INTRODUCTION:

Aspiration occurs when gastric or oropharyngeal fluids are secreted into the larynx and lower respiratory tract rather than being swallowed and absorbed by the esophagus and stomach. The severity of the condition is determined by the amount and duration of the aspirated substance, its chronicity, and how it interacts with the host's immune response.¹ While healthy people aspirate small quantities of oropharyngeal secretions while sleeping, still microaspiration is a significant pathogenetic process in most cases of pneumonia.² It is well known that aspiration pneumonia mainly requires macroaspiration of colonized oropharyngeal or upper gastrointestinal contents.³ Aspiration pneumonia is known by various names in healthcare surroundings such as Anaerobic pneumonia, Aspiration of vomitus, Aspiration pneumonitis, and Necrotizing pneumonia. Aspiration pneumonia is expected to play a more prominent role in mortality and morbidity, especially among the elderly, persons with dysphagia and weakened immune systems.^4–7Although research has made substantial strides in recognizing risk factors for aspiration pneumonia, healthcare providers are still facing challenges in diagnosing and treating this condition. This review will look into the literature underlying its identification, microbiology with implications for treatment, risk factors, and prevention.

Lung Microbiome:

The use of targeted polymerase chain reactions, sequencing of bacterial 16S ribosomal RNA genomes, and metagenomics has advanced our understanding of normal lower-airway microbiota in humans. The Human Microbiome Project has contributed to the understanding of the role of intestinal microbes in the establishment of mucosal immunity and how it plays a role in healthy and sick individuals. Studies of the lung microbiome have questioned present hypotheses about lung sterility and bacterial entry to the lungs by Aspiration. The function of the lung microbiome in health and disease and the pathophysiology of aspiration pneumoniaare still being investigated by researchers.^8,9

The bacteria that make up the lung microbiota modulates the immune tone of the airways and alveoli in the healthy individuals. Infection is caused due to host's immune reaction, inflammation, and tissue damage, rather than just the result of bacterial replication.^10,11 The lung microbiome's equilibrium is likely sustained by a balance of bacterial immigration and expulsion, as well as feedback loops. Bacterial migration from the oropharynx to the lungs is primarily accomplished by microaspiration, and bacterial expulsion is primarily accomplished by ciliary removal and coughing. Signals for bacterial growth may be suppressed or magnified by negative and positive feedback loops, respectively. An inflammatory incident may cause epithelial and endothelial damage, resulting in a positive feedback loop that promotes inflammation, disrupts bacterial homeostasis, and increases infection susceptibility.¹²

Aspiration pneumonia is related to the airway microbiome and may alter the lung microbiota, impairing pulmonary defenses. Macroaspiration, particularly in elderly and dysphagic patients with compromised bacterial elimination, due to decreased consciousness or a weakened cough reflex, could disrupt this immigration and elimination balance, further disrupting bacterial homeostasis,which triggers an increase in positive feedback loop that leads to acute infection.¹³ Bacterial colonies are present throughout the human oral cavity, but pathogenic bacteria present in healthy individuals. Gram-negative bacteria with high pathogenicity can enter the body and activate the pathway via fibronectin, which stimulates gram-negative receptors on underlying airway epithelial cells and is linked to acute infection in patients.¹⁴Earlier reports suggest that nearly all healthy patients endured Aspiration without complications or respiratory morbidity. Table 1 shows most common aerobes and anaerobes associated with aspiration pneumonia.

Table 1. Most common aerobes and anaerobes associated with aspiration pneumonia.

Anaerobes

Aerobes

1. Peptostreptococcus sp.

2. Proprionibacterium sp.

3. Peptococcus sp.

4. Prevotella sp.

5. Fusobacterium sp.

6. Veillonella sp.

7. Bacteroides sp.

1. Streptococcus pneumoniae

2. Staphylococcus aureus

3. Pseudomonas aeruginosa

4. Haemophilus influenza

5. α-Hemolytic Streptococcus

6. Pseudomonas aeruginosa

7. Escherichia coli

8. Streptococcus agalactiae

9. Methicillin-resistant S aureus

10. Streptococcus mitis

11. Streptococcus constellatus

12. Streptococcus salivarius

13. Klebsiella pneumoniae

14. Streptococcus milleri

15. Viridans streptococci

Clinical Presentation:

