Fat infiltration is moderately associated with disease activity in ankylosing spondylitis: a cross-sectional study: Lumbar paraspinal muscles in ankylosing spondylitis

To assess the association between lumbar paraspinal muscle (multifidus and erector spinae) and psoas muscle volume and fatty infiltration with clinical features in patients with ankylosing spondylitis (AS). This study included 35 patients with AS and 25 healthy controls (HC). Pain, disease activity, and functional status were assessed using the visual analog scale, Bath Ankylosing Spondylitis Disease Activity Index, and Bath Ankylosing Spondylitis Functional Index, respectively. Magnetic resonance imaging was used to compare cross-sectional area and fat infiltration at the L5/S1 level of the paraspinal muscles (multifidus, erector spinae) and psoas major muscles between the AS and healthy groups. The mean age of the patients included in the study was 31.54 ± 11.0 in the AS group and 29.04 ± 7.2 in the HC group (P > .05). The mean duration of disease in the AS group was 3.81 ± 2.8 years. There were no significant differences in age, body weight, height, or body mass index between the AS and HC groups. There was a significant positive correlation between fatty infiltration and disease duration (R = 0.4, P = .023) and Bath Ankylosing Spondylitis Functional Index scores (R = 0.3, P = .022). Increased fatty infiltration of lumbar paraspinal muscles is significantly associated with disease duration and functional disability in AS, indicating that muscle structural changes parallel clinical progression.