Community-acquired pneumonia (CAP) is the most common cause of death among infectious diseases, with a mortality rate ranging from 12% to 14% [1]. Pneumonia is associated with a particularly high mortality rate in elderly adults [1,2]. These patients often have longer hospital stays, more comorbidities, and greater disease severity than younger patients, necessitating consistent preparedness for adverse prognostic outcomes [2,3].

After CAP diagnosis, predicting its severity is crucial for appropriate treatment [4,5]. Failure to appropriately predict the severity or risk of CAP-related death can increase medical costs [6,7]. Patients who could be treated as outpatients might be unnecessarily hospitalized, and those requiring hospitalization might receive only outpatient treatment, leading to the deterioration of their condition [7,8]. The mortality rate was higher in patients with CAP who were initially treated as outpatients and later hospitalized, as well as in patients who were not initially admitted to the intensive care unit (ICU) despite high disease severity [9,10]. Accurately predicting the severity of pneumonia depends on the medical staff’s clinical judgment. Objective criteria such as the pneumonia severity index (PSI) and the CURB-65 (confusion, urea, respiratory rate, blood pressure, age ≥65 years) metric continue to be widely used to predict the prognosis of CAP [1,2]. However, those criteria can be ineffective in some clinical settings, particularly emergency departments. The PSI is difficult and inconvenient to apply in actual clinical settings because it requires the calculation of 20 different variables [1,7]. CURB-65 is simpler than the PSI, but its accuracy tends to decrease as patient age increases [11]. Therefore, a new criterion that is more applicable and accurate than existing criteria is needed to accurately predict the prognosis of CAP in practical clinical settings [7,12].

Sarcopenia, a progressive skeletal muscular disease associated with aging, is characterized by decreased muscle mass and function [13]. Skeletal muscle depletion can deteriorate respiratory muscle function, lead to diaphragm atrophy, impair swallowing muscles, and decrease coughing function, all of which can exacerbate pneumonia in patients [14,15]. Therefore, when deciding on effective treatments and predicting the prognosis of patients with pneumonia, it is beneficial to consider sarcopenic indicators, such as skeletal muscle depletion [15]. In general, magnetic resonance imaging (MRI) or dual-energy x-ray absorptiometry (DXA) is used to evaluate sarcopenia [16]; however, those procedures are difficult to use in emergency medical centers. Computed tomography (CT) can also be used to evaluate muscle mass and predict the prognosis of patients with a wide range of infectious diseases, including pneumonia [15–18]. The use of chest CT to diagnose pneumonia in patients visiting emergency medical centers has been steadily increasing, which means that skeletal muscle mass and composition can also be evaluated without requiring an additional test or cost [16]. Previous studies have used the paraspinal muscle index and muscle attenuation to evaluate sarcopenia and predict the prognosis of pneumonia [19,20]. However, the association remains unclear because only a few studies have examined it.

In this study, we evaluate the prognostic utility of paraspinal muscle measurements on chest CT as a predictor of 28-day mortality in elderly patients with CAP. We hypothesized that a high paraspinal muscle index and muscle attenuation would be associated with low 28-day mortality.

In this study, the paraspinal muscle index derived from chest CT in elderly patients with CAP was found to be associated with 28-day mortality, and it showed predictive performance superior to that of the PSI and CURB-65.

Sarcopenia causes age-related loss of skeletal muscle function and muscle mass in 6% to 22% of elderly adults [24]. Reduced muscle strength suggests sarcopenia, and the diagnosis is confirmed when the quantity or quality of muscle declines, with a reduction in physical function indicating severe sarcopenia [24,25]. The quantity and quality of sarcopenia are evaluated using various definitions and measurement methods [16,24]. Muscle mass can be measured using imaging techniques such as DXA, MRI, and CT [16]. Chest CT is commonly used in emergency medical centers to diagnose pneumonia. Therefore, sarcopenia can be quickly and simultaneously assessed by using CT to also measure muscle mass [15,20,25]. Assessing muscle depletion by measuring the muscles on both sides of the 12th thoracic vertebral spinous process has been proved useful in previous studies [19,20]. The paraspinal muscle area was associated with in-hospital mortality in critically ill patients with severe CAP admitted to the ICU [19], and the paraspinal muscle index showed superiority to CURB-65 in predicting ICU admission and mortality from pneumonia in patients older than 65 years [20].

In infectious diseases, such as pneumonia, muscle atrophy plays a more important role in prognosis than muscle composition [15,25]. Sarcopenia measured at the psoas muscle was associated with 90-day mortality in adult patients with sepsis [18]. Elderly patients tend to experience recurrent acute and chronic inflammation, which promotes cytokine production and reduces muscle mass through protein catabolism [15,25–27]. Proinflammatory cytokines are negatively correlated with muscle mass and strength, and high serum inflammatory markers have been linked to sarcopenia [27–29]. In addition, skeletal muscles regulate immune responses through myokine signaling and the expression of immunomodulatory surface molecules, and immune cells have a critical influence on muscle mass and function [29].