Often aspiration syndromes are caused by noninfectious microaspiration, which is often caused by gastroesophageal reflux disease. Chronic cough syndromes, bronchospasm, bronchiolitis obliterans in lung transplant recipients, and worsening of chronic fibrotic lung diseases, especially idiopathic pulmonary fibrosis and systemic sclerosis, are among these complications. Chronic microaspiration has also been linked to the development of pulmonary fibrosis. Microaspiration with exogenous compounds, such as fat particles that enter the lungs in chronic lipoid pneumonia, provides the most compelling evidence. It's also uncertain if chronic microaspiration of refluxed stomach contents causes clinically significant pulmonary fibrosis.¹⁵ These aspiration syndromes are broadly divided into Chemical pneumonitis, Bland Aspiration, Community-acquired (CAP), Hospital-acquired pneumonia (HAP), Anaerobic pleuropneumonia, and Aspiration pneumonia.

Chemical pneumonitis is caused by inhalation of irritating or poisonous substances to the lungs, which results in low blood oxygen levels, fever, tachycardia, and abnormal chest radiograph and lung examinations. Stomach acid is a generally inhaled toxic substance, so chemical pneumonitis can occur if a person inhales vomited stuff. When a person who vomits is not fully conscious, such as after a seizure, a drug or alcohol overdose, or when a person awakens from anesthesia, inhalation of vomit may occur. Laxative oils and hydrocarbons can also induce chemical pneumonitis. The severity of this can lead to acute respiratory distress syndrome (ARDS).¹⁶ All noninfectious microaspirations do not induce an inflammatory reaction in the lungs, so labeling them as pneumonitis is incorrect. Therefore, this time of Aspiration is termed as bland aspiration. It firstly includes the Aspiration of blood as a symptom of severe nose bleeding or vomiting of blood and theaspiration of enteral feedings. Bland Aspiration does not seem to be infectious, but blood and enteral feedings are excellent medium for harmful bacterial growth.¹⁷ CAP is characterized as a severe infection of the lung parenchyma contracted in the community, while HAP is defined as a lung infection contracted in a hospital. The severity of infection in CAP and HAP mainly depends upon host immune response and virulence of aspirated pathogen.¹⁸ It is reported that microaspiration is the main pathophysiologic process involved in CAP. CAP-causing microorganisms are colonized around the oropharynx or nasopharynx in non-hospitalized patients.¹⁹ Microaspiration of oropharyngeal, upper gastrointestinal, or subglottic contents has been shown to be the pathophysiology resulting in HAP, including ventilator-associated pneumonia (VAP).²⁰ Next is Aspiration pleuropneumonia, majorly caused by anaerobes. This pneumonia rarely occurs as it is not reported much nowadays because of improved accessibility to improved medical services and fluorinated water sources. This condition is characterized by foul-smelling sputum, excessive cough, and pus is associated with pulmonary infection.²¹

The present review is mainly focused on Aspiration pneumonia which develops during a large-volume aspiration of oropharyngeal or upper gastrointestinal contents with a pH strong enough to prevent chemical pneumonitis.¹ It is observed that patients getting treatment for CAP are more likely to contract Aspiration pneumonia, and its severity is primarily determined by the amount and quality of substances aspirated. In the hospitalized population, aspiration pneumonia accounts for 5% to 15% of pneumonia cases. And it is seen that this is the second most common diagnosis among the hospitalized patients, according to data provided by Medicare services. Therefore, there will be colossal healthcare and economic pressure as the incidence of Aspiration pneumonia rises. Even though Aspiration pneumonia is common in susceptible populations, there is a significant information deficit, primarily because of the complications in diagnosing aspiration pneumonia correctly. There is mounting evidence that Aspiration is more widespread within the healthy population than previously believed. This suggests that Aspiration pneumonia is caused due to combination of compromised host defenses and the virulence of aspirated pathogens, rather than just the aspiration case itself.²²

Risk Factors:

