Background
The number of patients with pneumonia is increasing with an increase in the population of older adults, and aspiration pneumonia accounts for the majority of cases [
1], especially in people aged ≥ 65 years [
2‐
4]. Patients with dysphagia are more likely to develop pneumonia at a rate of 1.6–11.9 times greater than patients without dysphagia [
1,
5‐
7]. Taylor et al. [
5] revealed that the 30-day mortality from pneumonia in patients at risk of aspiration was 17.2% when compared with 7.7% in patients not at risk. Therefore, in the 2017 Ministry of Health, Labor, and Welfare report, “aspiration pneumonia” was added as a category of cause of death [
8]. The number of deaths owing to aspiration pneumonia was 35,740 (mortality rate, 28.7%), with the disease ranking seventh among the causes of death [
8]. A meta-analysis conducted in Japan in 2016 reported that patients with aspiration pneumonia had poorer short- and long-term prognoses than those with other types of pneumonia [
9]. Even if the disease does not lead to death and is cured, a decline in physical function and an increase in the amount of assistance required in daily activities are inevitable [
3,
5,
6,
9,
10]. Aspiration pneumonia constitutes one of the lung diseases associated with dysphagia [
1‐
3,
5,
6]. The 2017 Japanese Respiratory Society’s guidelines for the management of pneumonia in adults define it as “pneumonia that occurs in cases in which dysphagia and aspiration have been proven (or are strongly suspected) [
11].” However, the concept itself remains vague worldwide, and global awareness is lacking; thus, neither diagnostic nor treatment strategies have been clearly established [
12,
13]. Therefore, understanding the pathology of aspiration pneumonia, early diagnoses, and establishing an appropriate treatment system are urgent clinical issues.
The Japanese Respiratory Society recommends pneumonia treatment by dividing it into community-acquired pneumonia (CAP), hospital-acquired pneumonia (HAP), and nursing and healthcare-associated pneumonia (NHCAP). Although guidelines have been published, several aspects of aspiration pneumonia overlap with those of HAP or NHCAP owing to patient characteristics, and the actual situation of aspiration pneumonia remains largely unknown. Teramoto et al. [
2] conducted a multicenter prospective study at 22 Japanese centers from 2004–2005. Aspiration pneumonia was reported in 394 (66.8%) of the 589 total cases of pneumonia; however, most epidemiological findings in previous studies constituted aggregated data on a facility-by-facility basis [
2,
3,
5,
9]. Therefore, the number of patients is small, and the accuracy of the incidence rate in a limited area and the various factors associated with the disease cannot be sufficiently investigated.
In Japan, all ambulance services are required by law to record prehospital transport data, such as age, sex, address, and recognition time (the time at which the ambulance was called) and hospital arrival. Nagasaki Prefecture launched the Nagasaki Practical Emergency Medical Liaison Committee in 1988 and introduced an original emergency transportation record system. The format was unified in 2004 and included additional information such as diagnostic codes, definitive diagnosis, and outcome at 1 week after ambulance transport. Overall, 13 main diagnostic codes are used by physicians. The average collection rate of the record has been reported to be 91.6%–93.1% [
14,
15]. This high collection rate of emergency transportation records offers accurate and reliable objective data and enables high-quality analysis of severe disease and trauma in a specific region. The aim of the present study was to elucidate the current incidence and trends of aspiration pneumonia in the Nagasaki Prefecture from 2005 to 2019 and clarify the characteristics of aspiration pneumonia using emergency transportation records.
Discussion
Japan has the largest population of older adults worldwide, and its disease structure is changing. In 2017, the Japanese Respiratory Society published the “Japanese Respiratory Society Guidelines for the Management of Pneumonia in adults” [
11] by integrating guidelines for CAP, HAP, and NHCAP. However, regarding aspiration pneumonia, only a list of risk factors is available, and almost no epidemiological evidence has been shown to elucidate the actual situation. This study provides valuable epidemiological knowledge by analyzing the incidence of aspiration pneumonia from emergency transportation record forms in the Nagasaki Prefecture.
Previous research examining the incidence of aspiration pneumonia in a specific limited area cannot be found. Our study revealed the actual number and incidence of aspiration pneumonia in the Nagasaki Prefecture from 2005 to 2019. A major strength of this study is that we analyzed large-scale data from original emergency transportation records, which have a high collection rate (93.1%) and include additional information at 1 week after ambulance transport. This methodology can supply reliable, objective data and enable high-quality analysis within a specific region. As a result, the annual number and incidence of aspiration pneumonia increased gradually since 2005 and reached 65.1 cases per 100,000 people in 2019. Nagasaki Prefecture, with an area of approximately 4,131 km
2 and a population of approximately 1,325,205 in 2019, is located in the southwestern part of Japan. The population of residents aged > 65 years has been increasing and reached 427,988 in 2019, accounting for 32.3% of the total population. The aging population is considered as a major factor leading to an increase in the incidence of aspiration pneumonia, which often occurs in elderly individuals [
3,
17‐
19], over time. Moreover, as the incidence in this study was calculated from the data of emergency transportation records, the potential incidence of aspiration pneumonia is estimated to be even higher. Thus, Nagasaki Prefecture can be regarded as a model of future population structures for developed countries, including Japan. Therefore, the medical significance of aspiration pneumonia, which has a high risk of leading to the requirement of nursing care or causing death, will increase in the near future.
