The insidious appearance of the dissecting aneurysm: Imaging findings and related pathophysiology. A report of two cases

Abstract

Intracranial dissecting aneurysms have been frequently reported to present with fairly challenging and time-variable imaging findings that can be mostly explained by the pathological mechanisms that underline the dissection. We present two cases of spontaneous dissecting aneurysm of the supraclinoid ICA, both clinically presenting with SAH, but characterized by different progression of clinical symptoms and imaging. However, in both cases an outpouch and a mild fusiform dilation of the supraclinoid ICA was present in the initial CTA performed after the occurrence of symptoms. These findings were well depicted by the MPR reformats performed retrospectively. We postulate that this finding may represent the point of initial transmural dissection and we recommend that careful analysis of the CTA MRP reformatted images should be performed in order to detect this finding promptly.

Introduction

Spontaneous intracranial dissecting aneurysms have been reported as a cause of subarachnoid hemorrhage (SAH) and stroke in young patients.^1–3 They occur most commonly in the vertebro-basilar circulation whereas the anterior circulation is less commonly involved.^4,5

The therapeutic strategy is based on the symptoms, such as SAH or ischemia, which are related to the radiologic features of the aneurysm, respectively: a fusiform dolicho-vessel, or a stenotic vessel secondary to intramural hemorrhage. Therefore, stenting with a flow diverter or stenting and coiling may be required in a fusiform aneurysm presenting with SAH, whereas medical treatment may be preferred if ischemia presents in the territory of local perforators of a dissecting vessel.⁶

Dissecting aneurysms are delusive entities characterized by a dynamic, often rapidly changing radiologic presentation, especially during the hyperacute phase, when the point of the initial parietal rupture and the extension of the dissection may not be visualized.^7,8 This challenging presentation can be explained by the pathological mechanisms that accompany the dissection, such as transmural dissection, leading to SAH, or sub-intimal dissection with sub-adventitial hemorrhage leading to the formation of an intramural hematoma. Depending on the efficacy of the repairing mechanisms of the intramural rupture, a narrowed vessel with adjacent ectatic segments, a fusiform dolicho-vessel, or a partially thrombosed aneurysm may form from the dissected vessel, which will be reflected into imaging. These patterns of presentation may rapidly evolve into each other depending on the instability of the dissection, thus resulting in a dynamic and evolving appearance.⁹

The variability of the imaging may lead to a misinterpretation of the findings and to delayed diagnosis and treatment.

In the present study we present two cases of dissecting aneurysm of the intracranial internal carotid artery (ICA) in which there was a discrepancy in computed tomography (CTA) and digital subtraction angiography (DSA) findings. Despite the fact that the clinical presentation of these two cases was rather similar, the evolution of the dissection differed enormously because of the different mechanisms involved in the pathogenesis of dissecting aneurysms.¹⁰

Materials and methods

This case report includes radiographic and clinical information in two patients. The data were analyzed retrospectively, after receiving the approval of our institutional review board (IRB).

Case 1

A 49-year-old male was admitted to the emergency department following sudden onset of severe headache. Brain CT scan performed after admission showed SAH (Figure 1(a)). CTA of the Circle of Willis showed a lateral outpouching of the supraclinoid segment of the right ICA, suspicious for aneurysm (Figure 1(b)).

Figure 1.

Initial computed tomography (CT) (a) and computed tomography angiography (CTA) (b) scan showing diffuse subarachnoid hemorrhage and a lateral outpouching of the supraclinoid segment of the right internal carotid artery (ICA) (white arrow on CTA). Initial digital subtraction angiography (DSA) (c) demonstrates a fusiform dilation of the supraclinoid segment of the right ICA (black arrow). CTA performed 12 days later (d) shows interval formation of a bulbous dissecting aneurysm of the right ICA (white arrow), confirmed by DSA (black arrow) (e). Follow-up DSA six months after endovascular treatment with a flow diverter confirms complete resolution of the aneurysm (f).

The DSA performed in our institution the day after clinical presentation showed a fusiform dilation involving the supraclinoid segment of the right ICA, which appeared to be rather different from the previous CTA findings (Figure 1(c)). The DSA, repeated five days later, showed a bulbous dilation of the supraclinoid right ICA, again different from previous DSA findings. CTA (Figure 1(d)) and DSA (Figure 1(e)) performed respectively 12 and 13 days after the onset showed a multilobular dissecting aneurysm of the supraclinoid right ICA. The dissecting aneurysm was successfully treated with a flow diverter a day later. The follow-up DSA at six months showed total occlusion of the dissecting aneurysm (Figure 1(f)).