Old age, dysphagia, diminished consciousness, neurologic conditions, esophageal motility disorders, altered mental status, usage of the enteral feeding tube, oral hygiene, male gender, smoking, diabetes mellitus, regular use of a proton-pump inhibitor or H2-blocker for gastric acid suppression, and use of antipsychotic or sedative drugs are contributing factors for aspiration pneumonia.^5,23As previously mentioned, most of these risk factors contribute to either the risk of aspiration or the compromise of the host's protective mechanisms, resulting in aspiration pneumonia. High frequency and large volume aspiration are two specific predisposing factors for aspiration pneumonia. Furthermore, influences that affect the resident bacterial flora, resulting in colonization by more virulent pathogens that are more likely to surpass the usual defensive mechanisms, play a role in clinical disease growth.^24,25

Dysphagia is the most major risk factor for aspiration pneumonia, which is caused by neurological disorders such as Parkinson's disease, Dementia, Stroke, and Multiple sclerosis. Dysphagia is not really indicative of aspiration, various individuals do not suffer from dysphagia but still aspirate, as seen in healthy individuals also.²⁶ In patients suffering from stroke, the occurrence of pneumonia is higher even if they are not suffering from dysphagia because of diminished consciousness. Because of decreased consciousness, individuals take more time to react as more stimuli are needed to cough.^27,28 Swallowing disorder is prevalent in patients who suffer from hyperinflated lungs and chronic obstructive pulmonary disease (COPD). Previous reports suggest that patients with COPD suffer from dysphagia and these patients need to avoid aspiration caused due to reduced laryngeal elevation at the time of swallowing.²⁹

Certain drugs can cause aspiration by interfering with the swallowing reflex. Antipsychotic drugs may influence the swallowing process by inhibiting dopamine and cause aspiration.³⁰ Proton-pump inhibitor (PPI) or H2-blocker used for gastric acid suppression also can lead to aspiration pneumonia. The cause of PPI-associated pneumonia is unknown, but it is believed to be due to an imbalance in the stomach's acidic environment, which allows acid-labile pathogenic bacteria to colonize in the stomach and then get aspirated into the lungs. It may also be possible because PPIs are responsible for reducing the acidity of the upper digestive tract by inhibiting gastric H⁺/K⁺-ATPase enzymes, which leads to an increase in bacterial invasion of the larynx, esophagus, and lungs.³¹

There is a strong correlation between altered mental status and increased aspiration, which can be because of trauma, poisoning, sedatives, and alcohol use. Vomiting and increased gastric reflux can also raise the risk of aspiration pneumonia in these individuals.³²

Esophageal motility disorders are mainly the result of some serious systemic disorders like scleroderma and polymyositis. Scleroderma is a category of rare diseases that cause the skin and connective tissues to harden and tighten. Polymyositis is a rare inflammatory condition that causes muscle weakness in the whole body. These disorders can compromise the host immune response directly or as a side effect of immunosuppressive therapy. These esophageal motility disorders may result in the aspiration of solid and liquid materials, and the former leads to severe bronchial impaction and post-obstructive pneumonia. Another disorder can occur due to vomiting caused by blockage of the small intestine. In this case, the stomach is no longer sterile and is filled with fluid containing bowel flora overgrowth. As a result, rather than the prevalent gram-positive/anaerobic oral flora, gram-negative bowel pathogens cause rapidly progressive aspiration pneumonia.³³

The use of an enteral tube has also been attributed to a higher risk of aspiration pneumonia. Various case studies have reported that all forms of enteral tubes, such as small and large-bore nasogastric tubes, postpyloric tubes, gastric tubes, and jejunal tubes, increase the risk of aspiration pneumonia. This condition may also be because of decreased gastric mobility, which diminishes gastric emptying and may lead to vomiting.³⁴

Microbiological conditions also influence aspiration pneumonia. This directly correlates to the body's capacity to regularly fight bacteria that enter the lower respiratory tract. Microbes that are unusual or more virulent may be more difficult to kill by the host's immune system. The use of antibiotics has the most significant impact on changes in normal oropharyngeal flora. Previous studies have shown that individuals with bad oral hygiene are more likely to be colonized by more virulent microbe density. So, patients with the gingival disease are more prone to aspiration pneumonia.³⁵ One cohort study has reported some risk factors other than those discussed above, like male gender, old age, smoking, diabetes mellitus, andmalnutrition.³⁶ Figure 1 shows the etiology and potential risk factors for pneumonia during macroaspiration.

