Background Non-radiographic axial spondyloarthritis (nr-axSpA) is characterized by lack of definitive radiographic sacroiliitis and is considered an early stage of ankylosing spondylitis (AS). MR is an ideal imaging tool for diagnosing early sacroiliitis and monitoring disease outcome and following patients after treatment interventions.
Objectives We aimed to compare MR-defined cross-sectional area and semi-quantitative grading of fatty degeneration of lumbar paravertebral muscles in patients with nr-axSpA and established AS.
Methods MRscans of lumbar spine taken on a 1.5T MR equipment were belong to the patients in our cohort who met ASAS classification criteria for axSpA and mNY criteria for AS. Patients’ anthropometric measurements, disease activity and functional measures and laboratory records were used in the analysis. Axial scans were blinded. Cross-sectional area (CSA) of lumbar paravertebral muscles including right and left multifidus (MF), erector spina (ES), psoas (PS) and vertebral body were measured. The CSA of the muscle was divided by the CSA of the vertebral corpus at the same level (muscle-vertebra ratio, MVr) to decrease bias caused by the relative body size of the patients. Fat infiltration within the paravertebral muscles (R and L MF and ES muscles at L4) was visually graded as follows; 0,normal= estimating less than 10% fat within the muscle; 1, slight=estimating more than 10% but less than 25% fat within the muscle; 2, moderate= more than 25% but less than 50% fat infiltration within the muscle; and 3, severe= more than 50% fat within the muscle. These were made on PACS using Infinitt software (INFINITT Healthcare CoLtd, Seoul,Korea). Intra-class correlation coefficient (ICC) was used for inter-rater reliability assessment.
Results Lumbar T2w MR scans of 14 patients (4 female) with nr-axSpA and 22 (4 female) patients with established AS were analyzed. Patients with nr-axSpA were younger (mean 28 vs 36 years) and had shorter symptom duration. Body mass index of patients were similar. CSA of MF, ES and PS muscles or the MVr (at L2, L3,L4 and L5 levels) were quite similar between patients with nr-axSpA and AS. However patients with AS had higher grades of fat infiltration compared to the patients with nr-axSpA (Right L4 grading of MF+ES muscles 1.4±0.73 vs 0.51±0.52, p=0.001; Left, 1.36±0.65 vs 0.38±0.50, p<0.0001). This difference remains significant after adjusting for age and symptom duration. Interestingly anthropometric measures of the spine were negatively correlated with grades of fat infiltration. There was no difference in CSA or grading between right and left site. The inter-rater reliability was good (ICC 0.75 and 0.85)
Conclusions This study demonstrated for the first time that patients with nr-axSpA differ from patients with established AS in terms of fatty infiltration of lumbar paravertebral muscles. Evaluation of paravertebral musculature should be considered when assessing the MR images of the lumbar spine. Semi-quantitative grading is reliable and fatty degeneration of paravertebral muscles seems to be related to chronicity and spinal functions in patients with uSpA and AS.
Disclosure of Interest None Declared