Lumbrical muscle enhancement on MRI and its association with rheumatoid arthritis

To investigate the significance of lumbrical muscle enhancement (LME) on magnetic resonance imaging (MRI) in rheumatoid arthritis (RA).

Blinded to the diagnoses, contrast-enhanced bilateral hand MRIs of patients with suspected early RA between 2014 and 2019 were reviewed by two observers for the presence and degree (weak/strong) of LME. The presence of other inflammatory findings was also noted. The patients were then stratified into RA (n = 41), control (n = 31), and other arthritides groups(n = 28) based on their final diagnoses in the hospital records within the following 12 months. Categorical variables were compared by chi-square test or Fisher's exact test. Differences among the groups were evaluated by one-way ANOVA or Kruskal-Wallis tests. When the p-value from the Kruskal-Wallis test was statistically significant, multiple comparison test was used to identify group differences. Correlations between LME and flexor tenosynovitis were evaluated by Spearman rank correlation test. The agreement between two observers was assessed by Cohen's Kappa (κ) statistic. P-value < 0.05 was considered as statistically significant.

There were 100 patients (88 females) with mean age of 47.2 ± 11.2. There were no significant differences for age or sex between groups (p = 0.17, p = 0.84, respectively). RA patients showed significantly more frequent (p < 0.001) and stronger LME (p = 0.001). There were no correlations between LME and flexor tenosynovitis (p > 0.05). Interrater agreement for the degree of LME on right and left sides was substantial (κ = 0.74, κ = 0.67, respectively).

RA patients demonstrated significantly more frequent and stronger LME with substantial interrater agreement. LME could constitute a subtle radiological clue for early RA.