Table 1

Characteristics of two SpA inception cohorts defined clinically and according to pelvic radiographs, and description of MRI lesions as recorded by independent evaluation of sacroiliac joint and spine scans

CohortA (n=62)B (n=88)
Groupnr-axSpAASNSBPHCnr-axSpAASNSBP
Number of subjects1991420312433
Demographic and clinical characteristics
 Male:female (% Male)11:8 (57.9)7:2 (77.8)3:11 (21.4)7:13 (35.0)17:14 (54.8)11:13 (45.8)17:16 (51.5)
 Age (years)32.3 (14.4)30.7 (11.4)*30.7 (14.1)30.6 (6.5)36.2 (12.1)41.5 (7.1)*33.6 (15.7)
 Symptom duration (years)1.2 (2.2)*4.0 (2.2)*NANA10.0 (14.0)*12.5 (13.5)*NA
 HLA B27 positive (%)11 (57.9)8 (88.9)ND†ND†24 (80.0)‡21 (87.5)10 (55.6)‡
 BASDAI (NRS)4.4 (3.1)5.4 (1.5)NANA3.5 (4.4)2.0 (3.4)‡NA
 BASFI (NRS)1.8 (3.9)*2.7 (1.5)NANA0.8 (2.3)*0.6 (2.8)‡NA
 CRP (mg/L)4.0 (5.0)§5.0 (8.5)§ND†ND†2.7 (5.2)8.0 (8.7)‡0.9 (0.9)‡
MRI lesions on subject level: n (%)
 Subjects with ≥1 SIJ lesion
  BMO16.0 (84.2)8.0 (88.9)3.8 (26.8)6.8 (33.8)15.3 (49.2)18.5 (77.1)9.0 (27.3)
  FI10.8 (56.6)6.3 (69.4)3.0 (21.4)4.5 (22.5)15.5 (50.0)20.8 (86.5)8.8 (26.5)
  ER12.0 (63.2)8.8 (97.2)0.8 (5.4)0.3 (1.3)11.0 (35.5)20.3 (84.4)4.5 (13.6)
  ANK0 (0)1.0 (11.1)0 (0)0 (0)1.0 (3.2)5.0 (20.8)0.3 (0.8)
  ≥1 SIJ lesion17.8 (93.4)9.0 (100.0)5.8 (41.1)9.3 (46.3)21.0 (67.7)24.0 (100.0)13.3 (40.2)
 Subjects with ≥1 spine lesion
  BMO12.3 (64.5)6.8 (75.0)9.0 (64.3)9.5 (47.5)16.3 (52.4)17.3 (71.9)12.3 (37.1)
  FI13.3 (69.7)7.3 (80.6)10.5 (75.0)14.0 (70.0)25.0 (80.6)20.8 (86.5)19.3 (58.3)
  ≥1 spine lesion15.8 (82.9)8.3 (91.7)12.8 (91.1)16.3 (81.3)27.5 (88.7)22.3 (92.7)23.0 (69.7)
 Subjects with ≥1 SIJ vs ≥1 spine lesion
  SIJ+/Spine+14.8 (77.6)8.3 (91.7)5.3 (37.5)7.3 (36.3)19.5 (62.9)22.3 (92.7)10.3 (31.1)
  SIJ+/Spine−3.0 (15.8)0.8 (8.3)0.5 (3.6)2.0 (10.0)1.5 (4.8)1.8 (7.3)3.0 (9.1)
  SIJ−/Spine+1.0 (5.3)0 (0)7.5 (53.6)9.0 (45.0)8.0 (25.8)0 (0)12.8 (38.6)
  SIJ−/Spine−0.3 (1.3)0 (0)0.8 (5.4)1.8 (8.8)2.0 (6.5)0 (0)7.0 (21.2)
MRI lesions on group level: mean (range)
 Number of SIJ quadrants
  BMO16.4 (0–52)21.5 (0–52)0.9 (0–7)¶1.0 (0–7)¶4.1 (0–39)11.4 (0–42)1.3 (0–21)¶
  FI6.7 (0–37)22.6 (0–51)1.1 (0–12)¶1.9 (0–24)¶8.8 (0–49)23.6 (0–57)3.2 (0–38)¶
  ER4.3 (0–26)16.1 (0–38)0.2 (0–5)¶0 (0–1)¶2.6 (0–27)12.2 (0–38)0.4 (0–8)¶
  ANK (joint halves)0 (0)1.9 (0–20)0 (0)0 (0)0.1 (0–8)3.3 (0–31)0 (0–2)
 Number of spine lesions
  BMO5.0 (0–30)11.3 (0–51)1.8 (0–7)1.8 (0–29)¶3.3 (0–52)7.1 (0–36)1.2 (0–12)
  FI5.3 (0–37)9.2 (0–43)4.1 (0–14)3.0 (0–18)7.9 (0–61)19.4 (0–124)3.7 (0–27)¶
  CIL3.4 (0–18)6.4 (0–22)1.3 (0–6)1.4 (0–18)¶2.2 (0–26)4.1 (0–22)0.9 (0–12)¶
  CFL3.9 (0–22)6.8 (0–31)3.2 (0–14)2.8 (0–18)6.5 (0–34)14.3 (0–55)3.0 (0–18)¶
  • Values for patient characteristics are the median (IQR) unless otherwise indicated.

  • Descriptive MRI data are the mean number (percentage) of subjects having a specific MRI lesion in ≥1 SIJ quadrant or in ≥1 spinal location over four readers, or the mean number (range) of SIJ quadrants or spinal locations affected by a specific MRI lesion over four readers. SIJ MRIs were assessed according to the MORPHO module,11 and spine MRI according to the CanDen module.17 ,18

  • *p Value ≤0.05 (Fisher's exact test) between cohorts A and B; individual p values for cohort A versus cohort B: median age in AS 30.7 years versus 41.5 years, p=0.009; median symptom duration in nr-axSpA 1.2 years versus 10.0 years, p<0.0001; median symptom duration in AS 4.0 years versus 12.5 years, p=0.0004; median BASFI in nr-axSpA 1.8 versus 0.8, p=0.03.

  • †The reason for not performing laboratory values in patients with NSBP and in HCs of cohort A is stated in the Discussion section.

  • ‡HLA B27 data are based on 30 patients with nr-axSpA; BASDAI, BASFI and CRP values are based on 22 patients with AS; in patients with NSBP, HLA B27 and CRP values are based on 18 patients; the prevalence of HLA B27 in patients with AAU in general is 50–60%.25

  • §CRP values are based on 17 and 8 patients with nr-axSpA and AS, respectively.

  • ¶p Value ≤0.05 (Wilcoxon test) for NSBP versus nr-axSpA and HC versus nr-axSpA for cohort A and cohort B separately.

  • ANK, ankylosis; AS, ankylosing spondylitis; BASDAI, Bath Ankylosing Spondylitis Disease Activity Index; BASFI, Bath Ankylosing Spondylitis Functional Index; BMO, Bone marrow oedema; CIL, corner inflammatory lesion (in central spinal MRI slices); CFL, corner fatty lesion (in central spinal MRI slices); CRP, C reactive protein (reference range ≤5 mg/L); ER, erosion; FI, fat infiltration; HC, healthy control; HLA B27, human leucocyte antigen B27; NA, not applicable; ND, not done; NRS, numerical rating scale; nr-axSpA, non-radiographic axial spondyloarthritis; NSBP, non-specific back pain; SIJ, sacroiliac joint; SpA, spondyloarthritis.