Idiopathic interstitial pneumonias (IIPs) are diffuse interstitial lung diseases of unknown cause. A useful mnemonic for the American Thoracic Society-European Respiratory Society (ATS-ERS) classification of IIPs is:
- All Idiopathic Chronic Lung Disease aRe Nonspecifically Patterned
The mnemonic reflects the frustration of trying to differentiate them on plain film. Indeed, diagnosis depends on HRCT findings, as well as clinical presentation. Even then, some cases require lung biopsy for definitive diagnosis.
- A: acute interstitial pneumonia (AIP). The only acute process in the list.
- I: idiopathic pulmonary fibrosis / usual interstitial pneumonia (IPF / UIP)
- C: cryptogenic organizing pneumonia (COP)
- L: lymphoid interstitial pneumonia (LIP)
- D: desquamative interstitial pneumonia (DIP)
- R: respiratory bronchiolitis–associated interstitial lung disease (RB-ILD)
- N: non-specific interstitial pneumonia (NSIP)
- P: pleuroparenchymal fibroelastosis (PPFE)
The first two, AIP and IPF/UIP, have a very poor prognosis. The next in the list, COP, is associated with a good response to steroids and good prognosis. LIP is more common in women (think "LIPstick"), in contrast to the others, which are either more common in men or have an equal gender distribution.
The following two, DIP and RB-ILD, are at the extreme opposite ends of a spectrum, associated with smoking, with good response to smoking cessation.
PPFE has a relatively poor prognosis, with frequent infectious complications and associated pneumothorax.
One caveat: the word chronic in the mnemonic should be taken with a pinch of salt. Some of the entities may have a subacute presentation, and one, AIP, presents acutely.
- 1. American Thoracic Society/European Respiratory Society International Multidisciplinary Consensus Classification of the Idiopathic Interstitial Pneumonias. This joint statement of the American Thoracic Society (ATS), and the European Respiratory Society (ERS) was adopted by the ATS board of directors, June 2001 and by the ERS Executive Committee, June 2001. Am. J. Respir. Crit. Care Med. 2002;165 (2): 277-304. Am. J. Respir. Crit. Care Med. (link) - Pubmed citation
- 2. Mueller-mang C, Grosse C, Schmid K et-al. What every radiologist should know about idiopathic interstitial pneumonias. Radiographics. 27 (3): 595-615. doi:10.1148/rg.273065130 - Pubmed citation
- 3, Travis WD, Costabel U, Hansell DM et-al. An official American Thoracic Society/European Respiratory Society statement: Update of the international multidisciplinary classification of the idiopathic interstitial pneumonias. Am. J. Respir. Crit. Care Med. 2013;188 (6): 733-48. doi:10.1164/rccm.201308-1483ST - Pubmed citation