Microthrombotic Lesions of Pulmonary Arterial Hypertension

Infrequent microthrombi, which are usually organizing, may be found in patients with familial and idiopathic pulmonary hypertension. They must be distinguished from marked intimal thickening and plexiform lesions. Microthrombi have intact muscle walls and elastic laminas, and multiple lumens. A stain for elastic tissue is very useful for distinguishing them from plexiform lesions, in which the media is at least partly destroyed.

Figure 1. Microthrombus

The following two images show examples of microthrombi. They are from a patient with thromboembolic pulmonary hypertension. Elastic van Gieson stain

Figure 2. Low-power View of Same Case with Multiple Organizing Microthrombi

Each vessel is surrounded by an intact media and elastic layers, and the organizing thrombus usually has more than one new channel. Elastic van Gieson stain

Comment: Whereas many cases of PAH (aside from PVOD) show microthrombotic lesions, cases showing only such lesions and no plexiform lesions have been described and are classified as the microthrombotic type [1]. Microthrombi are probably related to abnormalities of clotting found in these patients [2].

References:

1. Burke A, Farb A, Virmani R. The pathology of primary pulmonary hypertension. Mod Pathol 1991:4:269-282.

2. Herve P, Humbert M, Sitbon O, Parent F, Nunes H, Legal C, Garcia G, Simonneau G. Pathobiology of pulmonary hypertension. The role of platelets and thrombosis. Clin Chest Med 2001:22:451-458.

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