Percutaneous biopsy

Introduction Infective discitis is an inflammatory process involving the intervertebral disc space, often related to infection. It is a rare condition with an incidence of between 0.4 and 2.4 per 100,000 each year in the western world. There is a bimodal age distribution with the first peak in childhood and the second peak around the sixth decade. The lumbar spine is affected in up to 60% of cases, followed by the thoracic and cervical regions. Clinical presentation is often insidious and, therefore, delay in diagnosis for up to six months is not uncommon. Most patients complain of back or neck pain. Fever is present in up to two thirds of patients. In the majority of patients, the inflammatory markers (eg C-reactive protein) will be raised. The most common micro-organism isolated is staphylococcus aureus, implicated in 15-84% of non-tuberculous cases. This is followed in frequency by Gram negative bacilli (E.coli, proteus and pseudomonas) and enterococci. Tuberculous spondylodiscitis is often seen in the immunosuppressed, alcoholics, intravenous drug users and immigrants from sub-Saharan Africa, South East Asia and the Indian subcontinent. S. aureus is also the most commonly isolated pathogen in discitis, complicating invasive spinal procedures. Streptococci are frequently seen in association with a dental port of entry or endocarditis. In most cases, there is usually a preceding history of infection in the skin, soft tissues, genitourinary, respiratory and gastrointestinal tracts. From these sites, haematogenous spread of micro-organisms occur to the vertebral bodies. The organisms are deposited in the end-arterial arcades of the vertebral body metaphysis, resulting in bony ischaemia and infarction. The vertebral endplates are frequently involved and the infection may extend to involve the paraspinal soft tissues, the epidural space and adjacent vertebral bodies. Imaging Plain radiographic findings of infective discitis are not apparent until 2-8 weeks after the initial symptoms. Findings on plain radiographs include narrowing of the disc space, indistinct vertebral endplates and loss of height of the affected vertebral bodies (figure 1). There may also be an abnormal psoas shadow, mediastinal widening or retropharyngeal space enlargement, signs suggestive of paraspinal soft tissue involvement. Absence of these features, however, does not exclude the diagnosis. CT findings highly suggestive of infective discitis include anterior paravertebral soft tissue swelling with obliteration of paravertebral fat planes, fragmentation or erosions of vertebral end plates and paravertebral fluid collections (figure 2). MRI is the most sensitive (93-96%) and specific (92.597%) modality for diagnosis of discitis. It provides better definition of the paravertebral and epidural spaces and allows assessment of neural element compression. It may be able to differentiate between early post-operative fibrosis and infection. The initial inflammatory response to discitis involves bone marrow oedema. Features of infective discitis seen on MRI include: (a) Increased signal intensity of the disc and vertebral body on T2-weighted images; (b) Endplate and disc enhancement with Gadolinium contrast; (c) Presence of paraspinal or epidural inflammation. Epidural and soft tissue abscesses are readily seen on T2-weighted images and post-contrast sequences.