Pictorial ReviewMRI of the axial skeletal manifestations of ankylosing spondylitis
Introduction
Ankylosing spondylitis (AS) is a seronegative arthropathy of unknown aetiology affecting mainly young adults, with men more severely affected than women. Ninety percent of patients with AS exhibit HLA-B27 antigen, and although a quoted prevalence of 0.1–1% seems rather high, it is likely that many cases are mild, manifesting only as pain and stiffness with no other features.1
Clinically, the classical history is one of insidious onset low back pain early in the third decade with sacroiliitis commonly seen at presentation. The hip joints are involved in at least 20% of AS patients, with axial migration of the femoral head, prominent new bone formation and ankylosis.1 A peripheral polyarthritis with predilection for the lower limb occurs less commonly and may demonstrate a shaggy periosteal reaction and eventual ankylosis. Enthesopathy with sub-ligamentous bone resorption, iritis, aortic insufficiency and pulmonary fibrosis are further manifestations of the disease process.
However, AS is chiefly a disease of the spine resulting in flattening of the lumbar lordosis and exaggeration of thoracic kyphosis, together with a limitation in spinal flexion. A reduction in chest expansion is also a characteristic clinical finding. The radiographic features of this disease have been well described, and it is the aim of this pictorial review to illustrate the range of MRI appearances seen in the axial skeleton.
Section snippets
Sacroiliitis
Bilateral symmetrical sacroiliitis is commonly seen at presentation with changes occurring at both the synovial and ligamentous parts of the joint. Patchy periarticular osteoporosis with superficial erosions and foci of sclerosis are early changes, with the joint space initially widened as the erosions become more extensive. However, as the disease progresses further the erosions become sclerotic with consequent joint space narrowing and eventual fusion. As computed tomography (CT) demonstrates
Osteitis
Inflammation at the site of attachment of the anterior longitudinal ligament (ALL) to the vertebral bodies and discs gives rise to erosions at the anterior discovertebral margin. This is termed the Romanus lesion and is an early feature of the disease. The lesions tend to heal by proliferation of sclerotic bone producing vertebral body squaring due to loss of the normal concavity of the anterior vertebral body surface. Typical MR appearances include low SI on T1W images, with marked contrast
Syndesmophytosis
A later manifestation of the healing that follows the Romanus lesion is ossification of the annulus fibrosus, which is seen as vertically oriented bony outgrowths. These syndesmophytes predominate on the anterior and lateral aspects of the spine, and may eventually span the disc space. This ossification can spread to involve both the ALL and paraspinal tissues, and it is the later stages of this process that give rise to the appearance known as the “bamboo” spine. MRI shows thickening of both
Discovertebral lesions
Discovertebral erosion and destruction found in AS was originally described by Andersson in 1937 and is named after him. Discovertebral lesions were later classified by Cawley et al.7 into three types. Type I lesions involve the central portion of the discovertebral junction, which is covered by the cartilaginous end-plate and have the same appearance as Schmorl's node from any cause. MRI shows focal end-plate destruction with intra-osseous disc herniation and associated reactive end-plate
Disc calcification
Conventional teaching holds that the central or eccentric circular or linear calcifications commonly seen within the discs of patients with AS are represented by areas of signal void on MRI. However, there is evidence that some patients with long-standing disease show foci of increased SI on T1W images, which correspond to disc space calcification shown radiographically.12 The hyperintensity of the calcified discs is thought to be related to the physical structure of the calcium deposits and
Facet, costovertebral and costotranverse joint lesions
Erosions and consequent sclerosis of the facet joints throughout the spine leads to flattening of the normal lumbar lordosis, exaggeration of the normal thoracic kyphosis and straightening of the cervical spine. These changes can be difficult to detect on plain radiography especially in the thoracic and lumbar spine, but are well demonstrated on CT. The sclerosis at these joints eventually results in intra-articular calcification and consequent fusion. This, in addition to extra-articular
Ligamentous calcification
The ligamentum flavum, inter-spinous and supra-spinous ligaments are predominantly involved in AS. This is initially manifest on MRI as thickening of these structures. Ossification results in the ligaments assuming the signal characteristics of bone marrow, with increased SI on both T1W and T2W sequences (Fig. 13).
Atlanto-axial lesions
The synovial and ligamentous structures surrounding the odontoid peg are susceptible to inflammation, with consequent bony erosion producing shortening and irregularity of the peg (Fig. 14). Atlanto-axial subluxation may be seen rarely in AS and when present, is usually best demonstrated radiographically. Atlanto-axial fusion may occur late in the disease (Fig. 15).
Complications of spinal involvement
Even trivial falls can be catastrophic for AS patients, who are particularly susceptible to fracture as a consequence of their spinal rigidity and osteoporosis. Indeed, cervical spine fractures occur three times more commonly in these patients than in the general population.13 Fractures through the disc space, the weakest point in the ankylosed spine, are most common with the cervical spine being the most frequently affected region, followed by the thoracolumbar junction.14 The rigidity of the
Role of MRI in diagnosis and management
MRI is useful in both the diagnosis and management of patients with AS. The identification of Romanus lesions on sagittal MR images obtained for investigation of non-specific low back pain should raise the possibility of AS and prompt imaging of the sacroiliac joints.
MRI has also been used to assess the effect of tumour necrosis factor alpha (TNFα) blockade with drugs such as etanercept and infliximab in management of acute inflammatory lesions.17., 18. In the chronic stages of the disease, MRI
Conclusion
The role of MRI in imaging patients with AS has greatly expanded over recent years. This review has illustrated many of the more common imaging appearances that may be encountered.
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Early sacroiliitis in patients with spondyloathropathy: evaluation with dynamic gadolinium-enhanced MR imaging
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Comparison of radiography, computed tomography and magnetic resonance imaging in the detection of sacroiliitis accompanying ankylosing spondylitis
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A comparison of MR and CT in suspected sacroiliitis
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Marginal erosive discovertebral “Romanus” lesions in ankylosing spondylitis demonstrated by contrast enhanced Gd-DTPA magnetic resonance imaging
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Destructive diskovertebral lesions in ankylosing spondylitis
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