Acute interstitial pneumonitis

Acute interstitial pneumonia (AIP) is a rare and fulminant form of diffuse lung injury originally described by Hamman and Rich in 1935. AIP is classified as an idiopathic interstitial pneumonia (IIP), and among the IIPs, it has the most acute onset and rapidly progressive course. [1]

AIP is similar in presentation to the acute respiratory distress syndrome (ARDS) and probably represents a subset of cases of idiopathic ARDS. 

The chest imaging findings are like those seen in acute respiratory distress syndrome (ARDS). The chest radiograph reveals diffuse, bilateral, air-space opacification.  High-resolution computed tomographic (HRCT) scans typically show bilateral, patchy, symmetric areas of ground glass attenuation, often accompanied by airspace consolidation, septal thickening, and traction bronchiectasis.[1]

Other findings that may be seen include a predominantly subpleural distribution of disease and sparse honeycombing (less than 5 percent of the lung) 

If the initial minimally invasive tests do not yield an alternate diagnosis, a clinical decision must be made regarding whether to perform transbronchial, video-assisted (VATS), or open lung biopsy. Some experts favor obtaining a video-assisted lung biopsy early in the course in the hopes of identifying a treatable process. [1]

Some clinicians may choose to perform a transbronchial biopsy first to assess for sarcoidosis, lymphangitic carcinomatosis, acute eosinophilic pneumonia, and alveolar proteinosis before proceeding to VATS or open biopsy. For patients with early, milder disease this may allow a less invasive evaluation. On the other hand, when respiratory impairment is progressing rapidly, VATS or open lung biopsy will lead to a diagnosis more expeditiously.

Lung biopsy specimens are also examined for evidence of other specific causes of ARDS. As an example, areas of necrosis are typically not seen in AIP and suggest possible infection, infarct, or vasculitis. Well-formed granulomas, viral inclusions, and neutrophilic abscesses may suggest infection, while poorly formed granulomas suggest hypersensitivity pneumonitis.

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Alternative immunosuppressive therapies (eg, vincristine, cyclophosphamide, cyclosporine, and azathioprine) have been reported in case reports of AIP, although success was limited. In a patient who underwent single lung transplantation for AIP, the combination of cyclosporine, azathioprine, and prednisolone to suppress rejection led to partial improvement in the native lung. [1]

Multiple studies support rituximab's efficacy in severe, treatment-refractory ILD. In one retrospective series of 50 patients with severe, progressive ILD (excluding idiopathic pulmonary fibrosis), rituximab stabilized or improved lung function, with median FVC improvement of 6.7% following treatment. [2]

A phase 2 trial in therapy-refractory interstitial pneumonitis found that 71% of patients responded to rituximab, with higher pulmonary uptake of radiolabeled rituximab correlating with treatment response. [2]

Lung transplantation has been reported in patients with progressive interstitial lung disease thought to be due to AIP.