From the microscopic to the macroscopic: clinical–radiological–pathological correlation in pulmonary hypertension

Pulmonary hypertension (PH) is defined as the presence of a mean pulmonary arterial pressure >20 mmHg [1]. This simple haemodynamic definition encompasses a heterogenous collection of conditions. It is now appreciated that although treatable forms of PH are relatively rare, PH itself is not an uncommon entity, affecting ∼1% of the global population [1]. Current international guidelines describe five classification groups: group 1 (pulmonary arterial hypertension (PAH)), group 2 (PH associated with left heart disease), group 3 (PH associated with lung disease), group 4 (PH associated with pulmonary arterial obstructions) and group 5 (PH with unclear and/or multifactorial causes) figure 1 [1]. These groups are characterised by shared clinical, haemodynamic and radiological features which are underpinned by common histopathological changes. Pre-capillary PH (defined by a pulmonary arterial wedge pressure ≤15 mmHg and a pulmonary vascular resistance >2 Wood units (WU)) may result from remodelling, obstruction or loss of the pulmonary arterial bed while post-capillary PH (defined by a pulmonary arterial wedge pressure >15 mmHg) results from passive backwards transmission of elevated left heart filling pressures into the pulmonary circulation (isolated post-capillary PH) [2]. Some patients with post-capillary disease may develop subsequent pulmonary vascular remodelling with a subsequent increase in pulmonary vascular resistance to >2 WU (combined pre- and post-capillary PH). Three main classes of PAH therapies have been shown to be of benefit in group 1 disease (targeting the nitric oxide, endothelin-1 and prostacyclin pathways) while chronic thromboembolic PH can be effectively treated with pulmonary endarterectomy surgery together with important roles for balloon pulmonary angioplasty and pulmonary vasodilators [3, 4]. The optimal treatment in the majority of patients in groups 2, 3 and 5 is less well defined. Recent advances in our understanding of the histopathological changes in different forms of PH have the potential to further inform classification and patient management [5].

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FIGURE 1

Clinical classification of pulmonary hypertension (PH). PAH: pulmonary arterial hypertension; PVOD: pulmonary veno-occlusive disease; PCH: pulmonary capillary haemangiomatosis. Reproduced and modified from Kiely et al. [11] with permission, with updated classification reproduced from Humbert et al. [1] with permission.

It is, therefore, vital that clinicians who diagnose and manage patients with PH have a good understanding of the histopathological basis and radiological features associated with these different forms of PH. To this end, a series of reviews focusing on four of these diagnostic groups plus a bonus review specifically covering PAH with features of venous/capillary involvement (pulmonary veno-occlusive disease and pulmonary capillary hemangiomatosis) begins in this issue of the European Respiratory Review [6–10]. In these review articles, key histopathological and radiological patterns are explored using illustrative clinical case studies with particular attention being paid to the development of a systematic approach to radiological interpretation. It is hoped that the series will be an invaluable resource for all clinicians involved in the care of patients with PH.