
==== Front
Surg Neurol Int
Surg Neurol Int
Surgical Neurology International
2229-5097
2152-7806
Scientific Scholar USA

10.25259/SNI_493_2024
10.25259/SNI_493_2024
Case Report
Spontaneous disappearance of a small unruptured cerebral aneurysm in the clinoid segment of the internal carotid artery: A case report and literature review
Onodera Koki onoderakoki@gmail.com

Azekami Kuya cooya0204@yahoo.co.jp

Yahagi Noriyuki kanako-yahagi@cb.wakwak.com

Kimura Ryutaro ryutaro-kimura@nms.ac.jp

Kajimoto Ryuta kajimoto.ryuta@nihon-u.ac.jp

Yoshimura Masataka mstkktsm@yahoo.co.jp

Kohyama Shinya sk3821@5931.saitama-med.ac.jp

Department of Endovascular Neurosurgery, Saitama Medical University International Medical Center, Hidaka, Japan
* Corresponding author: Koki Onodera, Department of Endovascular Neurosurgery, Saitama Medical University International Medical Center, Hidaka, Japan. onoderakoki@gmail.com
2024
23 8 2024
15 29921 6 2024
31 7 2024
Copyright: © 2024 Surgical Neurology International
2024
https://creativecommons.org/licenses/by-nc-sa/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.
Background:

Various degrees of thrombosis have been reported in patients with giant aneurysms. However, small, unruptured aneurysms rarely resolve spontaneously. Herein, we report a case of a small unruptured aneurysm in the clinoid segment (C3) of the left internal carotid artery (ICA) that showed almost complete occlusion at the 1-year follow-up.

Case Description:

A 66-year-old woman developed a subarachnoid hemorrhage on the left side of the perimesencephalic cistern. Cerebral angiography performed on admission revealed no evidence of hemorrhage. Subsequent cerebral angiography on day 12 revealed a dissecting aneurysm on a branch of the superior cerebellar artery (SCA), and the patient underwent parental artery occlusion with 25% n-butyl-2-cyanoacrylate. The postoperative course was uneventful, and the patient was discharged on day 22 with a modified Rankin Scale score of 1. The 1 year follow-up cerebral angiogram demonstrated that the dissecting aneurysm in the SCA branch remained occluded. Notably, a small 2-mm unruptured aneurysm in the clinoid segment (C3) of the left ICA, which was present at the onset of subarachnoid hemorrhage, was almost completely occluded without intervention. Magnetic resonance angiography 1 year after spontaneous resolution of the aneurysm showed no apparent recurrence.

Conclusion:

This case highlights that even small, unruptured aneurysms can develop spontaneous occlusions.

Small aneurysm
Spontaneous disappearance
Thrombosis
Unruptured cerebral aneurysm
==== Body
pmc

INTRODUCTION

Intracranial aneurysms can partially or completely disappear on neuroimaging studies due to thrombosis.[6,7] In giant aneurysms, various degrees of thrombosis have been reported to occur in 50% of cases, with the rate of complete occlusion ranging from 13% to 20%.[17] The mechanism of this phenomenon is thought to involve slow and disturbed blood flow within the aneurysmal sac, which causes blood stagnation, resulting in thrombosis.[4,5] In contrast, unruptured non-giant aneurysms rarely resolve spontaneously.[1,4,5,16,18,19] Herein, we report a case of a small unruptured aneurysm in the clinoid segment (C3) of the left internal carotid artery (ICA) that showed almost complete occlusion without any intervention at the year follow-up.

