Introduction

The coronavirus disease 2019 (COVID-19) pandemic caused by the Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) represents a major challenge with an initially reported lethality rate of 2.3% in China, which reached up to 14.8% in infected patients older than 80 years.1 A high mortality rate affects older patients (>60 years of age), who represent about 90% of the overall death toll caused by the global COVID-19 pandemic.2 COVID-19 infection specifically threatens vulnerable, mostly multimorbid older adults.2-7

Assessment of muscle quantity could further help to identify patients at risk to develop fatal outcome during COVID-19. Particularly reduced muscle mass and deteriorated muscle function has been identified as detrimental condition among older-aged patients,8-10 which has relevant impact on outcome of various diseases.11-13 Pathological reduction of muscle mass is primarily caused by physical inactivity, catabolism, and age-dependent maladaptation.14 Reduced muscle quantity is an important finding associated with impaired mobility and muscle strength,15, 16 which is part of the frailty syndrome.17 Patients affected by frailty face increased vulnerability resulting from a decline in reserve and function that leads to an inadequate capacity to deal with extrinsic and intrinsic stressors.18 Frailty shows a prevalence of 15% in older patients (age >65 years) and a prevalence of even 25% in very old individuals (age >80 years)17 and should be considered in COVID-19 patients.

Radiological sectional imaging can be used reliably to measure muscle quantity and identify patients with low muscle mass.19 The cross-sectional assessment of several muscles at the lumbar vertebral level (L3) or assessment of the psoas muscles both normalized by the body height and the dual X-ray absorptiometry are currently the most widely used techniques.11-13, 20 However, radiological imaging is relatively cost intensive and cumbersome in patients requiring isolation for COVID-19.

Sonographic assessment of the psoas muscle area index (PMAI) as well as the thigh muscle thickness index (TMTI) are alternative approaches for bedside morphometry.21-24 Particularly, sonographic morphometry of the psoas muscle shows a good correlation (r > 0.9, P < 0.05) with corresponding radiologic sectional imaging.21, 22

The aim of this study was to apply ultrasound-based morphometry for a rapid, bedside assessment of muscle quantity in COVID-19 patients. In the absence of an adequate evaluation of the patient's strength and physical performance, fast and easy muscle assessment helps to identify patients at risk to develop fatal outcomes.

We quantified the dimensions of the Musculus psoas major and of the thigh muscle group in patients attending our clinic to obtain morphometric reference values and subsequently assessed sonographic muscle indices in hospitalized COVID-19 patients to predict outcome.

Patients and methods Experimental design and study aims

This prospective observational study followed a three-step design to evaluate different sonographic muscle indices representing muscle quantity as predictor for COVID-19 outcome. (I) Sonographic muscle indices were explored in patients without COVID-19 (reference cohort) in order to obtain benchmark muscle quantities regardless of the underlying acute or chronic pathology. Gender-specific medians of different muscle indices were defined as threshold value for low muscle quantity. (II) Sonographic muscle indices were analysed in a first cohort of COVID-19 patients. COVID-19 patients were stratified by muscle index threshold values to explore association with outcome (e.g. mortality) during a prospective 30 day follow up. (III) Sonographic muscle indices were subsequently validated in a second cohort of COVID-19 patients to confirm their association with mortality during a prospective 30 day follow up.

Additional analyses were designated to determine optimal threshold levels of sonographic muscle indices in COVID-19 patients to predict 30 day mortality. Also association of muscle quantity with other mortality risk factors of COVID-19 was explored. The flow chart and study design is shown in Supporting information, Figure S1.

Patient cohorts

Patients attending the University Medical Center between 20 March 2020 and 14 February 2021 were offered sonographic morphometry. Patients without COVID-19 were analysed from March to August 2020 and served as reference cohort. Two independent COVID-19 cohorts (COVID-19 cohort I + II) were subsequently assessed. The two COVID-19 cohorts consisted of hospitalized patients suffering from COVID-19 during the first and second infection wave in Germany. Patients of the first COVID-19 cohort were included between March and October 2020, and patients of the second COVID-19 cohort participated between November 2020 and February 2021. Patients suffering from degenerative muscular diseases were not approached for this study.

Ethics

Written informed consent was obtained from the participants or from their authorized representative prior to the inclusion in the study. The study was conducted in accordance with the ethical guidelines of the 1975 Declaration of Helsinki and its later amendments. The study protocol was approved by the ethics committee of the Landesärztekammer Rheinland-Pfalz (accession number 2020-15174).

