2026 年 13 巻 p. 117-121
Acute thrombosis is often observed following the rupture of an intracranial aneurysm, and optimal coil selection during endovascular coiling is challenging. A 75-year-old woman presented with a subarachnoid hemorrhage. Computed tomography angiography revealed aneurysms at the right anterior cerebral artery A2/3 junction and the right middle cerebral artery. Based on vessel wall imaging, the anterior cerebral artery aneurysm was diagnosed as the rupture site. Initial digital subtraction angiography demonstrated a 4.4 × 3.4 × 3.4 mm aneurysm with a 1.6 mm neck. Coil embolization was performed 1 day after diagnostic angiography. Preprocedural angiography revealed significant lumen shrinkage to 2.0 mm, probably due to aneurysmal thrombosis. A 4 mm framing coil was selected based on the initial digital subtraction angiography findings. Contrast extravasation occurred after coil deployment. Immediate protamine administration, blood pressure reduction, and coil embolization with smaller coils in the opacified aneurysm dome achieved hemostasis. Final angiography confirmed complete occlusion, without residual filling or distal thrombus migration. Postoperative computed tomography showed an intracerebral hematoma in the left frontal lobe, which subsequently resolved. The patient recovered without focal neurological deficits and was transferred to a rehabilitation hospital on day 18 with mild attention deficits. This case demonstrates the risk of intraprocedural rupture when coil sizing is selected based on pre-thrombosis dimensions rather than current lumen visualization in rapidly thrombosing aneurysms. When thrombosis reduces the lumen, selecting the coil size based on pre-thrombosis dimensions may increase the risk of intraoperative rupture. Coil size selection should match the currently visualized lumen.
Ruptured intracranial aneurysms affect approximately 9 per 100,000 persons annually and remain associated with substantial morbidity and mortality despite advances in treatment.1) Treatment options include microsurgical clipping and endovascular coil embolization. The International Subarachnoid Aneurysm Trial demonstrated favorable outcomes for coiling in selected ruptured aneurysms.2) However, in the case of ruptured aneurysms, acutely thrombosed intracranial aneurysms pose additional challenges. Thrombus can dynamically alter the angiographic lumen, cause unpredictable coil behavior, and increase the risk of complications, including coil migration, recanalization, and intraprocedural rupture (IPR).3)
IPR is associated with increased morbidity and mortality rates. Risk factors for intraprocedural rerupture during coil embolization of ruptured intracranial aneurysms include irregular morphology, aneurysm diameter <3.4 mm, time from symptom onset to intervention >2 days, and cerebral vasospasm during embolization.4) However, coil sizing in rapidly thrombosing aneurysms has not been adequately addressed in the literature.
During treatment, coil selection is conventionally guided by the angiographically visible lumen. However, in rapidly thrombosing aneurysms, the discrepancy between the visible lumen and the thrombus-inclusive aneurysm morphology may be misleading. Reports of aneurysms that thrombose and shrink angiographically within 1 day are rare,5) and IPR due to coil sizing based on prior measurements has not been described. Here, we report a case of coil embolization for a rapidly thrombosing ruptured aneurysm resulting in IPR during the deployment of a framing coil selected based on the diameter measured on initial digital subtraction angiography (DSA). The patient involved in this case report provided informed consent.
A 75-year-old woman presented with a sudden headache and subarachnoid hemorrhage on computed tomography (CT) (Figure 1A). On initial examination, her Glasgow Coma Scale score was 15 (E4V5M6) with no focal neurological deficits. CT angiography revealed aneurysms at the right anterior cerebral artery (ACA) A2/3 junction and the right middle cerebral artery (Figure 1B and C). Given that vessel wall magnetic resonance imaging demonstrated enhancement of the aneurysm at the right ACA A2/3 junction, this lesion was considered a ruptured aneurysm (Figure 1D and E). For further assessment of the aneurysm and branching arteries, right carotid injection was performed, and a distal ACA aneurysm measuring 4.4 × 3.4 × 3.4 mm with a 1.6 mm neck was observed (Figure 1F).
Head CT on admission showing diffuse subarachnoid hemorrhage in the basal cisterns and bilateral sylvian fissures (A). CT angiography reveals aneurysms at the right ACA A2/3 junction (B, C, arrows) and at the right MCA bifurcation (B, C, arrowhead). Vessel wall magnetic resonance imaging demonstrating wall enhancement of the aneurysm at the right ACA A2/3 junction (D, E, arrows), identifying this lesion as the ruptured aneurysm. Initial digital subtraction angiography showing the right ACA aneurysm measuring 4.4 × 3.4 × 3.4 mm with a 1.6 mm neck (F).
