Thursday, January 21, 2010

SUBACUTE HEMATOGENOUS OSTEOMYELITIS

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SUBACUTE HEMATOGENOUS OSTEOMYELITIS

Compared with acute osteomyelitis, subacute hematogenous osteomyelitis has a more insidious onset and lacks the severity of symptoms, which makes the diagnosis of this disorder difficult. Subacute osteomyelitis is relatively common. Because of the indolent course of subacute osteomyelitis, diagnosis typically is delayed for more than 2 weeks. Systemic signs and symptoms are minimal. Temperature is only mildly elevated if at all. Mild-to-moderate pain is one of the only consistent signs suggesting the diagnosis. White blood cell counts generally are normal. The ESRis elevated in only 50% of patients, and blood cultures usually are negative. Even with an adequate bone aspirate or biopsy specimen, a pathogen is identified only 60% of the time. Plain radiographs and bone scans generally are positive .          

The indolent course of subacute osteomyelitis is thought to be the result of increased host resistance, decreased bacterial virulence, or the administration of antibiotics before the onset of symptoms. It is speculated that the combination of an organism of low virulence with a strong host response may allow the inflammation to persist in bone without producing significant signs or symptoms. Nevertheless, correct diagnosis largely depends on clinical suspicion and radiographic findings. 
Table   --  Subacute Osteomyelitis
Type
Gledhill Classification
Robert et al. Classification
Differential Diagnosis
I
Solitary localized zone of radiolucency surrounded by reactive new bone formation
Ia—Punched-out radiolucency
Langerhans' cell histiocytosis


Ib—Punched-out radiolucent lesion with sclerotic margin
Brodie abscess
II
Metaphyseal radiolucencies with cortical erosion

Eosinophilic granuloma; osteogenic sarcoma
III
Cortical hyperostosis in diaphysis; no onion skinning
Localized cortical and periosteal reaction
Osteoid osteoma
IV
Subperiosteal new bone and onion skin layering
Onion skin periosteal reaction
Ewing sarcoma
V

Central radiolucency in epiphysis
Chondroblastoma
VI

Destructive process involving vertebral body
Tuberculosis; osteogenic sarcoma
 A radiographic classification of subacute hematogenous osteomyelitis was described by Gledhill and modified by Roberts et al. (Table). Differentiating these lesions from a primary bone tumor sometimes can be difficult. The diagnosis often must be established by an open biopsy and culture. Purulent material is not always obtained on biopsy, but granulation tissue is a common finding. S. aureus and Staphylococcus epidermidis are the predominant organisms identified in subacute osteomyelitis. 

Ross and Cole recommended biopsy and curettage followed by treatment with appropriate antibiotics for all lesions that seem to be aggressive. For lesions that seem to be a simple abscess in the epiphysis or metaphysis, biopsy is not recommended. These lesions, which are characteristic of subacute hematogenous osteomyelitis, should be treated with intravenous antibiotics for 48 hours followed by a 6-week course of oral antibiotics. They suggested open biopsy and curettage only for aggressive-appearing lesions or for lesions that do not respond to antibiotic treatment alone.



Brodie Abscess
A Brodie abscess is a localized form of subacute osteomyelitis that occurs most often in the long bones of the lower extremities of young adults. Before physeal closure, the metaphysis is most often affected. In adults, the metaphyseal-epiphyseal area is involved. Intermittent pain of long duration is the presenting complaint, along with local tenderness over the affected area. On plain radiographs, a Brodie abscess generally appears as a lytic lesion with a rim of sclerotic bone, but it can have a markedly varied appearance. Careful evaluation of plain films is mandatory because a Brodie abscess can be easily mistaken for a variety of neoplasms.
Organisms of low virulence are believed to cause the lesion. S. aureus is cultured in 50% of patients; in 20%, the culture is negative. This condition often requires an open biopsy with curettage to make the diagnosis. The wound should be closed loosely over a drain.