Acute interstitial pneumonitis

Last revised by Yacoob Omar Carrim on 05 Feb 2023

Acute interstitial pneumonitis (AIP), also known as Hamman-Rich syndrome, is a rapidly progressive non-infectious interstitial lung disease of unknown etiology. It is considered the only acute process among the idiopathic interstitial pneumonias.

AIP has a similar clinical presentation and histological features to those seen in acute respiratory distress syndrome (ARDS), showing extensive diffuse alveolar damage (DAD). Both conditions likely represent the same pathology, with AIP probably accounting for some of the idiopathic cases of ARDS. 

Truly idiopathic AIP tends to occur in those without pre-existing lung disease and typically affects middle-aged adults (mean ~50 years 5). However, in certain conditions such as leflunomide-induced acute interstitial pneumonia, patients have pre-existing lung disease.

Clinical features are varied. Patients often have a history of an antecedent illness such as a viral upper respiratory infection. Common initial symptoms include myalgia, arthralgia, pyrexia, chills, and malaise. Severe exertional dyspnea develops over a matter of days to weeks 13.

AIP is characterized histologically by diffuse alveolar damage (DAD) 2 and is indistinguishable from acute respiratory distress syndrome (ARDS). The alveolar damage comprises three phases:

  • an acute exudative phase

  • a subsequent organizing phase

  • a final fibrotic phase

The clinical context is vital for correct image interpretation.

Nonspecific and often shows bilateral patchy airspace opacification.

In Acute Interstitial Pneumonia (AIP) there may be diffuse, patchy ground-glass opacities or hazy areas of increased lung attenuation, often accompanied by interlobular septal thickening.

During the initial stages, AIP can have features similar to acute respiratory distress syndrome (ARDS), which include:

The condition usually progresses to respiratory failure that requires mechanical ventilation and corticosteroid therapy. Even despite mechanical ventilation, it often carries a grave prognosis with >70% mortality at ~3 months 1.

The clinical features of AIP were first described by L Hamman and A Rich in 1935 8, and the pathological processes were first described by A L Katzenstein et al. in 1986 3.

Considerations in the early stages include:

Other considerations include:

For a more general differential, consider:

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Cases and figures

  • Case 1
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