Reference Cohort
The correlations of MI and MG values with age in the reference cohort is plotted in Figure 1A and 1B. The MI and MG values were significantly higher in males than in females. Previously, we found that the L4 psoas muscle index (PMI) value peaked in the 2nd to 3rd decade of life(15). Interestingly, other MIs, including the T12 skeletal muscle index (SMI), T12 dorsal muscle index (DMI), and L3 SMI, peaked in later stages of life. All MIs were significantly correlated with age after the age of 40, with L-spine MI values showing a steeper declining trend than T-spine indices (Supplemental Table 1). Comparing the MI and MG values in the same muscle group, the MG values had a steadier declining trend with increasing age (Figure 1B). All MG values had significantly stronger correlations with age than did the MI values (r range from -0.55 to -0.45, all p < 0.05) after the age of 40.
Correlations between the eight different CT-based muscle metrics were analyzed (Supplemental Table 2). The MI and MG values had moderate to strong linear relationships between the thoracic (T12) and lumbar (L3, L4) vertebrae, and the relationships were strongest between the L4 PMI and the L4 PMG (r = 0.89) and the weakest between the L4 PMG and the T12 DMI (r = 0.5).
Definitions of Low Muscle Index(MI) and Muscle Gauge(MG) Values
Four hundred sixty-two adults (356 males and 106 females) aged 18 to 40 were analyzed to generate Asian mean reference values for each metric. We adopted the EWGSOP consensus recommendations to define low MI or MG values as those that were two standard deviations (SD) below the sex-specific mean reference values in a healthy, young adult cohort(22). A stricter definition (very low MI or MG) was described as 2.5 SD below the sex-specific reference values. The sex-specific cutoff values for each metric are summarized in Supplemental Table 3. The study cohort was further analyzed using these cutoff values.
Study Cohort
The demographics of the geriatric patients presenting with abdominal emergencies are shown in Table 1, with 660 males and 532 females included. The mean age of the cohort was 76.16 ± 7.42 years old. Male patients had a lower body mass index (BMI) than female patients (23.34 ± 3.71 vs. 24.09 ± 4.14 kg/m2, p=0.001). Seventy-three patients (6.1%) were hemodynamically unstable before surgery. The most common comorbidities were hypertension (54.4%), followed by diabetes mellitus (24.7%), the presence of a malignancy (14.8%), and previous cerebrovascular accident (9.6%). The prevalence of previous myocardial infarction, end-stage renal disease, chronic obstructive pulmonary disease, and cirrhosis were less than 5% in the cohort. Most of the patients underwent surgery due to hollow organ perforation (22.2%), biliary disease (20.4%), bowel obstruction (19.9%), and appendicitis (16.4%), with 83 patients (7.0%) diagnosed with ischemic bowel disease. One-third of the patients (n=381) underwent laparoscopic surgery. Overall complication and mortality rates in the cohort were 11.6% and 8.5%, respectively. The mean length of hospital stay (LOS) was 15.27 ± 16.56 days, and the mean length of ICU stay (ICU LOS) was 4.04 ± 8.41 days. Notably, male MI and MG values were significantly higher than female MI and MG values (all p<0.001).
Univariate regression analysis was performed to evaluate prognostic factors for the overall complication and mortality rates (Table 2). The variables associated with increased postoperative complications were older age (odd ratios[OR] 1.04, p<0.001), unstable hemodynamics (OR 1.90, p=0.039), COPD (OR 2.96, p=0.003), ESRD (OR 2.29, p=0.015), previous cerebrovascular accident (OR 3.78, p<0.001), very low L4 PMI value (OR 2.52, p<0.001), low L4 PMG value (OR 1,77, p=0.020), very low L4 PMG value (OR 2.25, p<0.001), very low T12 SMG value (OR 2.59, p<0.001), low L3 SMI value (OR 1.79, p=0.013), very low L3 SMI value (OR 2.62, p<0.001), very low L3 SMG value (OR 2.39, p<0.001) and very low T12 DMG value (OR 2.69, p<0.001).
