Aspiration pneumonia results from inhalation of stomach contents or secretions of the oropharynx leading to lower respiratory tract infection. In many healthy adults, very small quantities of aspiration occur frequently but the normal defence mechanisms (cough, lung cilia) remove the material with no ill effects. However aspiration may cause:¹

• Chemical pneumonitis: chemical irritation of the lungs, which may progress to acute respiratory distress syndrome and/or bacterial infection. Acute aspiration of gastric contents into the lungs can produce an extremely severe and sometimes fatal illness. This has been termed Mendelson's syndrome and can complicate anaesthesia, particularly during pregnancy.
• Obstruction: large volumes of aspirated material may lead to obstruction of the respiratory tract.
• Bacterial infection: infection of the lower airways may lead to empyema, lung abscess, acute respiratory failure and acute lung injury. Persistent aspiration pneumonia is often due to anaerobes and it may progress to lung abscess or even bronchiectasis.

The usual site for an aspiration pneumonia are the apical and posterior segments of the lower lobe of the right lung. If the patient is supine then the aspirated material may also enter the posterior segment of the upper lobes.

Epidemiology

• Common and may account for up to 15% of patients with community acquired pneumonia.
• Aspiration pneumonia is relatively common in hospital and usually involves infection with multiple bacteria, including anaerobes.
• More common in men, young children and the elderly.

Pathogens¹

Pathogens of community acquired aspiration pneumonia are often the normal flora of the oropharynx including:

• Streptococcus pneumoniae
• Staphylococcus aureus
• Haemophilus influenzae
• Anaerobes, e.g. Peptostreptococcus, Fusobacterium and Prevotella spp.
• Streptococcus ‘milleri’ group

Pathogens of nosocomial aspiration pneumonia include:

• Oral anaerobes - as above
• Gram-positive cocci, e.g. Peptostreptococcus spp., Peptococcus spp.
• Gram-negative bacilli, e.g. Enterobacteria (Klebsiella pneumoniae, Escherichia coli, Enterobacter spp.), Pseudomonas aeruginosa.
• Meticillin-resistant Staphylococcus aureus (MRSA)

Risk factors for aspiration pneumonia¹

In the absence of a tracheo-oesophageal fistula, significant aspiration usually occurs only during periods of impaired consciousness, with reflux oesophagitis with an oesophageal stricture, or in bulbar palsy.

• Impaired consciousness: drug or alcohol abuse, general anaesthesia, seizures, sedation, acute stroke, central nervous system lesions, head injury
• Swallowing disorders: oesophageal stricture, dysphagia, stroke, bulbar palsy, pharyngeal disease (e.g. malignancy), neuromuscular disorders (e.g. multiple sclerosis)
• Other: tracheo-oesophageal fistula, ventilator-associated pneumonia, nasogastric feeding tube,² periodontal disease, gastro-oesophageal reflux,³ post gastrectomy, tracheostomy

Presentation

• Non-specific symptoms, e.g. fever, headache, nausea, vomiting, anorexia, myalgia, weight loss
• Cough
• Dyspnoea
• Pleuritic chest pain
• Purulent sputum
• Signs may include tachycardia, tachypnoea, decreased breath sounds and dullness to percussion over areas of consolidation, pleural friction rub
• Severe infection may lead to hypoxia and septic shock

Differential diagnosis

Other causes of respiratory distress, including:

• Other causes of pneumonia
• Bronchiolitis
• Croup
• Epiglottitis
• Foreign body in respiratory tract
• Asthma
• Cardiovascular disease

Investigations

• Blood count: neutrophil leucocytosis
• Electrolytes and renal function: dehydration, electrolyte imbalance
• Blood culture
• Blood gases
• Culture of sputum:
  Many aspiration pneumonias are mixed aerobic-anaerobic infections.⁴
  • In patients with bacterial aspiration pneumonia may show organisms normally resident in the pharynx (see list above).
• Chest x-ray:
  • Right middle and lower lung lobes are the most common sites.
  • Aspiration when upright may cause bilateral lower lung infiltrates.
  • Right upper lobe often shows consolidation in alcoholics who aspirate in the prone position.
• Lung CT and bronchoscopy are only very occasionally required.

Management

• Mechanical obstruction: removal of object normally by bronchoscopy.
• Tracheal suction if seen early.
• Intubation with positive pressure ventilation may be required.
• Bacterial infection of lower airways (the choice of antibiotics will be influenced by any recent previous antibiotic treatment, microbiology culture results and the patient’s condition):
  • Initial empirical antibiotic therapy while awaiting culture results.
  • Antimicrobial therapy should be based on patient characteristics, the setting in which aspiration occurred, the severity of pneumonia, and available information regarding local pathogens and resistance patterns.⁴
  • Community acquired aspiration pneumonia is often initially treated with either clindamycin, or amoxicillin plus metronidazole.¹ See our Pneumonia article for indications for hospital admission.
  • Hospital acquired aspiration pneumonia: a suitable combination in patients who have not already recently been treated with these antibiotics is cefuroxime plus metronidazole.¹
• The role of steroids is uncertain and not of proven benefit.

Complications

• Untreated, bacterial aspiration pneumonia may progress to lung abscess or bronchiectasis.
• Acute respiratory distress.

Prognosis

Depends on the underlying cause, general well-being of the patient, presence of complications, speed of diagnosis and effective treatment.

Prevention

• Keep the head of the bed at a 30 degree angle: reduces the risk or aspiration pneumonia in those at risk.
• Nasogastric feeding for at risk patients, e.g. poor gag reflex, dysphagia.

Document references

1. The British Society for Antimicrobial Chemotherapy; Aspiration pneumonia.
2. Gomes GF, Pisani JC, Macedo ED, et al; The nasogastric feeding tube as a risk factor for aspiration and aspiration pneumonia. Curr Opin Clin Nutr Metab Care. 2003 May;6(3):327-33. [abstract]
3. DeLegge MH; Aspiration pneumonia: incidence, mortality, and at-risk populations. JPEN J Parenter Enteral Nutr. 2002 Nov-Dec;26(6 Suppl):S19-24; discussion [abstract]
4. Johnson JL, Hirsch CS; Aspiration pneumonia. Recognizing and managing a potentially growing disorder. Postgrad Med. 2003 Mar;113(3):99-102, 105-6, 111-2. [abstract]

Internet and further reading

• Swaminathan A; Pneumonia, Aspiration. eMedicine, March 2008.

Acknowledgements