Muscle attenuation is related to the lipid composition of the muscle [30]. When the intramuscular lipid concentration increases by 1 g/100 mL, muscle attenuation on CT decreases by 1 HU [30]. Increased lipid content in skeletal muscle is an age-related process that can lead to mitochondrial dysfunction and decrease oxidative metabolism [31,32]. Predicting prognosis using muscle attenuation has not shown consistent results. Boutin et al. [33] reported that the paraspinal muscle index and muscle attenuation could predict survival times in elderly patients with hip fractures. However, in a study by Ahn et al. [34], muscle attenuation did not show significant results in predicting in-hospital mortality or ICU admissions in elderly patients who underwent surgery for proximal femoral fractures. In addition, a study of critically ill patients with severe CAP admitted to the ICU reported that the value obtained by multiplying the muscle index and attenuation produced more significant results than muscle attenuation alone [19]. Similar results were observed in patients who underwent fenestrated-branched endovascular aortic repair; the multiplied value was more significant for survival than attenuation alone [35]. Intramyocellular and intermuscular fat cannot be distinguished on CT, and using contrast agents for CT can affect muscle attenuation [16]. Lipids stored in muscles are associated with energy metabolism, obesity, insulin resistance, and diabetes [30]. Therefore, muscle attenuation is assumed to reflect sarcopenia less accurately than the muscle index. In this study, muscle attenuation did not adequately reflect patient prognosis.

Predicting disease severity and providing appropriate treatments to patients with CAP who visit emergency medical centers is crucial; the PSI and CURB-65 are widely used for this purpose [7]. Although the PSI showed higher accuracy in predicting 30-day mortality than CURB-65, the PSI procedure is complicated and time-consuming in clinical settings because it considers many variables [7,36,37]. In contrast, CURB-65 is simpler to use than the PSI, but its predictive power decreases as the patient’s age increases [11,38]. Similar to previous studies, the patients enrolled in this study were 65 years or older, and the predictive power of CURB-65 for 28-day mortality was lower than that of the muscle index. It is reasonable to use the PSI or muscle index to evaluate the prognosis of elderly patients with CAP, but the muscle index, which can be measured rapidly, is more useful in an emergency medical center where time is limited.

This study has several limitations. First, this was a retrospective study conducted at a single regional emergency medical center, which might have caused selection and recall bias. The relatively small study population also limits the generalizability of the results, which should be verified in multicenter, large-scale, prospective studies. Second, errors might have occurred because the muscle area and attenuation were measured manually. To minimize such errors, two physicians unaware of each patient's baseline characteristics independently conducted the measurements twice. The reliability of the muscle measurements between the two reviewers, as measured by ICC, was excellent. Third, this study included patients who underwent chest CT within 48 hours before or after hospital admission. The variation in the timing of the CT scan could thus be up to 96 hours, which might have influenced the results. However, 74.6% of patients underwent chest CT within 12 hours before admission. Only nine patients had the CT scan either 24 hours before or 24 hours after admission. No statistically significant differences in CT timing were observed between the survivor and nonsurvivor groups. Fourth, only patients diagnosed with CAP using chest CT were included in this study. Patients who were too unstable to undergo chest CT at initial admission or were diagnosed with pneumonia through other clinical findings and imaging, such as an x-ray or abdominal CT, were not included in the study. Fifth, our hospital has no specific chest CT indications for pneumonia patients, which might limit the study's generalizability. Sixth, we assessed only the need for ICU admission; detailed assessments such as ICU mortality, ICU-free days, and ventilator-free days were not included in this study. Further studies addressing those associations in an ICU setting are required.

Despite those limitations, this study has confirmed that sarcopenia, measured on CT as the paraspinal muscle index, is a prognostic factor in elderly patients with CAP. This distinctive study is the first to demonstrate the superiority of the paraspinal muscle index in predicting 28-day mortality over the already validated PSI. Because chest CT is commonly used to diagnose pneumonia, assessing sarcopenia using chest CT can be a simple and effective method that does not require additional costs or tests, making it suitable for clinical practice, including in emergency medical centers.

In conclusion, a high paraspinal muscle index in elderly patients with CAP is associated with low 28-day mortality, and the predictive power of the muscle index is higher than that of PSI or CURB-65. The paraspinal muscle index is expected to be useful for predicting prognosis because it can be measured easily and quickly without a cost burden in emergency medical centers.