Medical conditions associated with increased overall mortality were older age (OR 1.07, p<0.001), unstable hemodynamics (OR 4.45, p<0.001), cirrhosis (OR 7.91, p<0.001), ESRD (OR 3.78, p<0.001), previous cerebrovascular accident (OR 3.02, p<0.001), and the presence of a malignancy (OR 2.31, p=0.001). The morphometric variables associated with increased mortality included low and very low L4 PMI values (OR 1.85, p=0.027; OR 3.27, p<0.001), low and very low L4 PMG values (OR 2.57, p=0.002; OR 4.52, p<0.001)), very low T12 SMI value (OR 2.91, p=0.009), very low T12 SMG value (OR 3.73, p<0.001), low and very low L3 SMI values (OR 1.95, p=0.011; OR 2.61, p=0.001), low and very low L3 SMG values (OR 3.01, p=0.013; OR 6.35, p<0.001), low and very low T12 DMI values (OR 2.29, p=0.008; OR 2.94, p=0.001), and very low T12 DMG value (OR 5.72, p<0.001). Diagnoses of hollow organ perforation, bowel obstruction, and ischemic bowel disease were associated with significantly higher overall complication and mortality rates than diagnoses of appendicitis and biliary disease (all p<0.05). In the cohort, the laparoscopic approach was a protective factor in geriatric abdominal emergencies.
Patients were stratified by their L4 PMG values, and their characteristics are summarized in Table 3; 536 patients (45.0%) were defined as having a normal PMG value, 287 patients (24.1%) had a low PMG value, and 369 patients (31.0%) had a very low L4 PMG value. The low and very low L4 PMG groups were characterized by an older age (76.63 ± 7.16 and 78.99 ± 7.41 vs 73.97 ± 6.84 years old, p<0.001) and lower BMI (23.06 ± 3.84 and 22.78 ± 3.62 vs 24.62 ± 3.96 kg/m2, p<0.001) than the normal L4 PMG group. The majority of patients with normal L4 PMG values underwent surgery due to biliary disease (n=137, 25.6%), appendicitis (n=107, 20.0%), bowel obstruction (n=92, 17.2%), and hollow organ perforation (n=90, 16.8%). The most common indications for surgery in the low L4 PMG group were bowel obstruction (n=65, 22.6%), hollow organ perforation (n=64, 22.3%), biliary disease (n=55, 19.2%) and appendicitis (n=54, 18.8%). In the very low L4 PMG group, hollow organ perforation (n=111, 30.1%) was the most common diagnosis, followed by bowel obstruction (n=80, 21.7%), biliary disease (n=51, 13.8%) and appendicitis (n=35, 9.5%). A higher proportion of hemodynamically unstable patients were identified in the low and very low L4 PMG groups than in the normal group (6.3% and 10.3% vs 3.2%, p<0.001).
There was no significant difference in the prevalence of comorbidities among the three groups. However, 21.7% of patients (n=80) with very low L4 PMG values had a history of malignancy, which was significantly higher than the proportion in the groups with low (n=44, 15.3%) and normal (n=56, 10.4%) index values. More than 40% of normal L4 PMG patients underwent laparoscopic surgery. Meanwhile, more than 70% of patients with low L4 PMG and very low L4 PMG received emergency laparotomy (p<0.001).
Patients with low and very low L4 PMG values had worse surgical outcomes than those with normal L4 PMG values, with significantly longer LOS and ICU LOS and higher overall complication and mortality rates (all p<0.001). Notably, the overall mortality rate of the very low L4 PMG group was nearly fourfold that of the normal group (14.9% vs. 3.9%).
Among the CT-measured muscle metrics, L4 PMG and L3 SMG showed the highest predictive power for complications and mortality in univariate analysis. To avoid multicollinearity and because it is easy to use in the clinical environment, L4 PMG was used for multivariate analysis.
The results of multivariate regression analysis for overall complications and mortality are shown in Table 4. Independent risk factors for increased overall complications were previous cerebrovascular accident (OR 3.03, p<0.001), hollow organ perforation (OR 2.27, p=0.043), bowel ischemia (OR 3.45, p=0.007), laparotomy (OR 3.13, p=0.001), and very low L4 PMG value (OR 2.11, p=0.002). Meanwhile, older age (OR 1.05, p=0.006), cirrhosis (OR 12.46, p<0.001), old cerebrovascular event (OR 2.83, p=0.001), bowel ischemia (OR 13.12, p<0.001), laparotomy (OR 3.125, p=0.011), low L4 PMG value (OR 2.24, p=0.021) and very low L4 PMG value (OR 3.41, p<0.001) were independent risk factors for increased overall mortality.