Background: Intensive care unit-acquired weakness (ICU-AW) is a common complication among critically ill patients, including those with COVID-19. While viral myopathy and established ICU-related risk factors predispose patients with COVID-19 to muscle dysfunction, few studies have directly compared muscle wasting and weakness between ICU populations with and without COVID-19 using both structural and functional assessment modalities. Methods: This was a small, non-concurrent, propensity score–matched ICU study which compared muscle wasting and strength in patients with and without COVID-19 who remained in the ICU for ≥ 7 days. Muscle thickness was assessed using ultrasound (US), body composition using bioelectrical impedance analysis (BIA), and functional strength using handgrip dynamometry. Measurements were performed on ICU days 1, 5, and 7. To reduce baseline differences, propensity score matching was applied using illness severity, nutritional risk, and mechanical ventilation parameters. Results: In total, 143 patients were included (101 without COVID-19, 42 with COVID-19). After propensity score matching, 23 pairs were analysed. US revealed significant within-group reductions in muscle thickness over time in both matched cohorts, with no statistically significant between-group differences. BIA-derived phase angle (PhA) values were consistently lower in patients with COVID-19; however, between-group differences in PhA change lost statistical significance after matching. Handgrip dynamometry revealed a significantly higher incidence of muscle weakness in patients with COVID-19 initially, but this difference was non-significant. Absolute and residual strength remained similar between groups. Conclusions: ICU patients both with and without COVID-19 experienced comparable degrees of muscle wasting and weakness when adjusted for baseline characteristics. ICU-AW appears more closely associated with the severity of critical illness and ICU treatments than with SARS-CoV-2 infection itself. US measurements appeared less affected by differences in fluid balance, whereas BIA-derived phase angle was more closely related to hydration status. Handgrip dynamometry provided a simple, objective measure of functional muscle strength at ICU discharge.

Background: Critically ill patients experience rapid muscle wasting during their ICU stay. Ultrasound (US) and bioelectrical impedance analysis (BIA) are widely used to assess muscle mass; however, their accuracy may be affected by fluid balance alterations. This study aimed to compare the reliability of US and BIA in detecting muscle loss under varying fluid balance conditions in ICU patients. Methods: In this prospective observational study, adult ICU patients with an ICU stay of ≥7 days were evaluated on Days 1, 5, and 7. Muscle thickness was measured using US, and phase angle (PhA) using BIA. Cumulative fluid balance, C-reactive protein (CRP), and lactate levels were recorded. Patients were stratified according to cumulative fluid balance. Results: A total of 143 ICU patients were included in the final analysis. US demonstrated a progressive decrease in muscle thickness (−3.54% ± 10.90% from Day 1 to Day 5 and −7.56% ± 11.82% from Day 1 to Day 7 (both p < 0.0001)), whereas BIA showed no significant change in PhA. Positive fluid balance significantly reduced PhA compared with the negative balance group, p < 0.001, whereas no statistically significant effect on US measurements was detected. CRP > 200 mg/L was associated with greater US-detected muscle loss on Day 5, while lactate > 2.5 mmol/L was associated with lower PhA. Conclusions: Ultrasound reliably identified structural muscle wasting in critically ill patients, with no statistically significant effect of fluid balance detected in this cohort. Furthermore, ultrasound measurements were associated with inflammation-related muscle loss. In contrast, BIA was strongly influenced by hydration and perfusion status, limiting its ability to assess true muscle mass loss in the ICU setting.