Pulmonary Hypertension Under the Microscope: What Histopathology Means for Diagnosis and Treatment

Pulmonary hypertension (PH) comprises a heterogeneous group of conditions characterized by elevated mean pulmonary artery pressure and is classified into 5 groups according to underlying etiology. Across PH subtypes, pathologic remodeling of the pulmonary microvasculature, including arterioles, capillaries, and venules, drives hemodynamic burden, right ventricular dysfunction, and clinical outcomes. Although certain structural features are conserved, distinct histopathologic patterns shape group-specific pathophysiology, treatment responses, and prognosis.

Experimental models (eg, monocrotaline, hypoxia) have revealed key mechanisms of medial hypertrophy, distal neomuscularization, intimal fibrosis, and recanalization, but also highlight differences in cellular pathways and vascular compartments involved. Human histologic studies confirm these changes and demonstrate group-specific signatures: venule-predominant remodeling in group 2 PH, dense occlusive venous fibrosis in pulmonary veno-occlusive disease, arterial-dominant lesions in group 1 pulmonary arterial hypertension, and mixed arterial-venous changes in chronic thromboembolic pulmonary hypertension. These structural differences help explain variable therapeutic responses; for example, vasodilators are effective in precapillary arteriolar predominant disease but may precipitate pulmonary edema in venous-predominant PH phenotypes. In chronic thromboembolic pulmonary hypertension, distal arteriolar pathology is a therapeutic target; however, venous remodeling may underlie heterogeneity of outcomes. Emerging noninvasive markers, including echocardiographic indices such as tricuspid annular plane systolic excursion/right ventricular systolic pressure and exercise hemodynamics, may provide physiological surrogates of microvascular disease; however, direct correlations with histopathology remain limited.

Pulmonary vascular remodeling varies substantially across PH groups, with distinct arterial and venous signatures that influence hemodynamics, treatment response, and outcomes. Integrating histopathologic insights with hemodynamic phenotyping and noninvasive assessments may enable earlier detection of pulmonary vascular disease and support more precise, mechanism-directed therapeutic strategies.