Acute interstitial pneumonia (AIP), a form of idiopathic interstitial pneumonia, equally affects apparently healthy men and women, usually those > 40 years.
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 of acute interstitial pneumonia 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 of acute interstitial pneumonia is suspected in patients with symptoms, signs, and chest x-ray findings of acute respiratory distress syndrome (ARDS—eg, diffuse bilateral airspace opacification). 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.
High-resolution CT (HRCT) supports the diagnosis of AIP, but definitive diagnosis 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.
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) confirms a diagnosis of AIP. 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, acute eosinophilic pneumonia, and cryptogenic organizing pneumonia. However, the risks and benefits of surgical lung biopsy must be carefully considered.
Treatment of acute interstitial pneumonia is supportive and usually requires mechanical ventilation, often using the same methods as used for ARDS (including low tidal volume ventilation). Corticosteroid therapy is generally used, but efficacy has not been established.
Mortality is > 60%; most patients die within 6 months 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.