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 . 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 : 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 :
- 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
- pleural effusion: ~30%
- atelectatic changes: ~25%