Pulmonary vascular disease and pulmonary hypertension

Pulmonary hypertension (PH) is a devastating condition that ultimately leads to right heart failure and death, if untreated. The morphological correlate for clinically relevant PH is pulmonary vascular disease that may concern all vascular compartments of the lung: pulmonary arteries, capillaries and veins, but also systemic lung vessels, commonly known as bronchial arteries and vasa vasorum. The recent diagnostic PH classification at the PH World Symposium in Nice, France, redefined five PH groups based on pathophysiological, histological and clinical differences. This article reviews the rationale of histology/pathology and, partly, pathobiology that has led to these different categories of PH. Even if aetiology and initial pathophysiology discriminate these groups to some extent, it has become clear that several disease-relevant pathomechanisms are shared between the groups, leading to comparable therapeutic molecular targets, such as phosphodiesterase-5 inhibitors for PAH (group 1 in the Nice classification) and soluble guanylate cyclase agonists in CTEPH (group 4 of the Nice classification). When considering PH group 2 (PH due to left heart disease), its histological appearance, with remodelled small pulmonary veins and arteries, and its haemodynamics in some patients suggest an overreacting pre-capillary vasculature in response to left heart failure. Thus the still-used descriptor ‘orphan disease' might one day no longer be used in relation to PH. Indeed, PH is a condition that represents a global health issue, and hence is worth a closer look and a thorough definition from the perspective of the pathologist.