Acute interstitial pneumonitis

Acute interstitial pneumonitis (AIP) is a rapidly progressive non infectious interstitial lung disease of unknown aetiology. It is considered the only acute process of the idiopathic intersitial pneumonias.

Epidemiology

It tends to occur in those without pre-existing lung disease. Typically affects middle age adults (mean ~  50 years ⁵)

Clinical presentation

Clinical features are often wide. 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 dyspnea develops over a matter of days. 

Pathology

Characterised histologically by diffuse alveolar damage (DAD) ².  The alveolar damage comprises of three phases, an acute exudative phase, a subsequent organising phase and a 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 film

Non specific and often shows bilateral patchy airspace opacification

HRCT

During intialy stages 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 parechymal 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,

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 diagnoses

Considerations in early stages include

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

For the more general differential, consider

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