Acute interstitial pneumonia (AIP) is an idiopathic version of the acute respiratory distress syndrome (ARDS—see Acute Hypoxemic Respiratory Failure (AHRF, ARDS)) .
AIP, a form of idiopathic interstitial pneumonia (see Overview of Idiopathic Interstitial Pneumonias), equally affects apparently healthy men and women usually > 40 yr.
AIP is defined histologically by organizing diffuse alveolar damage, a nonspecific pattern that occurs in other causes of lung injury unrelated to idiopathic interstitial pneumonia. The hallmark of organizing diffuse alveolar damage is diffuse, marked alveolar septal edema with inflammatory cell infiltration, fibroblast proliferation, occasional hyaline membranes, and thickening of the alveolar walls. Septa are lined with atypical, hyperplastic type II pneumocytes, and airspaces are collapsed. Thrombi develop in small arteries but are nonspecific.
Symptoms consist of the abrupt onset of fever, cough, and shortness of breath, which in most patients increase in severity over 7 to 14 days, progressing to respiratory failure.
Diagnosis is suspected in patients with symptoms, signs, and chest x-ray findings of ARDS (eg, diffuse bilateral airspace opacification). Diagnosis is supported by high-resolution CT (HRCT) but usually requires biopsy. HRCT shows bilateral patchy symmetric areas of ground-glass attenuation and sometimes bilateral areas of airspace consolidation in a predominantly subpleural distribution. Mild honeycombing, usually affecting < 10% of the lung, may be present. Routine laboratory tests are nonspecific and generally not helpful.
Diagnosis is confirmed by surgical lung biopsy showing diffuse alveolar damage in the absence of known causes of ARDS and diffuse alveolar damage (eg, sepsis, drugs, toxins, radiation, viral infection). Acute exacerbation of underlying lung disease (in particular acute exacerbation of idiopathic pulmonary fibrosis) must be considered and may explain some cases previously characterized as AIP. Biopsy is often required to distinguish AIP from diffuse alveolar hemorrhage syndrome, acute eosinophilic pneumonia, and cryptogenic organizing pneumonia.
Treatment is supportive and usually requires mechanical ventilation, often using the same methods as used for ARDS (including low tidal volume ventilation—see Treatment). Corticosteroid therapy is generally used, but efficacy has not been established.
Mortality is > 60%; most patients die within 6 mo of presentation, and death is usually due to respiratory failure. Patients who survive the initial acute episode may recover complete pulmonary function, although the disease may recur.
Last full review/revision April 2013 by Harold R. Collard, MD
Content last modified November 2013