Acute Interstitial Pneumonia - Diffuse Alveolar Damage

Differential Diagnosis

Usual Interstitial Pneumonia	Acute Interstitial Pneumonia
Temporal heterogeneity	Temporally uniform
Honeycomb change prominent	Honeycomb change only in late stage
Patchy distribution with subpleural accentuation	Diffuse, uniform throughout
Hyaline membranes not seen	Hyaline membranes prominent in early stage

Nonspecific Interstitial Pneumonia	Acute Interstitial Pneumonia
Predominantly interstitial	Prominent intra-alveolar component
No hyaline membranes	Hyaline membranes prominent in early stage
Intra-alveolar organization rare and focal	Prominent intra-alveolar organization in intermediate stage
Honeycomb change rare	Diffuse honeycomb change may be seen in late stage
Insidious onset, chronic disease	Acute onset

Cryptogenic Organizing Pneumonia	Acute Interstitial Pneumonia
Patchy distribution	Diffuse
No hyaline membranes	Hyaline membranes prominent in early stage and fragments in organizing phase
No collagen deposition, architecture preserved	Diffuse honeycomb change may be seen in late stage

Acute Interstitial Pneumonia / Diffuse Alveolar Damage	Acute Eosinophilic Pneumonia
Eosinophils infrequent	Prominent eosinophils in lavage and biopsy specimens
Not steroid responsive, poor prognosis	Steroid responsive, no relapses

Diffuse alveolar damage pattern may be seen in acute eosinophilic pneumonia

Specific causes of DAD pattern must always be ruled out
- These include infection, aspiration, shock, toxins, burns, transfusions and drugs
DAD pattern may be superimposed on a chronic process such as UIP in the setting of an acute exacerbation
- Defining feature
  - Hyaline membranes, intact or organizing
- Features that should suggest superimposed AIP/DAD
  - Squamous metaplasia of respiratory bronchioles
  - Fibrin thrombi in small arteries
  - Type II pneumocyte hyperplasia
- Clinical and radiographic history is important