Atypical pneumonia refers to the radiological pattern associated with patchy inflammatory changes, often confined to the pulmonary interstitium, most commonly associated with atypical bacterial aetiologies such as Mycoplasma pneumoniae, Chlamydophila pneumoniae and Legionella pneumophilia. Viral and fungal pathogens may also create the radiological and clinical picture of atypical pneumonia.
Atypical pneumonia makes up a significant proportion of community-acquired pneumonia.
Mycoplasma pneumoniae (see mycoplasma pneumonia) is the causative organism in up to 20% of community acquired pneumonia and is often seen in paediatric populations and young adults 3. It is especially associated with patients living in close community settings.
Chlamydophila pneumoniae is the causative organism in up to 10% of community-acquired pneumonia, and similarly to Mycoplasma pneumoniae it often affects paediatric populations and young adults.
Specific causative organisms have other associated epidemiological associations; for example, Legionella pneumophila (see Legionella pneumonia) infection is associated with immunocompromised patients and exposure to contaminated aerosolised water (for example, from air conditioning systems). Coxiella burnetii infection (Q fever pneumonia) is associated with exposure to livestock 4.
The presentation of atypical pneumonia is often similar to the presentation of more 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
Despite these characteristic features, there is often considerable overlap between the clinical features of typical bacterial pneumonia and atypical pneumonia 2.
Atypical pneumonia may be caused by a variety of pathogens. The most common cause of atypical pneumonia is Mycoplasma pneumoniae.
Other aetiological agents include 1:
- Chlamydophila pneumoniae
- Legionella pneumophilia
- viruses including influenza, respiratory syncytial virus, rhinoviruses, varicella viruses and adenovirus
- Coxiella burnetii (the causative organism of Q fever pneumonia)
Infection prompts an immune response, necrosis and inflammation. In atypical pneumonia, the inflammation is often confined to the pulmonary interstitium and the interlobular septa; this causes the characteristic radiological features of atypical pneumonia. As there is often no exudate in the alveolar air spaces, consolidation is less common sign in atypical pneumonia than in bacterial pneumonia of more typical causative organisms.
Because the inflammation is often limited to the pulmonary interstitium and the interlobular septa, atypical pneumonia has the radiographic features of patchy reticular or reticulonodular opacities. These opacities are especially seen in the perihilar lung 5. Subsegmental and sometimes segmental atelectasis from small airway obstruction may occur. The radiographic features are often more extensive than what is suggested clinically.
Atypical pneumonia has a pattern of focal ground-glass opacity in a lobular distribution. Involvement is often diffuse and bilateral 9. There may also be evidence of pleural effusion. Bronchial wall thickening is another common CT finding 6.
Diffuse ground glass nodules in a centrilobular pattern are often present, although they progress to a soft tissue density as the infection and inflammation progress 9.
In Mycoplasma pneumoniae infection, airspace consolidation is common. HRCT is sensitive for nodules, which are seen in ~90% of patients 7.
In Legionella pneumophila infection, residual scarring may persist after resolution of the infection 8.
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