Morphometric measurement

Patients of the reference cohort underwent muscle morphometry during routine diagnostic ultrasonography as indicated by the underlying medical condition. COVID-19 patients received muscle morphometry during sonographic assessment of respiratory distress and infection within 48 h after admission.

Sonography was either performed at the ultrasound ward with an Aplico i800 ultrasound system (Canon, Tokyo Japan) or was performed as a bedside exam under isolation conditions using an ACUSON Freestyle ultrasound system (Siemens Healthcare, Erlangen, Germany) with a wireless ultrasound probe.

All patients lay in supine position for sonographic examination as body position could affect muscle dimensions.25 The psoas muscle was depicted in the frontal plane above the iliac crest and the psoas muscle thickness was measured at the level of the caudal kidney pole to obtain reproducible results (Figure 1A).

Sonographic muscle quantification. Transversal sonographic B-mode section (A) acquired at the height of the iliac cristae. The lower kidney pole, psoas muscle thickness (PMT), psoas muscle area (PMA) and vertebrae are indicated. Transversal sonographic B-mode section (B) of the upper thigh middle, showing the thigh muscle group (Musculus vastus intermedius, Musculus rectus femoris) and the femur. Thigh muscle thickness without compression (TMT) and with compression (cTMT) by the sonography probe are indicated. All morphometric variables are obtained from a person in supine position.

The extensor muscle thickness of the lower limb, including the M. rectus femoris and M. vastus intermedius, was measured in the frontal plane midway between the spina iliaca anterior and the cranial edge of the patella (Figure 1B). The combined extensor muscle thickness was assessed between the femur bone and the subcutaneous fat tissue, whereby the subcutaneous fat tissue was not included in the measurement.

Based on these measurements the psoas muscle thickness index (PMTI), PMAI,21, 22 and the TMTI were calculated as published.21, 26 The PMTI is obtained from the average bilateral psoas muscle thickness indexed to the body height of the patient (mm/m). PMAI incorporates the average bilateral psoas muscle area indexed to the squared body height of the patient (mm2/m2). The average bilateral psoas muscle area for the PMAI is approximated from the M. psoas diameter by the formula for a circular surface (Figure 1A).21 The TMTI represents the average bilateral extensor muscle thickness of the lower limb (M. rectus femoris and the M. vastus intermedius) indexed by the body height of the patient (mm/m). The same thigh muscle group dimensions measured under compression with the ultrasound detector resulted in the compressed TMTI (cTMTI) (Figure 1B).26

Clinical parameters

Demographic data and comorbidities of patients were prospectively obtained from the hospital information system and chart reviews. All patients with COVID-19 were followed for a maximum of 30 days after hospital admission and mortality was assessed during this follow up period. Later events were not considered. Severity of COVID-19 was classified into an uncomplicated, complicated and critical clinical course. Patients with uncomplicated disease required no monitoring or oxygen supplementation, whereas patients affected by complicated COVID-19 were in need for oxygen supplementation and critical COVID-19 patients received invasive ventilation.27 SARS-CoV-2 infection was confirmed by polymerase chain reaction (PCR) from respiratory samples (RealStar SARS-CoV-2 RT-PCR Kit 1.0, Altona Diagnostics GmbH, Hamburg, Germany).

Statistical analysis and sample size calculation

Statistical analysis was carried out in order to identify an association between muscle quantity and COVID-19 outcome. A flow chart of statistical analyses is incorporated in Figure S1. Quantitative data are expressed as median with interquartile range (IQR). Pairwise comparisons for quantitative variables were performed with an unpaired t test or Mann–Whitney U test, accordingly. Categorical variables are given as frequencies and percentages, respectively. Categorical variables of two or more patient groups were compared by χ2 test. Univariate survival was assessed by Kaplan–Meier blot and log-Rank comparison. Receiver operating characteristic-area under the curve (ROC-AUC) and Youden index were used to identify the optimal cutoff values for mortality prediction by muscle indices. Binary logistic regression was applied to explore multivariable associations with the 30 day morality rate. SPSS Version 26 (IBM, SPSS Statistics, Armonk NY, USA) was used for the statistical evaluation. The significance level was set at <0.05.

A case number planning was carried out based on the reported association of muscle indices with severe COVID-19 courses.28 A case number of 54 patients was calculated in order to show an influence of the muscle quantity on the course of a COVID disease. The case-finding design is based on a test strength of 95% and an alpha error of 0.01 for the prediction of mortality by sonographic muscle quantification. Detailed information regarding the sample size calculation can be found in the supporting information.

