Pneumonia is a common, expensive (in 2002, the total cost was $8 billion to treat pneumonia in the United States), and often serious infection with considerable morbidity and mortality. The major burden of pneumonia in a community is borne by the elderly. Successful management of pneumonia, in any patient, but especially in the elderly requires considerable skills.
From the viewpoint of the pathologist, pneumonia is an inflammatory response in the lung caused by an infectious agent that involves the alveoli and terminal bronchioles. It is manifested by increased weight of the lungs, replacement of the normal lung sponginess by consolidation and alveoli filled with white blood cells, red blood cells, and fibrin (Figures 126-1 and 126-2).
Photograph of a lung with pneumonia (white area) involving the entire upper lobe.
Photomicrograph of lung showing alveoli filled with inflammatory exudate. Magnification: 445x.
The clinician defines pneumonia as a combination of symptoms (fever, chills, cough, pleuritic chest pain, sputum), signs (hyper or hypothermia, increased respiratory rate, dullness to percussion, bronchial breathing, aegophony, crackles, wheezes, pleural friction rub), and an opacity (opacities) on a chest radiograph (Figure 126-3). In addition, laboratory findings; such as, increased white blood cell count and decreased level of oxygen saturation, may also be part of the definition.
Gram stain sputum showing many polymorphonuclear leucocytes and gram-positive diplococci (S. pneumoniae was recovered on culture of this specimen). Magnification: 1000x.
The epidemiologist or clinical trialist defines pneumonia as two or more of the symptoms listed above, one or more of the physical findings listed above and a new opacity on chest radiograph that is not because of a condition other than pneumonia (such as, congestive heart failure, vasculitis, pulmonary infarction, atelectasis, or drug reaction).
Pneumonia may also be categorized according to the site of acquisition—community, hospital (nosocomial) or nursing home. Some authorities categorize nursing home acquired pneumonia as community-acquired, while others insist it more closely resembles nosocomial pneumonia and should be labeled institutionally acquired pneumonia. However, nursing homes or long-term care facilities have residents who range from fully functional to those who are bedridden. We prefer to consider nursing home/long-term care facility acquired pneumonia as a separate category.
It is useful for the practicing clinician to remember definitions for the certainty with which an agent can be implicated as the cause of the pneumonia. An agent is said to be the definite cause of pneumonia if it is isolated from blood (although some blood isolates, such as coagulase negative staphylococci, are usually contaminants and not pulmonary pathogens), from pleural fluid or from pulmonary tissue; if it isolated ...