Atypical pneumonia refers to the radiological pattern associated with patchy inflammatory changes, often confined to the pulmonary interstitium, most commonly associated with atypical bacterial etiologies 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 pediatric populations and young adults 3. It is especially associated with patients living in close community settings.
Chlamydophila pneumoniae (Chlamydia pneumonia) is the causative organism in up to 10% of community-acquired pneumonia, and similarly to Mycoplasma pneumoniae it often affects pediatric 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 aerosolized 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 etiological agents include 1:
- Chlamydophila pneumoniae
- Legionella pneumophilia : Legionella pneumonia
- 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.
- 1. Kumar V, Abbas AK, Fausto N et-al. Robbins & Cotran Pathologic Basis of Disease (Robbins Pathology). Saunders. ISBN:B005WV2Q86. Read it at Google Books - Find it at Amazon
- 2. Clinical Infectious Disease. Cambridge University Press. ISBN:0521871123. Read it at Google Books - Find it at Amazon
- 3. File TM. Community-acquired pneumonia. Lancet. 2003;362 (9400): 1991-2001. doi:10.1016/S0140-6736(03)15021-0 - Pubmed citation
- 4. Marrie TJ. Coxiella burnetii pneumonia. Eur. Respir. J. 2003;21 (4): 713-9. Pubmed citation
- 5. Lange S, Walsh G. Radiology of Chest Diseases. TIS. ISBN:B005UG7V10. Read it at Google Books - Find it at Amazon
- 6. Miyashita N, Sugiu T, Kawai Y et-al. Radiographic features of Mycoplasma pneumoniae pneumonia: differential diagnosis and performance timing. BMC Med Imaging.9 (1): 7. BMC Med Imaging (full text) - doi:10.1186/1471-2342-9-7 - Free text at pubmed - Pubmed citation
- 7. Reittner P, Müller NL, Heyneman L et-al. Mycoplasma pneumoniae pneumonia: radiographic and high-resolution CT features in 28 patients. AJR Am J Roentgenol. 2000;174 (1): 37-41. doi:10.2214/ajr.174.1.1740037 - Pubmed citation
- 8. Yagyu H, Nakamura H, Tsuchida F et-al. Chest CT findings and clinical features in mild Legionella pneumonia. Intern. Med. 2003;42 (6): 477-82. Pubmed citation
- 9. Elicker BM, Webb WR. Fundamentals of High-Resolution Lung Ct. Lippincott Williams & Wilkins. (2013) ISBN:1451184085. Read it at Google Books - Find it at Amazon