H1N1 influenza

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.

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

See also

• viral pneumonia