CASE REPORT

A 66-year-old woman without a notable medical history was brought to her primary physician with a sudden onset of occipital pain while swimming. She was referred to our hospital after a computed tomography (CT) scan showed a subarachnoid hemorrhage on the left side of the perimesencephalic cistern [Figure 1a]. On admission, she was alert with no neurological deficits such as hemiplegia. Blood tests revealed no abnormalities in coagulation. Digital subtraction angiography (DSA) performed on the same day revealed an aneurysm measuring 1.9 mm at the neck and 2 mm deep in the clinoid segment of the left ICA [Figure 1b]. However, other sources of hemorrhage were suspected due to the inconsistency in hematoma distribution and site of the aneurysm, in addition to its oval shape. On day 12 after onset, subsequent DSA revealed no morphological changes in the ICA aneurysm, and angiographic vasospasm was not apparent. However, a dissecting aneurysm measuring 2 mm was found on a branch of the left superior cerebellar artery (SCA), which was thought to be the source of the bleeding, and parent artery occlusion was performed urgently with 25% n-butyl-2-cyanoacrylate [Figure 1c and d]. Magnetic resonance imaging (MRI) the next day showed scattered infarcts in the left SCA region, but the postoperative course was favorable. She was discharged with a modified Rankin scale score of 1 on day 22 after the onset. Follow-up DSA performed 1 year after endovascular treatment showed no recurrence of the dissecting aneurysm in the SCA. Notably, although the aneurysm of the left ICA was untreated, spontaneous, almost complete occlusion was achieved, with a slight bulge remaining at its neck [Figure 2a]. Magnetic resonance angiography 1 year after the disappearance of the aneurysm showed no apparent recurrence [Figure 2b].

Figure 1: (a) Computed tomography scan on admission showing subarachnoid haemorrhage on the left side of the perimesencephalic cistern. (b) Three-dimensional digital subtraction angiography (3D-DSA) performed on admission showing a saccular aneurysm measuring 1.9 mm at the neck and 2 mm in depth in the left internal carotid artery (ICA). (c and d) 3D-DSA on day 12 before and after parent artery occlusion with 25% n-butyl-2-cyanoacrylate performed for a dissecting aneurysm in the branch of the left superior cerebellar artery.

Figure 2: (a) One year follow-up three-dimensional digital subtraction angiography showing spontaneous, almost complete occlusion of the Internal carotid artery (ICA) aneurysm with a slight bulge remaining at its neck. (b) Magnetic resonance angiography one year after disappearance of the aneurysm showing no apparent recurrence.

DISCUSSION

Spontaneous resolution of cerebral aneurysms is common in ruptured aneurysms,[7] dissecting aneurysms,[20] giant aneurysms,[7] traumatic aneurysms,[9] mycotic aneurysms,[2] revascularization surgery for Moyamoya disease,[11] treatment of arteriovenous malformation,[13] and flow alteration for aneurysms,[15] but rarely occurs in small unruptured aneurysms.[1,4,5,16,18,19] The main etiology is thought to be thrombosis, and it is well known to occur in giant aneurysms. Thrombosis is associated with the morphological features of giant aneurysms with large dome volumes and narrow necks.[7,16] It is thought that slow and turbulent blood flow induces blood flow stagnation, leading to thrombosis. Computational fluid dynamics demonstrated the existence of a shear-rate threshold below which a thrombus could form within the aneurysm.[14,16] Thrombosis is also relatively common in ruptured aneurysms, and complete thrombosis occurs in 1–2% of cases.[8] The mechanism can be related to hypotension, vasospasm, and damage to the arterial walls.[7] Other etiologies proposed to cause thrombosis include intimal injury due to intra-aneurysmal turbulent flow, hemodynamic changes in the parent artery, and increased coagulability.[7]

However, the spontaneous disappearance of small unruptured aneurysms is rare, and the underlying mechanism has not been clarified. Despite the lack of configuration features that predispose patients to thrombus formation, ischemic events have been reported in small unruptured saccular aneurysms, and distal clot embolization from the aneurysmal sac is the most common mechanism.[6] This fact suggests that thrombosis is associated with the radiological disappearance of small, unruptured aneurysms.[1,5] To the best of our knowledge, eight cases of the spontaneous disappearance of unruptured aneurysms, including our case, have been reported [Table 1].[1,4,5,16,18,19] Seven of the eight were anterior circulation aneurysms, consistent with the characteristics of complete spontaneous thrombosis in unruptured non-small aneurysms.[16] Classifying these small aneurysms based on their position within the cerebral vasculature,[3] six were sidewall type, and two were bifurcation type. The most common site was the clinoid segment of the ICA in three cases, all of which were classified as the side-wall type.[18,19] A study of computational fluid dynamics using 10 mm diameter porous media modeling demonstrated that bifurcation-type aneurysms have great variation in intra-aneurysmal flow and require a higher volume embolization rate to achieve a given volume of stagnant flow during coiling procedures when this type of aneurysm is more oblique to the parent artery.[12] On the other hand, the volume embolization rate was independent of the aneurysm angle for side-wall type counterparts.[12] Bifurcation-type aneurysms may be less prone to thrombosis, and these results may also apply to small aneurysms.