Diagnosis:

Aspiration pneumonia is diagnosed mostly based on the patient's history of sickness, medical health history, vital signs, and chest radiograph. Chest x-rays will show an infiltration in the dependent lung segments, such as superior or posterior basal segments of a lower lobe or the posterior segment of an upper lobe. A cavitary lesion can be visible on a chest x-ray in the case of an aspiration-related lung abscess. A circular lesion filled with fluid or with an air-fluid level would be visible on contrast-enhanced computed tomography (CT), which is more reliable and specific for lung abscess.

Figure 1. The etiology and potential risk factors for pneumonia during macroaspiration.

Aspiration pneumonia can be distinguished from chemical pneumonitis and other lung infections based on clinical characteristics.¹ Chemical pneumonitis is normally a more visible occurrence due to the vast amount of stomach contents used to induce it. Furthermore, the clinical feature of chemical pneumonitis is extreme hypoxemia, which occurs almost instantly and may lead to a fatal lung damage. Bronchitis, frothy sputum, and chest radiographs with patchy infiltrates on both sides are common in these Chemical pneumonitis patients. So, chest radiograph is very much helpful in distinguishing between aspiration pneumonia and chemical pneumonitis. Frequent throat clearing or a wet-sounding cough after eating can be signs of ongoing aspiration. On some occasions, patients may showno such symptoms, and the best way to know whether aspiration is present is to perform barium esophagography to rule out a swallowing disorder.³⁷ Diagnosis of CAP due to aspiration and anaerobic pleuropneumonia is also very difficult as clinical features are similar. And molecular diagnosis shows that anaerobes are the causative agents in 15% of CAP cases.³⁸

Because of the difficulty in diagnosing aspiration pneumonia due to similar clinical characteristics with other aspiration syndrome, attempts have been made to use biomarkers to differentiate it. One study has reported that the intensity of microbial invasion appears to be linked to serum concentrations of procalcitonin, but there was no difference in culture-negative and culture-positive patients.³⁹ Similarly, pepsinogen is also used as biomarker in one study, the presence of pepsinogen in tracheal secretions is suggestive of aspiration.⁴⁰ The levels of amylase in bronchoalveolar lavage have been linked to clinical risk factors for aspiration.⁴¹ Accurate diagnosis is critical for medication safety, not only to avoid multidrug-resistant infections but also to reduce the risk of antibiotic-related side effects. The key is to diagnose microbes as soon as possible using culture. Both aerobic and anaerobic techniques must be used to prepare cultures. The most successful culture source will be a tracheal aspirate or bronchoscopy collected from within the bronchial tree.^42–45 Although more convenient to obtain, sputum cultures are unable to distinguish pathogens in the oropharynx from those in the lower lung areas, which are more likely to be infected by the oropharynx.⁴⁵ For better anaerobic culture, empyema and abscesses would require careful anaerobic handling of the samples. Furthermore, certain bacteria are unable to develop in culture.⁸ Newer approaches involving PCR for different bacteria, on the other hand, have demonstrated improved yields.⁴⁶

Prevention:

Prevention of aspiration pneumonia mainly includes treating the risk factors. There are a variety of preventive strategies available, including strengthening swallowing, speech therapy, improving oral hygiene, facilitating airway clearing, lowering the dosage and number of agents involved with aspiration, treatments to enhance frailty and immune status, and encouraging exercise. A variety of minor trials have looked at using drugs to shield the airway through the cough reflex. Angiotensin-converting enzyme inhibitors (ACEIs) are the most interesting medications being researched because of their function in degrading substance P and bradykinin, which are cough reflex stimulants. One case study reported that patients taking ACEIs have a lower risk of aspiration pneumonia.⁴⁷ Since enteric tube feeding poses a risk of aspiration, researchers have compared different tube feeds to reduce this risk. And comparing various tubes suggested that post-pyloric feed tubes are the best ones to be used in patients with gastric dysmotility.³³ Supplementing of folic acid increases the swallow reflex, according to a non-randomized study. Another randomized trial has shown that cilostazol, a phosphodiesterase inhibitor widely used in peripheral vascular disease, helps enhance swallow reflex. Amantadine has also been shown to reduce the risk of pneumonia in stroke patients by 20%.^48–51