Results Reference patients

Aiming to define gender-specific reference muscle indices, a total of n = 136 patients without COVID-19 were assessed by ultrasound of whom 32.4% (44/136) were hospitalized. The main diagnoses requiring sonographic work up were liver diseases (50.7%, 69/136), malignant diseases (12.5%, 17/136), infectious diseases (9.6%, 13/136), and cardiovascular entities (6.9%, 9/136). Patients included into the reference cohort were male in 55.9% (n = 76/136), showed a median age of 60 years (49–69 years), and also presented with common comorbidities (Table 1).

Table 1. Patient characteristics Characteristics Reference cohort COVID-19 Cohort I COVID-19 Cohort II Combined COVID-19 Cohorts I + II Patients (N) 136 58 55 113 Age (years) 60 (49–69) 67 (54–79) 70 (57–79) 69 (57–79) <65 years 87 (64.0%) 26 (44.8%) 20 (36.4%) 46 (40.7%) 65–80 years 39 (28.7%) 18 (31.0%) 22 (40.0%) 40 (38.4%) >80 years 10 (7.4%) 14 (24.1) 13 (23.6%) 27 (23.9%) Gender (male/female) 76/60 (55.9%/44.1%) 31/27 (53.4%/46.6%) 38/17 (69.1%/30.9%) 69/44 (69.1%/30.9%) BMI (kg/m2) 26.9 (23.6–30.3) 27.2 (24.0–33.4) 25.7 (22.9–27.8) 26.1 (23.7–30.3) BMI (>30 kg/m2) 38 (27.9%) 21 (36.2%) 8 (14.5%) 29 (25.7%) Pre-existing comorbidities Arterial hypertension 50 (36.8%) 34 (58.6%) 39 (70.9%) 73 (64.6%) Cardiovascular disease 37 (27.2%) 20 (34.5%) 34 (61.8%) 54 (47.8%) Diabetes mellitus 29 (21.3%) 13 (22.4%) 25 (45.5%) 38 (33.6%) Chronic respiratory disease 10 (7.4%) 6 (10.3%) 17 (30.9%) 23 (20.4%) Cerebrovascular disease 12 (8.8%) 14 (24.1%) 14 (25.5%) 28 (24.8%) COVID-19 severity Mild NA 28 (48.3%) 16 (30.9%) 44 (38.9%) Complicated NA 17 (29.3%) 19 (34.5%) 36 (31.9%) Critical NA 13 (22.4%) 20 (34.5%) 33 (29.2%) Deceased NR 7 (12.1%) 11 (20.0%) 18 (15.9%) Medical care Hospital care 44 (32.4%) 58 (100%) 55 (100%) 113 (100%) Hospital care duration (days) NR 13 (8–21) 19 (10–31) 16 (8–25) Intensive care NR 13 (22.8%) 16 (29.1%) 29 (25.9%) Intensive care duration (days) NR 25 (8–37) 17 (9–39) 20 (9–38) Invasive ventilation NR 12 (21.1%) 11 (20%) 23 (20.5%) Invasive ventilation duration (days) NR 15 (5–35) 22 (7–38) 18 (6–35) Vasopressor therapy NR 8 (14.3%) 10 (18.2%) 18 (16.2%) Renal replacement therapy NR 3 (5.2%) 6 (10.9%) 9 (8.9%) ECMO therapy NR 1 (1.8%) 2 (3.6%) 3 (2.7%) Dexamethasone therapy NR 22 (37.9%) 31 (56.4%) 53 (46.9%) Anticoagulation None NR 3 (5.2%) 4 (7.3%) 7 (6.3%) Prophylactic LMWH NR 39 (68.4%) 30 (54.5%) 69 (61.6%) Therapeutic LMWH NR 15 (26.3%) 21 (38.1%) 36 (32.1%) Oral anticoagulation NR 0 (0.0%) 0 (0.0%) 0 (0.0%) BMI, body mass index; LMWH, low molecular weight heparin, ECMO, extracorporeal membrane oxygenation, NA not applicable, NR, not rated. Patient characteristics presented as median (interquartile range) or number (%).