Table 1: Summary of cases of spontaneous disappearance of small unruptured cerebral aneurysms.

The interval between the identification and disappearance of small aneurysms ranges from 1 to 15 years, and the degree of disappearance varies.[1,4,5,16,18,19] According to Choi et al. and Begley et al.,[4,5] small unruptured aneurysms disappeared only partially, and Yokoya et al. reported the disappearance of a small unruptured aneurysm in the C3 segment of the ICA; however, three-dimensional DSA revealed that a slight bulge of the aneurysmal neck remained, as observed in the present case.[19] Moreover, Akimoto et al. reported that small unruptured aneurysms of the distal anterior cerebral artery regressed and eventually disappeared over 5 years.[1] In our case, magnetic resonance angiography was performed 6 months after diagnosis of the aneurysm; however, due to its small size, morphological changes over time could not be assessed.

Yamada et al. showed the spontaneous disappearance of small unruptured aneurysms in the clinoid segment of the ICA with intra-aneurysmal T2 high intensity, suggesting thrombosis.[18] Neuroimaging features of thrombotic aneurysms are characterized by various high densities within the aneurysms on CT and heterogeneous T1 and T2 signals on MRI, representing differences in the timing of thrombus formation.[10] However, in small thrombotic aneurysms, these features may be missed due to imaging resolution issues. In our case, the aneurysm was too small for CT and MRI to show any findings indicating a thrombus. However, it could still be a thrombosed aneurysm.

There is no consensus regarding the management of small thrombotic aneurysms. Vandenbulcke et al. reported that none of the 13 (0%) thrombotic aneurysms measuring <10 mm ruptured, although one of 6 (17%) measuring 10-20 mm and 2 (100%) larger than 20 mm thrombotic aneurysms ruptured, and that aneurysm size was the factor predicting rupture.[16] The risk of rupture is low in small thrombotic aneurysms, whereas large or giant thrombotic aneurysms are at risk of rupture, and therapeutic intervention is recommended. However, even small thrombotic aneurysms can cause ischemic events.[6,7] According to Cohen et al., ischemic events due to small thrombotic aneurysms occurred in 3 of 4000 stroke patients (0.075%).[6] Although small thrombosed aneurysms often have a low thrombus volume, resulting in a good neurological prognosis, antiplatelet therapy is recommended to prevent recurrent ischemic events.[6,16]

In addition, Akimoto et al. reported a small unruptured aneurysm of the distal anterior cerebral artery that completely occluded spontaneously but recurred 2 years later,[1] linking it to a potential risk of recanalization in thrombosed aneurysms.[16] Long-term follow-up is required even if the spontaneous disappearance of small unruptured aneurysms is observed.

CONCLUSION

Here, we report the spontaneous disappearance of a small unruptured aneurysm in the clinoid segment of the ICA. The etiology of this phenomenon has not yet been elucidated. However, neuroimaging findings suggested that this may be related to thrombosis. Long-term follow-up is required due to the potential risk of recanalization and ischemic stroke, in addition to clarifying the etiology.

Ethical approval

The Institutional Review Board approval is not required.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Use of artificial intelligence (AI)-assisted technology for manuscript preparation

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Disclaimer

The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Journal or its management. The information contained in this article should not be considered to be medical advice; patients should consult their own physicians for advice as to their specific medical needs.

How to cite this article: Onodera K, Azekami K, Yahagi N, Kimura R, Kajimoto R, Yoshimura M, et al. Spontaneous disappearance of a small unruptured cerebral aneurysm in the clinoid segment of the internal carotid artery: A case report and literature review. Surg Neurol Int. 2024;15:299. doi: 10.25259/SNI_493_2024
==== Refs
REFERENCES

