Etiology
In most cases, lung injury results from aspiration of oropharyngeal material that contains a mixture of aerobic and anaerobic bacteria. Patients most at risk are those who have swallowing dysfunction, gastroesophageal reflux, neurologic disease, or mechanical and device-related impairment of the upper digestive tract (such as a tracheostomy, endotracheal tube, or a nasogastric or percutaneous feeding tube).[12][13] The aspiration event is usually not witnessed and does not necessarily lead to pneumonia.
Bacteria enter the lung owing to aspiration of colonized secretions from the oropharynx. Aspiration of secretions occurs in nearly 50% of healthy adults, but protective mechanisms such as coughing, ciliary action in the airways, low bacterial volume in normal secretions, and humoral and cellular defence mechanisms prevent aspiration from causing frequent episodes of pneumonia. Impairment of any of these mechanisms could result in aspiration pneumonia.[8]
The bacteriology depends on the setting in which aspiration occurs and also reflects the oral flora of the patient; in this regard, it is influenced by periodontal disease. The bacteriology has changed over the past five decades. Older studies reported anaerobes (45% to 48%) alone or in combination with aerobes (41% to 46%) with Bacteroides species, Porphyromonas species, Prevotella melaninogeica, Fusobacterium species, and anaerobic gram-positive cocci being predominantly isolated.[1] However, more recent studies show bacteriology similar to non-aspiration pneumonia.[2][8][14][15] The reasons for this change include concerns that older studies may have used methods that over-represented anaerobic infections. However, it is likely that the incidence of anaerobic pleuropulmonary syndrome (a later presentation of cavitary pneumonia or empyema associated with prior loss of consciousness and poor dental hygiene) has been decreasing over time, due to improvements in dental care and earlier access to care for pneumonia and aspiration syndromes.
Pathophysiology
Aspiration causes an inflammatory reaction called pneumonitis. This can result in damage to the lung parenchyma. Damage to lung parenchyma causes an inflammatory reaction that can lead to symptoms such as fever, cough, or elevation of white cell count. A secondary bacterial infection can occur because of this damaged parenchyma, leading to aspiration pneumonia. Pneumonia is likely a disruption of the microbiome rather than an infection of a sterile organ.[16]
The basal segments of the lower lobes are more commonly affected in a patient who aspirates while sitting upright.[8] The posterior segments of the upper lobes or the apical segments of the lower lobes are affected in a patient who aspirates while recumbent.
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