Background: Low skeletal mass, often present at hospital admission, has been associated with poor prognoses.

Aim: To explore the association between computed tomography (CT)-derived skeletal muscle mass at the lumbar level and short- and long-term mortality in critically ill patients.

Methods: Following PRISMA 2020 guidelines, we included studies on critically ill adults (≥ 18 years) hospitalized in intensive care units (ICU) that measured CT-derived skeletal muscle mass at the lumbar vertebral level within ± 7 days of ICU admission. The primary outcome was mortality, categorized as short-term (including ICU, hospital, 28- and 30-day mortality) and long-term (> 30 days) mortality. MEDLINE and Embase databases were searched without date restrictions. Study screening was performed using Rayyan, data extraction was guided by a custom-designed tool, and quality assessment was performed using the JBI Cohort Study Checklist. A meta-analysis was conducted, focusing on studies that reported short- and long-term mortality among patients with preserved and reduced skeletal muscle. A prevalence meta-analysis was also performed for studies that reported the size of subgroups with low muscle mass.

Results: Out of 1248 unique records, 35 studies met the inclusion criteria, involving 9366 participants. The majority were retrospective, single-centre studies conducted on four continents and included heterogeneous populations such as patients with sepsis, COVID-19 and trauma. Sample sizes ranged from 36 to 939, with a wide age range, from 40 to 70 s, and a predominance of male patients (62%). Skeletal mass was most commonly reported as skeletal muscle index at the third lumbar vertebra. Studies reported mainly short-term mortality on day 28 or 30. Long-term mortality, measured at 90 days, 6 months, and 1 year, was evaluated in 11 studies. Meta-analyses revealed that low skeletal muscle mass area and index were significantly associated with increased risks of both short (OR = 2.33, CI 1.90-2.87, I²  = 41.39%)-and long-term mortality (OR = 2.67, CI 1.45-4.92, I²  = 62.24%). The overall prevalence of low muscle mass was 42% (CI 34-49%, I²  = 98.2%).

Conclusions: CT-assessed skeletal muscle mass at the lumbar level on admission to ICU is associated with both short- and long-term mortality. It may serve as a prognostic marker in critically ill patients. Standardized protocols for measuring and defining low skeletal muscle mass in this population are essential to improve comparability across studies.