Key-words:
Spontaneous disappearance - thrombosis - unruptured cerebral aneurysm
Introduction
Saccular intracranial aneurysms can often have a variable degree of thrombosis. Although spontaneous thrombosis including partial thrombosis in giant aneurysms is a relatively common vascular event,[[1]],[[2]],[[3]] the occurrence of this phenomenon in nongiant saccular aneurysms is rarely reported.[[4]],[[5]],[[6]] Here, we report two cases of a small unruptured saccular aneurysm showing a radiographic spontaneous disappearance and present a brief literature review related to these cases.
Case Reports
Case 1
A 64-year-old female visited our hospital for a brain checkup in November, 2011. An aneurysm located at the left distal anterior cerebral artery was found on magnetic resonance angiography (MRA) [[Figure 1]]a. Three-dimensional computed tomography (CT) angiography performed at the same time showed no vessel stenosis around the aneurysm [[Figure 2]]. The size of the aneurysm was 4.9 mm in diameter. Laboratory tests revealed no abnormalities, including those of the coagulation system. Follow-up MRA, conducted every 6 months, showed no remarkable change of the aneurysm for 3.5 years after the initial diagnosis. MRA performed 4 years after the initial diagnosis showed a slight reduction in the aneurysm size [[Figure 1]]b. The aneurysm completely disappeared the following year [[Figure 1]]c. MRA performed 2 years after the disappearance of the aneurysm showed a recurrence at the same site [[Figure 1]]d. MRA performed 1 year after the recurrence showed a slight enlargement of the aneurysm [[Figure 1]]e.
Figure 1: (a) Magnetic resonance imaging at first examination showing a small unruptured aneurysm (arrow) arising from the distal anterior cerebral artery. (b) Magnetic resonance imaging performed 4 years after the initial diagnosis showing a slight reduction in aneurysm size. (c) Magnetic resonance imaging performed 5 years after the initial diagnosis showing complete disappearance of the aneurysm. (d) Magnetic resonance imaging performed 2 years after disappearance showing spontaneous recanalization of the aneurysm. (e) Magnetic resonance imaging performed 1 year after recanalization showing a slight enlargement of the aneurysm
Figure 2: Three-dimensional computed tomography angiography performed at the initial diagnosis showing no vessel stenosis around the aneurysm
Case 2
A 57-year-old female visited our hospital for a brain checkup in October, 2006. A saccular aneurysm located at M1–M2 bifurcation of the right middle cerebral artery was found on MRA [[Figure 3]]a. The size of the aneurysm was 2.4 mm in diameter. Laboratory tests revealed no abnormalities including those of the coagulation system. Follow-up MRA, conducted once a year, showed no remarkable change for 11 years [[Figure 3]]b, but showed spontaneous disappearance of the aneurysm 13 years after the initial diagnosis [[Figure 3]]c. Further radiographic follow-up is planned.
Figure 3: (a) Magnetic resonance imaging at first examination showing a small unruptured aneurysm (arrow) arising from the bifurcation of the middle cerebral artery. (b) Magnetic resonance imaging performed 11 years after the initial diagnosis showing unremarkable change of the aneurysm size. (c) Magnetic resonance imaging performed 13 years after the initial diagnosis showing complete disappearance of the aneurysm
Discussion
Spontaneous radiographic disappearance of intracranial aneurysms has been reported to occur under special conditions including postsubarachnoid hemorrhage aneurysms,[[7]] giant/fusiform aneurysms,[[8]] dissecting aneurysms,[[9]] mycotic aneurysms,[[10]] neoplastic aneurysms,[[11]] traumatic aneurysms,[[12]] aneurysms associated with moyamoya disease[[13]] or arteriovenous malformation,[[14]] aneurysms associated with the parent artery occlusion,[[8]] and aneurysms associated with surgical interventions.[[15]] Intimal repair around the aneurysm can occur in a small number of cases,[[10]],[[11]],[[13]],[[14]] but the major etiology of this phenomenon is thought to be spontaneous intra-aneurysmal thrombosis. It is common in giant intracranial aneurysms, occurring in approximately 50% of cases.[[16]] The natural history of intracranial aneurysms is a dynamic process thought to be governed by a delicate balance between thrombogenesis and thrombolysis. The exact mechanism behind the intra-aneurysmal thrombosis is unknown, but the turbulent blood flow within an aneurysm is a well-known fact, which can account for complete or partial thrombosis of the aneurysm, together with endothelial injury. Despite the fact that spontaneous thrombosis is more frequently associated with giant rather than nongiant aneurysms,[[3]] the latter comprise the majority of cerebral ischemia cases related to unruptured aneurysms.[[17]] Therefore, spontaneous thrombosis occurring in a small unruptured aneurysm may be tightly related with a radiologic disappearance.
Spontaneous radiologic disappearance or reduction in size of a small unruptured saccular aneurysm was reported to be rare. To the best of our knowledge, only five cases[[4]],[[5]],[[6]] including ours have been reported previously [[Table 1]]. The average age at initial diagnosis was 62.6 years and all aneurysms were located in the anterior cerebral circulation. The average size of the aneurysms was 3.8 mm in diameter. All cases showed a spontaneous disappearance or reduction in size on MRA or CT angiography without any intervention; the mean interval between the initial diagnosis and the radiographic disappearance was 7.0 years. The first case (case 1) of those presented is the initial case to show spontaneous radiographic disappearance and is also followed by spontaneous reappearance of a small unruptured saccular aneurysm.
Table 1: Summary of cases showing spontaneous disappearance of a small unruptured aneurysm
In current clinical practice, most patients with small unruptured saccular aneurysms are managed conservatively. However, as in our first case, even radiologically occult aneurysms have a risk of recanalization and of possible future rupture. Therefore, this observation should be kept in mind, and strict and vigilant radiographic follow-up is essential, even in patients with small unruptured saccular aneurysms.
Conclusion
We report two cases of small unruptured saccular aneurysms showing spontaneous disappearance on MRA; one showed a spontaneous disappearance also followed by spontaneous reappearance on MRA. Therefore, even radiologically disappeared small unruptured aneurysms still have a risk of recanalization and of possible future rupture. Strict follow-up including serial imaging examinations is required.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form, the legal guardian has given his consent for images and other clinical information to be reported in the journal. The guardian understands that names and initials will not be published and due efforts will be made to conceal identity, but anonymity cannot be guaranteed.
