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Definition and Classification of Pneumonia

When the word pneumonia is used in medical practice, it almost always refers to a syndrome caused by acute infection, usually bacterial, that is characterized by clinical and/or radiographic signs of consolidation of a part or parts of one or both lungs. The use of the term has however been greatly extended to include non-bacterial infection of the lungs caused by a wide variety of microorganisms. Pneumonitis is occasionally used as a synonym for pneumonia, particularly when inflammation of the lung has resulted from a non-infectious cause such as in chemical or radiation injury.

For practical purposes the classification of pneumonia should be both anatomical, in which the descriptive terms used communicate the extent and distribution of the process in the lung or lungs, and causal, in which the responsible microorganism is named. When, as is often initially the case, the infectious cause is not known it is useful to consider whether the pneumonia is community-acquired or nosocomial (hospital-acquired). It is also useful to consider whether the pneumonia may have resulted from pharyngeal aspiration and whether or not it is occurring in an immunocompromised host.

The anatomical terms used will indicate whether the pneumonia involves one or more entire lobes or whether the process is confined to a segment or segments. In its most confined form, pneumonia may be subsegmental. Such anatomical descriptions are in life entirely dependent upon the chest radiographic appearances which show the extent of pneumonia more accurately than can be gauged by physical examination. Early clinicians distinguished between bronchopneumonia and lobar pneumonia in pathological terms. Bronchopneumonia was regarded as a complication of bronchitis in which the inflammatory process was confined to the territory of a small or terminal bronchus and the lung lobule subtended by it, hence the alternative term lobular pneumonia. Lobar pneumonia, on the other hand, frequently occurred de novo and was characterized by an inflammatory out-pouring or exudation of fluid extending throughout most of a lobe or lobes.

It is commonplace for the term lobar pneumonia to be used when there is clinical and radiographic evidence of confluent consolidation occupying the greater part of one or more lobes of one or both lungs. The term segmental pneumonia is used when such consolidation is not extensive enough to occupy most of a lobe but corresponds more closely to the anatomy of a bronchopulmonary segment in one or more lobes. Where the area of radiographic shadowing is even more confined, then subsegmental pneumonia is an appropriate descriptive term, although this still implies a confluent and localized process. Where subsegmental shadowing is patchy

(non-confluent) and poorly localized, being scattered throughout part or the whole of one or both lungs, the term bronchopneumonia remains entirely acceptable.

This anatomical classification is complementary but subservient to causal classification, being of only limited value in establishing the likely infective agent, for although lobar pneumonia is usually caused by Streptococcus pneumoniae it can be caused by many other microorganisms besides, as indeed can all other anatomical types. Every reasonable effort should therefore be made to establish the identity of the pathogenic organism responsible for pneumonia in each patient, in order that specific antimicrobial therapy can be directed against it, for without this information some patients will not recover who otherwise would have done so.

The causal organism can only be guessed at when the patient is first seen and it is useful in this respect to classify the patient as having either community-acquired pneumonia or nosocomial pneumonia, as the spectrum of infecting organisms is different in each case. Nosocomial pneumonia is a particular problem in post-operative patients and in those treated in intensive care units, the latter group being highly susceptible to lower respiratory tract infection. The differing types of causal organisms found in the hospital situation are due in part to the altered immunocompetency of patients who are afflicted with other serious disease, in part to the alteration of bacterial flora that results from treatment with antibiotics and in part to the instrumentation or intubation of the upper airways of patients. Such hospital-acquired infections are more frequently due to aerobic gram-negative bacilli and Staphylococcus aureus than are those acquired in the community.