ACA: anterior cerebral artery; CT: computed tomography; MCA: middle cerebral artery
Coil embolization was performed under general anesthesia on the following day. An 8 Fr OPTIMO Flex guiding catheter (Tokai Medical Products, Kasugai, Aichi, Japan) was placed in the right cervical internal carotid artery. Systemic heparinization was initiated with a 4,000 IU bolus followed by 1,000 IU/h infusion to maintain a target activated clotting time (ACT) of >250 s (baseline ACT, 165 s). The ACT immediately following bolus injection was 384 s. Preprocedural angiography revealed that the diameter of the aneurysm lumen decreased from 4.4 mm to 2.0 mm, suggesting acute intra-aneurysmal thrombosis (Figure 2A). The opacified portion of the aneurysm appeared to be a small, wide-necked lesion. However, filling the opacified portion with small coils can lead to unstable framing or distal coil migration due to intra-aneurysm thrombolysis. Therefore, the first frame coil was selected based on the size of the aneurysm as measured on the initial DSA. A Phenom 17 microcatheter (Medtronic, Dublin, Ireland) was navigated into the aneurysmal neck using a Synchro-14 microwire (Stryker Neurovascular, USA) without touching the aneurysm dome. A Target 360 Soft Coil (4 mm× 8 cm; Stryker Neurovascular, Kalamazoo, MI, USA) was then deployed. Throughout the deployment of the first coil, no coil protrusion through the aneurysm wall or herniation beyond the aneurysm dome was observed, and the coil did not extend beyond the pre-thrombosis diameter of the aneurysm at any point. After the first coil placement, angiography revealed contrast extravasation extending along the corpus callosum (Figure 2B). Immediately after the event, protamine sulfate (50 mg administered intravenously over 10 min) was administered to reverse heparinization, along with blood pressure reduction using intravenous nicardipine, aiming for a systolic target of 100-120 mmHg. Thereafter, a 3-6 mm× 6 cm Variable-Range Frame Coil (VFC) and 3 1-3 mm× 3 cm VFCs (MicroVention, Aliso Viejo, CA, USA) were deployed, resulting in complete obliteration of the aneurysm without distal thromboembolic migration (Figure 2C). Throughout the surgery, no abnormal hypertension or tachycardia was observed.
Preprocedural angiography on the day of treatment demonstrated a marked reduction of the aneurysmal lumen to 2.0 mm in diameter (A), suggesting progressive intra-aneurysmal thrombosis. Angiography immediately after deployment of the first 4 mm framing coil showing contrast extravasation (B) extending along the corpus callosum, indicating intraprocedural rupture. The aneurysm visualized at rupture measured approximately 6 mm in diameter. Final angiography after completion of coil embolization with five coils showing no contrast extravasation or residual aneurysmal filling (C).
Postoperative CT revealed a high-density area in the left medial frontal lobe and basal cistern (Figure 3A). As hemostasis was achieved, general anesthesia was discontinued on postoperative day 1. The patient began rehabilitation without any limb weakness or aphasia. Magnetic resonance imaging on postoperative days 2 and 8 (Figure 3B) confirmed no recanalization of the aneurysm.
Immediate postoperative computed tomography demonstrating high-density areas in the left medial frontal lobe and basal cistern, consistent with intraprocedural hemorrhage (A). Magnetic resonance imaging on postoperative day 8 confirms complete occlusion of the aneurysm (arrow) without recanalization (B).
The patient was physically independent but exhibited mild attention deficits (Mini-Mental State Examination, 29/30; Frontal Assessment Battery, 15/18) and thus transferred to a rehabilitation hospital on postoperative day 18.
This case illustrates an underappreciated hazard in the endovascular treatment of ruptured aneurysms. Rapid thrombosis can dramatically alter aneurysm morphology within hours, rendering the initial measurements unreliable and potentially dangerous for procedural planning. The aneurysm lumen decreased from 4.4 mm to 2.0 mm within 1 day, illustrating how dynamic thrombosis can distort the angiographic appearance.