It's also crucial to maintain our natural host defense mechanisms like the gastrointestinal environment. Medications that disrupt gastric pH balance, such as histamine-2 receptor antagonists or proton-pump inhibitors, should be used cautiously.⁵² Some research reports have shown that routine oral care reduced the chances of aspiration pneumonia. Bacteria found on the plaque of patients with reduced oral hygiene are similar to those found in lung aspirates in patients with aspiration pneumonia. And there was more significant colonization of Streptococcus aureus and Enterobacter sp. which are possible causative agents of pulmonary infections.^53,54 One other study reported that better oral hygiene improved cough reflex, another risk factor for aspiration.⁵⁵Sothese findings highlight the importance of maintaining proper oral hygiene as a means of avoiding aspiration pneumonia.

Treatment:

Currently, there is no significant research reporting the benefits of antibiotic usage for aspiration pneumonia. With time treatment regimens have changed as known infections have moved from anaerobes to aerobes. The majority of the reports focus on people who are in nursing homes. Eight studies were evaluated and compared with each other. To date, treatment with penicillin G, ampicillin/sulbactam, clindamycin, cephalosporin, azithromycin, and carbapenems are among the options. Antibiotic choice is influenced by the location of acquisition as well as risk factors for infection with multidrug-resistant pathogens. Treatment with broad-spectrum antibiotics in the previous 90 days and hospitalization for at least five days are both risk factors.⁴³ Treatment with ampicillin-sulbactam, a carbapenem, or fluoroquinolone is beneficial for the majority of patients with community-acquired pneumonia. Only when the risk of primarily anaerobic infection is high, such as in patients with serious periodontal disease and necrotizing pneumonia or lung abscess, clindamycin is used.⁵⁶ A similar regimen is used for hospital-acquired infections with a low chance of multidrug-resistant pathogens. If resistance is a problem, broad-spectrum treatment with piperacillin–tazobactam, cefepime, levofloxacin, imipenem, or meropenem is needed, either alone or in combination. An aminoglycoside or colistin can be used as part of combined therapy in cases of multidrug-resistant infection, with the addition of vancomycin or linezolid if the patient has reported nasal or respiratory colonization with methicillin-resistant Streptococcus aureus.⁵⁷ As data suggests, patients suffering from CAP,HAP,and VAP with a good clinical response and no evidence of extrapulmonary infection need 5 to 7days of treatment and longer treatment for patients with necrotizing pneumonia, lung abscess, or empyema. To date, there is no report supporting the use of glucocorticoids for treatment, so it is not recommended to use glucocorticoids for treating aspiration pneumonia. Potential drug-related side effects, such as Clostridium difficile colitis and antibiotic resistance, should be considered when choosing a treatment. More detailed research is required to decide the best antibiotic regimens for aspiration pneumonia and the length of therapy.⁵⁸

CONCLUSION:

Aspiration pneumonia is a common severe disease that can be difficult to detect and differentiate from other aspiration syndromes. Aspiration pneumonia can be distinguished by identifying the risk factors for aspiration such as cough reflex, oral microbiology, and the swallowing mechanism. And clinical and radiographic tools can be used for proper diagnosis. And the condition should be treated in the appropriate clinical settings. Antibiotics can be used to cure aspiration pneumonia, but no glucocorticoids should be used as reports suggest. Patients who are at risk of aspiration should take preventative steps. Diet treatments for dysphasia, oral treatment, post-pyloric tube feedings, and the use of a post-pyloric tube can all help avoid aspiration pneumonia without causing morbidity. For mechanically ventilated patients, a semi-recumbent posture is- recommended.

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Received on 08.10.2024 Revised on 29.11.2024

Accepted on 04.01.2025 Published on 28.02.2025

Available online from March 03, 2025

Asian J. Pharm. Res. 2025; 15(1):65-71.

DOI: 10.52711/2231-5691.2025.00011

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