Acute Interstitial Pneumonia AIP

The term idiopathic interstitial pneumonias refers to a group of seven entities with distinct histologic patterns idiopathic pulmonary fibrosis (IFF), characterized by the pattern of usual interstitial pneumonia (UIP) nonspecific interstitial pneumonia (NSIP) cryptogenic organizing pneumonia (COP) respiratory bron-chiohtis-associated interstitial lung disease (RB-ILD) desquamative interstitial pneumonia (DIP) lymphoid interstitial pneumonia (LIP) and acute interstitial pneumonia (AIP). 

Fig. 26.12a,b. Acute interstitial pneumonia (AIP) in a 58-year-old patient, a Axial CT image shows bilateral ground-glass opacities in a geographic distribution (arrow). Consolidation is seen in the more dependent lung (arrowheads). Small, coexisting bilateral pleural effusions are present, b (see next page)... 

Acute exacerbations of IPF are characterized by rapid development of cough, dyspnea, hypoxemia, and worsening pulmonary infiltrates in patients with known IPF (29,148-151). Presentation is similar to acute respiratory distress syndrome (ARDS) (29,148,149,151,152). The cardinal histological feature is DAD superimposed on a background of UIP (149,151). Idiopathic acute interstitial pneumonia (AIP) (28,152) exhibits similar clinical and histological features as acute exacerbations of IPF, but lacks the requisite features of UIP. High-dose intravenous (IV) pulse methylprednisolone has been used to treat acute exacerbations of IPF, but data on treatment are limited to anecdotal cases and small series (29,148,149,151). This entity is reviewed in chapter 15 and will not be further discussed here. 

Abbreviations-. HRCT, high-resolution computed tomography AIP, acute interstitial pneumonia. 

Figure 7 HRCT images demonstrating diffuse patchy involvement of ground glass opacities, consolidation, and reticulation in a patient with AIP. Abbreviations HRCT, high-resolution computed tomography AIP, acute interstitial pneumonia.

The histologic features of acute interstitial pneumonitis (AIP) are the various phases of DAD. Initially, an increase of capillary leukocytes including neutrophils, macrophages, and lymphocytes are seen. Within a day, interstitial edema becomes more evident and type I pneumocytes undergo type II metaplasia and become swollen. As the alveolar wall becomes necrotic, fibrin and cellular residue forms a layer of eosinophiUc material that lines the alveolar wall, the so-called hyaline membrane. Hyaline membranes are the defining feature of DAD. As the disease process progresses, polyps of organizing pneumonia develop and tend to replace the hyaline membranes (Fig. 4) (3,27,28). 

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