Acute interstitial pneumonia

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

Epidemiology

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

Clinical presentation

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

Pathology

AIP is characterised histologically by diffuse alveolar damage (DAD) ². The alveolar damage comprises of three phases:

• acute exudative phase
• subsequent organising phase
• final fibrotic phase

Histological features are very similar with that of the adult respiratory distress syndrome (ARDS).

Radiographic features

The correct clinical context is vital for image interpretation.

Plain radiograph

Nonspecific and often shows bilateral patchy airspace opacification.

CT

During the initial stages, AIP can have similar features to adult respiratory distress syndrome (ARDS). Features include:

• areas with ground-glass attenuation: generally tends 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
• lung parenchymal architectural distortion
• 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 tends to carry 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 have other organ involvement 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