Interstitial and viral pneumonia. Etiology, pathogenesis, microscopic & gross pathomorphology
Interstitial pneumonia
Interstitial pneumonia (aka Nonspecific interstitial pneumonia) is a rare disorder affecting the alveoli, it is a type of interstitial lung disease (other types include idiopathic pulmonary fibrosis and nonspecific interstitial pneumonitis (this is autoimmune)).
Interstitial pneumonia is a chronic idiopathic bilateral lung diseases. It can be one of two forms, a cellular pattern (with mild chronic interstitial inflammation in uniform pattern) or a fibrosing pattern (with patchy interstitial fibrosis).
H&E, interstitial pneumonia (NSIP, cellular pattern): the alveolar walls are uniformly thickened by lymphocytes and plasma cells; the alveolar spaces stay mostly free of exudate.
Image: “Non-specfic interstitial pneumonia (NSIP), cellular variant” by Yale Rosen, CC BY-SA 2.0, via Wikimedia Commons.
Patients will present with cough of several months, possibly with dyspnea, fatigue and clubbing.
Viral pneumonia
Viruses such as influenza A and B, respiratory syncytial virus and human parainfluenza virus (in children) can all cause pneumonia (rare viral agents include Adenovirus and SARS).
Viral pneumonia is generally community acquired pneumonia, but morphologically it is an interstitial pneumonia, unlike the alveolar exudate of bacterial pneumonia.
Remember: in viral and other atypical pneumonias (e.g. Mycoplasma) the alveolar septa are widened by oedema and lymphocytes, macrophages and plasma cells, while the alveoli stay largely free of exudate; severe cases form hyaline membranes.