H1N1 influenza is a strain of influenza that notably resulted in a pandemic in 2009. It was referred to colloquially as 'swine flu' due to the origin of the virus, but it was also named H1N1/09 virus. A specific but different strain of H1N1 (called H1N1 influenza A) was the cause of the Spanish flu, one of the worst global pandemics in recorded human history ref. This article refers to the disease resulting from the H1N1/09 virus.
There can be a wide spectrum of clinical syndromes ranging from being asymptomatic to fulminant viral pneumonia, respiratory failure and death.
It is caused by a type of influenza A virus of swine origin.
It can have a variable presentation and a number of features have been described 2: In the majority of cases the chest radiograph can be normal.
- initial chest radiographs usually show central or peripheral pulmonary ground-glass opacities (GGO) and consolidations that have a patchy or nodular appearance
- multizonal and bilateral peripheral opacities are associated with adverse prognosis
- it should be stressed that a normal radiograph cannot exclude a poor outcome
Described imaging spectrum is broad, with each individual feature being non-specific on its own 2,3:
- ground-glass attenuation: ~70% region (most common)
- bronchial wall thickening: ~45% (second most common)
- confluent consolidation: ~30%
- patients are thought to have worse clinical progression
- typically tends to be peripheral consolidation involving the lower lobes
- can also frequently involve all lobes 4
- pleural effusion: ~30%
- atelectatic changes: ~25%
- 1. Agarwal PP, Cinti S, Kazerooni EA. Chest radiographic and CT findings in novel swine-origin influenza A (H1N1) virus (S-OIV) infection. AJR Am J Roentgenol. 2009;193 (6): 1488-93. doi:10.2214/AJR.09.3599 - Pubmed citation
- 2. Aviram G, Bar-Shai A, Sosna J et-al. H1N1 influenza: initial chest radiographic findings in helping predict patient outcome. Radiology. 2010;255 (1): 252-9. doi:10.1148/radiol.10092240 - Pubmed citation
- 3. Shim SS, Kim Y, Ryu YJ. Novel influenza A (H1N1) infection: chest CT findings from 21 cases in Seoul, Korea. Clin Radiol. 2011;66 (2): 118-24. doi:10.1016/j.crad.2010.07.011 - Pubmed citation
- 4. Elicker BM, Schwartz BS, Liu C et-al. Thoracic CT findings of novel influenza A (H1N1) infection in immunocompromised patients. Emerg Radiol. 2010;17 (4): 299-307. doi:10.1007/s10140-010-0859-x - Free text at pubmed - Pubmed citation
- 5. Ajlan AM, Quiney B, Nicolaou S et-al. Swine-origin influenza A (H1N1) viral infection: radiographic and CT findings. AJR Am J Roentgenol. 2009;193 (6): 1494-9. doi:10.2214/AJR.09.3625 - Pubmed citation
- 6. El-Badrawy A, Zeidan A, Ebrahim MA. 64 multidetector CT findings of influenza A (H1N1) virus in patients with hematologic malignancies. Acta Radiol. 2012;53 (6): 662-7. doi:10.1258/ar.2012.120038 - Pubmed citation
- 7. Choi MJ, Lee YS, Lee JY et-al. Novel influenza A (H1N1) virus infection in children: chest radiographic and CT evaluation. Korean J Radiol. 2010;11 (6): 656-64. doi:10.3348/kjr.2010.11.6.656 - Free text at pubmed - Pubmed citation
- 8. Lee EY, McAdam AJ, Chaudry G et-al. Swine-origin influenza a (H1N1) viral infection in children: initial chest radiographic findings. Radiology. 2010;254 (3): 934-41. doi:10.1148/radiol.09092083 - Pubmed citation
- 9. Edson Marchiori, Gláucia Zanetti, Giuseppe D'Ippolito et-al. Swine-Origin Influenza A (H1N1) Viral Infection: Thoracic Findings on CT. (2012) American Journal of Roentgenology. 196 (6): W723-8. doi:10.2214/AJR.10.5109 - Pubmed
- 10. McEwen RE, Scriven JE, Green CA et-al. Chest radiography findings in adults with pandemic H1N1 2009 influenza. (2010) The British journal of radiology. 83 (990): 499-504. doi:10.1259/bjr/24252450 - Pubmed