Osteomyelitis

Definition

Infection of the bone, usually starting in the medullary area.

Pathogenesis

The majority of cases in children are secondary to hematogenous spread. There may also direct inoculation of bacteria by penetrating injury or surgical intervention, or rarely, spread from an adjacent focus into the bone.

Infection normally begins in the metaphysis of long bones (especially the tibia, femur,  and humerus), often after trauma, but any bone can be affected. More commonly affected areas have a high density of slow-flowing sinusoidal vessels that allow for traumatic thrombosis and infarction, creating a possible nidus for infection that is then seeded by transient bacteremia.

In older children and adolescents, infection is usually confined to the metaphyseal cortex. However in neonates, the thin cortex allows for rupture into surrounding soft tissue and joint space.

Etiologies

S. aureus is by far the most common organism. MRSA has been associated with more severe complications including myositis, polymyositis, intraosseous or subperiostel abcess, DVT, septic emboli, multiple sites of infection, and chronic infection.

Strep pneumonae, group A strep (especially when complicating varicella) and  are also common. H. influenza was common before vaccination, but is now rare.

Some clinical situations may suggest certain bacteria. In neonates, the most common causes are group B strep  and gram negative enterics. In patients with hemoglobinopathies, salmonella should be considered.  Infection as the result of puncture wounds, particularly to the foot, suggests pseudomonas. Multiple sites of infection suggests MRSA. Infection as the result of continguos spread is often polymicrobial.  Chronic infection, which is rare in children, is more likely to be caused by MRSA or gram negative enterics.

In 50% of cases, a bacterial cause is never identified.           

Clinical Presentation

Most children present within 2 weeks of infection with sudden onset of fever, irritability, local erythema, swelling, tenderness, joint effusion, and decreased range of motion, but these symptoms vary by age.

 Infants- irritable when affected bone is touched and may have pseudoparalysis secondary to susceptibility to local spread. There is often swelling and redness of the area. Remember that neonates especially may present with an increase or decrease in body temperature.

Toddlers- may have limp and point tenderness. There may be fever.

Older children- limp and fever. Most cases occur before  the age of 5 years.

Differential Diagnosis

Fever and bone pain is osteomyelitis until proven otherwise.

Differential includes:

  • Septic joint
  • Trauma or fracture
  • Leukemia or bone malignancy
  • Rheumatic etiology
  • Bone infarction associated with Sickle Cell disease
  • Toxic Synovitis

Diagnosis

Diagnosis of osteomyelitis is clinical, based on its rapid onset, presence of fever, localizing symptoms and systemic illness. Lab tests and imaging may help support diagnosis, rule out other causes, and guide treatment.

Labs

 Elevated ESR, CRP and WBC can support diagnosis. CRP can be followed for response to treatment, so having a baseline measurement is important.

Cultures

             Blood cultures should be drawn but will be positive only 50% of the time. If negative, bone biopsy or aspirate should be cultured for microbe and its susceptibilities. This can be obtained under CT or ultrasound guidance.

Imaging

Plain films may not be positive for 10-14 days or longer, but can help to rule out malignancy or fracture.

MRI findings are present in 3-5 days but are non-specific, including edema of the bone marrow. Evidence of an abcess can be seen using gadolinium contrast. It is especially helpful for visualizing pelvic or vertebral lesions. MRI is also useful for visualizing local spread and may be needed if to guide surgeon if surgery is indicated.

Technetium bone scans are very sensitive but have low specificity. They are helpful if infection is poorly localized or if multiple sites of infection are suspected.

Ultrasound can be helpful in patients with sickle cell anemia to differentiate between periosteal abcess and infarction.

Treatment

Empiric antibiotic treatment is aimed at s. aureus and depends on local resistance patterns. Treatment should be adjusted if an organism can be identified and its susceptibilities are known. Children with bacteremia and multiple sites of infection should be managed in the PICU.

Antimicrobial Choice for empiric therapy

If <10% of community acquired infections are MRSA, parenteral nafcillin or oxacillin, followed by PO cephalexin or dicloxacillin will cover all major pathogens.

If > 10% of community isolates are MRSA, then vancomycin is standard of care. Clindamycin can be used if <10% of community isolates have inducible resistance (D test). Neither clindamycin nor vancomycin will cover Kingella Kingae, which is susceptible to nafcillin or oxacillin, and 2nd or 3rd gen cephalosporins.

If  Pseudomonas is a concern (due to a puncture wound) , ceftazidime or antipseudomonal penicillin plus an aminoglycoside should be used.

If Salmonella  is a concern (presence of hemoglobinopathy), use  ampicillin/amoxicillin if susceptible, otherwise cefotaxime or deftriaxone should be used.

Length of treatment

In otherwise healthy children, treatment is 4 days parenteral, then oral for a total of 4 weeks, on until ESR and CRP are within the normal range. In immunocompromised children, longer parenteral treatement is indicated, and a total of 6 weeks or more or treatment may be needed.

Surgical management

May be needed in some cases, particularly those involving MRSA:

Drainage  is indicated if subperiosteal or intraosseous abcess is present. This can usually be done by IR.

Surgical debridement is indicated for contiguous infection, direct inoculation, or chronic infection.

Hyperbaric oxygen can be attempted if refractory to above treatments.

References

  1. Roy Dennis. Osteomyelitis Pediatrics in Review October 1995
  2. Lew Daniel, Waldvogel Francis. Osteomyelitis. NEJM Vol 336 No.14 April 3. 1997
  3. Sharif I. Current Treatment of Osteomyelitis. Pediatrics in Review January 2005
  4. Kaplan, Sheldon. Osteomyelitis in Children. Infect Dis Clin N Am 19 (2005) 787-797
  5. Hatzenbuehler, J and T Pulling. Diagnosis and Management of Osteomyelitis. Am Fam Physician. 2011;84(9):1027-1033

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