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 caused by a direct chemical insult due to the aspirated material or by a primary or secondary bacterial infection.

The most common predisposing factors for aspiration in adults are alcoholism, stroke and other neuromuscular disorders, seizures, and loss of consciousness.

Pathophysiology: Aspirated foreign material may cause obstruction in the tracheobronchial tree from the level of the glottis to distal bronchi. Most commonly, the aspirated material lands in the posterior segment of upper lobes and superior segment of lower lobes typically in supine positioned patients; therefore, these sites are most commonly the locations of aspiration pneumonia. 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 position during the event. If aspiration occurs when a person is upright, the opacities usually are in the right lower lobe. If the individual aspirates in the supine position, the material tends to accumulate in the upper lobes.

The primary sites for damage by chemical or microbial aspirates are the small airway and alveoli in which the delicate structures are particularly prone to infections 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. Some recent studies have revealed the key role played by interleukin-8 in the process.

Frequency:

• In the US: 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.

Mortality/Morbidity: The mortality and morbidity rates of aspiration pneumonia vary greatly from chronic indolent (possibly becoming granulomatous) infection to acute overwhelming sepsis and acute respiratory distress syndrome (ARDS) associated with rapid death. Patient 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.

• A hydrogen ion concentration (pH) lower than 2.5 causes considerable damage, including hemorrhagic tracheobronchitis and pulmonary edema. Massive aspiration of 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.

Race: No evidence suggests that race is an independent risk factor in aspiration pneumonia.

Sex: No strong evidence suggests an increased risk in either sex.

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. People of any age with alcoholism are more susceptible to aspiration than the general population is. 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.

Clinical Details: 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, alcohol or other drug intoxication, general anesthesia, seizures, trauma, and metabolic derangement such as hypoglycemia

   

• Neuromuscular disorders such as degenerative neuromuscular disorders, 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 (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: Anaerobes (oral flora) cause infection in outpatients, and aerobes (gut, skin, upper airway, and hospital flora) cause infection in inpatients. 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. In children or patients who are mentally challenged, 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:

• Aspiration of 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: Patients who aspirate paraffin oil or hydrocarbons can be asymptomatic, or they can develop direct chemical pneumonitis. Pneumonitis may have different radiographic manifestations such as multifocal scattered consolidation, chronic segmental or lobar consolidation, or focal masslike opacities.

   

• Lentil aspiration: Usually, elderly patients present with diffuse, poorly defined nodular opacities (1-3 mm to 1 cm), with branching. These are best demonstrated on CT scans. These findings may indicate vegetable material aspiration.

   

• Recurrent aspirations: Patients may develop repeated recurrent consolidation in the same lobe or segment or may have nonresolving opacities resulting from an acute or chronic process.

   

• Gastrografin aspiration: This form of chemical aspiration can rapidly progress 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. One case report exists of only 50 mL in aspirate volume, quickly leading to acute pulmonary edema and arrest.

   

• Hydrocarbon/petroleum aspiration: This pneumonitis can occur in fire-eaters, among others. This cause usually involves an accidental aspiration of petroleum during the performance of fire-eating. Chest radiographs can show basal lung infiltrates followed by pneumatocele formation. Radiologic resolution of the pneumatoceles occurs within 2-12 months.

Preferred Examination: Traditionally, posteroanterior (PA) and lateral chest radiographs have been used to diagnose 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. Still, chest radiography is by far the most commonly used imaging test to evaluate aspiration pneumonia. Chest radiography is readily available and inexpensive.

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 radiograph, although to date, no large study has been performed to compare MRI with CT for the evaluation of aspiration.

Limitations of Techniques: CT is the most sensitive method for evaluation of aspiration pneumonia and its complications; however, chest radiographs usually adequately demonstrate lung consolidation, atelectasis, and abscess formation.

CT scanning is the best method for diagnosing aspiration pneumonia, an abscess, or an empyema. Although CT scans are more sensitive and specific than radiographs, plain images remain the most practical first-line imaging study. Many factors affect the initial appearance of the radiograph, including the patient’s hydration status, his or her ability to mount 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.

DIFFERENTIALS

Acute Respiratory Distress Syndrome
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.

X-RAY

Findings: AP portable chest images may demonstrate bilateral opacities in the middle or lower lung zones. On PA and lateral images, the opacities may be localized to the posterior segments of upper lobes or to the superior segments of lower lobes. Alternatively, the images may indicate a more extensive distribution of disease.

Chest CT can demonstrate the abnormal lung opacities earlier and in more detail than can plain chest radiographs. An intratracheal or intrabronchial foreign body can be identified on CT scans, as can any associated atelectasis/consolidation or effect of partial obstruction such as focal overaeration. Specific aspirates such as fat or opaque material can be identified and even measured on CT images, though not routinely.

Complications from aspiration pneumonia (eg, abscess formation, lung necrosis, empyema) are well depicted on CT scans. Long-term complications, such as obliterative bronchiolitis, are diagnosed best with high-resolution CT (HRCT).

Degree of Confidence: Traditionally, most physicians have depended on plain radiographs, which have moderate-to-good specificity and sensitivity.

CT has better sensitivity and specificity and should be used to more promptly diagnose aspiration pneumonia, to determine its cause, and to detect its complications earlier.

False Positives/Negatives: False-negative findings are associated with subtle or early findings in a clinical course. False-positive findings usually occur when the clinical history is unclear. The findings in aspiration pneumonia are not specific; pulmonary edema, pneumonias from other causes and neoplasm are in the differential diagnosis.