Aspiration pneumonia overview

# Overview
Aspiration pneumonia is a common pneumonia among patients with risk factors such as neurologic diseases, dysphagia, swallowing dysfunction, altered mental status, COPD, and hospitalization. Microaspiration and macroaspiration of different materials are the primary cause of aspiration pneumonia. The mechanism behind damage of lung due to aspiration of depends on the content of aspirate and the response of lung tissue to the content. Chemical pneumonitis usually occurs following aspiration of materials that are toxic to pulmonary tissue. In case of oropharyngeal secretions the damage is due to bacteria infecting and inducing inflammation in lung tissues. Foreign body aspiration might present acutely with mechanical obstruction or chemical pneumonitis. Lipoid pneumonia is caused by aspiration of mineral oil when used for constipation treatment. Patients might present with acute chest pain, cough, fever, and sweating. Less common symptoms of aspiration pneumonia include fatigue, nausea and vomiting, diarrhea, and dyspnea. Chemical pneumonitis usually develop after aspiration of gastric acid and might present acutely within two hours. Rapid clinical recovery or worsening of respiratory distress and hypoxemia might happen. Bacterial infection following aspiration is slower that other community-acquired pneumonia and might be acute, subacute, or chronic. Complications of aspiration pneumonia include segmental or lobar pneumonia, bronchopneumonia, bronchiectasis, lung abscess, empyema, respiratory failure, bacteremia, and shock. Diagnosis of aspiration pneumonia is by history and symptoms following aspiration or in patients with risk factors. Chest x-ray is used to diagnose aspiration pneumonia. Bronchoscopy is used to remove foreign body from the respiratory tract. Antibiotic therapy such as ampicillin-sulbactam, amoxicillin-clavulanate, or clindamycin for 7 days is required for chemical pneumonitis and bacterial infection. Supportive ventilation might be required in patients with respiratory failure following chemical pneumonitis. Clearing respiratory tract by suction is helpful to prevent the extent of pulmonary damage. Effective measures for the primary prevention of aspiration pneumonia include dietary habit changes, maintaining oral hygiene, postural maneuvers, and medications such as H2 antagonists, metoclopramide, mosapride, amantadine, or cilostazol.

# Historical Perspective
The literature on aspiration pneumonia came into knowledge of medical society along with the discovery of pneumonia. During 1893, Veillon was first to write about the role of anaerobic bacteria in aspiration pneumonia. The major breakthrough came when x-ray was invented by Roentgen in 1896.

# Classification
Aspiration pneumonia is a part of aspiration syndrome which is consist of four classes depending on nature of aspirated substance including foreign body aspiration, chemical pneumonitis, bacterial infection, and lipid pneumonia. Aspiration pneumonia depends on the duration of systems might be classified into two groups of acute and chronic.

# Pathophysiology
Aspiration pneumonia is a common pneumonia among patients with risk factors including neurologic diseases. Microaspiration and macroaspiration of different materials are the main cause of aspiration pneumonia. The mechanism behind damage of lung due to aspiration of depends on the content of aspirate and the response of lung tissue to the content. Host factors including mucociliary clearance, cough reflex, and immune system might be probably impaired. Chemical pneumonitis usually occurs following aspiration of materials that are toxic to pulmonary tissue. There might be no bacterial or viral organisms involved. It is mostly associated with aspiration of gastric acid. In case of oropharyngeal secretions the damage is due to bacteria infecting and inducing inflammation in lung tissues. Foreign body aspiration might present acutely with mechanical obstruction or chemical pneumonitis. Lipoid pneumonia is caused by aspiration of mineral oil when used for constipation treatment. Following oil aspiration there is an inflammatory response with regional edema and acute cough, fever, and dyspnea. Patients with genetic syndromes and paralysis of lower cranial nerves might be prone to aspiration pneumonia. On gross pathology, different aspirated particles might be seen. On microscopic histopathological analysis, aspirated material fragments, inflammation, fibrosis, and skeletal muscle fibers might be seen.

# Causes
Aspiration pneumonia is caused by aspiration of different particles including secretions, gastric contents or any foreign material which reaches lung parenchyma and damages lung tissue by inflammation. Microorganisms that are responsible for aspiration pneumonia include S. aureus, S. pneumoniae, Enteric bacilli, Hemophilus species, Neisseria species, M. catarrhalis, P. aeruginosa.

# Differentiating Aspiration Pneumonia from Other Diseases
Aspiration pneumonia must be differentiated from other diseases that cause productive cough, fever, and dyspnea.

# Epidemiology and Demographics
The incidence and prevalence of aspiration pneumonia are underestimated. It is mostly because of similarities between pneumonias from different causes and lack of specific marker to distinguish pneumonias from each other. The incidence of aspiration pneumonia is approximately 300,000 to 600,000 individuals annually in the United States. The prevalence of aspiration pneumonia is approximately 5,000 to 15,000 per 100,000 individuals admitted in the hospital due to community acquired pneumonia. The mortality rateof aspiration pneumonia is approximately 10.6-21%. The incidence of aspiration pneumonia increases with age; the median age at diagnosis is 70-80 years. Males are more commonly affected by aspiration pneumonia than females. There is no racial predilection to aspiration pneumonia.

