Spontaneous Disappearance of Intracranial Unruptured Saccular Aneurysms: Two Case Reports

Abstract

Spontaneous disappearance of small unruptured intracranial aneurysms is exceedingly rare, as most reported cases describe resolution in the context of rupture, giant aneurysms, or dissecting lesions. We report two cases of small saccular aneurysms that vanished during long-term magnetic resonance angiography follow-up. In Case 1, a 3.6 × 3.4-mm right internal carotid artery aneurysm demonstrated gradual reduction followed by complete disappearance, coinciding with proximal hemodynamic alteration. In Case 2, a 3.6 × 2.9 mm aneurysm of the right middle cerebral artery remained stable for 7 years before disappearing following asymptomatic M2 branch occlusion. These observations suggest that spontaneous thrombosis may occur even after prolonged morphologic stability, particularly when hemodynamic conditions shift toward reduced flow or altered shear stress. Recognition of this phenomenon is important for follow-up strategies, and additional imaging details are provided in the supplementary material.

Keywords

intracranial aneurysm - disappearance - spontaneous - thrombosis

Introduction

Intracranial aneurysms exhibit dynamic morphological changes throughout their natural history.[1] [2] [3] [4] Spontaneous disappearance has been reported mainly after rupture, in giant aneurysms, or in dissecting aneurysms.[5] [6] [7] In contrast, disappearance of small unruptured saccular aneurysms is extremely uncommon. We describe two such cases involving the internal carotid artery and middle cerebral artery.

Table 1 Summary of previously reported cases and two present cases involving spontaneous disappearance of small unruptured intracranial aneurysms

No.	Authors	Year	Age/Sex	Location	Types	Duration until disappearance	Morphology	Status of obliteration	Recurrence	Follow-up period after disappearance
1	Choi et al[15]	2012	69/M	MCA	Bifurcation	3 y	4.5 mm in diameter	Partial disappearance	N/A	N/A
2	Yokoya et al[4]	2020	53/F	ICA	Side wall	12 y	3.0 mm at neck	Almost complete	N/A	N/A
3	Akimoto et al[1]	2020	64/F	ACA	Side wall	3 y	4.9 mm in diameter	Complete disappearance	Yes	2 y
4	Akimoto et al[1]	2020	57/F	MCA	Bifurcation	13 y	2.4 mm in diameter	Complete disappearance	N/A	N/A
5	Yamada et al[18]	2010	53/F	ICA	Side wall	19 mo	3.0 mm in diameter	Complete disappearance	N/A	N/A
6	Begley et al[2]	2023	65/F	PCA	Side wall	15 y	4.2 × 2.4 mm in diameter	Partial disappearance	N/A	N/A
7	Onodera et al[10]	2024	66/F	ICA	Side wall	1 y	1.9 mm at neck, 2.0 mm at depth	Almost disappearance	N/A	1 y
8	Hwang et al[3]	2025	53/F	MCA	Bifurcation	12 d	2 mm in diameter	Complete disappearance	No	N/A
9	Vandenbulcke et al[9]	2021	73/F	ICA	Side wall	54 mo	2.5 mm at the dome	Almost disappearance	No	4 mo
10	Present case 1	2025	66/F	ICA	Side wall	3 y	3.6 × 3.4 mm in diameter	Complete disappearance	No	2 y
11	Present case 2	2025	74/F	MCA	Side wall	8 y	3.6 × 2.9 mm in diameter	Complete disappearance	No	2 y

Abreviativos: ACA, anterior cerebral artery; ICA, internal carotid artery; MCA, middle cerebral artery; PCA, posterior cerebral artery.

Case Report

Case 1

A 66-year-old woman experienced transient twitching of the left lower eyelid. Magnetic resonance angiography (MRA) demonstrated a 3.6 × 3.4 mm aneurysm at the C3 portion of the right internal carotid artery ([Fig. 1A]; [Fig. 2A]). Mild proximal stenosis was also noted. Because the aneurysm measured less than 5 mm, scheduled MRA follow-up was performed. After 2.5 years, slight shrinkage was observed ([Fig. 1B]; [Fig. 2B]). Following the development of hypertension and initiation of antihypertensive therapy, the aneurysm disappeared completely within 6 months ([Fig. 1C]; [Fig. 2C]). Source images confirmed intra-aneurysmal thrombosis ([Fig. 1D]; [Fig. 2D]). Digital subtraction angiography was recommended but declined. No recurrence was noted over 4 years.

Fig. 1 (A) ICA aneurysm at first detection; (B) Slight reduction at 2.5 years; (C) Complete disappearance at 3 years; (D) Persistent absence for 4 years. ICA, internal carotid artery.

Fig. 2 Time-of-flight magnetic resonance angiography (TOF-MRA) source images demonstrating the spontaneous disappearance of the aneurysm. Serial TOF-MRA source images obtained at (A) baseline, (B–D) during follow-up, show a gradual reduction in aneurysmal signal intensity, followed by complete disappearance. These source images support true aneurysm resolution rather than an imaging artifact, suggesting intra-aneurysmal thrombosis or flow-related signal loss.

Case 2

A 74-year-old woman underwent magnetic resonance imaging for transient extremity numbness. A 3.6 × 2.9 mm aneurysm of the right M2 inferior trunk was incidentally identified ([Fig. 3A]). Annual MRA demonstrated no change for 7 years ([Fig. 3B]). During the eighth year, asymptomatic occlusion of the M2 inferior branch occurred, followed by complete disappearance of the aneurysm ([Fig. 3C]). A small subacute infarction was detected but remained clinically silent. Carotid ultrasound showed no significant vascular disease. No recurrence was noted at 9-year follow-up ([Fig. 3D]).

Fig. 3 (A) M2 inferior trunk aneurysm at detection; (B) Stable for 7 years; (C) Disappearance with M2 occlusion; (D) No recurrence at 9-year follow-up.

Discussion

Only a limited number of small (≤5 mm) unruptured aneurysms have been reported to disappear spontaneously during follow-up.[1] [2] [3] [4] [8] [9] Including our cases, 11 such instances have been published.[1] [2] [3] [4] [9] [10] [15] [18] A female predominance is notable, and most aneurysms arise from the internal carotid artery or the middle cerebral artery ([Table 1]). Several mechanisms have been proposed, including intra-aneurysmal thrombosis, stagnation of blood flow, and reduced shear stress.[9] [10] [11] [12] [13] [14] Hemodynamic alterations, such as proximal stenosis (Case 1) or distal branch occlusion (Case 2) likely contributed. Even long-stable aneurysms may thrombose suddenly. Previous reports of spontaneous regression support this concept.[15] [16] [17] [18] [19] Side-wall aneurysms are reportedly more susceptible to flow stagnation and thrombosis than bifurcation aneurysms.[20] Although small thrombosed aneurysms rarely rupture, distal embolization may occur, and long-term follow-up remains essential.

Conclusion

These cases highlight the potential for spontaneous thrombosis and disappearance of small unruptured aneurysms. Sudden disappearance may occur even after prolonged radiological stability, particularly when hemodynamic factors shift. Continued imaging surveillance is therefore recommended.

Conflict of Interest

None declared.

• References

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Publication History

Article published online:
03 February 2026