Serial ultrasound assessment of flexor carpi ulnaris and biceps brachii cross-sectional area in critically ill patients: A prospective cohort study

Background Serial ultrasound measurements may help characterize muscle wasting in critically ill patients. The flexor carpi ulnaris (FCU) is a superficial forearm muscle that may provide an additional assessment site. We described longitudinal FCU and biceps brachii (BB) cross-sectional area (CSA) changes and explored between-muscle differences and clinical associations. Methods This prospective two-center observational study enrolled 23 intensive care unit (ICU) patients; 20 were included in the analysis (median age, 57 years; range, 33–82). CSA was measured on days 0, 5, 7, and 14 after ICU admission. Analyses used Friedman tests, exact conditional Wilcoxon signed-rank tests with Bonferroni correction, and exploratory Spearman correlations. Post hoc between-muscle comparisons assessed participant-level percentage changes. Results Mean FCU CSA decreased from 1.36 to 1.08 cm² and mean BB CSA from 6.08 to 5.09 cm² over 14 days, corresponding to reductions of 20.9% and 16.4% in the unrounded group means. The median paired difference in percentage change (FCU minus BB) was −11.9 percentage points on day 7 (unadjusted p = 0.0023), meeting the Bonferroni threshold of 0.05/3. Differences on days 5 and 14 did not meet this threshold (p = 0.1429 and p = 0.2024). Baseline CSA values were correlated (ρ = 0.845, p < 0.001), as were 14-day percentage changes (ρ = 0.549, p = 0.0122); correlation p-values were unadjusted. Conclusions Serial ultrasound documented decreases in FCU and BB CSA in this small cohort, with an exploratory direct comparison identifying a greater proportional FCU decrease on day 7. These findings support further evaluation of FCU ultrasound as an additional muscle-monitoring measure. Its value for earlier detection of muscle wasting or diagnosis of ICU-acquired weakness requires direct validation.