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Friday, 22 April 2011

Influenza A - H1N1


  • Influenza A - causes influenza in birds.
  • Single stranded RNA virus
  • Subtypes labeled according to hemaglutinin (H1-16), and neuraminidase (N1-9) present on the surface envelope of the virus
Some overviews of studies related to radiologic findings in influenza A and H1N1 patients:

8 patients evaluated
A rapidly progressive pneumonia with high mortality rate was observed especially for cases with late admission. The major radiologic abnormalities were extensive pneumonic infiltration with segmental and multifocal distribution, mostly located in lower zones of the lung. No pleural effusion and hilar lymphadenopathy was noted.

RSNA: H1N1: Initial Chest Radiographic Findings in Helping Predict Patient Outcome
179 H1N1 influenza patients, 39 had findings related to influenza, Characteristic imaging findings included the following: ground-glass (69%), consolidation (59%), frequently patchy (41%), and nodular (28%) opacities. Bilateral opacities were common (62%), with involvement of multiple lung zones (72%). Findings in four or more zones and bilateral peripheral distribution occurred with significantly higher frequency in patients with adverse outcomes compared with patients with good outcomes.
Extensive involvement of both lungs, evidenced by the presence of multizonal and bilateral peripheral opacities, is associated with adverse prognosis. Initial chest radiography may have significance in helping predict clinical outcome but normal initial radiographs cannot exclude adverse outcome.

Previous studies have reported chest radiographic findings of consolidation and ground-glass opacity in patients with swine-origin influenza A (H1N1). Results Of 159 H1N1 influenza patients, The predominant radiographic findings were consolidation (93% of all patients, 91% of adults and 100% of children) and ground- glass opacity (74% of all, 77% of adults and 57% of children). Pulmonary emboli were detected by CT in 2/9 (22%) patients. Conclusion The pandemic 2009 swine influenza causes common and widely distributed infiltrates on chest radiographs among hospitalized patients, which are not peculiar to usual respiratory viral infections, and these findings can not be differentiated from usual bacterial pneumonia.

PubMed - a literature review:
Novel influenza A (H1N1) at the origin of the 2009 pandemic flu developed mainly in subjects of less than 65 years contrary to the seasonal influenza, which usually developed in elderly patients of more than 65 years. Influenza A(H1N1) can arise in serious forms within 60 to 80% of cases a fulminant acute respiratory distress syndrome (ARDS) "malignant and fulminant influenza" in subjects without any comorbidity, which makes the gravity and the fear of this influenza. The fact that this influenza A (H1N1) can develop in healthy young patients and evolve in few hours to a severe ARDS with a refractory hypoxemia gave to the foreground the possible interest of the recourse to extracorporeal oxygenation (ECMO) in some selected severe ARDS (5- 10%). A bacterial pneumonia was associated to H1N1 influenza in approximately 30% of the cases as at admission in ICU or following the days of the admission justifying an early antibiotherapy associated to the antiviral treatment by oseltamivir (Tamiflu). Obesity, pregnancy and respiratory diseases (asthma, COPD) seem to be associated to the development of a severe viral pneumonia due to influenza A (H1N1) often with ARDS. Older age, high APACHE II and SOFA scores and a delay of initiation of the antiviral treatment by oseltamivir are associated to higher morbidity and mortality.


(Text sources:, PubMed, RSNA, EJR )
(Images: )

Questions remain unanswered:
- What are the age groups at risk?
-What are the typical CXR, CT features?
- What are the dynamics of CXR, CT features?
- Frequently associated comorbidities?
- When should the Radiologist start thinking of H1N1?

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