Disappearance of a small unruptured intracranial aneurysm: A case report and brief literature review

Introduction

Intracranial aneurysms are relatively common and present in up to 3% of the general population. ¹ 30% of patients with one aneurysm will be found to have multiple. ² Aneurysms are classically found within the circle of Willis, with a preference for the anterior circulation compared to the posterior one. Although consensus regarding a size-based classification of aneurysms is still lacking, aneurysms that are <5 mm are considered small with a low annual risk of rupture, approximately 0.36% per year.^3–5 The International Study of Unruptured Intracranial Aneurysms reported that the 5-year cumulative risk of rupture in anterior circulation aneurysms was 0% in aneurysms less than 7 mm, and the corresponding rupture rate for similar aneurysms in the posterior circulation was 2.5%. ⁵ Disappearance of intracranial aneurysms has been previously reported; however, this often occurs in the ruptured setting or in the setting of giant aneurysms. It is thought to be related to spontaneous thrombosis of the aneurysm. The spontaneous shrinking or disappearance of a small unruptured aneurysm is extremely uncommon. ⁶ We present a case of a small, unruptured, intracranial aneurysm that was stable across 11 years of follow-up yet spontaneously regressed on all follow-up imaging at year 15. A waiver for consent was obtained and all patient information de-identified.

Case presentation

A 65-year-old woman with a medical history of hypertension and hyperlipidemia presented in 2006 with aneurysmal subarachnoid hemorrhage (SAH) from a ruptured right posterior communicating (PCOM) artery aneurysm. She underwent endovascular embolization of the PCOM aneurysm and during digital subtraction angiography (DSA) was noted to have a small, left P1 aneurysm in addition to the PCOM aneurysm.

A remnant of the embolized PCOM aneurysm and the P1 aneurysm were both were seen on follow-up angiography at 7 months (Figure 1(a)) and on 3-D time-of-flight (TOF) MRA at 16 months (Figure 1(b)). Both studies described stability of the left P1 aneurysm which measured 2–3 mm. The patient did not receive any follow-up until May 2015 when she underwent angiography which demonstrated the previously documented P1 aneurysm (Figure 1(c)). This aneurysm was oval-shaped, superoposteriorly projecting and incorporating the proximal left P1 segment (Figure 1(d)). It was measured to be 3.2 mm × 2.4 mm with a wide neck. Of note, the previously seen ruptured PCOM aneurysm remnant was enlarged compared to prior imaging and decision was made to place a flow-diverting stent across the recurrence of the PCOM aneurysm. The aneurysm was unchanged during DSA on September 2015, 3-D TOF MRA on February 2016, and DSA on May 2017 (Figure 2).OPEN IN VIEWER

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Figure 2.

Timeline summarizing the left P1 aneurysm appearance on follow-up imaging between initial diagnosis after subarachnoid hemorrhage (SAH) in 2006 and last follow-up in 2022. All imaging was completed either on 3-D time-of-flight (TOF) contrast-enhanced magnetic resonance angiography (MRA) or digital subtraction angiography (DSA).

In December 2021, she underwent follow-up 3-D TOF MRA which demonstrated no evidence of left P1 aneurysm (Figure 3(a)). Formal angiography was planned for January 2022 and showed almost complete disappearance of the left P1 aneurysm, now measuring 1.3 mm × 1.1 mm (Figure 3(b)).OPEN IN VIEWER

Discussion

Spontaneous resolution of an aneurysm is thought to be associated with thrombosis of the aneurysm sac. Thrombosis of both ruptured and unruptured aneurysms has been previously reported in the literature with an incidence of 1–3%. ⁷ It most commonly occurs in the setting of subarachnoid hemorrhage after aneurysm rupture, in giant aneurysms, or in partially thrombosed aneurysms. Thrombosis is associated with giant cerebral aneurysms due to the size of the aneurysm dome creating slow and turbulent blood flow that leads to stasis and thrombosis. When thrombosis occurs, imaging findings can mimic intra-axial neoplasms and patients may undergo oncologic workup with attempted resection of the lesion. ⁸

