) .Acute interstitial pneumonia (AIP), a form of idiopathic interstitial pneumonia Overview of Idiopathic Interstitial Pneumonias Idiopathic interstitial pneumonias (IIPs) are interstitial lung diseases of unknown etiology that share similar clinical and radiologic features and are distinguished primarily by the histopathologic... read more , 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 Acute Hypoxemic Respiratory Failure (AHRF, ARDS) Acute hypoxemic respiratory failure is defined as severe hypoxemia (PaO2 (See also Overview of Mechanical Ventilation.) Airspace filling in acute hypoxemic respiratory failure (AHRF) may result... read more .
Diagnosis of Acute Interstitial Pneumonia
High resolution CT (HRCT)
Sometimes lung biopsy
Diagnosis of acute interstitial pneumonia is suspected in patients with symptoms, signs, and chest x-ray findings of acute respiratory distress syndrome Acute Hypoxemic Respiratory Failure (AHRF, ARDS) Acute hypoxemic respiratory failure is defined as severe hypoxemia (PaO2 (See also Overview of Mechanical Ventilation.) Airspace filling in acute hypoxemic respiratory failure (AHRF) may result... read more (ARDS—eg, diffuse bilateral airspace opacification). Acute exacerbation of underlying lung disease (in particular acute exacerbation of idiopathic pulmonary fibrosis Idiopathic Pulmonary Fibrosis Idiopathic pulmonary fibrosis (IPF), the most common form of idiopathic interstitial pneumonia, causes progressive pulmonary fibrosis. Symptoms and signs develop over months to years and include... read more 
) 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 supports a diagnosis of AIP in the right clinical setting. Acute exacerbation of underlying lung disease (in particular acute exacerbation of idiopathic pulmonary fibrosis Idiopathic Pulmonary Fibrosis Idiopathic pulmonary fibrosis (IPF), the most common form of idiopathic interstitial pneumonia, causes progressive pulmonary fibrosis. Symptoms and signs develop over months to years and include... read more ) must be considered and may explain some cases previously characterized as AIP. Biopsy is often required to distinguish AIP from diffuse alveolar hemorrhage Diffuse Alveolar Hemorrhage Diffuse alveolar hemorrhage is persistent or recurrent pulmonary hemorrhage that originates from the lung parenchyma (ie, the alveoli) as opposed to the airways. There are numerous causes, but... read more 
, acute eosinophilic pneumonia Acute Eosinophilic Pneumonia Acute eosinophilic pneumonia (AEP) is a disorder of unknown etiology characterized by rapid eosinophilic infiltration of the lung interstitium. (See also Overview of Eosinophilic Pulmonary Diseases... read more , and cryptogenic organizing pneumonia Cryptogenic Organizing Pneumonia Cryptogenic organizing pneumonia is an idiopathic condition in which granulation tissue obstructs alveolar ducts and alveolar spaces with chronic inflammation occurring in adjacent alveoli.... read more 
. 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 Treatment Acute hypoxemic respiratory failure is defined as severe hypoxemia (PaO2 (See also Overview of Mechanical Ventilation.) Airspace filling in acute hypoxemic respiratory failure (AHRF) may result... read more ). Corticosteroid 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 Treatment reference Acute interstitial pneumonia (AIP) is a rare and fulminant idiopathic pulmonary disorder that manifests similarly to acute respiratory distress syndrome ( ARDS) . Acute interstitial pneumonia... read more ). 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
