Thrombi & Emboli

Background

A vascular thrombus is a blood clot that forms in a blood vessel or the heart and remains attached to its place of origin. If it remains in place (and the patient is still alive) it will gradually undergo a process of infiltration and organization. 

An embolus is a detached intravascular mass (solid, liquid, or gas) that is carried by the blood to a site distant from its point of origin. The most common embolus is a thromboembolism from a deep vein thrombosis (most commonly resulting in a pulmonary thromboembolism – commonly shorthanded as a “pulmonary embolism”). 

But the type and source of emboli are extremely varied, being composed of many substances and arising from many possible locations. These include clotted blood, fat, bone marrow, tumor, air, amniotic fluid, etc. This article lists some of the more common and/or important thrombotic/embolic phenomena at autopsy. 

Importantly, the characteristic features of true antemortem thrombi are covered below in this article, as well as in the combined article on Deep Vein Thrombosis and Pulmonary Embolism.

Jump to:

• Deep Vein Thrombosis/Pulmonary thromboembolism 
• Atheromatous emboli (see also: Atherosclerosis)
• Fat emboli
• Gas emboli
• Amniotic fluid emboli
• Tumor emboli
• Other foreign substances (e.g. injection drug excipient lung disease)

A few general notes:

• Systemic emboli usually arise from cardiovascular sources (80%) including deep veins, aortic aneurysms and ulcerated aortic atherosclerotic plaques. 
• Emboli traveling to the brain should be suspected in cases with multifocal, hemorrhagic lesions predominantly at the gray-white junction (except in fat emboli syndrome which will go to the white matter). 
• Although rarely needed, fibrin thrombi (seen in many conditions) are highlighted by PAS. Alternatively, trichrome can assist with aging fibrin thrombi (fresh fibrin = orange/yellow, mature is red, and old blue).

Image: PAS-positive fibrin thrombus of the central retinal artery in the optic nerve. (Image credit: Meagan Chambers/University of Washington).

Characteristic features of true antemortem thrombi/thromboemboli which are not typically seen in postmortem thrombi include:

• Polymorphonuclear (neutrophil) nuclear debris
• Lines of Zhan (layers of platelets – which can be seen on H&E or highlighted on CD61)
• Adherence to the wall (which can be appreciated on gross examination as well as histologically)
• Re-epithelialization of the clot by vessel endothelium
• Infiltration of fibroblasts. (This last one takes time and so the clot must be in place for some time before this can be seen)

The following portal vein thrombus in a liver demonstrates most of these above features:

Image: Low power view of a portal vein thrombus.

Image: Close up showing layering of red blood cells, fibrin, and platelets (lines of Zahn).

Image: CD61 staining demonstrating thin layers of platelets (lines of Zhan).

Image: close up of neutrophil nuclear debris (arrows). Of note, Krywanczyk 2023 (below) describes abundant neutrophil debris seen at low power as more suggestive for a true thrombus compared to sparse neutrophil debris as seen here. 

Image: Close up of adherent edge of thrombus with vessel wall (arrows aligned along the interface).

Image: Infiltrating fibroblasts can be seen in the thrombus in subacute thrombi/thromboemboli.

(Image credits: Meagan Chambers/Stanford University)

Recommended References

• Krywanczyk AR, Tan CD, Rodriguez ER. Histologic and Immunohistochemical Features of Antemortem Thrombus Compared to Postmortem Clot: Updating the Definition of Lines of Zahn. Arch Pathol Lab Med. 2023 Nov 1;147(11):1241-1250. doi: 10.5858/arpa.2022-0147-OA. PMID: 36626295.