The syndrome approach

This is based on the assumption that different pathogens cause distinct and non-overlapping clinical syndromes. The terms "typical" and "atypical" pneumonia were adopted to describe these syndromes. Typical pneumonia was caused by the pneumococcus and was said to present with pyrexia of greater than 39°C, pleuritic chest pain, a lobar distribution of consolidation, and an increase in immature granulocytes. Features of atypical pneumonia included a more gradual onset and a diffuse interstitial or alveolar pattern on the plain chest radiograph.

Numerous studies, however, have shown that clinical overlap between the different pathogens is great and that no single or combination of symptoms and plain chest radiology will reliably differentiate between the different pathogens.1 In severe CAP the situation is even more difficult; case series of severe pneumococcal, staphylococcal, and legionella pneumonia show no reliable distinguishing features. In a recent series of 84 patients requiring ICU admission for severe legionella pneumonia, 39% had only unilateral radiographic changes at presentation.14 Hyponatraemia is often quoted as a sign of legionella pneumonia but in this series14 hyponatraemia was strongly associated with poor outcome, suggesting that it is a marker of disease severity rather than disease type.

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