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.
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Clinical presentation
There can be a wide spectrum of clinical syndromes ranging from being asymptomatic to fulminant viral pneumonia, respiratory failure, and death.
Pathology
It is caused by a type of influenza A virus of swine origin.
Radiographic features
Plain radiograph
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
CT
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%
- pneumothorax/pneumomediastinum
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