Background/Purpose: A recent consensus statement based on a systematic literature review by the Assessment of SpondyloArthritis International Society suggested the presence of ≥3 corner inflammatory lesions (CIL) or of several corner fat lesions (CFL) as candidate definitions for a positive MRI of the spine in axial spondyloarthritis (SpA) [1]. The goals of this study were to determine data-driven cut-off values for spinal CIL and CFL yielding a specificity ≥90%, and to evaluate their diagnostic utility in non-radiographic axial SpA (nr-axSpA) and ankylosing spondylitis (AS).

Methods: The study sample comprised 2 independent cohorts A/B of 130 consecutive patients with back pain ≤50 years newly referred to 2 university clinics, and 20 healthy controls (HC). Patients were classified according to clinical examination and pelvic radiography as having nr-axSpA (n=50), AS (n=33), or mechanical back pain (MBP; n=47). Spinal MRI were assessed by 4 blinded readers according to the standardized CanDen module. Readers recorded bone marrow edema and fat infiltration in the central and lateral compartment of 23 discovertebral units. We calculated cut-off values for CIL and CFL to obtain ≥90% specificity and the corresponding area under the curve (AUC) with confidence interval (CI). Finally, we tested the diagnostic utility (mean sensitivity/specificity over 4 readers) of cut-off values for spinal MRI as proposed in the literature (≥3 CIL [1] and ≥5 CFL [2]) for nr-axSpA and AS patients in both cohorts.

Results: In cohorts A/B, ≥3 CIL were reported in 43.4%/25.0% of nr-axSpA patients, 61.1%/42.7% of AS patients, and 25.0%/10.6% of MBP patients (and in 17.5% of HC in cohort A). Corresponding numbers for ≥5 CFL were 31.6%/43.5%, 47.2%/54.2%, and 32.1%/24.2% (and 23.8%). For cohorts A/B, the rounded lesion cut-offs to obtain ≥90% specificity were 3/2 CIL and 7/10 CFL, respectively. The corresponding AUC for CIL were 0.69 (CI 0.49-0.84) and 0.69 (CI 0.47-0.85) in the 2 cohorts, and for CFL 0.60 (CI 0.43-0.75) and 0.71 (CI 0.56-0.82), respectively. The diagnostic utility of the spinal thresholds of ≥3 CIL and of ≥5 CFL was low in both cohorts when comparing nr-ax SpA versus MBP.

Diagnostic utility of candidate definitions for a positive MRI of the spine in cohorts A/B

Lesion cut-off	Mean   Sensitivity	Mean   Specificity	Positive   LR	Negative   LR
nr-axSpA vs MBP
≥3 CIL	0.43/0.25	0.75/0.89	1.74/2.36	0.75/0.84
≥5 CFL	0.32/0.44	0.68/0.76	0.98/1.80	1.01/0.75
≥7 CFL	0.21/0.34	0.86/0.82	1.47/1.86	0.92/0.81
≥10 CFL	0.12/0.21	0.89/0.90	1.11/2.13	0.99/0.88
AS vs MBP
≥3 CIL	0.61/0.43	0.75/0.89	2.44/4.03	0.52/0.64
≥5 CFL	0.47/0.54	0.68/0.76	1.47/2.23	0.78/0.61
≥7 CFL	0.36/0.47	0.86/0.82	2.53/2.58	0.75/0.65
≥10 CFL	0.19/0.45	0.89/0.90	1.81/4.55	0.90/0.61

LR: Likelihood Ratio

Conclusion:

In this controlled study, the definitions of a positive spinal MRI proposed in a recent consensus statement showed low diagnostic utility in nr-axSpA. While a cut-off of ≥2/≥3 CIL for a positive spinal MRI was optimal, the threshold for CFL was as high as 10.

References. [1] Hermann KG et al. ARD 2012;71:1278. [2] Bennett A et al. ARD 2010;69:891.

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