Atypical pneumonia

Last revised by Liz Silverstone on 25 Dec 2023

Atypical pneumonia is a term used inconsistently through time and in different parts of the world. Unless clarified, the term is an unhelpful addition to radiology reports. It generally refers to non-lobar pneumonias that do not respond to beta-lactam antibiotics and that cause upper and lower respiratory tract disease and constitutional symptoms 10.

Streptococcus pneumoniae, Haemophilus influenzae and Staphylococcus aureus are 'typical' pneumonias. Defining 'atypical' pneumonias is much more difficult! The etiological agents are bacteria and viruses. Globally, the agents vary depending on variation in local pathogens. Definitions are inconsistent and have changed over time. The etiological agent is not identified in about 60% of pneumonias.

Pneumonia remains a leading cause of death and 'atypical' pneumonias are significant causes of outbreaks and pandemics. Recognition is an important public health issue.

The presentation overlaps with that of typical bacterial pneumonia. However, there are some clinical features that are more characteristic of atypical pneumonia 2:

  • more pronounced constitutional symptoms such as a headache and myalgia

  • a low-grade fever

  • persistent dry cough

  • more insidious onset and protracted clinical course

  • lack of consolidation

  • a mixture of upper and lower respiratory tract symptoms and signs

  • extra-pulmonary disease

Atypical bacterial pneumonias in immunocompetent hosts may account for 15% of community acquired pneumonias (CAP). Organisms commonly include:

Zoonoses such as:

Viruses with person-person transmission such as:

and Hanta virus (animal vector).

Bacterial and viral co-infection is common.

Some radiologists would include fungi and TB. The radiology report should make your findings and interpretation clear and offer guidance for further management.

Pulmonary inflammation may be airway and interstitial predominant with radiographic features of bronchopneumonia, peribronchial opacity, patchy reticulonodular or hazy opacities. These opacities are especially seen in the lower lobe(s) or parahilar lung 5. Subsegmental and sometimes segmental atelectasis from small airway obstruction may occur. The radiographic features are often more extensive than expected from the clinical features. 

Atypical pneumonia has a pattern of focal ground-glass opacity in a lobular distribution. Involvement may be focal or diffuse and bilateral 9. There may also be evidence of pleural effusion. Bronchial wall thickening is another common CT finding 6

Diffuse centrilobular ground glass nodules may progress to soft tissue density and consolidation as the infection and inflammation progress 9

  • the term was first introduced in the 1930s for pneumonias with unusual presentations when few infecting organisms were recognized

  • in the 1940s the meaning was modified to include pneumonias with gradual onset and constitutional as well as respiratory symptoms

  • during World War II the term was extended to include outbreaks of pneumonia with extrapulmonary symptoms

  • post war, the definition could imply unknown etiology, resistance to beta-lactam antibiotics e.g. penicillin, non-lobar lung disease and organisms that were difficult to diagnose by culture 10

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