Introduction
Aspiration pneumonia. CT scan through the lower-lobe bronchi demonstrates a metallic object in the left lower-lobe bronchus. The patient had aspirated a filling, which had fallen out of one of his teeth. The patient underwent bronchoscopy, and the foreign body was removed. The patient was treated with antibiotics for the pneumonia, which eventually resolved. Incidentally, a small pleural effusion on the right side was due to minimal congestive heart failure (CHF).
Aspiration pneumonia. Lateral radiograph in an 84-year-old patient confirms the location of the abnormality in the left lower lobe.
Background
Aspiration is defined as entry of a foreign substance, solid or liquid, into the respiratory tract or inhalation of fumes and vapors. Aspiration pneumonia is an infectious process caused by aspirated oropharyngeal flora. Aspiration pneumonitis, which is caused by a direct chemical insult due to the aspirated material, is technically a different entity, but is often referred to as aspiration pneumonia.
The most common predisposing factors for aspiration in adults are alcoholism, stroke and other neuromuscular disorders, seizures, and loss of consciousness.
Related eMedicine topics:
Pneumonia, Aspiration (Emergency Medicine)
Pneumonia, Aspiration (Pulmonology)
Pathophysiology
Aspirated foreign material may cause obstruction in the tracheobronchial tree from the level of the glottis to distal bronchi. The mechanical obstruction impedes the usual mucosal cleansing mechanism, leading to increased vulnerability to seeded pathogens.
The distribution of aspirated material in the lung depends on the person's positioning during the event. If the individual aspirates while supine, which is the most common scenario, the aspirated material typically lands in the posterior segment of the upper lobes and the superior segment of the lower lobes. If aspiration occurs when the person is upright, the opacities usually are in the right lower lobe. With aspiration in the prone position, which is common among alcoholics, the material tends to be in the right upper lobe.
The primary sites for damage by chemical or microbial aspirates are the small airways and alveoli. These delicate structures are particularly prone to infection and inflammation.
The acute inflammatory-phase response (possibly to an infection) involves massive recruitment of neutrophils, with the systemic elaboration of various cytokine-mediated cascades. Interleukin-8 appears to be an important mediator of this inflammatory process.
Frequency
United States
As many as 45% of healthy individuals experience aspiration, which usually involves small amounts of saliva, particularly while they sleep at night. However, clinically significant aspiration occurs in less than 4% of individuals.
Aspiration pneumonia has been reported in 20.6% of patients with health care–associated pneumonia and in 3.0% of those with community-acquired pneumonia.1 The incidence of perioperative aspiration in adults undergoing elective surgery is estimated at 1 per 2000-3000 general anesthesias.2 Approximately 10% of patients hospitalized for drug overdose develop aspiration pneumonia.3
Mortality/Morbidity
Outcomes of aspiration pneumonia vary greatly, from chronic indolent infection (possibly becoming granulomatous) to acute overwhelming sepsis and acute respiratory distress syndrome (ARDS) associated with rapid death. The outcome depends on the nature (quality and quantity) of the aspirate and the baseline health condition of the patient.
The consequences of aspiration include bronchial obstruction, infection, and direct chemical destruction of tissues.
- Aspirate with a pH lower than 2.5 causes considerable damage, including hemorrhagic tracheobronchitis and pulmonary edema. Massive aspiration of an acid substance, such as gastric contents, leads to the development of diffuse bilateral lung parenchymal opacities.
- Infection is typically caused by the aspiration of oral flora, especially in patients with poor oroperiodontal hygiene. Patients who are chronically intubated are prone to infection from gram-negative flora. Aspirated material in the lungs may lead to pneumonia, abscess, and empyema.
- Large particles can cause acute airway obstruction with lobar or segmental atelectasis. Smaller particles can cause acute focal inflammation, which can evolve into chronic granuloma and scarring, manifested by nodular and linear opacities, best seen on CT. Often, the above features are commingled.
Age
The prevalence is directly linked to specific risk factors related to age, neuromuscular conditions, and changes in mental status. Young children may aspirate foreign objects. Some studies have suggested that individuals older than 70 years are at greater risk for complications of aspiration in the intensive care setting, especially after intubation.
Anatomy
The body's natural defenses against aspiration include normal swallowing, closing of the glottis, and the cough reflex.
Swallowing is a complex act that requires the coordination of muscles in the buccolabial area; the tongue; the palate; the pharynx; the larynx; and, finally, the esophagus. Neurally, swallowing is controlled by the sensory (afferent) and motor (efferent) branches of cranial nerves IX and X.
Below the glottis, the cough reflex is stimulated by the presence of foreign material in the airway. Coughing is an attempt to forcefully expel a substance up and out of the airway.
Presentation
The clinical history is important in diagnosing aspiration pneumonia. The nature of the aspirated material, the quantity of aspirated material, and the time course of the event influence the size and distribution of the lung parenchymal abnormality.
Patients most susceptible to aspiration are those with the following conditions:
- Altered mental status due to stroke, intoxication from alcohol or other drugs, general anesthesia, seizures, trauma, and metabolic derangement such as hypoglycemia
- Neuromuscular disorders such as degenerative neuromuscular disease, muscular dystrophy, or Guillain-Barré syndrome
- Anatomic or structural abnormalities such as local tumor, esophageal stricture, achalasia, tracheoesophageal fistula, or gastroesophageal reflux disease
Clinical manifestations depend on the nature of the aspirate. Some common and well-studied types of aspirates include the following:
- Gastric content (Mendelson syndrome): Asthmalike manifestations (if the aspirate is purely liquid) or obstruction (if medium-to-large particles are involved) may occur from aspirate. Other manifestations may include dyspnea, tachycardia, wheezing, rhonchi, pulmonary edema, hemorrhagic tracheobronchitis, hypotension, oxygen desaturation (in severe form), or cardiopulmonary arrest.