Sonographic assessment of the psoas muscle identified a median PMTI of 18.4 mm/m (16.2–20.2 mm/m) and a median PMAI of 268.4 mm2/m2 (207.3–321.8 mm2/m2). Thigh muscle dimensions resulted in a median cTMTI of 9.3 mm/m (6.8–12.0 mm/m) under compression and a median TMTI of 21.0 mm/m (16.9–26.1 mm/m) without compression, respectively.

Given that baseline muscle quantity is dependent on gender,22 muscle indices were analysed for male and female patients accordingly (Table 2A). The gender related differences were significant for median PMAI (male: 291.1 vs. female: 260.6 mm/m2, P = 0.037) and median PMTI (male: 19.2 vs. female: 18.2 mm/m, P = 0.040). However, no significant differences were observed for median cTMTI (male: 9.4 vs. female: 8.9 mm/m, P = 0.676) and median TMTI (male: 21.2 vs. female: 20.5 mm/m, P = 0.998).

Table 2. Morphometric variables Panel a Reference cohort Morphometric variables Total Male Female Patients (N) 136 76 60 Body height (cm) 170 (165–178) 176 (170–182) 167 (163–170) PMA (mm2) 780.3 (606.1–965.3) 881.6 (708.4–1024.9) 683.7 (534.1–831.3) PMAI (mm2/m2) 268.4 (207.2–321.8) 291.1 (224.8–328.4) 260.6 (191.5–297.8) PMT (mm) 31.5 (27.8–35.0) 33.5 (30.0–36.0) 29.5 (26.0–32.5) PMTI (mm/m) 18.4 (16.2–20.2) 19.2 (16.9–20.4) 18.2 (15.6–19.4) TMT (mm) (no compression) 35.8 (28.5–45.0) 37.0 (28.8–46.5) 33.8 (27.5–43.3) TMTI (mm/m) (no compression) 21.0 (16.9–26.1) 21.2 (16.8–25.6) 20.4 (16.9–26.2) cTMT (mm) (compression) 15.8 (11.5–20.8) 16.3 (12.0–21.3) 15.0 (11.0–20.0) cTMTI (mm/m) (compression) 9.2 (6.8–12.0) 9.4 (6.8–12.2) 8.9 (6.8–12.0) Panel b COVID-19 Cohort I Morphometric variables Total Male Female Patients (N) 58 31 27 Body height (cm) 170 (165–176) 176 (173–180) 166 (160–170) PMA (mm2) 719.0 (497.9–1018.7) 730.8 (573.3–1114.1) 702.2 (436.0–974.7) PMAI (mm2/m2) 240.4 (176.4–331.7) 246.9 (197.6–330.0) 224.1 (155.8–367.1) PMT (mm) 30.2 (25.0–36.0) 30.5 (27.0–37.5) 29.9 (23.6–35.0) PMTI (mm/m) 17.5 (14.9–20.5) 17.7 (15.6–20.5) 16.9 (14.0–21.5) TMT (mm) (no compression) 25.3 (19.5–33.0) 25.0 (18.5–32.5) 25.5 (20.0–37.0) TMTI (mm/m) (no compression) 15.1 (11.5–20.6) 14.2 (10.6–18.6) 15.8 (12.2–21.7) cTMT (mm) (compression) 11.8 (9.0–15.7) 10.5 (9.0–15.0) 13.0 (9.0–17.0) cTMTI (mm/m) (compression) 6.8 (5.1–9.6) 6.2 (5.0–8.8) 7.8 (5.9–9.8) Panel c COVID-19 cohort II Morphometric variables Total Male Female Patients (N) 55 38 17 Body height (cm) 175 (166–188) 176 (173–180) 165 (160–170) PMA (mm2) 788.9 (573.3–1,125-5) 913.0 (594.2–1164.4) 683.7 (510.9–962.9) PMAI (mm2/m2) 258.3 (194.0–373-9) 289.2 (195.5–373.9) 251.5 (176.8–353.7) PMT (mm) 31.5 (27.0–37.5) 34.0 (27.5–38.5) 29.5 (25.5–35.0) PMTI (mm/m) 18.13 (15.7–21.8) 19.0 (15.8–21.8) 17.8 (15.0–21.2) TMT (mm) (no compression) 24.0 (10.4–31.0) 23.5 (20.5–31.5) 24.0 (20.5–28.0) TMTI (mm/m) (no compression) 14.4 (11.2–18.0) 13.6 (10.2–18.0) 15.4 (11.5–16.5) cTMT (mm) (compression) 12.0 (8.5–16.0) 12.5 (8.5–15.0) 11.0 (9.0–16.0) cTMTI (mm/m) (compression) 6.8 (5.0–9.2) 6.9 (4.8–8.6) 6.5 (5.6–9.4) Panel d Combined COVID-19 Cohorts I + II Morphometric variables Total Male Female Patients (N) 113 69 44 Body height (cm) 172 (166–180) 176 (173–180) 165 (160–170) PMA (mm2) 730.8 (5,435–1078.4) 744.2 (575.7–1114.1) 688.4 (472.9–968.8) PMAI (mm2/m2) 251.5 (190.1–353-7) 259.9 (196.3–341.6) 238.9 (172.7–361.5) PMT (mm) 30.5 (26.2–37.0) 32.0 (27.5–37.9) 29.5 (24.4–35.0) PMTI (mm/m) 17.7 (15.6–21.1) 18.2 (15.8–20.8) 17.3 (14.8–21.4) TMT (mm) (no compression) 24.5 (20.0–32.0) 24.0 (19.5–31.5) 24.8 (20.3–32.3) TMTI (mm/m) (no compression) 14.6 (11.5–19.2) 13.8 (10.9–18.0) 15.5 (11.9–20.3) cTMT (mm) (compression) 12.0 (9.0–16.0) 11.5 (9.0–15.0) 12.3 (9.0–16.5) cTMTI (mm/m) (compression) 6.8 (5.1–9.4) 6.7 (5.0–7.1) 7.5 (5.7–9.8) BMI; body mass index; cTMTI, compressed tight muscles thickness index; PMA, psoas muscle area; PMAI, psoas muscle area index; PMTI, psoas muscle thickness index; TMT, tight muscles thickness. Patient characteristics are presented as median (interquartile range) or number (%). COVID-19 patients

