Osteomyelitis

Osteomyelitis: It is an inflammatory process caused by infectious organism that can lead to bone destruction. It should be considered first when skin or soft-tissue infections overlie bone and when localized bone pain accompanies fever or sepsis.

Etiology: Classified based on causative agent, route of infection to bone, duration of infection, anatomic location, local and systemic host factors influencing the pathogenesis and outcome.

    • Acute hematogenous OM is caused most frequently by S. aureus (50%). Acute hematogenous OM, accounts for ~20% of cases of OM. Primarily affects children in whom the long bones are infected (tibia, femur, or humerus). In older adults, and IDU, in whom the spine is the most common site of infection.

      • Group A streptococci is a common pathogen in children

      • Group B streptococci and E. coli are common during the neonatal period.

      • Unusual cases of hematogenous OM include disseminated histoplasmosis, coccidioidomycosis, and blastomycosis in endemic areas.

      • Hematogenous OM with Mycobacterium bovis has been reported following intravesicular instillaiton of BCG for cancer of the bladder.

    • Vertebral OM may be due to E. coli, and other enteric bacilli in ~25% of cases. S. aureus, or Mycobacterium TB or brucellosis are also common. Pseudomonas, Serratia, and Candida albicans infections are associated with IDU. Sources of bacteremia include UTI in men >50 years, dental abscesses, soft tissue infections, and contaminated IV lines, but source of bacteremia is not evident in more than half of the patients.

      • DM requiring insulin inj, recent invasive medical procedure, HD, and IDU carry an increased risk of spinal infection.

      • Pt who have h/o degenerative joint spine involving the spine, report an episode of trauma preceding the onset of infection.

      • Penetrating injuries and surgical procedures involving the spine may cause nonhematogeneous vertebral OM or infection localized to a disk.

    • OM associated with a contiguous focus of infection may be due to S. aureus (>50%), GNB, coagulase-neg staph (surgical-site inf), or anaerobes (inf sacral decub ulcers). Usually polymicrobial.

      • ~80% of all cases of OM in adults.

      • Penetrating injuries, bites, puncture wounds, and open fractures; surgical procedures; and direct extension from adjacent soft tissues.

      • Generalized vascular insufficiency and presence of FB are predisposing factors.

      • A special type of contiguous-focus OM occurs in the setting of PVD, and nearly always involves the small bones of the feet of adults with DM. It is responsible for many thousands of amputations per year. It is often during the evaluation of a non-healing foot ulcer, a swollen toe, or acute cellulitis that the radiography provides the first evidence of OM. If bone is palpable during examination of the base of an ulcer with a blunt surgical probe, OM is likely.

      • Infections are mostly polymicrobial with Staph, Strep, enteric organisms, anaerobes.

      • S. aureus is common organism for post-op inf. Coagulase-negative Staph are common pathogens after implantation of orthopedic appliances, also seen in sternal OM s/p cardiac surgery.

      • P. aeruginosa in nail puncture wounds

      • Pasteurella multocida infection commonly follows cat bites.

    • OM in presence of ortho devices is most often caused by S. aureus or coagulase-negative Staph. species.

    • OM assoc w/ Hb-pathies is most often caused by S. aureus or Salmonella species.

    • Chronic OM is usually assoc w/ a sequestrum of necrotic bone and may involve gram-negative pathogens as well as S. aureus.

Dx:

  • First changes in bone - periosteal reaction - is not evident until at least 10 days to 2 weeks, after onset of infection. Lytic changes can be detected only after 2 - 6 wks, when 50-70% of bone density has been lost. Rarely, a well-circumscribed lytic lesion, or Brodie's abscess, is seen in a child who has been in pain for several months but has had no fever. Hence, diagnosis is made by detection of bone exposed through the skin ulcer or by imaging with plain films, bone scintigraphy, or MRI.

  • CT, MRI may demonstrate epidural, paraspinal, retropharyngeal, mediastinal, retroperitoneal, or psoas abscesses that originate in the spine. Epidural abscess may evolve suddenly or over several week; the classic clinical presentation is spinal pain progressing to radicular pain and/or weakness. Irreversible paralysis can occur if this is not recognized. MRI is the best test for detection of epidural abscess and should be performed in all cases of vertebral OM accompanied by subjective weakness or objective neurologic abnormalities.

  • Bx and cultures of the affected bone should be performed (before initiation of antimicrobials, if possible) for pathogen-directed therapy. BC are positive in only 20-50% of the time.

  • ESR and CRP are usually markedly elevated and can be used to monitor the response to therapy.

Tx:

  • If causative organism is not identified, empiric therapy should be selected to cover S. aureus (oxacillin or vancomycin) and any other likely pathogens

  • Cure typically requires at least 4-6 weeks of high-dose antimicrobial therapy. Parenteral therapy should be given initially; oral regimens may be considered after 2 - 3 weeks only if the pathogen is susceptible and adequate bactericidal levels can be achieved.

  • Acute hematogenous OM. In the absence of vascular insufficiency or a FB, this disease can be treated with antimicrobial therapy alone.

