02. Osteomyelitis

Overview

Acute (generally symptomatic for < 2 weeks) or chronic (generally > 2 weeks duration) infection of the bone.

Risk Factors

  • Diabetes (5% lifetime risk)
  • Bacteremia 
  • Skin and soft tissue infections 
  • Trauma

Etiology 

  • Contiguous-focus: most common type. Occurs after injury or extension from a soft tissue infection. Risk factors include prosthetic joints, pressure ulcers, NSG and trauma.
    • Organisms: S. aureus and Staph epi (in prosthetic joints) are most common; GNRs, anaerobes and polymicrobial infections are also seen.
  • Diabetic foot infections: subtype of contiguous-focus osteo but with vascular insufficiency
    • Organisms: mixed infections are common. Staph, Strep, Enterococcus, CoNS, GNRs, and anaerobes.
  • Hematogenous osteomyelitis (e.g., vertebral): more common in IVDU, sickle cell disease (SCD), DM, and among the elderly. Usually metastatic spread of existing infection due to transient or continuous bacteremia.
    • Organisms: S. aureus is most common, GNRs (including Pseudomonas), Strep, TB; Salmonella can be seen in SCD.

Evaluation

Clinical presentation:

  • Fevers in <50%, pain is common (may not be present with diabetic neuropathy), localized infection with cellulitis, sinus tract, back pain.
  • Vertebral osteo: extension of infection may lead to epidural and subdural abscess, meningitis, and cord compression. Site: Lumbar (60%) > thoracic (30%) > cervical (10%). Endocarditis may be associated with vertebral disease in up to 30% of patients. Back pain and spinal tenderness are seen in >80% of patients.
  • In diabetic foot ulcers, if you can probe an ulcer down to bone, osteomyelitis is highly likely (sensitivity 66%, PPV 85%).
  • High risk DM ulcers are: >3mm deep or 2 cm2, >2 wks duration, ESR >70.

Diagnosis:

  • Work up: CBC, ESR/CRP, blood cultures, imaging, +/- bone bx.
    • WBC elevated in <50% cases, ESR and CRP elevated in 70-75%.
    • Blood cultures are positive in <50-60% of cases. Higher yield with vertebral osteo and hematogenous spread.
    • Superficial wound cultures and sinus tract cultures are unreliable and don’t correlate well with bone cultures.
  • Imaging:
    • Plain films have low sensitivity, may be more diagnostic after 2-3 weeks.
    • MRI is preferred with >90% sensitivity and specificity.
    • Radionuclide bone scan combined with tagged WBC scan is diagnostically useful when MRI cannot be performed (e.g., iron-based prosthetic joints, valves, clips, etc.), but is less specific.
    • Spinal TB: classically involves the lower thoracic and upper lumbar vertebrae, starting along the anterior aspect and spares the disc space in the early phases of disease (but this pattern is not always seen). It may also involve multiple distinct levels (aka "skip lesions"), and can be associated with “scalloping” of anterior portions of vertebrae. 
  • Consider fungi and acid-fast bacilli cultures when onset is insidious, bacterial cultures are negative, or risk factors are present.
  • Bone aspiration/biopsy (CT guided FNA) with gram stain and culture is necessary for microbiologic diagnosis and antibiotic therapy. Note that bones may have patchy involvement and false negatives are common: the yield of needle biopsy is only ~50-60%. If a patient is undergoing biopsy, try to send for pathology as well as bacterial, fungal, and AFB stains and cultures. If enough sample can be obtained, save extra tissue on hold in the micro lab for universal PCR if cultures are negative.

Management 

Unless patient is critically ill (or impending neurologic complication), DO NOT START ANTIBIOTICS until adequate deep tissue/bone cultures are obtained.

  • In general, ID should be consulted for all documented osteo cases to guide diagnostic work-up and antibiotics; surgical specialists can help with surgical management or obtaining cultures.
  • Treatment varies depending on pathogen(s) and drug-resistance(s).
  • Prolonged antibiotic therapy (at least 6 weeks) is usually required for osteomyelitis. Often at least the initial course is given IV, but there is now more data for using oral antibiotics for treatment of bone and joint infections (e.g., the OVIVA trial). The decision on whether to use oral antibiotics and which agents to use should be made in conjunction with the ID team. Factors to consider are the infecting organism and its susceptibility, clinical details of the case (e.g. presence of an epidural abscess or prosthetic material, plan for surgical debridement), and contraindications to oral agents (in particular rifampin). A common oral antibiotic used in this situation is a fluoroquinolone (with addition of rifampin for S. aureus).
  • Antibiotics plus surgical debridement and removal of devitalized tissue are necessary for treating chronic osteomyelitis and contiguous-spread osteomyelitis.
  • Vertebral body osteo and epidural abscess may require urgent NSG decompression.

Key Points

  •  Document a neurological exam in patients with back pain and suspected osteomyelitis.
  •  Inability to probe to bone does NOT rule out osteomyelitis.
  •  Avoid antibiotics before biopsy unless the patient is unstable.
  •  Up to 14% patients with vertebral osteo will relapse within 1 year.
  •  Normal ESR or CRP does not rule out osteomyelitis.

References

Berbari EF, Kanj SS, Kowalski TJ, et al, Infectious Diseases Society of America. 2015 Infectious Diseases Society of America (IDSA) Clinical Practice Guidelines for the Diagnosis and Treatment of Native Vertebral Osteomyelitis in Adults. Clin Infect Dis. 2015 Sep 15;61(6):e26-46.

Li HK, Rombach I, Zambellas R, et al. Oral versus Intravenous Antibiotics for Bone and Joint Infection. N Engl J Med. 2019 Jan 31;380(5):425-436.

Lew DP, Waldvogel FA. Osteomyelitis. Lancet 2004 364:369-379.

Mandell GL, Bennett JE, Dolin R. Principles and Practice of Infectious Diseases, Sixth Edition. Churchill Livingstone;2005.

Zimmerli W. Clinical Practice: Verebral Osteomyeliitis. N Eng J Med. 2010. Mar 18;362:1922-1029.

Termaat MF, Raijmakers PG, Scholten HJ, et al. The accuracy of diagnostic imaging for assessment of chronic osteomyelitis. J Bone Joint Surg Am 2005;87:2464-2471.

Papadakis M, McPhee S. Current Medical Diagnosis and Treatment 2015. Fifty-Fourth Edition. Lange: 2015.

Key words: Osteomyelitis, Diabetic foot ulcer, vertebral osteomyelitis, Spinal Tuberculosis