An interstitial lung pattern is a regular descriptive term used when reporting a plain chest radiograph. It is the result of the age-old attempt to make the distinction between an interstitial and airspace (alveolar) process to narrow the differential diagnosis.
A re-read of the timeless work of Benjamin Felson in 1979, "A new look at pattern recognition of diffuse pulmonary disease" 1, explains the difficulty of describing a disease process as purely interstitial. The problem is that despite processes starting in the pulmonary interstitium, by the time they appear on a radiograph, there is almost certainly a degree of airspace involvement.
He cites several examples including:
- lymphangitic carcinomatosis: while the process creates a linear pattern, by the time it is recognisable on a radiograph, "at autopsy the spreading neoplasm usually appears to have cut a tornado-like path through all compartments of the pulmonary tissue"
- viral pneumoniae: while these are widely considered to be interstitial, chickenpox pneumonia is a predominantly alveolar process with airspace opacification on the chest radiograph
- pneumocystis pneumonia starts as an interstitial process, but by the time the radiograph is taken, histological examination demonstrates extensive alveolar involvement
The solution from the paper is the use of terms that describe the appearance on the chest radiograph rather than attempting to outline the pathological location of the abnormality.
The list of gamuts he describes are:
- bronchial disease, e.g. mucoid impaction, bronchiectasis
- discrete miliary nodules
- Kerley lines
- small irregular shadows - reticular, reticulonodular
- vascular abnormality
The importance here is to reduce the confusion that arises when trying to be dogmatic about separating pathology into interstitial or airspace pathology. Take the example of a cavity - it could arise from airspace or interstitial disease:
- cavitating pneumonia occurs as a result of airspace pathology but it is unreasonable to say that the interstitium is not involved
- cavitation of a tuberculous nodule - the nodule is initially interstitial, but with growth involves the airspaces, and as it enlarges, cavitation occurs