Andersson lesion | Radiology Signs

Answer:

The Andersson lesion is a destructive discovertebral lesion in ankylosing spondylitis, seen as irregular erosion and sclerosis of the vertebral endplates with narrowing or widening of the disc space, and on MRI as marrow oedema and fluid-like signal spanning the disc and adjacent endplates. Two mechanisms are recognised and the distinction is clinically important. The inflammatory lesion is a primary spondylodiscitis of the disease itself and is usually central, multiple and painless or mildly symptomatic. The mechanical or pseudarthrosis type is far more significant: it is a transdiscal or transvertebral stress fracture through an ankylosed, rigid, osteoporotic spine, typically at the thoracolumbar junction, often after trivial injury, and it may involve the posterior elements, in which case it constitutes a three-column unstable injury with a real risk of neurological deficit and progressive kyphosis. Because a fused spine behaves like a long bone, any new or changed back pain in ankylosing spondylitis should be imaged with CT for the posterior elements and MRI for marrow, epidural haematoma and cord compression, and a normal radiograph must not end the assessment. Infective spondylodiscitis is the main differential and is separated by paravertebral and epidural abscess, systemic features and a positive culture, with biopsy where doubt persists.

Why is it called so?

The lesion is named after the Swedish radiologist Andersson, who described the destructive discovertebral change in ankylosing spondylitis.

Pathophysiology

Chronic enthesitis in ankylosing spondylitis leads to syndesmophyte formation, ossification of the annulus and the spinal ligaments and eventually to complete ankylosis, which converts the flexible segmented spine into a single rigid lever arm. At the same time, immobility and inflammation produce marked osteoporosis. Loads that a normal spine would distribute across many mobile segments are concentrated at any residual mobile level or at a point of stress, and the bone fails there. Continued motion across the fracture prevents union and produces a pseudarthrosis, with reactive sclerosis, erosion and fluid, which is why the appearance mimics infection. The inflammatory form, by contrast, results from direct extension of the inflammatory process into the discovertebral junction.

Alternative names: Andersson spondylodiscitis; pseudarthrosis of the ankylosed spine

Other associated named signs: Shiny corner sign, the bamboo spine, the dagger sign and the trolley track sign

References

1. Kim SK, Shin K, Song Y, Lee S, Kim TH. Andersson lesions of whole spine magnetic resonance imaging compared with plain radiography in ankylosing spondylitis. Rheumatol Int. 2016;36(12):1663-1670. PMID: 27460819.