This study showed that preoperative sarcopenia-related masseter muscle loss could be a risk factor for postoperative pneumonia after esophagectomy for esophageal cancer, and that the OS rate may be significantly lower for patients with postoperative pneumonia than for patients without pneumonia. Little is known about the relationship between postoperative pneumonia and masseter muscle loss in the field of gastrointestinal surgery. To the best of our knowledge, this is the first report to demonstrate the effects of MMS on postoperative pneumonia. Postoperative pneumonia following esophagectomy is the most common complication; however, it is known to be the most serious complication along with anastomotic leakage¹².

In a previous study¹the authors retrospectively analyzed 484 cases of curative resection for esophageal cancer, and 8 postoperative pneumonia occurred in 108 (22.3%) patients. Patients with pneumonia were divided into two groups according to the treatment period (within 7 days and after days). Independent factors risk for acute pneumonia were older age, respiratory disease comorbidity, cT3-4 cases, prolonged operation time, and posterior mediastinal reconstruction. DFS was significantly lower in patients with acute pneumonia than in those without pneumonia (p = 0.0002). However, all Existing risk factors in their study are perioperative rather than intervenable patient factors.

In contrast, dysphagia is the clinical symptom of swallowing dysfunction. Untreated dysphagia leads to malnutrition, pneumonia, and poor quality of life. The etiology of swallowing dysfunction includes central nervous system disorders, such as those of the cortex, basal and brain stems, and efferent/afferent peripheral nerve dysfunction (motor and sensory or muscle function)¹⁴. In 2014, Wakabayashi first proposed the concept of sarcopenic dysphagia¹⁵. The definition of sarcopenic dysphagia is “dysphagia due to sarcopenia in both generalized skeletal muscles and swallowing-related muscles,” and those without sarcopenia in the whole body are excluded¹⁶.

Sarcopenic dysphagia is a reversible syndrome and an essential concept that can be improved through early interventions. Loss of muscles of swallowing is related to aging and associated with swallowing dysfunction¹⁶. Tongue thickness evaluated using ultrasonography¹⁷ and pharyngeal wall thickness measured using magnetic resonance imaging¹⁸ have been reported as surrogate assessments of swallowing-related muscles; however, both are not routine examinations in gastrointestinal surgery and are difficult to apply in clinical practice. Swallowing consists of the (1) oral phase, (2) pharyngeal phase, and (3 ) esophageal phase. The masseter muscle is an important skeletal muscle during mastication in the oral phase¹⁶; the masseter muscle area can be easily evaluated using preoperative CT. Furthermore, the masseter muscles are predominantly composed of type I muscle fibers, also known as slow muscle fibers, which are more strongly atrophied by disuse than by aging¹⁶. Therefore, we focused on the relationship between masseter muscle loss due to malnutrition or dysphagia before esophagectomy and postoperative pneumonia, which is a common complication of esophagectomy.

In the subgroup analysis of this study, factors such as low FEV_1.0 and RLNP, which are difficult to intervene, were significantly associated with acute pneumonia. These results were consistent with previous reports^1,4,5,6,7. Interestingly, these factors were not associated with an increased risk of subacute phase pneumonia (respectively: p = 0.363, p = 0.537). Although there was no significant difference, likely owing to a small sample size, MMS tended to be a risk factor not only for acute phase pneumonia but also for subacute phase pneumonia (respectively: p = 0.077, p = 0.078). Subacute phase pneumonia is a serious complication that reduces quality of life, prolongs hospital stay, and adversely affects the continuation of chemotherapy¹⁹. Improving MMS through preoperative intervention may contribute to reducing postoperative pneumonia for esophageal cancer. In addition, MMS is an independent risk factor for decreased OS, and may contribute not only to pneumonia but also to OS improvement.

In this study, a low L3-PMI alone (sarcopenia group) or masseter muscle loss without sarcopenia was not a risk factor for postoperative pneumonia; however, significant postoperative pneumonia was noted in the MMS group with both low L3-PMI and low MMI. Furthermore, there was a positive correlation between L3-PMI and MMI (r = 0.531, p = 0.004) and the area under the ROC curve was higher for L3-PMI plus MMI than for L3-PMI alone and MMI alone (0.696 vs. 0.674 vs. 0.665, respectively). These results supported the concept of sarcopenic dysphagia proposed by Wakabayashi et al.¹⁶.

Evaluation of sarcopenic dysphagia using both masseter muscles and psoas muscles might be a useful indicator with higher diagnostic ability in predicting postoperative pneumonia following esophagectomy.

In addition to conventional exercise and nutrition therapy for sarcopenia, postoperative pneumonia following esophagectomy and OS of esophageal cancer can be improved through the following factors: oral care, including denture adjustments or improvement of oral hygiene^20,21,22,23; early chewing and swallowing-related muscle training aimed at recovery of tongue pressure and masseter muscle function. Specifically, several reports suggest that bilateral chewing exercises using a NOSICK exerciser (NOSICK EXERCISER, HIFEELWORLD Inc.) are effective in improving occlusal force and masseter muscle thickness in healthy elderly individuals^{twenty four}. The NOSICK exerciser is a U-shaped oral exercise device that was developed for rehabilitation purposes and the performance of masticatory muscle exercises in patients with jaw joint disorders; the elasticity of three small springs inside the device allows resistance during chewing. This device might be effective for masticatory muscle exercises in patients with esophageal cancer.

Our work has some limitations. First, this study was a single-center retrospective study with a small number of patients. Second, the definition of sarcopenia in this study was based on muscle mass alone; however, the European Working Group on Sarcopenia in Older People²⁵ recommends that muscle strength or walking speed should also be considered in this diagnosis. Third, the cut-off values ​​for L3-PMI or MMI remain controversial. Specifically, the cut-off values ​​for L3-PMI determined in a large number of Asian adults were used in this study, considering race and sex. Finally, patient nutritional status was only evaluated using the preoperative Glasgow Prognostic Score, while postoperative oral intake, weight loss, or occlusal clinical force were not assessed. There are few clinical studies on masseter muscle sarcopenia , and further large-scale studies including other ethnicities and races are warranted in the future.

In conclusion, preoperative masseter muscle loss due to sarcopenia is significantly associated with postoperative pneumonia in patients who underwent esophagectomy for esophageal cancer. Preoperative MMI in combination with L3-PMI may serve as a useful postoperative indicator for predicting the incidence of pneumonia in patients with esophageal cancer.