Objective The aim of our study was to compare the
magnetic resonance imaging (MRI)-defined cross-sectional
area and semi-quantitative grading of fatty degeneration of
lumbar paravertebral muscles in patients with non-radiographic
axial spondyloarthritis (nr-axSpA) and established
ankylosing spondylitis (AS).
Methods Cross-sectional area (CSA) of lumbar paravertebral
muscles, including the right and left multifidus
(MF), erector spina (ES), psoas (PS), vertebral body and
muscle:vertebra ratio (MVr), was measured. Fat infiltration
of the paravertebral muscles was graded semi-quantitatively.
Results The CSA of the MF, ES and PS muscles and the
MVr (at the L2, L3, L4 and L5 levels) were quite similar
between patients with nr-axSpA (n = 14) and AS
(n = 22). However, patients with AS had higher grades of
fat infiltration than those with nr-axSpA (Right L4 grading
of MF ? ES muscles: 1.4 ± 0.73 vs. 0.51 ± 0.52,
respectively, p = 0.001; left L4 grading: 1.36 ± 0.65 vs.
0.38 ± 0.50, respectively, p\0.0001). This difference
remained significant after adjusting for age and symptom
duration. The inter-rater reliability was good (intraclass
correlation coefficient 0.75 and 0.85).
Conclusions This is first study demonstrating that MRIdefined
fatty degeneration differs between patients with
nr-axSpA and established AS. Semi-quantitative grading is
reliable, and fatty degeneration of paravertebral muscles
seems to be related to chronicity and spinal functions in
patients with nr-axSpA and AS.