Sonographic muscle indices were subsequently explored in hospitalized patients suffering from COVID-19 (COVID-19 Cohort I). The first COVID-19 cohort showed a balanced gender distribution (male/female, n = 31/27) and a median age of 67 years (54–79 years). They presented with comorbidities, such as arterial hypertension (n = 34/58, 58.6%), cardiovascular disease (n = 20/58, 34.5%) and diabetes mellitus (n = 13/58, 22.4%). Obesity (BMI > 30 kg/m2) was present in 36.2% (n = 21/58) (Table 1). Throughout hospitalization, the patients developed a mild (n = 28/58, 48.3%), complicated (n = 17/58, 29.3%), or critical (n = 13/58, 22.4%) course of COVID-19. Seven COVID-19 patients (n = 7/58, 12.1%) died during a follow up of 30 days (Table 1).

Sonographic assessment of the psoas muscle revealed a median PMAI of 240.4 mm2/m2 (176.4–331.7 mm2/m2) and a median PMTI of 17.5 mm/m (14.9–20.5 mm/m). Thigh muscle assessment showed a median TMTI of 15.1 mm/m (11.5–20.6 mm/m) and a median cTMTI of 6.8 mm/m (5.1–9.6 mm/m). All median morphometric indices of the COVID-19 patients were lower compared with the reference cohort (Table 2, Panel b).

The second independent COVID-19 cohort, which primarily originated from the second COVID-19 wave in Germany, was assessed to validate our findings (COVID-19 Cohort II). The second COVID-19 cohort showed no significantly different baseline characteristics compared to the first COVID-19 cohort (Table 1). Eleven patients (n = 11/55, 20.0%) of the second COVID-19 cohort died during a follow up of 30 days (Table 1). In line with our previous findings, the second COVID-19 cohort also presented with lower morphometric indices compared to the reference cohort (Table 2, Panel c).

Muscle quantity associated with outcome of COVID-19 patients

We explored the impact of muscle quantity on prognosis in COVID-19 patients. Hence, median muscle indices derived from the reference cohort were used as cutoff value to stratify COVID-19 patients (Table 2). COVID-19 patients with a PMAI and PMTI below the gender-specific median showed a significantly higher mortality during a 30 day follow up (both log rank p = 0.025) (Figure 2A and 2C), whereas COVID-19 patients with a TMTI or cTMTI above the gender-specific median had a higher 30 day mortality (both log rank P > 0.05) (Figure 2E and 2G).

Patient survival according to muscle indices. Kaplan–Meier survival curves are displayed for COVID-19 Cohort I (n = 58) (A, C, E, G) and COVID-19 Cohort II (n = 55) (B,