Osteomyelitis, Staphylococcus aureus

Key Features

• S aureus is the cause of approximately 60% of all cases of osteomyelitis

• Osteomyelitis may occur by

  • Direct inoculation (eg, from an open fracture or as a result of surgery or other procedure)

  • Extension from a contiguous focus of infection or open wound

  • Hematogenous spread (more common)

Clinical Findings

• Acute, abrupt local symptoms and systemic toxicity; or insidious onset of vague pain over the site of infection, progressing to local tenderness and constitutional symptoms (fever, malaise, anorexia, night sweats)

• Fever is absent in one-third or more of cases

• Back pain

  • Often the only symptom in vertebral osteomyelitis

  • May be associated with an epidural abscess and spinal cord compression

Diagnosis

• Isolation of S aureus from the blood (60%), bone, or a contiguous focus of a patient with symptoms and signs of focal bone infection

• Bone biopsy and culture should be considered if blood cultures are sterile

• Inflammatory markers (C-reactive protein, erythrocyte sedimentation rare) are typically elevated

• Bone scan and gallium scan can identify the site of bone infection

• Spinal infection (unlike malignancy) traverses the disk space to involve the contiguous vertebral body

• MRI

  • Slightly less sensitive than bone scan but has a specificity of 90%

  • Indicated when epidural abscess is suspected in association with vertebral osteomyelitis

• ¹⁸F-FDG-PET/CT may be useful in the diagnosis of vertebral osteomyelitis as well as the detection of other metastatic sites of infection

Treatment

• Recommended antibiotic duration is 4–6 weeks or longer

• Intravenous regimens are advised during the acute phase

  • Nafcillin or oxacillin, 9–12 g/day in six divided doses, is the drug of choice

  • Cefazolin, 2 g every 8 hours is an alternative

  • Vancomycin, 30 mg/kg/day intravenously divided in two or three doses, may be used for penicillin-allergic patients or in those with infections due to methicillin-resistant strains of S aureus

• In patients who are responding clinically, oral step-down therapy to a rifampin combination regimen appears to be as effective; primary oral therapy may be effective but consultation with an infectious diseases specialists is recommended

• The role of newer agents, daptomycin or linezolid, remains to be defined

• Surgical treatment is often indicated under the following circumstances:

  • Staphylococcal osteomyelitis with associated epidural abscess and spinal cord compression

  • Other abscesses (psoas, paraspinal)

  • Extensive disease

  • Recurrent or persistent infection despite standard medical therapy