Age- and sex-specific data indicated that aspiration pneumonia is approximately 1.2 times more common in males (55.1%) than in females (44.9%), and most of the cases occurred in patients aged ≥ 70 years (82.2%). The mean age at onset was 83.0 years (81.0 years for males and 85.6 years for females). Aspiration pneumonia is more common in people aged ≥ 75 years and in males, which is consistent with the results of several previous studies [
2,
3,
19]. Teramoto et al. [
2] reported that 80.1% of cases (306 of 382 cases) of pneumonia occurring in people aged ≥ 70 years were aspiration pneumonia. Furthermore, although there are more males aged < 90 years, the number of females with aspiration pneumonia tends to increase among people aged > 90 years, which is thought to reflect the male–to-female ratio in the population of older people aged > 90 years. Furthermore, the changes over time by age group indicate that the increase rate is particularly remarkable among those aged ≥ 80 years. The short- and long-term prognosis after aspiration pneumonia is poor [
3,
5,
9,
10], with a 1-month mortality rate of 17.2% [
5] and a 3-month mortality rate of 38.6% [
3]. Therefore, in countries where the elderly population is expected to increase, it is important to enhance effective preventive measures in anticipation of the increase in patients with aspiration pneumonia.
To the best of our knowledge, no previous studies have demonstrated the number of cases of aspiration pneumonia by month or day of the week. The present study revealed that there were no significant differences in the number of cases of aspiration pneumonia by month or day of the week. Unlike epidemic pneumonia caused by bacterial or viral infections, aspiration pneumonia develops when secretions, such as food and saliva, are aspirated into the deep respiratory trachea owing to a decline in swallowing function. Thus, aspiration pneumonia is thought to occur at a constant rate, regardless of the season or day of the week. In contrast, the count of emergency calls was relatively low from night to early morning and became frequent during the daytime. Aspiration is classified into overt aspiration and silent aspiration depending on the presence or absence of coughing or choking. Aspiration pneumonia is typically caused by silent aspiration without these symptoms [
20]. As silent aspiration includes microaspiration that occurs during sleep [
21,
22], it is speculated that a considerable number of aspirations also occur at night. However, elderly people, who often develop aspiration pneumonia, do not show typical symptoms and are less aware of it. In particular, in patients with cerebrovascular or central nervous system disorders, there is a high possibility that surrounding caregivers may notice abnormalities and call for an ambulance. Consequently, the recognition of cases may have increased during the daytime, especially around 9 to 11 am.
No previous study has focused on the location of occurrence and outcome after 1 week in the area that was covered in this study. Regarding the location of occurrence, “house” (39.8%) was the most common, followed by “hospital and clinic” at 16.5%, “nursing homes (special care)” at 14.8%, and “nursing homes (others)” at 13.5%. In terms of indoor locations, 91.7% of the incidents occurred in living rooms, patient rooms, bedrooms, and other living rooms. Generally, it is considered that patients who develop aspiration pneumonia are elderly individuals, have many comorbidities, and repeatedly experience aspiration owing to a decline in swallowing function [
23,
24]. In other words, aspiration pneumonia is more likely to occur in terminally ill or elderly people who are in nursing homes or who are almost bedridden in their own rooms. The present study found that almost 90.5% of patients were still hospitalized (80.9% of the patients were treated in the receiving hospital and 9.6% were transferred to another hospital) 1 week after ambulance transport. This observation indicates that most cases of aspiration pneumonia require hospitalization for 1 week or more. In the early stages of pneumonia owing to aspiration, there are only a few symptoms or only mild systemic symptoms such as a mild fever, fatigue, and loss of appetite. Nonetheless, in cases with immune dysfunction, infection could expand and lead to severe pneumonia [
1,
3]. The Adult Pneumonia Treatment Guidelines also recommend that a treatment strategy should be formulated based on a comprehensive assessment of the risk of resistant bacteria, general condition (terminal stage/senility), and severity. In this study, more than 90% of patients were hospitalized 1 week after ambulance transport and 6.5% of patients had a fatal outcome. Therefore, it is necessary to formulate a treatment strategy carefully considering the background factors.
Relying on the data from emergency transportation records in this study, several potential limitations must be considered. First, patients from the same institution or those admitted to the hospital by other means could not be counted, raising the possibility of duplicates owing to the same patient being transported via different ambulances on separate occasions. Second, as a methodological limitation, we could not ascertain the percentage of patients with aspiration pneumonia transported by ambulance out of the total number of patients. Moreover, considering the evolving awareness and diagnostic criteria for aspiration pneumonia over time, it is plausible that improvements in recognition and clarity could impact the accuracy of diagnoses. These limitations may introduce bias, and consequently, the results of this study could not directly identify the exact risk factors of aspiration pneumonia. We analyzed the actual situation of aspiration pneumonia occurring within a specific region using a relatively large number of objective data obtained from emergency transportation records with a high response rate. To the best of our knowledge, this is the first study to focus on aspiration pneumonia from an epidemiological perspective. Therefore, the findings of the present study may compensate for such deficits, and it is important to utilize these findings for developing effective preventive measures against aspiration pneumonia.
Conclusions
The present study investigated 8,321 cases of aspiration pneumonia from 2005 to 2019 in the Nagasaki Prefecture using emergency transportation records, and analyzed the current incidence, trends, and characteristics. The mean patient age at occurrence was 83.0 years, and the annual incidence per 100,000 population increased from 12.4 in 2005 to 65.1 in 2019. Males (55.1%) were more commonly affected than females (44.9%), and 82.2% of cases involved patients aged ≥ 70 years. Aspiration pneumonia occurred at a constant rate regardless of the season or day of the week, and frequently in houses (39.8%) and facilities for elderly individuals (40.8%). At 7 days after admission, 80.9% of patients remained hospitalized, and 6.5% had died. The findings of the present study should be utilized to valid preventive measures against aspiration pneumonia.
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