Choose wisely: imaging for diagnosis of axial spondyloarthritis

Abstract

Objective: To assess the diagnostic accuracy of radiography (X-ray, XR), CT and MRI of the sacroiliac joints for diagnosis of axial spondyloarthritis (axSpA).

Methods: 163 patients (89 with axSpA; 74 with degenerative conditions) underwent XR, CT and MR. Three blinded experts categorised the imaging findings into axSpA, other diseases or normal in five separate reading rounds (XR, CT, MR, XR +MR, CT +MR). The clinical diagnosis served as reference standard. Sensitivity and specificity for axSpA and inter-rater reliability were compared.

Results: XR showed lower sensitivity (66.3%) than MR (82.0%) and CT (76.4%) and also an inferior specificity of 67.6% vs 86.5% (MR) and 97.3% (CT). XR +MR was similar to MR alone (sensitivity 77.5 %/specificity 87.8%) while CT+MR was superior (75.3 %/97.3%). CT had the best inter-rater reliability (kappa=0.875), followed by MR (0.665) and XR (0.517). XR +MR was similar (0.662) and CT+MR (0.732) superior to MR alone.

Conclusions: XR had inferior diagnostic accuracy and inter-rater reliability compared with cross-sectional imaging. MR alone was similar in diagnostic performance to XR+MR. CT had the best accuracy, strengthening the importance of structural lesions for the differential diagnosis in axSpA.

Keywords: ankylosing; low back pain; magnetic resonance imaging; spondylitis.

Figure 1

Flow chart of patient inclusion. After excluding 19 patients with missing imaging data, a total of 163 patients were included, 89 with the final diagnosis of axSpA. The image datasets were anonymised into five different chunks: radiography, MR and CT alone and MR combined with XR, and MR combined with CT. The datasets were separately presented to the readers. axSpA, axial spondyloarthritis; XR, X-ray.

Figure 2

Frequency of positive and negative findings in radiography (XR), CT, MRI and combinations with resulting diagnostic accuracy values. Numbers are percentages of positive imaging results in patients with and without axSpA. axSpA, axial spondyloarthritis; LR−/+, negative/positive likelihood ratio; SE, sensitivity; SP, specificity.

Figure 3

Imaging examples. (A) Female patient with osteitis condensans (23 years old, HLA-B27 negative, normal CRP). Radiography suggests bilateral erosions and joint space blurring (arrows) with mild sclerosis. However, cross-sectional imaging shows no erosions but some bone marrow oedema (arrows) and sclerosis (arrowheads) consistent with the final diagnosis. (B) Male patient with axSpA (53 years old, HLA-B27 positive, long history of back pain). Radiography shows only mild blurring of the joint space (arrowheads) and capsular calcification (arrow) and was deemed negative by all readers. However, MR and CT show extensive ankylosis (arrowheads) with preservation of only a small portion of the joint space, suggesting advanced axSpA. (C) Female patient with mechanical joint disease (34 years old, HLA-B27 negative, normal CRP). Radiography and T1W MR show extensive sclerosis (arrowheads) and irregularities (arrows) on the left side, MR-STIR extensive bone marrow oedema (arrowheads) and joint fluid (arrow). Both were misclassified by the readers as positive for axSpA. In this patient, only CT ruled out erosions (arrow) and confirmed the diagnosis of osteitis condensans and iliosacral complex as an anatomical variant. (D) Male patient with axSpA (40 years old, HLA-B27 positive, normal CRP). Radiography shows only minor irregularities (arrows) and was deemed negative. MR shows small cysts (arrows) and minor irregularities (arrowhead) as well as some bone marrow oedema on STIR but was judged negative by two of the three readers. Only CT shows very tiny erosions, confirming the diagnosis of axSpA (arrows). axSpA, axial spondyloarthritis; CRP, C reactive protein; STIR, short-tau inversion recovery; XR, X-ray.

Figure 4

Clinical scenarios. (A) the current clinical standard (MR in patients with mNYC negative XR) shows the highest sensitivity but only poor specificity. (B) XR considered positive if sacroiliitis grade three or four unilaterally is present. This increases the specificity, but MR still must be performed in nearly 75% of patients. (C) MR alone outperforms the scenarios with XR as imaging of first choice showing better overall diagnostic accuracy. (D) CT as first-line imaging showed the best diagnostic accuracy and specificity. However, only 3% of CT-negative patients are positive when adding MR, calling into question, whether the additional MRI is beneficial, whatsoever. axSpA, axial spondyloarthritis; DA, diagnostic accuracy; FN, false-negative; FP, false-positive; mNYC, modified New York Criteria; NPV: negative predictive value; PPV: positive predictive value; TN, true-negative; TP, true positive; SE: sensitivity; SP: specificity; XR, X-ray.