What is aspiration pneumonia?

Definition of Aspiration Pneumonia

Aspiration pneumonia is a bacterial infection of the lung parenchyma caused by aspiration of contaminated oropharyngeal secretions that passes below the true vocal folds into the trachea, resulting in pulmonary infection. 1, 2

Key Distinguishing Features

Aspiration Pneumonia vs. Chemical Pneumonitis

• Aspiration pneumonia is an infectious process caused by bacteria from oropharyngeal secretions, with symptoms persisting beyond 24-48 hours and worsening without antibiotic treatment 2
• Chemical pneumonitis is a sterile inflammatory injury from aspiration of gastric acid or other toxic substances that typically resolves within 24 hours without antibiotics 2
• At 24 hours post-aspiration event: improving symptoms indicate chemical pneumonitis (supportive care only), while persistent or worsening symptoms indicate aspiration pneumonia requiring immediate antibiotics 2

Aspiration vs. Penetration

• Aspiration occurs when material passes below the true vocal folds into the trachea 1
• Penetration is when material enters only the laryngeal area to the level of the true vocal folds 1

Diagnostic Criteria

Clinical Definition

Aspiration pneumonia is diagnosed when both of the following are present:

1. Inflammatory findings in the lungs (new infiltrate on chest radiograph with fever, cough, and auscultatory findings such as rales or evidence of pulmonary consolidation) 1, 3

2. Evidence of aspiration, which includes either:

   • Overt/witnessed aspiration event 3, 4
   • Strong suspicion of aspiration based on abnormal swallowing function, dysphagia, or presence of risk factors for silent aspiration 3, 4

Risk Factors Required for Diagnosis (Silent Aspiration)

When no witnessed aspiration event occurs, diagnosis requires:

• One or more risk factors for oropharyngeal aspiration: depressed consciousness, stroke, dysphagia, impaired laryngeal sensation, bed-bound status, post-surgical state, mechanical ventilation 1, 2, 4
• Plus one or more risk factors for oral bacterial colonization: malnutrition, smoking, poor oral hygiene, dry mouth, multiple medications, sedative use 1, 4

Epidemiology and Clinical Significance

• Aspiration pneumonia is the most common form of hospital-acquired pneumonia among adults, occurring in 4-8 per 1,000 hospitalized patients 1
• Mortality rates range from 20% to 65%, significantly higher than community-acquired pneumonia 1, 5
• In healthy adults, 45% aspirate during sleep based on radioisotope-tracer studies, but most do not develop pneumonia due to intact host defenses 2

Pathophysiology

Primary Mechanism

• Aspiration of oropharyngeal organisms is the most important mechanism for both nosocomial and community-acquired pneumonia 2
• Bacteria invade the lower respiratory tract primarily through aspiration, though other routes include inhalation of aerosols, hematogenous spread, and possibly bacterial translocation from the gastrointestinal tract 2

Anatomic Considerations

• The posterior right lower lobe is the most common site for aspiration pneumonia, particularly when patients are supine 6

Microbiology

Community-Acquired Aspiration Pneumonia

• Causative organisms are bacteria residing in the oral cavity: pneumococcus, Haemophilus influenzae, Staphylococcus aureus, and anaerobes 3
• Modern microbiology demonstrates that aspiration pneumonia is rarely solely an anaerobic infection; aerobic and mixed cultures are common 7, 5

Hospital-Acquired/Nursing Home Aspiration Pneumonia

• Additional pathogens include aerobic gram-negative bacilli (including Pseudomonas aeruginosa) and Staphylococcus aureus (including MRSA) 8, 1
• The 2019 IDSA/ATS guidelines do not recommend routine anaerobic coverage for suspected aspiration pneumonia in inpatient settings, except when lung abscess or empyema is suspected 8

Clinical Presentation

Typical Symptoms

• Fever, cough, and auscultatory findings such as rales or evidence of pulmonary consolidation 1, 2

Atypical Presentations

• Elderly patients are less likely to report typical symptoms, leading to delayed diagnosis 6
• Neonates and infants may present with apnea and increased swallowing frequency rather than coughing 1, 2

Important Clinical Caveats

• Silent aspiration is common and frequently occurs without coughing, particularly in patients with stroke (22-38% demonstrate aspiration on videofluoroscopic swallow evaluation) 1, 6
• Aspiration pneumonia is difficult to distinguish from other pneumonia syndromes based on clinical presentation alone 7
• The presence of dysphagia or witnessed aspiration does not automatically indicate bacterial pneumonia—chemical pneumonitis must be ruled out first by observing clinical course over 24 hours 2