symptom onset occurred within 7 days of medical intervention in each of the 11 patients, and the offending pathogen was isolated from bone specimens in 9 (82%); the 2 children with sterile aspirates were pretreated with oral antimicrobials (Table 1). Treatment included surgical exploration of the sternum in all patients followed by intravenous and then oral antibiotics. Complete recovery without morbidity was uniform.Table 1: Reported cases of primary pediatric sternal osteomyelitisThe pathogenesis of PSO is presumably by the hematogenous route. Because of active growth in the metaphysis of the body of the sternum, this is the most frequently involved site of PSO affecting patients <25 years of age. An antecedent history of trauma is absent in the majority of cases. S. aureus is the most frequently isolated pathogen excluding patients with hemoglobinopathies, such as SCD, where Salmonella sp. is more likely . Pseudomonas sp. is rarely isolated from pediatric patients with PSO unless there is a history of intravenous illicit drug use. 1, 2 Other unusual pathogens causing PSO include Mycobacterium tuberculosis, which is even more indolent in presentation and occurs where tuberculosis is endemic, 12 and fungal pathogens in patients with immunodeficiency. 13 The sternum in children is composed of multiple segments or sternebrae separated by fibrocartilage analogous to the vertebrae in the spine. Noninfectious processes such as malignancy or trauma are usually confined to the bony tissue with sparing of the disc space. As can be seen from Figure 2, inflammation of the intervening fibrocartilage was evident highlighting the probability of an infectious process. A timely diagnosis and prompt initiation of treatment can prevent significant morbidity and long term sternal defects. Whereas scintigraphy with three phase 99mTc-MDP has good sensitivity (84 to 100%) and specificity (70 to 96%) for the diagnosis of osteomyelitis in children, MRI has comparable sensitivity and possibly greater specificity. 10, 11 Scintigraphy is indicated in the initial assessment of most patients with suspected osteomyelitis, but MRI should be considered if surgical evaluation is likely; if the spine, pelvis or sternum is involved; or where an appropriate response to medical therapy has not occurred. 10 In our patient the MRI provided not only a specific diagnosis but also excellent preoperative delineation of the extent of soft tissue and bony involvement. Optimal therapy for PSO remains elusive because of its rarity and the lack of clinical experience. From this review of published cases it appears that a combination of surgery, to obtain material for culture and to drain and remove of infected tissue, and antimicrobial therapy is warranted to assure the best outcome. Empiric antibiotics should include an antistaphylococcal agent, such as nafcillin or vancomycin, combined with gentamicin for more severe infection and perhaps a third generation cephalosporin if the child has underlying SCD. Duration of therapy will be guided by clinical response to treatment, but 4 to 6 weeks of parenteral therapy seems appropriate. 5–8 PSO remains an infrequently encountered infection in the pediatric patient. Where MRI techniques are available they provide an advantage over routine bone scanning in evaluating this serious skeletal infection.