Giant M1 segment aneurysm: Illustration of the importance of cross-sectional ratio of aneurysmal neck and the parent artery

Introduction

M1 segment aneurysms are rare, comprising 2%–7% of the middle cerebral artery (MCA) aneurysms.[1,2,3,4] Their surgical management can be complicated due often to a broad neck, large size, and associated atherosclerosis of the involved segment.[5] A variety of complex surgical methods have been described for treating these aneurysms.[1] In this article, we present the case of a ruptured, giant M1 segment aneurysm which was clip occluded. Postoperatively, a delayed clip-related parent artery stenosis was diagnosed, leading to a successful clip re-adjustment 23 h of the primary clipping. This technical report highlights the challenges of microsurgical clipping of giant M1 segment aneurysms and the importance of taking into consideration the “cross sectional ratio” of aneurysm neck to the parent artery circumference and not merely the closure line concept, in such giant aneurysms.

Case Presentation

A 58-year-old female presented with a thunderclap headache 3 days before clinical presentation. On clinical and radiological evaluation, she was found to have a ruptured giant left M1 segment aneurysm with a World Federation of Neurological Societies Grade I subarachnoid hemorrhage [Figure 1]. Following admission, we instituted the standard medical management including oral administration nimodipine at the dose of 60 mg every 4 hourly and this was continued for a total of 21 days. During the surgery, the aneurysm was found proximal to the origin of the lenticulostriate perforators, involving almost the entire length of the M1 segment. The aneurysm neck and lenticulostriate artery (LSA) perforators were dissected after trapping the aneurysm with temporary clips. The neck of the aneurysm had a yellow discoloration suggesting atherosclerosis. Two long permanent clips were applied across the neck, along the anticipated outline of the original MCA caliber (closure line). Intra-operative indocyanine green (ICG) angiogram showed a complete occlusion of the aneurysm and filling of all the branches and perforators at the base [Video 1].

Figure 1.

The preoperative images of the patient; (a) Axial magnetic resonance view showing giant thrombosed aneurysm at the left anterior sylvian fissure (yellow arrow), the hyperintense subarachnoid hemorrhage is present adjacent to the aneurysm (white arrow); (b) Diffusion-weighted imaging sequence showing the thrombosed aneurysm; (c and d) Lateral view of computed tomography angiogram, showing a giant aneurysm from the M1 segment (red arrow), the middle cerebral artery (MCA) bifurcation can be seen distal to the aneurysm (blue arrow); (e) Coronal view and (f) Lateral view of digital subtraction angiography, showing giant thrombosed aneurysm involving the left M1 segment with filling of the distal MCA

In the immediate postoperative period, she was fully conscious and neurologically intact. She was being managed medically with clinical examination for any signs of vasospasm. After 18–20 h of the surgery, she developed right hemiplegia. An angiogram was done at this point with the intent to rule out vasospasm and if needed treat with intra-arterial vasodilators. Angiogram showed a narrowing of the M1 segment beneath the clip and opacification of the distal MCA circulation from the pial collaterals [Figure 2]. Intraarterial nimodipine was given at appropriate dose. However, there was no improvement in the arterial narrowing. Therefore, the M1 segment narrowing was thought to be mechanical, aneurysm clip induced.

Figure 2.

Postoperative angiogram, left internal carotid artery (ICA) run (a) Left anterior-oblique view showing two clip artifacts (yellow arrow); (b and c) Lateral angiogram showing no immediate filling of the distal middle cerebral artery (MCA) (blue arrow) and delayed filling of the distal MCA from pial collaterals; (d) Coronal angiographic view, showing filling of distal MCA from pial collaterals (red arrow) and normal filling of the left anterior cerebral artery (ACA); (e) Coronal view after right ICA run, showing normal filling of right MCA and right ACA with no flow across the anterior communicating artery onto left ACA; (f) Coronal view following, left vertebral artery run

She was taken to the operation theater for re-exploration within 5 h of the deficit. By that time 23 h had elapsed from the time of primary clipping. At the second surgery, the proximal clip was removed, and the distal clip was readjusted to leave a uniform, residual neck, allowing a slight increase in the cross section of the newly reconstructed MCA. ICG angiogram showed adequate and satisfactory filling of the M1 segment and its branches [Video 2].

Postoperatively, the hemiparesis gradually improved and her power improved to 3/5 in the right upper and lower limbs at discharge. At 6 weeks’ follow-up, she had a power of grade 4/5 and could almost independently perform her daily activities.

