Imaging in Axial Spondyloarthritis




Magnetic Structural changes in axial Spondyloarthritis (axSpA) are best identified by conventional radiographs, while magnetic resonance imaging (MRI) is considered the gold standard for assessment of inflammatory changes. Imaging of the axial skeleton is crucial for the diagnosis but also for classification to non-radiographic axSpA according to the 2009 ASAS classification criteria. Despite the existing definitions for a positive MRI for the sacroiliac joints and the spine, these predefined lesions can also be seen in other conditions, representing pitfalls and false-positive conclusions in patients with similar clinical symptoms who do not have SpA. Diagnosis of SpA should result from the combination of clinical, laboratory and imaging findings.







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    Magnetic resonance imaging (MRI) is considered the gold standard for assessment of inflammatory changes. Furthermore, there is increasing evidence that structural changes can also be detected by MRI.


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    Computed tomography is currently the most precise imaging method to depict erosions, sclerosis, and ankylosis – however its use is limited due to relatively high radiation exposure.


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    Imaging is an important but not solitary tool for making a solid diagnosis in patients with inflammatory or degenerative diseases of the axial skeleton.



Key Points


Introduction


The concept of axial spondyloarthritis (axSpA) includes 2 subgroups, ankylosing spondylitis (AS) and nonradiographic axial SpA (nr-axSpA), which are differentiated on the basis of presence or absence of definite structural changes in the sacroiliac joints (SIJ) while no clear decision has been made regarding such changes in the spine. The disease that mainly affects the axial skeleton is characterized by inflammatory (sacroiliitis, spondylitis) and osteoproliferative changes (syndesmophytes, ankylosis). Sacroiliitis, spondylitis, and spondylodiscitis are considered almost pathognomonic for axSpA, because the main differential diagnosis is infection, which occurs relatively rarely in developed countries. How inflammation and new bone formation are linked has remained a matter of debate for several years. Osteodestructive changes such as erosions in both the SIJ and the spine may also occur, although in the spine this is a rare event.


Imaging of the axial skeleton is important for the diagnosis, classification, and monitoring of patients with axial SpA. Whereas structural changes are best identified by conventional radiographs, magnetic resonance imaging (MRI) is considered the gold standard for the assessment of inflammatory changes. Furthermore, there is increasing evidence that structural changes can also be detected by MRI. Computed tomography (CT) has also been used, especially for the SIJ, and is considered the gold standard for the assessment of structural changes in these joints.




Use of imaging for the classification of axial SpA


For several decades, conventional radiographs of the axial skeleton have been essential for the diagnosis and classification of AS, mainly because they are an essential feature of the modified New York criteria. However, the complicated S-shaped anatomy of the SIJ but also the lack of ability of conventional radiographs to detect inflammation have caused problems for differentiation between positive and negative findings regarding a diagnosis of AS and axSpA. This quandary has contributed to the substantial delay in the diagnosis. Another issue that may cause problems with the use of conventional radiographs is that with increasing age, degenerative changes in the axial skeleton become more prevalent. Conventional radiographs are currently used in daily practice to detect structural changes but in the future, CT may also be increasingly used to better detect structural changes in the axial skeleton. In general, CT can be helpful in doubtful cases of chronic changes and for the differential diagnosis against other frequent spinal disorders such as diffuse idiopathic skeletal hyperostosis (DISH).


Scintigraphy has been widely used in the past to diagnose patients with axSpA. However, because of its low sensitivity and specificity, scintigraphy is no longer recommended for routine use in such patients.




Use of imaging for the classification of axial SpA


For several decades, conventional radiographs of the axial skeleton have been essential for the diagnosis and classification of AS, mainly because they are an essential feature of the modified New York criteria. However, the complicated S-shaped anatomy of the SIJ but also the lack of ability of conventional radiographs to detect inflammation have caused problems for differentiation between positive and negative findings regarding a diagnosis of AS and axSpA. This quandary has contributed to the substantial delay in the diagnosis. Another issue that may cause problems with the use of conventional radiographs is that with increasing age, degenerative changes in the axial skeleton become more prevalent. Conventional radiographs are currently used in daily practice to detect structural changes but in the future, CT may also be increasingly used to better detect structural changes in the axial skeleton. In general, CT can be helpful in doubtful cases of chronic changes and for the differential diagnosis against other frequent spinal disorders such as diffuse idiopathic skeletal hyperostosis (DISH).


Scintigraphy has been widely used in the past to diagnose patients with axSpA. However, because of its low sensitivity and specificity, scintigraphy is no longer recommended for routine use in such patients.




Use of imaging for diagnosing AXIAL SpA by conventional radiographs


Conventional Radiographs of the Sacroiliac Joints


As already mentioned, quantification of structural changes is used in the modified New York criteria for the diagnosis of established AS ( Box 1 ).



Box 1




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    Grade 0: normal


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    Grade 1: suspicious changes


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    Grade 2: minimal changes: small localized areas with erosion or sclerosis, without involvement of the joint width


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    Grade 3: definite changes: moderate or advanced sacroiliitis with 1 or more of: erosions, evidence of sclerosis, widening, narrowing, or partial ankylosis


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    Grade 4: severe changes: total ankylosis



Modified New York criteria


Differential Diagnoses and Pitfalls of Conventional Radiographs of the Sacroiliac Joints in Axial Spondyloarthritis


Examples of the most frequent clinical problems are provided here to illustrate the main difficulties and pitfalls in the diagnosis of axSpA.


Extensive sclerosis and osteitis condensans ilii


Extensive sclerosis at the sacral or iliac side of the SIJ can lead to misdiagnosis of AS ( Fig. 1 ). It is important that the joint margins are assessed for the occurrence of erosions and for joint width. Triangular-shaped sclerosis of the iliac side of the SIJ manifests as osteitis condensans ilii, especially in women after pregnancy, although the condition may also occur in men.




Fig. 1


Extensive sclerosis on the iliac part of both sacroiliac joints as a sign of osteitis condensans iliis (triangular hyperostosis) as a differential diagnosis of a bilateral sacroiliitis, in a 49-year-old woman with clinical signs of low back pain.


Diffuse idiopathic skeletal hyperostosis (DISH, Forestier disease)


The SIJ can be irregularly shaped, including some sclerosis mimicking sacroiliitis and also showing bony bridges crossing both sides of the joint ( Fig. 2 ).


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Oct 1, 2017 | Posted by in RHEUMATOLOGY | Comments Off on Imaging in Axial Spondyloarthritis

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