Acute interstitial pneumonia (AIP) is a rare and fulminant idiopathic pulmonary disorder that manifests similarly to acute respiratory distress syndrome (ARDS). Clinical features include fever, cough, and dyspnea. Diagnosis is by imaging and sometimes, lung biopsy. Treatment is with glucocorticoids and mechanical ventilation. Prognosis is poor; mortality is > 50%.
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 and Signs of Acute Interstitial Pneumonia
Symptoms of acute interstitial pneumonia consist of the abrupt onset of fever, cough, and shortness of breath. In most patients, hypoxia increases in severity over 7 to 14 days, progressing to respiratory failure that usually requires mechanical ventilation. Some patients may experience prodromal symptoms of myalgias, fatigue, and chills. Signs include tachypnea and diffuse crackles are often heard on auscultation over bilateral lung fields.
Diagnosis of Acute Interstitial Pneumonia
High resolution CT (HRCT)
Laboratory tests
Sometimes lung biopsy
Diagnosis of acute interstitial pneumonia is suspected in patients with symptoms, signs, and chest radiograph 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 or an underlying myositis-associated interstitial lung disease) 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 generally nonspecific. A marked leukocytosis may be present on a complete blood cell count. Arterial blood gas often show moderate to severe hypoxemia with a ratio of partial pressure of arterial oxygen to fractional inspired oxygen (PaO2/FaO2) < 200, which is in the range of acute respiratory distress syndrome. Bronchoscopy is typically preformed to exclude other causes of respiratory failure. Bronchoalveolar lavage fluid is nonspecific and typically shows marked neutrophilia.
Surgical lung biopsy showing diffuse alveolar damage in the absence of known causes of ARDS and diffuse alveolar damage supports a diagnosis of AIP. Acute exacerbation of underlying lung disease (in particular acute exacerbation of idiopathic pulmonary fibrosis or an underlying myositis-associated interstitial lung disease) must be considered and may explain some cases previously characterized as AIP. When bronchoscopy is non-diagnostic, surgical lung biopsy may be needed 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
Supportive care
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).
Glucocorticoid therapy is generally used, but efficacy has not been established.
Mortality is > 50%; most patients die within 6 months of presentation, and death is usually due to respiratory failure (1). Patients who survive the initial acute episode may recover complete pulmonary function, although the disease may recur.
Treatment reference
1. Travis WD, Costabel U, Hansell DM, et al. An official American Thoracic Society/European Respiratory Society statement: Update of the international multidisciplinary classification of the idiopathic interstitial pneumonias. Am J Respir Crit Care Med 2013;188(6):733-748. doi:10.1164/rccm.201308-1483ST
