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Aspiration pneumonia is often caused by an incompetent swallowing mechanism, such as occurs in some forms of neurological disease (a common cause being strokes) or while a person is intoxicated. An iatrogenic cause is during general anaesthesia for an operation and patients are therefore instructed to be nil per os (NPO) for at least four hours before surgery.
Whether aspiration pneumonia represents a true bacterial infection or a chemical inflammatory process remains the subject of significant controversy. Both causes may present with similar symptoms.
Aspiration of foreign material (often the stomach contents) into the lung can be a result of disorders that affect normal swallowing, disorders of the esophagus (esophageal stricture, gastroesophageal reflux), or decreased or absent gag reflex in unconscious or semi-conscious individuals. Old age, dental problems, use of sedative drugs, anesthesia, coma, and excessive alcohol consumption are also causal or contributing factors. The response of the lungs depends upon the characteristics and amount of the aspirated substance. The more acidic the material, the greater the degree of lung injury, although this may not necessarily lead to pneumonia.
The injured lungs may become infected with multiple species of anaerobic bacteria or aerobic bacteria. A collection of pus may form in the lung. A protective membrane may form around the abscess.
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