1The diagnostic utility of MRI in spondyloarthritis
Introduction
Spondyloarthritis (SpA) comprises a group of diseases, which are phenotypically distinct although they share several clinical, biochemical and radiological characteristics. The clinical presentation of SpA is heterogeneous and no single symptom or clinical, imaging or laboratory finding is pathognomonic for the disease [1]. During the last three decades, different classification criteria based on combinations of various symptoms, clinical, imaging and laboratory findings have guided clinicians to identify patients with SpA [2], [3], [4], [5]. Until recently, confidence in a diagnosis of axial SpA after clinical and biochemical examination was often primarily determined by the presence of sacroiliitis on radiography. This major role of pelvic radiographs for making a diagnosis of SpA was one of the principal reasons for a diagnostic delay of several years [6], because radiography captures only post-inflammatory structural changes and is unable to depict early inflammatory lesions in the sacroiliac joints (SIJs) [7]. By contrast, magnetic resonance imaging (MRI) is more sensitive and superior to scintigraphy [8], [9], computed tomography (CT) [8] and radiography [8], [9], [10] to detect active sacroiliitis in patients with inflammatory back pain who show normal SIJs on pelvic radiographs or equivocal SIJ changes not compatible with definite radiographic sacroiliitis.
The emergence of tumour necrosis factor-alpha (TNFα) inhibitors, a powerful treatment option in patients with ankylosing spondylitis (AS) [11], [12], [13], [14], has called for new and more sensitive criteria to facilitate early diagnosis and treatment. This need to identify SpA patients early in the disease course was addressed by the Assessment of SpondyloArthritis international Society (ASAS) classification criteria for axial *[15], *[16] and for peripheral SpA [17], which were published in 2009 and 2011, respectively. In contrast to previous classification criteria for SpA, the ASAS criteria are the first to include findings on MRI of the SIJ. In these criteria, inflammatory MRI features are weighted equally with structural radiographic changes for detection of sacroiliitis, and imaging has become one of the two major criteria for axial SpA *[15], *[16]. Thus, MRI has become an important tool in daily practice for evaluating patients with early SpA suspected on clinical grounds.
The aims of this review are to: 1) describe the evidence for the use of MRI as a diagnostic aid in SpA, 2) to discuss the strengths and weaknesses of MRI as a diagnostic tool and 3) to summarise how to use MRI in daily routine to facilitate early diagnosis of SpA. This review is primarily based on studies that aimed to investigate the diagnostic utility of MRI in patients with early SpA including patients with back pain due to other reasons and healthy subjects, and it mainly focusses on studies published within the last 5 years.
Section snippets
The ASAS classification criteria for axial and peripheral SpA
According to the ASAS classification criteria for axial SpA [16], a patient with chronic back pain of more than 3 months' duration and symptom onset before the age of 45 years can be classified as having axial SpA if either sacroiliitis is present on imaging (pelvic radiographs or SIJ MRI) and at least one additional clinical or laboratory feature is present (so-called ‘imaging arm’), or if human leucocyte antigen B27 (HLA-B27) is positive and at least two additional SpA-associated features are
The MRI definition of the ASAS classification criteria for axial and peripheral SpA
As sacroiliitis on MRI had not been defined formally before, an ASAS/OMERACT (Outcome Measures in Rheumatology Clinical Trials) MRI working group of radiologists and rheumatologists was established [18]. The aim of the group was to identify and describe MRI findings in sacroiliitis and to reach a consensus on which features are essential for its definition. The group reviewed MRI scans from patients with axial SpA and healthy subjects, and provided a consensus statement (Table 1). A ‘positive’
Development and validation of the ASAS classification criteria for axial SpA
The ASAS classification criteria for axial SpA were developed by a two-step approach *[15], *[16]. In the first study, 20 ASAS experts reviewed standardised clinical records including clinical, biochemical and imaging data from 71 real-life patients referred to a SpA centre with the diagnosis of possible axial SpA [16]. The reviews were performed twice in random order, first excluding information on SIJ MRI (i.e., active inflammation) and second including this information. In this study, a
Development and validation of the ASAS classification criteria for peripheral SpA
The ASAS classification criteria for peripheral SpA were developed using two sets of pre-specified candidate criteria, which were developed by the use of cases from study 1 (described above) and clinical experience [17]. These candidate criteria were tested and modified based on clinical, laboratory and imaging data of 266 consecutive patients referred with peripheral arthritis or enthesitis or dactylitis and suspicion of peripheral SpA. The local rheumatologist diagnosed 176 (66.2%) of the
Diagnostic utility of the ASAS definition and subsequent proposals for a positive SIJ MRI
A series of studies in an international collaboration (the MORPHO studies, named after the similarity of light reflection patterns on the opened wings of rainforest Morpho butterflies with bright BME signal of sacroiliitis on MRI) [19], *[20], *[21] aimed at assessing the diagnostic utility of SIJ MRI to differentiate patients with early axial SpA from patients with non-specific back pain (NSBP) and healthy subjects. The study also compared the diagnostic utility of the ASAS MRI definition with
Diagnostic utility of MRI inflammation of the spine
The diagnostic utility of MRI inflammation of the spine has been investigated in two reports. The first was a retrospective cross-sectional study comprising 174 unselected patients with a mean age of 52.5 years (SD: 17.3; range: 19–91 years), who had an MRI scan of the spine performed for back pain within a 5-year period in a single hospital [29]. The physicians diagnosed 64 (37%) patients with SpA (including 20 with AS), 45 (26%) patients with degenerative spinal disease and 45 (26%) patients
The diagnostic utility of MRI in peripheral SpA
No studies have investigated the diagnostic utility of MRI in peripheral SpA.
General considerations of strengths and weaknesses of MRI in SpA
A major advantage of MRI in axial and peripheral SpA is its ability for detailed assessment of all structures involved in the disease process, such as synovial membrane, cartilage, joint capsule, entheses, ligaments, tendons and bone, with high spatial resolution and good contrast, whereas CT and radiography mainly visualise bone. MRI provides information on both disease activity and structural damage in the same examination, without exposing patients to ionising radiation (a major disadvantage
How and when should MRI be applied for the diagnostic assessment of SpA?
In patients presenting with symptoms indicative of axial SpA (i.e., chronic back pain for more than 3 months and symptom onset before the age of 45 years and at least one SpA-associated feature) or peripheral SpA (arthritis, or entesitis or dactylitis and at least one SpA-associated feature), imaging may be very helpful to confirm the diagnosis in an early disease stage *[15], *[17]. According to the ASAS classification criteria, findings on radiography and MRI of the SIJs are weighted equally
Future directions
Future research should focus on further validation of inflammatory and structural lesions on SIJ and spinal MRI in patients with clinically suspected SpA, in large, prospective, long-term follow-up studies. Most studies investigating the diagnostic utility of MRI in patients with SpA have applied conventional T1w and STIR MRI sequences, but alternative sequences that are more sensitive to detect inflammation [44] and erosions [45] may provide additional information. The MRI evaluation in
Conflict of interest statement
Dr. Pedersen, Dr. Weber and Dr. Østergaard reported no potential conflicts of interest relevant to this article.
Acknowledgements
The authors thank Carol Urfer and Christian Streng, Medical Documentation, Balgrist University Hospital, Zurich, Switzerland, for their technical assistance with some of the figures. Dr. Pedersen's research is funded by the Danish Council for Independent Research in the Medical Sciences.
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