Classifying interstitial lung diseases in a fractal lung: a morphologist's view "anno Domini 2000"

Interstitial lung diseases (ILDs) remain a challenging problem for the pathologist. New insights in aetiology and pathogenesis, new diagnostic tools and successful research have led to a renewed interest in ILDs during the last few years, and highlighted the need for a novel classification, particularly of the chronic and/or idiopathic categories of interstitial pneumonias. The present paper compares the terminology of the latter categories in current and previous classifications and briefly discusses the pathological basis for the classifications of ILDs in general, and for the idiopathic interstitial pneumonias (IIPs) in particular. The difference between high versus low morphological specificity determines the pathological classifications. The classification of lIPs relies upon a pattern recognition taking temporal and spatial distribution into consideration. The last section of this paper discusses recent research opposing the conventional pathological approach, analogous to the mechanical two-compartment model of the lung, in which a discontinuity is considered between these two compartments, and thus, a distinction is made between interstitial lung diseases with and without bronchiolitis. In the recent "fractal" concept, the continuity of the lung architecture is emphasized: the lung is a so-called fractal tree with noninteger dimensions. In this fractal model, an interstitial lung disease effects a peripheral part of the pulmonary fractal tree and this may or may not include bronchioles.