Growth and rupture mechanism of partially thrombosed aneurysms

Abstract

Within the group of giant and large aneurysms the subgroup of the so-called "partially thrombosed" aneurysms can be differentiated according to clinical and neuroimaging findings. The present study was carried out to determine the site of bleeding of these aneurysms and what implications concerning their pathomechanism can be drawn from these findings. Twenty patients aged two to 77 (mean 44) years who exhibited a partially thrombosed aneurysm that had recently bled were included. Images (MRI including T1 pre- and postcontrast and T2 weighted images in multiple planes, CT and digital subtraction angiography) and patients' charts were reviewed. MRI showed an onion-skin appearance of the thrombus in 19 patients, rim enhancement of the aneurysm wall (either partial or complete) in 17, and a perifocal edema in 16 patients. The acute hemorrhage was typically crescent-shaped and located at the periphery of the aneurysm, distant from the perfused lumen of the aneurysm within the thrombosed part of the aneurysm. The current denomination "partially thrombosed" intracranial arterial aneurysms leads to the presumption that thrombus is present endoluminal whereas in fact the site of hemorrhage is within the vessel wall. A more accurate nomination would, therefore, be "aneurysms with intramural hemorrhage". The enhancing wall and the edematous reaction of the adjacent brain parenchyma might be a sign for an inflammatory pathomechanism which is reinforced by histological and pathophysiological studies. This disease should be regarded as a clinical entity separate from saccular or non-thrombosed giant or large aneurysms.

Figure 1

MRI (Proton Density in Frame A,T1 post contrast enhancement in Frame B) in a patient with a giant partially thrombosed aneurysm demonstrates onion-skin appearance of the thrombus and dense rim enhancement of the aneurysm wall.

Figure 2

This patient became symptomatic due to brain stem compression, MRI (T1 pre contrast enhancement in frames A and D, post-contrast in frames B and E and T2-weighted images in frame C) shows the typical findings of a partially thrombosed aneurysm of the V4 segment. Methemoglobin (T1 hyperintense on pre-contrast images) as a sign for an acute bleeding is present at the rim of the aneurysm far from the perfused part that is in this case located medially. T2-weighted images show perifocal edema and the onion-skin layer of mural thrombus of different ages.

Figure 3

This patient became symptomatic with an intraparenchymal bleeding that was observed posterior and superior to a mass lesion in the left basalganglia. The mass lesion extended to the basal cisterns and could be identified as a partially thrombosed carotid bifurcation aneurysm. T1-weighted images pre-contrast (A,C) and post contrast (B,D,E) demonstrate that the site of the bleeding is distant from the perfused part of the aneurysm that is located more anterior and inferior. There is partial rim enhancement, and T2-weighted images (F,G) demonstrate an extensive perifocal edema.

Figure 4

Non-enhanced CT (frame A) shows a hyperdense mass lesion in the MCA cistern with perifocal edema. On T1-weighted non-enhanced images (frame B, G and H) a crescent shaped hyperintensity on the lateral wall of the lesion suiting intramural hemorrhage can be seen. The source of the bleeding is not close but distant to the perfused aneurysm (that is demonstrated on T1 weighted images post contrast (frames I-L). There is clot/intramural hematoma inbetween the site of the bleeding and the aneurysm lumen. After contrast administration, a contrast enhancing ring surrounding the aneurysm can be perceived.

Figure 5

Schematic representation of aneurysms resulting from dissecting processes