- Infectious aspirate: Predominant pathogens in community-acquired aspiration pneumonia are Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, and Enterobacteriaceae. Gram-negative organisms, including Pseudomonas aeruginosa, predominate in patients with hospital-acquired aspiration pneumonia.3 Patients in whom ventilators are used for more than 48 hours are at increased risk for pneumonia, abscess, empyema, and ARDS.
- Obstructive aspirate: Signs and symptoms depend on the size and location (level) at which the aspirate lodges. Patients may present with atelectasis, wheezing, stridor, and hypoxia. Children or mentally challenged patients with occult foreign body aspiration can present with unilateral new-onset wheezing or recurrent and frequent pneumonia.
Aspirate subtypes as noted in the literature include the following:
- Water (near drowning): Currently, no significant difference is believed to exist between saltwater aspiration and freshwater aspiration. Patients present with dyspnea. On chest radiographs, initially scattered opacities progress to diffuse bilateral opacities.
- Mineral oil: Exogenous lipoid pneumonia may result from aspiration of mineral oil used as a laxative, especially in children or the elderly. Patients may be asymptomatic or may develop direct chemical pneumonitis. Pneumonitis may have different radiographic manifestations such as multifocal scattered consolidation, chronic segmental or lobar consolidation, or focal masslike opacities.
- Leguminous vegetables: Pneumonia from aspiration of leguminous vegetables, termed lentil aspiration pneumonia, usually occurs in elderly patients. Radiologically, these cases present as diffuse, poorly defined nodular opacities (1-3 mm to 1 cm), with branching, which are best demonstrated on CT scans.
- Recurrent aspirations: Patients may experience recurrent consolidation in the same lobe or segment or may have nonresolving opacities resulting from an acute or chronic process.
- Oral contrast medium: Aspiration of Gastrografin can produce rapid progression to respiratory failure. Patients may develop immediate cyanosis, respiratory distress, and pulmonary edema resulting in cardiopulmonary arrest. Diffuse bilateral opacities typically are seen on radiographs. In one case report, aspiration of only 50 mL led to rapid onset of acute pulmonary edema and arrest.4 Aspiration of barium sulfate usually has minimal clinical consequences, because barium sulfate is inert, but fibrosis has been reported on later CT studies.5 Massive barium sulfate aspiration may be fatal, however.6
- Hydrocarbon/petroleum: Accidental aspiration of petroleum may occur during the performance of fire-eating.7 Chest radiographs can show basal lung infiltrates followed by pneumatocele formation. Radiologic resolution of the pneumatoceles occurs within 2-12 months.
Preferred Examination
Chest radiography is readily available and inexpensive and is by far the most commonly used imaging test to evaluate aspiration pneumonia. Traditionally, posteroanterior (PA) and lateral chest radiographs have been recommended for imaging aspiration pneumonia and its complications. However, because many patients are not able to cooperate for PA and lateral imaging, anteroposterior (AP) portable images have been more commonly used for diagnosis.
CT scanning is the best method for diagnosing aspiration pneumonia, an abscess, or an empyema. CT precisely delineates the location of the lobar or segmental opacity. A foreign body in the tracheobronchial tree and associated atelectasis or consolidation can be defined with relative ease on CT scans. Aspiration of specific material such as fat or contrast material can sometimes be determined by measuring the tissue attenuation on CT scans. Esophageal abnormalities may also be seen on CT images without the need for contrast material. Necrosis, cavity formation, and empyema are all complications of aspiration pneumonia that are seen better and earlier with CT than with plain radiography.
The patient's swallowing mechanism can be studied by using fluoroscopy with a contrast agent. This is a real-time evaluation of the swallowing process that is often performed in conjunction with speech therapy.
MRI is more sensitive than plain radiography. To date, no large study has been performed to compare MRI with CT for the evaluation of aspiration.
Limitations of Techniques
Radiography remains the most practical first-line imaging study for patients with suspected aspiration pneumonia. Chest radiographs usually adequately demonstrate lung consolidation, atelectasis, and abscess formation. However, CT scanning is more sensitive and specific than radiography.
Many factors affect the appearance on initial studies, including the patient's hydration status, his or her ability to mount an adequate inflammatory response, and the nature and amount of aspirate. Days may pass before aspiration is visible on imaging studies. Aspiration must be considered in patients with a suggestive history, especially when results of other studies (eg, biopsy of pulmonary mass) do not yield sufficient information concerning a particular lesion.
Patient Education: For excellent patient education resources, visit eMedicine's Pneumonia Center. Also, see eMedicine's patient education article Chemical Pneumonia.
Differential Diagnoses
Acute Respiratory Distress Syndrome
Asthma
Atelectasis, Lobar
Pneumonia, Typical Bacterial
Pneumonia, Viral
Pulmonary Edema, Noncardiogenic
Other Problems to Be Considered
Non-aspiration pneumonia
Pulmonary edema
ARDS
Pulmonary hemorrhage
Alveolar proteinosis
If the pattern is lobar or segmental atelectasis, intrabronchial malignancy (usually squamous cell carcinoma) is also a differential consideration, as is bronchoalveolar carcinoma.
More on Aspiration Pneumonia |
 Overview: Aspiration Pneumonia |
| Imaging: Aspiration Pneumonia |
| Follow-up: Aspiration Pneumonia |
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| References |
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References
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Further Reading
Keywords
aspiration pneumonia, chemical insult, bacterial infection, fume inhalation, vapor inhalation, aspirated foreign material
