Acute interstitial pneumonitis

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.

Terminology 

AIP has a similar clinical presentation and histological features of those seen in the adult 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. 

Epidemiology

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 presentation

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.

Pathology

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

• an acute exudative phase
• a subsequent organizing phase
• a final fibrotic phase

Radiographic features

The clinical context is vital for correct image interpretation.

Plain radiograph

Nonspecific and often shows bilateral patchy airspace opacification.

CT

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

Differential diagnosis

Considerations in early stages include:

• adult respiratory distress syndrome (ARDS): can involve other organs ⁹
• infectious multifocal pneumonia
  • atypical: e.g. mycoplasma pneumonia
  • bacterial pneumonia
  • severe acute respiratory syndrome (SARS)

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:

• differential diagnosis of diffuse air space consolidation
• differential diagnosis of ground-glass opacification