# Risk Factors
Common risk factors in the development of aspiration pneumonia include dysphagia, swallowing dysfunction, altered mental status, COPD, and hospitalization. Less common risk factors in the development of aspiration pneumonia include medications, esophageal motility disorders, vomiting, enteral feeding, oropharyngeal colonization, male sex, and smoking.

# Screening
There is insufficient evidence to recommend routine screening for aspiration pneumonia.

# Natural History, Complications, and Prognosis
Aspiration pneumonia occurs following aspiration of different materials and particles. Natural history, complications, and prognosis are different for each category. Chemical pneumonitis usually develop after aspiration of gastric acid and might present acutely within two hours. Rapid clinical recovery or worsening of respiratory distress and hypoxemia might happen. Bacterial infection following aspiration is slower that other community-acquired pneumonia and might be acute, subacute, or chronic. Foreign body aspiration might present acutely with mechanical obstruction or chemical pneumonitis. Patients might present acutely with inflammation and cough, fever, and dyspnea. However, they might be asymptomatic and present with an incidental mass on radiographs. Complications of aspiration pneumonia include segmental or lobar pneumonia, bronchopneumonia, bronchiectasis, lung abscess, empyema, respiratory failure, bacteremia, and shock.

# Diagnosis

## Diagnostic Study of Choice
Aspiration pneumonia is mainly diagnosed based on clinical presentation. The CURB-65 and the eCURB scoring systems are used to evaluate and predict mortality in patients with pneumonia. However, they are not helpful in aspiration pneumonia.

## History and Symptoms
Patients with aspiration pneumonia may have a positive history of predisposing condition or altered level of consciousness. The most common symptoms of aspiration pneumonia include chest pain, cough, fever, and sweating. Less common symptoms of aspiration pneumonia include fatigue, nausea and vomiting, diarrhea, and dyspnea.

## Physical Examination
Patients with aspiration might appear normal or toxic. Physical examination of patients with aspiration pneumonia is usually remarkable for fever, tachypnea, hypotension, crackles, decreased breath sounds, and increased tactile fremitus.

## Laboratory Findings
Different laboratory tests might be used in patients with aspiration pneumonia. Sputum analysis including gram stain and culture must be done in patients with cough. ABG may show acute hypoxemia and decreased mixed venous oxygen saturation. CBC shows leukocytosis with left shift or leukopenia, anemia, or thrombocytopenia.

## Electrocardiogram
There are some non-specific findings on ECG of a patient with chronic aspiration pneumonia which include sinus tachycardia, minor nonspecific ST-segment or T-wave changes, right atrial enlargement, QRS abnormalities like right axis deviation, and presence of S1S2S3.

## X-ray
Chest x-rays may be helpful in the diagnosis of aspiration pneumonia. Findings on an chest x-ray suggestive of aspiration pneumonia include lobar pneumonia, areas of opacity, unilateral consolidation, air bronchogram, or cavitation.

## Echocardiography and Ultrasound
In some cases, ultrasound is used for the diagnosis and follow-up of a patient with aspiration pneumonia, for a guided thoracocentesis and to quantify the amount of pleural effusion.

## CT scan
A chest CT scan might be used in patients with aspiration pneumonia if a chest x-ray is not conclusive. CT findings may include lobar consolidation, ground-glass opacities, bronchiectasis, atelectasis, pleural effusion, and consolidation. A chest CT can also help to assess reasons for therapy failure and complications, such as lung abscess, and pleural effusions.

## MRI
Chest MRI may be helpful in the diagnosis of aspiration pneumonia. Findings on MRI suggestive of aspiration pneumonia include defining the nature of aspirated particle and extend of lung injury, atelectasis, consolidation and opacities.

## Other Imaging Findings
Bronchoscopy with bronchoalveolar lavage is useful to obtain samples for gram stain and culture in patients with certain conditions, such as immunocompromised patients, ICU admission, or antibiotic failure.

## Other Diagnostic Studies
Videofluoroscopic swallow study (VFSS) might be used to evaluate swallowing difficulties.

# Treatment

## Medical Therapy
There are different approaches for different classes of aspiration pneumonia. Pneumonitis and bacterial infection require antibiotic therapy, while foreign body aspiration and mechanical obstruction may need invasive interventions. Chemical pneumonitis must be treated supportively. Immediate clearing the respiratory tract from aspirated material and fluid by suction must be the first step if the diagnosis of aspiration is definite. Pharmacologic medical therapy for aspiration pneumonia includes antibiotics such as ampicillin-sulbactam, amoxicillin-clavulanate, or clindamycin for 7 days. Alternative regimens include combination of metronidazole with penicillin G, amoxicillin, ceftriaxone, or cefotaxime. Positive pressure ventilation with 100% oxygen to support pulmonary function is sometimes required.

## Surgery
Surgical intervention is not recommended for the management of aspiration pneumonia. However, interventional techniques are used to remove foreign body from the respiratory tract. Flexible or rigid bronchoscopy is indicated in patients with observed aspiration or chronic wheezing.

## Primary Prevention
Effective measures for the primary prevention of aspiration pneumonia include dietary habit changes, maintaining oral hygiene, postural maneuvers, and medications such as H2 antagonists, metoclopramide, mosapride, amantadine, or cilostazol.

## Secondary Prevention
There are no established measures for the secondary prevention of aspiration pneumonia.