Ultrasound assessment of muscle atrophy and its association with functional outcomes in critically ill patients: a systematic review and meta-analysis

Critically ill patients experience rapid skeletal muscle wasting due to immobility and systemic inflammation. Traditional imaging modalities—including computed tomography, magnetic resonance imaging, and dual-energy X-ray absorptiometry—can accurately quantify muscle mass but are impractical for monitoring critically ill patients because of cost, radiation, and logistical constraints. Ultrasound offers a bedside, radiation-free, and repeatable alternative capable of assessing both quantitative (cross-sectional area, muscle thickness) and qualitative (echointensity, pennation angle) muscle changes. This systematic review and meta-analysis synthesizes existing evidence regarding ultrasound-derived measures of muscle atrophy monitoring in critically ill adults. A systematic search identified 69 eligible studies evaluating lower-limb muscle ultrasound in the ICU. Meta-analysis demonstrated substantial, progressive atrophy. By Day 7 of ICU admission, the rectus femoris cross-sectional area showed a decline of 15.76% (95% CI − 18.06 to − 13.47, I2 = 88.72%), while quadriceps muscle thickness exhibited a parallel, yet smaller, reduction of 11.00% (95%CI − 14.38 to − 7.62, I2 = 90.63%). Echointensity tended to increase, while pennation angle decreased, reflecting compositional and architectural deterioration. Considerable heterogeneity may reflect differences in patient characteristics, anatomical landmarks, and ultrasound technical factors. Limited data linked early ultrasound changes with ICU-acquired weakness. However, after excluding an outlier, pooled analysis of Day 1–3 rectus femoris cross-sectional area changes showed a statistically significant greater muscle loss in patients who developed ICU-acquired weakness to those who did not (mean difference: − 3.58%; 95% CI − 5.95 to − 1.22, I2 = 0%). Ultrasound detects early, rapid muscle wasting and concurrent quality degradation in critically ill adults. However, very low certainty of evidence due to heterogeneity and methodological limitations hinders clinical translation. Standardized protocols and prospective validation studies are needed to clarify ultrasound’s potential clinical utility in ICU muscle monitoring.