The presence of a lateral outpouch was clearly revealed by the CTA multiplanar reconstruction (MPR) reformats of the intracavernous and supraclinoid ICA segments, performed retrospectively on the first CTA images (Figure 1(b)).

Although the diagnosis of blister aneurysm could have been considered at presentation, the mild fusiform dilation associated with the lateral outpouch, which later evolved into a dolicho-fusiform dilation, would rule out the hypothesis of blister aneurysm. Blister aneurysms are usually not associated with a change in the caliber of the parent artery.¹¹

Case 2

A 49-year-old male was admitted to another institution after the sudden onset of severe headache and right facial droop. CT scan performed after the admission showed SAH and areas of ischemia in the left middle cerebral artery (MCA) and anterior cerebral artery (ACA) territory with small foci of hemorrhagic transformation (Figure 2(a)). CTA that was performed almost six days after the acute onset of symptoms demonstrated significant diffuse vasospasm, most severely affecting the left proximal MCA and ACA. There was no evidence of intracranial aneurysm. At that time, the patient was transferred to our institution because of worsening of the clinical condition with right hemiplegia related to the uncontrollable cerebral vasospasm. DSA performed after admission in our institution, six days after the initial presentation (Figure 2(c)), confirmed the severe vasospasm of the M1 and A1 portion of the left MCA and ACA. A severely stenotic supraclinoid portion of the left ICA was also noted associated with a focal, mildly laterally protruding, dilation. No intracranial aneurysm was detected. Follow-up magnetic resonance angiography (MRA) performed three days later showed residual vasospasm in the proximal left ACA and MCA; however, the focal stenosis in the supraclinoid portion of the left ICA, previously noted, was resolved. Owing to the lack of visualization of any intracranial vascular abnormality, the patient was kept in the neurological intensive care unit; however, he had new, acute clinical worsening 11 days later. CT scan showed a left frontal intracerebral hematoma, probably related to rebleeding (Figure 2(d)). CTA was immediately repeated and it showed a irregular dissecting aneurysm of the supraclinoid segment of the left ICA, with a maximum diameter of 7.1 mm, which was confirmed on the conventional DSA (Figure 2(e)) and successfully treated with coiling (Figure 2(f)). Stenting with a flow diverter was not performed to avoid the risk of rebleeding due to the required antiplatelet treatment, since the patient had just experienced a second SAH and he had a left frontal hematoma, likely due to hemorrhagic transformation of an ischemic lesion due to vasospasm.

Figure 2.

Initial computed tomography (CT) (a) and computed tomography angiography (CTA) (b) scan showing diffuse subarachnoid hemorrhage and a focal stenosis with adjacent lateral outpouch (white arrow on CTA) of the supraclinoid segment of the left internal carotid artery (ICA). Initial digital subtraction angiography (DSA) (c) confirms the CTA findings (black arrow); there is also associated vasospasm at the left A1 and A2 segments. CT (d) scan performed 19 days later shows a large left frontal hematoma. The repeated DSA (e) shows an aneurysm associated with a mild focal dilation of the artery (black thick arrow) that is delimited by proximal and distal narrowing of the vessel (black thin arrows). Control DSA (f) after endovascular treatment shows complete occlusion of the aneurysm with coils.

The CTA MPR reformats performed retrospectively on the first CTA were able to identify the focal stenosis and adjacent outpouch involving the supraclinoid segment of the ICA (Figure 2(b)). The dissecting aneurysm on the conventional DSA corresponded to the same location of the original outpouch documented by CTA.

Discussion

Intracranial arterial dissection is classically described on DSA imaging by the appearance of a double lumen, a focal vessel wall irregularity or a fusiform dilation preceded by a stenotic segment. The primary cause of arterial dissection is an acute disruption of the internal elastic lamina (IEL), which leads to an intramural hemorrhage, as proposed by Mizutani et al. in their classification.^10,12

According to Krings and Choi, the presentation of a dissecting aneurysm underlies different pathological mechanisms: If the sub-intimal hematoma disrupts the entire wall of the vessel, the clinical presentation will be that of SAH. In this case an acute, widespread disruption of the IEL occurs that corresponds to a type I dissecting aneurysm in the classification proposed by Mizutani. If blood enters the sub-intimal space following the IEL, rupture different consequences will be possible: (1) If the hematoma expands in the sub-intimal space and then ruptures into the vessel lumen, distal embolic events may occur. (2) If the hematoma expands between the media and intima, the vessel lumen becomes narrow or occluded and the patient presents with ischemia. The hematoma may also involve perforators and occlude them. (3) After occluding the vessel the intramural hematoma may re-canalize and expand the dissection longitudinally and laterally, thus leading to the formation of a partially thrombosed aneurysm with channels of fresh blood within it, which corresponds to a type III dissecting aneurysm of the Mizutani classification, characterized by the fragmentation of the IEL combined with multiple dissections. All these different aspects of the dissecting aneurysm cannot be divided into compartments as they are all interconnected and may dynamically transform into each other depending on the efficacy of the repairing mechanism of the intramural thrombus.⁹

