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.
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. The alveolar damage comprises three phases:
- an acute exudative phase
- a subsequent organizing phase
- a final fibrotic phase
Histological features are very similar to those of adult respiratory distress syndrome (ARDS).
The clinical context is vital for correct image interpretation.
Nonspecific and often shows bilateral patchy airspace opacification.
During the initial stages, AIP can have features similar to adult respiratory distress syndrome (ARDS), which include:
- areas with ground-glass attenuation: generally tend to be bilateral and symmetrical 10
- traction bronchiectasis: can be seen in ~80% of cases during the course of the disease 4 and correlates with disease duration 2
- parenchymal architectural distortion of the lung
- air space consolidation: may have a slight predilection towards the dependent portions 5
Treatment and prognosis
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.
History and etymology
- clinical features first described by L Hamman and A Rich in 1935 8
- pathological processes first described by A L Katzenstein et al. in 1986 3
Considerations in early stages include:
- adult respiratory distress syndrome (ARDS): can involve other organs 9
- infectious multifocal pneumonia
Other considerations include:
- an acute interstitial pneumonitis process triggered by certain medications, e.g. leflunomide-induced acute interstitial pneumonia
For a more general differential, consider:
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- 8 Hamman, L, Rich, A Fulminating diffuse interstitial fibrosis of the lungs. Trans Am Clin Clinatol Assoc 1935;51,154-163
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