1 Akimoto Y Yanaka K Onuma K Nakamura K Takahashi N Ishikawa E Spontaneous disappearance of an intracranial small unruptured aneurysm on magnetic resonance angiography: Report of two cases Asian J Neurosurg 2020 15 1055 8 33708689
2 Allen LM Fowler AM Walker C Derdeyn CP Nguyen BV Hasso AN Retrospective review of cerebral mycotic aneurysms in 26 patients: Focus on treatment in strongly immunocompromised patients with a brief literature review AJNR Am J Neuroradiol 2013 34 823 7 23064596
3 Baharoglu MI Lauric A Gao BL Malek AM Identification of a dichotomy in morphological predictors of rupture status between sidewall-and bifurcation-type intracranial aneurysms J Neurosurg 2012 116 871 81 22242668
4 Begley SL White TG Khilji H Katz J Dehdashti AR Disappearance of a small unruptured intracranial aneurysm: A case report and brief literature review Neuroradiol J 2023 36 621 4 36647339
5 Choi CY Han SR Yee GT Lee CH Spontaneous regression of an unruptured and non-giant intracranial aneurysm J Korean Neurosurg Soc 2012 52 243 5 23115669
6 Cohen JE Gomori JM Leker RR Thrombosis of non-giant unruptured aneurysms causing ischemic stroke Neurol Res 2010 32 971 4 20433779
7 Cohen JE Itshayek E Gomori JM Grigoriadis S Raphaeli G Spektor S Spontaneous thrombosis of cerebral aneurysms presenting with ischemic stroke J Neurol Sci 2007 254 95 8 17258773
8 Kim HJ Kim JH Kim DR Kang HI Thrombosis and recanalization of small saccular cerebral aneurysm: Two case reports and a suggestion for possible mechanism J Korean Neurosurg Soc 2014 55 280 3 25132936
9 Morón F Benndorf G Akpek S Dempsy R Strother CM Spontaneous thrombosis of a traumatic posterior cerebral artery aneurysm in a child AJNR Am J Neuroradiol 2005 26 58 60 15661701
10 Nguyen HS Doan N Eckardt G Gelsomino M Shabani S Brown WD A completely thrombosed, nongiant middle cerebral artery aneurysm mimicking an intra-axial neoplasm Surg Neurol Int 2015 6 146 26425396
11 Ni W Xu F Xu B Liao Y Gu Y Song D Disappearance of aneurysms associated with moyamoya disease after STAMCA anastomosis with encephaloduro myosynangiosis J Clin Neurosci 2012 19 485 7 22281383
12 Otani T Nakamura M Fujinaka T Hirata M Kuroda J Shibano K Computational fluid dynamics of blood flow in coil-embolized aneurysms: Effect of packing density on flow stagnation in an idealized geometry Med Biol Eng Comput 2013 51 901 10 23529587
13 Redekop G TerBrugge K Montanera W Willinsky R Arterial aneurysms associated with cerebral arteriovenous malformations: Classification, incidence, and risk of hemorrhage J Neurosurg 1998 89 539 46 9761046
14 Ribeiro de Sousa D Vallecilla C Chodzynski K Corredor Jerez R Malaspinas O Eker OF Determination of a shear rate threshold for thrombus formation in intracranial aneurysms J Neurointerv Surg 2016 8 853 8 26215274
15 Takemoto K Tateshima S Rastogi S Gonzalez N Jahan R Duckwiler G Disappearance of a small intracranial aneurysm as a result of vessel straightening and in-stent stenosis following use of an Enterprise vascular reconstruction device J Neurointerv Surg 2014 6 e4 23378433
16 Vandenbulcke A Messerer M Starnoni D Puccinelli F Daniel RT Cossu G Complete spontaneous thrombosis in unruptured non-giant intracranial aneurysms: A case report and systematic review Clin Neurol Neurosurg 2021 200 106319 33268195
17 Whittle IR Dorsch NW Besser M Spontaneous thrombosis in giant intracranial aneurysms J Neurol Neurosurg Psychiatry 1982 45 1040 7 7175528
18 Yamada Y Kinjo T Ohki M Kayama T Spontaneous thrombosis of an unruptured internal carotid artery aneurysm: A case report Surg Cereb Stroke 2010 38 114 8
19 Yokoya S Hino A Oka H Rare spontaneous disappearance of intracranial aneurysm World Neurosurg 2020 134 452 3 31765869
20 Zhang YS Wang S Wang Y Tian ZB Liu J Wang K Treatment for spontaneous intracranial dissecting aneurysms in childhood: A retrospective study of 26 cases Front Neurol 2016 7 224 27999562