  • OM associated with vascular insufficiency (e.g., DM patients) is seldom cured by drug tx alone; revascularization, debridement, or amputation is often needed. Inf are generally polymicrobial, including anaerobes.

  • OM in the presence of orthopedic devices is rarely eradicated by antimicrobials alone. Cure typically requires removal of the device. When removal is impossible, the addition of rifampin, 300 mg PO tid, is recommended. Long-term, suppression antimicrobial therapy may be needed.

  • Salmonella OM may require surgical tx and administration of a 3G cephalosporin or ciprofloxacin.

  • Chronic OM. Eradication requires a combination of medical and surgical treatment to remove the persistent nidus of infection. Long-term, suppressive antimicrobial therapy can be used if surgery is not feasible. Hyperbaric oxygen may be a useful adjunctive therapy.

Scans are performed using a gamma camera at three points following tracer injection: immediately after injection (blood flow phase), 15 minutes after injection (blood pool phase), and four hours after injection (osseous phase).

In the setting of osteomyelitis, there is intense uptake in all three phases. In contrast, in the setting of cellulitis there is increased activity only in the first two phases and normal or mild diffuse increased activity in the third phase.

Other radionuclide scans: Gallium scan, Indium-labelled WBC (more specific).

US may detect subperiosteal fluid collection or soft tissue abscess adjacent to bone, but largely supplanted by CT and MRI.

MRI is sensitive for acute OM. Detects changes in the water content of bone marrow before disruption of cortical bone. Test of choice for vertebral OM. Use may be limited by a metallic FB.

CT has limited role in acute OM. In chronic OM is good for detection of sequestra, cortical destruction, soft tissue abscesses, and sinus tracts. Use is limited in the presence of metallic FB.

  • Best initial test: X-ray.

  • If the X-ray does not show anything, do MRI. You must lose >50% of the calcium content in the bone before the X-ray becomes abnormal

  • Earliest finding of OM on X-ray: periosteal elevation.

  • Most accurate diagnostic test is a bone biopsy and culture.

  • MRI is superior to bone scan. Bone scan is poor to distinguish between infection in the bone vs infection in soft tissue above it.

  • MRI and bone scan are equally sensitive. They both have an equal ability to exclude OM, if they are normal. MRI, however, is far more specific.

  • Don't swab the wound overlying area of suspected OM, it will send you on a wild goose chase. If you treat on this basis, chances are 50/50 that you are treating the right organism.

  • If patients have no fever, or ▲ WBC, check ESR. If ESR is still markedly high after 4-6 weeks of therapy, further treatment and possibly surgical debridement is required.

  • ESR is the best method of following a response to therapy.

  • CRP is always elevated.

  • Check blood cultures, if the patient looks septic. OM spreads both hematogenously as well as contiguous spread from overlying tissue.

Three phase bone scan — Three phase bone scans uses a radionuclide tracer that accumulates in areas of bone turnover and increased osteoblast activity (such as technetium-99m bound to a phosphorus containing compound).

Selection of ABx for Tx of Acute OM:

  • S. auerus

    • (MSSA):

      • Naficillin or oxacillin, 2 g IV q4h.

      • Alternatives: Cefazolin, 1 g IV q8h; ceftriaxone, 1 g IV q24h; clindamycin, 900 mg IV q8h

      • PCN, 3-4 million U IV q4h

    • (MRSA):

      • Vancomycin, 15 mg/kg IV q12h; rifampin, 300 mg PO q12h.

      • Alternatives: Clindamycin, 900 mg IV q8h; linezolid 600 mg IV or PO q24h (anectodal reports of efficacy); daptomycin, 4-6 mg/kg IV q24h (anectodal)

  • Streptococci (including S. milleri, beta-hemolytic streptococci):

      • PCN, 3-4 million U IV q4h. Alternatives: Cefazolin, ceftriaxone, clindamycin.

  • Gram-negative aerobic bacilli:

    • E. coli, other "sensitive" species: Ampicillin, 2 g IV q4h; cefazolin, 1 g IV q8h.

    • Alternatives: Ceftriaxone, 1 g IV q24h; parenteral or PO FQ (cipro, 400 mg IV or 750 mg PO q12h). (Do not administer FQ with divalent cations (Ca, Mg, Fe, Al, as they block the drug's absorption)

    • Pseudomonas. aeruginosa: ESBL agent (piperacillin, 3-4 g IV q4-6h; or ceftazidime, 2 g IV q12h) + tobramycin, 5-7 mg/kg q24h).

    • Alternatives: Substitute parenteral or PO FQ for beta-lactam agents (if Pt. allergic) or for tobramycin (in relation to nephrotoxicity).

  • Enterobacter spp. other "resistant" species: ESBL agent IV or FQ IV or PO as above.

  • Mixed infections possibly anaerobic bacteria: Ampicillin/sulbactam, 1.5-3 g IV q6h; piperacillin/tazobactam, 3.375 g IV q6h. Alternatives: Carbapenem ABx or a combination of a FQ + clindamycin (as above) or metronidazole, 500 mg PO tid.