Key-words:
Arterial dissection - internal elastic lamina - nonbranching aneurysm
Introduction
The majority of cerebral saccular aneurysms arise at arterial bifurcations, and cerebral
aneurysms occurring at nonbranching sites are relatively rare. Previous studies have
demonstrated that nonbranching site aneurysms are typically associated with arterial
dissection and take the form of so-called blood blister-like aneurysms.[[1]] However, there are some reports of true saccular aneurysms at nonbranching sites
of the internal carotid artery (ICA),[[2]] it is not clear how their formation differs from bifurcation aneurysms. We recently
encountered two cases of a saccular aneurysm arising at nonbranching sites and examined
the pathological findings. We discuss the pathogenesis and formation mechanism of
nonbranching cerebral aneurysms.
Case Reports
Case 1
A 79-year-old male with a history of diabetes mellitus and hypertension presented
with sudden headache and leg weakness without any head trauma. He showed confusion
and had mild paresis in his left leg. Computed tomography (CT) of the head showed
an acute subdural hematoma in the interhemispheric fissure and subarachnoid hemorrhage
[[Figure 1]]a. CT angiography showed a 4-mm saccular aneurysm on a branch of the right pericallosal
artery [[Figure 1]]b. After confirming that the aneurysm was not infectious, we performed trapping
of the aneurysm on day 5. The intraoperative findings showed that the aneurysmal neck
was torn at its origin [[Figure 1]]c. H and E staining of the resected aneurysm showed the lesion was composed of mostly
thrombi, and no obvious vascular structures, such as the internal elastic lamina (IEL)
or adventitia, were observed [[Figure 1]]d. The pathogenesis of aneurysm formation was considered to be mild arterial wall
injury caused by the edge of the falx cerebri.
Figure 1: (a) Head computed tomography showing acute subdural hematoma in the interhemispheric
fissure. (b) Aneurysm located on a branch of the right pericallosal artery demonstrated
by three-dimensional reconstructed computed tomography angiography. (c) Intraoperative
image just after removal of the aneurysm (arrow). The aneurysmal neck was torn at
its origin (arrowhead). (d) H and E staining of the resected aneurysm (x10). No obvious
vascular structures such as internal elastic lamina or adventitia are visible
Case 2
The patient was a 67-year-old male with a history of diabetes mellitus and hypertension.
A 15-mm unruptured aneurysm was incidentally found in the left middle cerebral artery
(MCA) at a nonbranching site of the M2 inferior trunk, where the MCA sharply curved
[[Figure 2]]a and [[Figure 2]]b. Neck clipping through the pterional approach was performed, and the aneurysmal
dome was resected. The intraoperative findings showed a firm aneurysmal neck and severe
atherosclerosis on the dome [[Figure 2]]c. Pathological findings showed that the intima was entirely hyalinized and thickened
[[Figure 3]]a. Although Elastica van Gieson stain typically shows the IEL as a black-purple
membrane, no such staining was observed in the aneurysmal wall [[Figure 3]]b. Pathologically, this aneurysm looked like an ordinary bifurcation aneurysm on
a branching site.
Figure 2: (a) Right internal carotid artery angiography shows a 15 mm saccular aneurysm on
the left middle cerebral artery-M2 inferior trunk (arrowhead). (b) Three-dimensional
reconstructed image showing the aneurysm originating from the convex portion of the
sharply curved vessel (arrow). (c) Intraoperative image showing the aneurysm (arrow)
with severe atherosclerosis on the aneurysmal dome (arrowhead)
Figure 3: (a) H and E staining of the resected aneurysm (x10) shows a considerably thickened
and hyalinized intima. (b) Elastica van Gieson staining section shows lack of internal
elastic lamina on the aneurysmal wall
Discussion
We encountered two different types of nonbranching aneurysms. Pathologically, one
was a pseudoaneurysm, and the other was a firm saccular aneurysm similar to those
seen on branching sites. Anterior wall aneurysms comprise 0.9%–6.5% aneurysms in the
ICA,[[3]] of which a previous study suggested there are two types, differing in shape, histological
features, and treatment requirements.[[2]] One is a blood blister-like type, showing a small hemispherical bulge located at
the anterior wall of the supraclinoid portion of ICA. Most patients with this type
of aneurysm suffer massive subarachnoid hemorrhaging because the aneurysmal walls
are thin and fragile.[[3]] Pathologically, abrupt termination of the IEL is observed at the area adjacent
to the rupture point, which is not composed of collagenous tissue, as seen in an ordinary
aneurysm. The other type is the saccular type, which has a saccular dome with an obviously
firm neck similar to ordinary aneurysms arising at arterial bifurcations. Unlike with
the blood blister-like type, neck clipping is possible for this type of aneurysm as
in ordinary bifurcation aneurysms. We believe that this classification system can
be applied to cerebral arteries other than the ICA, such as in our two cases.
Mizutani et al. reported that saccular-shaped arterial dissections could develop by
tearing of the IEL at nonbranching sites.[[4]] They classified aneurysms unrelated to the branching zones into four types, of
which type 4 aneurysms appear saccular shaped. However, pathologically, these have
an abruptly disrupted IEL without intimal thickening, and lack IEL on the dome. Case
1 and blood blister-like aneurysms are consistent with type 4 of this classification
system.
The question remains of how an aneurysm like the one in Case 2 is formed. We hypothesize
that if the IEL tears but does not rupture, it can become a firm saccular aneurysm
or “non-branching true aneurysm.” A previous study suggested that when the IEL is
injured, the lesion is covered by endothelium in 2–4 days, and neointima is formed
in about 3 months.[[5]] It was also reported that fibroblastic tissue can thicken the aneurysmal wall.[[6]] Based on these facts, nonbranching true aneurysms can develop, as shown in [[Figure 4]]. When the IEL is torn by mild wall injury, such as shear stress or mild trauma,
but does not rupture, the inner layer of the thin aneurysmal wall is first covered
by endothelium. Neointima is then formed, and finally, the aneurysmal wall becomes
thickened by fibroblastic tissue and hyalinization. The fact that blood blister-like
aneurysms are only found in cases of arterial rupture, contrary to arterial dissection
which has nonhemorrhagic onset, leads us to propose that unruptured blood blister-like
aneurysms can develop into nonbranching true aneurysms.
Figure 4: Scheme of the development of nonbranching true aneurysm. When arterial dissection
occurs (a), the luminal surface is covered by endothelium in 2-4 days (b). Then, neointima
gradually forms over 3 months (c). Finally, fibroblastic tissue thickens the aneurysmal
wall and a nonbranching true aneurysm has formed (d)
As a pathological feature, ordinary aneurysms show fibrosis and hyalinization of the
arterial wall, and fragmented and/or missing IEL.[[7]] The nonbranching true aneurysm presented in Case 2 had pathological findings similar
to ordinary aneurysms, but the difference between these cases is the mode of occurrence.
Ordinary aneurysms occur because of a congenital medial defect at the bifurcation,[[7]] while nonbranching true aneurysms may be due to IEL injury caused by mild arterial
wall damage. [[Table 1]] shows a proposed concept of aneurysm classification.
Table 1: Proposed concept of aneurysm classification
In conclusion, aneurysms arising from nonbranching sites take the form of either pseudoaneurysms
or true saccular aneurysms. The latter of which have similar pathological findings
to bifurcation aneurysms. This fact and previous studies propose that nonbranching
true aneurysms can develop from unruptured blood blister-like aneurysms caused by
IEL injury due to mild arterial wall damage. Further reports will help to elucidate
the precise mechanism of aneurysm formation at nonbranching sites.
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