Because of the hemodynamic dynamicity of the pathological mechanisms, the imaging presentation of spontaneous dissecting aneurysms is known to be variable and instable. Furthermore, a previous study by Ota et al.⁸ demonstrated that the orifice of entry of a dissecting aneurysm can be contained inside the unruptured portion of the aneurysm or can extend into it. This finding differs drastically from the description of dissecting aneurysm made by Mizutani, in which the rupture site was located just above the orifice of the entrance. The authors concluded that current diagnostic imaging cannot precisely characterize the entry point of the dissection.

Moreover, while the mechanism of development of the relatively frequent vertebral dissecting aneurysm is mostly known,^13–15 spontaneous dissections of the anterior cerebral circulation are rare entities and still poorly understood, with the MCA being the most common site of dissection, followed by the ICA and the ACA.^16–20

Our two cases well represent the variability over the course of presentation of dissecting aneurysms. However, both patients showed an outpouch over a fusiform dilation of the supraclinoid ICA, which was revealed retrospectively by CTA reformats. We hypothesized that these outpouches may represent the point of initial transmural rupture of the vessel, which may explain why this finding is evident in both our patients, presenting with SAH, despite the different evolution of the dissection. Neither hyperdensity on plain CT scan nor hyperintensity on T1-weighted MR images were noted at the site of dissection, which might indicate the presence of an intramural thrombus.²¹ We also observed that the suspected point of transmural rupture was contained in the fusiform dilation in the first case and appeared just distal to the dilation in the second case. This finding may sustain what Ota et al.⁸ described.

After review of the literature we found that the majority of cases of dissecting aneurysm of the intracranial ICA were located in the supraclinoid segment.^{13,18,21–24} However, our two cases illustrate the use of CTA in the early detection of the outpouch in the reformatted MPR images as the possible site of the initial transmural rupture of the dissecting aneurysm.

Furthermore, the supraclinoid segment is the preferential site of formation of blister-like aneurysm: a poorly understood entity, also presenting after SAH, consisting of a fragile fibrin layer in focal arterial segments, that has been found to be mostly associated with dissection.^11,25

The ICA arises from the third aortic arch at 4 mm to 5 mm of the embryological state and the two intracranial ICAs are temporally connected through plexiform channels.²⁶ Supraclinoid ICA dissection mostly occurs just distal to the site of division of the primitive ICA (immediately distal to the ophthalmic artery).^23,24.27 Therefore we hypothesize that the supraclinoid ICA segment represents a preferential site of dissection because a point of local weakness of the IEL may occur after the division of the primitive ICA.

Wall vessels imaging using 3 T MRI is an emerging technique for the visualization of intracranial vascular abnormalities including vasculitis, atherosclerotic disease and dissecting aneurysms.^28,29 This advanced technique is not available in our institution for clinical investigation.

We believe it is important to underline the role that CTA has played in our cases after thin-slice MPR images of the supraclinoid portion of the ICA were reviewed. The MPR reformatted images were able to identify vascular irregularities, which could otherwise be easily overlooked even on conventional DSA images. CTA with appropriate thin MPRs should be obtained in cases of suspected dissecting or blister aneurysms presenting with SAH. The finding of an outpouch or irregular vessel wall associated with a fusiform dilation should raise the suspicion of a transmural rupture that may lead to the formation of a dissecting aneurysm.

Conclusions

In conclusion, we propose that intracranial ICA dissection should be considered when an outpouch irregularity is noted associated with a fusiform dilation of the supraclinoid ICA in patients presenting with SAH. This finding should be considered even if the dilation of the ICA is mild. We suggest that the role of CTA with MPR reformats may be crucial to disclose this finding in the early phases of the clinical presentation.

Our report highlights the importance of performing follow-up imaging and/or DSA in patients with an aneurysmal pattern of SAH presenting with initially negative DSA because of the insidious evolution of these lesions and their significant morbidity. We recommend close surveillance in these patients with serial CTA. In case the morphology of the irregularities changes abruptly on CTA MPR reformatted images, conventional DSA should be performed to assess the lesion and eventually proceed to treatment.³⁰

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.