We selected a 4 mm framing coil based on the initial DSA measured 1 day before treatment for the following reasons. First, if coils are undersized relative to the true aneurysm wall, they may become embedded within the soft thrombus rather than maintaining stable apposition against the actual aneurysm wall, potentially leading to coil compaction, incomplete occlusion, and delayed aneurysm growth. Second, the goal of coil embolization is to promote organized thrombosis and fibrosis of the entire aneurysm sac, and coils that are too small may fail to achieve adequate packing. Third, considering only the 2.0 mm opacified portion as the target might have made stable framing difficult or caused distal coil migration due to blood flow. However, oversizing relative to the intra-aneurysmal lumen that opacified immediately before coil embolization resulted in extravasation from the aneurysm.
Based on fluid-structure interaction modeling studies, thrombi adhering to the aneurysm wall generally exhibit a low risk of detachment under physiological flow conditions. In contrast, centrally located thrombi with insufficient wall contact may migrate or become destabilized by hemodynamic forces.6) In the present case, selecting a slightly oversized coil may have altered the mechanical balance within the aneurysm, causing displacement or compression of the centrally located intraluminal thrombus. Such thrombus compression can transiently increase local wall stress and precipitate rupture. Throughout the deployment of the first coil, no coil protrusion through the aneurysm wall or herniation beyond the aneurysm dome was observed, suggesting that the coil did not directly penetrate the thrombus to contact the wall. Instead, the mechanism was likely mechanical compression of the thrombus against the aneurysm wall, which generated excessive wall stress. On the DSA obtained at the time of rupture, the aneurysm measured approximately 6 mm, implying that thrombus formation had already started at the time of the initial angiogram and that the fresh thrombus was insufficiently organized to provide adequate structural support against coil-induced compression forces.
The limited evidence available for rapidly thrombosing aneurysms suggests that intentional undersizing to match the currently visualized lumen is a viable strategy. Iwabuchi et al.5) reported a case of an angiographically occult anterior communicating artery aneurysm measuring 5 mm on CT angiography but poorly visualized on angiography due to intraluminal thrombosis. They intentionally selected an undersized 3 mm coil based on the visible neck portion, resulting in initial hemostasis without acute rerupture; however, recanalization occurred, requiring subsequent retreatment. During the retreatment procedure, the authors deliberately chose an undersized 3 mm coil based on a visualized dome diameter of approximately 4 mm, completing the procedure without complications and achieving a favorable long-term outcome. This approach suggests that coils with a smaller diameter reduce the risk of IPR and enable safe completion of the procedure. Chang et al.7) described an anterior communicating artery aneurysm that underwent spontaneous near-complete thrombosis during coil embolization, leaving only a small remnant of the aneurysm neck. They inserted a considerably smaller 2 mm coil into the remaining stump, achieving complete occlusion that remained stable at 1-year follow-up without recanalization. The authors postulated that tight packing near the aneurysm neck may have promoted sustained thrombosis and prevented recanalization.
These cases of acute thrombosis fundamentally differ from those of chronic partially thrombosed intracranial aneurysms (PTIAs). In chronic PTIAs, the thrombus typically undergoes organization with fibrosis over months to years, resulting in an organized intraluminal thrombus that forms a solid mass and provides greater structural support.3) In contrast, acute thrombosis that occurs within hours to days after aneurysm rupture consists of fresh, soft, and mechanically unstable clots that may not provide an adequate cushioning effect when compressed by coils that exceed the aneurysm diameter. The predominant mechanisms of recurrence in chronic PTIAs are migration of coils into the thrombus and coil compaction.8) Although pathological confirmation was not available in our case, the rapidity of thrombosis within 1 day and the response of the aneurysm to coil insertion suggest that the thrombus was fresh and unorganized. Therefore, in acutely thrombosing ruptured aneurysms, conservative coil sizing matched to the currently visualized lumen appears safer than aggressive sizing based on pre-thrombosis dimensions. However, meticulous radiographic follow-up is required to recognize subsequent aneurysm recanalization. In conclusion, this case illustrates a critical and previously undescribed hazard: coil sizing based on pre-thrombosis dimensions may not be safe when rapid thrombosis contracts the visualized lumen over several hours. Coil selection should match the currently visualized contrast lumen in patients with acutely thrombosed aneurysms.
We would like to thank Editage (www.editage.jp) for English language editing.
All authors declare that there are no conflicts of interest (COIs) concerning this article according to the criteria of The Japan Neurosurgical Society (JNS). The authors (TS, YA, and YO), who are members of JNS, have registered online self-reported COI disclosure statement forms through the website for JNS members.