Factors associated with thrombosis are reflective of classic risks for coagulation and include arterial wall damage, vasospasm of the parent vessel, and relative hypotension. Thrombosed aneurysms are best diagnosed on MRA or CTA as these modalities can image the vessel wall, rather than with DSA which can only characterize the lumen, if patent. Ruptured aneurysms are naturally unstable with endothelial disruption occurring at the time of injury that increases the potential for thrombosis. The spontaneous resolution of intracranial aneurysm has also been reported with flow related aneurysms in the setting of treated arteriovenous malformations.^9,10 Changes in hemodynamic flow are thought to lead to spontaneous regression of aneurysms, if shear forces on the vessel wall no longer maintain intra-aneurysm blood flow and allow for stasis.^11,12

Literature review reveals, to the best our knowledge, only six^11–15 reported cases of disappearance of a small unruptured intracranial aneurysm (Table 1). As with our patient, all aneurysms documented were <5 mm. Unlike the other six patients, we describe a posterior circulation aneurysm, although all occur within the circle of Willis. All the cases showed either a reduction in size or complete disappearance on formal angiography, MRA, or CTA without any intervention. Although >50% of the cases noted complete disappearance,^11,13–15 one of the reported cases noted recurrence 2 years after documented disappearance ¹¹ and two cases (including our patient) only had a partial reduction in size. ¹² Given the unknown pathophysiology that led to the disappearance of these aneurysms and the documented possibility of recurrence at the same location, observation and follow-up should continue even in patients with small and unruptured aneurysms given the risk of recanalization and possibility of growth or rupture.^15,16

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Table 1.

Reported cases of spontaneous disappearance of a small, unruptured, intracranial aneurysm.

Author, year	Patient age, sex	Aneurysm location	Aneurysm size	Duration of interval until disappearance	Notes
Akimoto et al., 2020 11	64F	Distal ACA	4.9 mm diameter	5 years	Recurrence after 2 years
Akimoto et al., 2020 11	57F	M1-M2 bifurcation	2.4 mm diameter	13 years	–
Choi et al., 2012 12	69M	MCA	4.5 mm diameter	3 years	Near complete occlusion
Takemoto et al., 2013 13	32F	Anterior choroidal	1.1 mm depth	4 months	Persistent disappearance at 1 year
Yamada et al., 2010 14	70F	ICA	3 mm diameter	2 years	–
Yokoya et al., 2020 15	53F	Clinoid ICA (C3)	2.96 × 4.08 mm	12 years	–
Present case	65F	P1	4.2 × 2.4 mm	15 years	Near complete occlusion

The proposed etiology of small, unruptured aneurysm disappearance is thought to be linked to spontaneous thrombosis, as it is with giant aneurysms, although the factors that encourage thrombosis in these small sacs are less understood.^11,12 Small aneurysms have less potential for stasis, although this is increased in aneurysms with large diameters and narrow necks. Since unruptured aneurysms are not subjected to the endothelial injury associated with rupture, other factors must trigger the coagulation process. Although these aneurysms were all untreated, they did use of formal angiography with intra-arterial contrast injections to monitor growth. It has been proposed that angiographic procedure could contribute to the spontaneous thrombosis and regression via the introduction of turbulent flow, vasospasm, or pro-thrombotic materials. ¹² Therefore, the risk of recanalization could remain high after treatment if the return to previous hemodynamic forces encourages thrombolysis and aneurysm regrowth. Patients with spontaneous disappearance of intracranial aneurysms should be followed with a combination of invasive (angiography) and non-invasive monitoring (MRA, CTA) to monitor changes in aneurysm size and risk of future rupture. Given that the patient described in this report went over 4 years without imaging follow-up prior to the documented regression of her aneurysm, it is impossible to know if the spontaneous reduction happened slowly or acutely within that period. This further supports the need for consistent follow-up in these patients which could improve our understanding of the aneurysm regression process.

Conclusion

While intracranial aneurysms are prevalent, the spontaneous disappearance of non-giant and unruptured aneurysms is rare and infrequently described. This report discussed a case of a small, unruptured left P1 aneurysm that remained stable throughout 11 years of follow-up but demonstrated spontaneous and near-complete resolution in the 15th year. A literature review found only six similar cases of disappearing and unruptured intracranial aneurysms <5 mm, with our case uniquely describing a disappearance in the posterior circulation. The unknown etiology in these clinical scenarios warrants the need for continued close observation and follow-up of patients given the risk of recurrence after disappearance. Data collection from follow-up of such patients will be crucial in further understanding the underlying nature of such disappearing aneurysms.