Discussion

There were three main highlights of this report:

1. Clipping technique in giant M1 segment aneurysms

2. Postoperative clip-related parent artery stenosis and delayed successful clip readjustment

3. Need to leave a residual neck in wide necked, atherosclerotic aneurysms to preserve parent artery patency.

M1 segment aneurysms are very rare, comprising 2%–7% of MCA aneurysms.[1,2,3,4] These generally arise from the early branches of MCA or the LSA perforators. Paulo et al.[6] reported a large experience with these aneurysms and noted that the M1 segment was generally longer in such patients (16.5 mm, to where it is normally 11.8 mm).[7] They noted that the majority of these aneurysms were small, unlike our case. Various surgical techniques have been described for such aneurysms including tandem clipping, picket fencing, high flow bypass with blind pouch creation and even aneurysmectomy with artery reconstruction.[1] While these techniques are useful, the technical complexity and often additional costs may be prohibitive in many centers. We adopted a simpler treatment strategy. As both surgical videos showed, once the neck was defined and the proximal/distal parent arteries were in vision, an appropriately sized clip applied across the neck sufficed to exclude these aneurysms. The high flow inside MCA, mandates a second clip, applied distal to the first clip to prevent a possible future aneurysm regrowth.[8]

In our patient, the ICG angiogram showed a satisfactory flow in the parent artery after the first clipping. Despite this, our patient developed ischemia underscoring the fallacy of ICG as an intraoperative means of determining blood flow in the arteries. It provides a semiquantitative measure for blood flow unlike a handheld Doppler ultrasound.[9,10,11] Intraoperative electro-physiological monitoring, another indirect means to determine adequacy of parent artery flow during aneurysm clipping, determines the viability of the motor and sensory pathways.[12] Therefore, these above methods clearly have limitations. Although these modalities have significantly increased the patency rates during aneurysm surgery, a subset of patients may become ischemic and necessitate re-exploration.[12,13] The rate of ischemic complications related to aneurysm clipping has been reported to range from 0.9% to 5.6%, depending on whether SSEP was used for monitoring.[12] In addition, cerebral vasospasm is an important cause of ischemia in ruptured intracranial aneurysms like our case.[14,15]

Postclipping parent artery stenosis can occur due to clip-related factors such as occlusion of part of the vessel,[16] rotation of the clip,[17] irregular shape of dome and neck,[18] and nonclip-related factors such as vasospasm, calcification, development of thrombus clip migration, kinking, and dislodgment of atheroma at the neck.[19,20] Therefore, caution is necessary during neck clipping of such atherosclerotic aneurysms.

We demonstrated a successful outcome following clip readjustment undertaken 23 h after the primary clipping and 5 h after the detection of the neurological deficit. The pial collaterals maintained the retrograde flow into MCA and provided us the necessary time window for the intervention, like acute ischemic strokes. Delayed re-adjustment of the clip has been known to reverse the neurological deficits caused be reduction in blood supply to the penumbra up to 72 h[16,19] as the vessel is not completely occluded and the reduction in blood supply is only partial.

The third and perhaps most important point of this discussion is the concept of “cross sectional ratio.” Conventionally, we understand the neck and its dimension in along the long axis of the parent artery, both anatomically and angiographically [Figure 3a]. Thus, “closure line” is an important target for aneurysm surgeons during such surgeries. However, large to giant wide necked aneurysms take up a significant circumference of the parent arterial wall. Therefore, we believe the neck of such aneurysms must be understood in terms of the cross-sectional ratio of the aneurysm neck to the parent artery [Figure 3b]. The importance of this “cross sectional ratio” concept can be understood by the following physics principle:

Figure 3.

(a) The cross-section of the aneurysm (a) along with the parent vessel (p) before clipping. (b) The cross-section of the aneurysm and parent vessel after applying the clip, at first surgery, showing reduction in diameter of the parent vessel. (c) The new configuration of the M1 segment after clip readjustment and leaving the residual neck of aneurysm and maintaining the diameter of the parent vessel

Physics of flow

This can be better understood by applying the Hagen-Poiseuille law, which describes the relationship between the flow rate of a fluid through a cylindrical pipe and the various factors that influence it. It is particularly relevant for the laminar flow of incompressible, Newtonian fluids (such as water or blood in arteries) through a pipe (like a blood vessel).[21]

The law is mathematically expressed as:

Here, Q is the volume flow rate (volume per unit time), ΔP = Pressure difference between the two ends of the pipe, r = Radius of the pipe, η = Dynamic viscosity of the fluid and L = Length of the pipe. Flow rate (Q) is directly proportional to the fourth power of the radius (r). This means even small changes in the radius of the vessel or pipe significantly affect the flow rate. Applying this principle to aneurysm occlusion, a small reduction in the circumference of the vessel can lead to a significant reduction in the flow across the vessel. Therefore, keeping in mind the above principle, one can easily understand the need to apply clips distal to the closure line without causing parent artery narrowing [Figure 3b].

What we propose in this article is that we maintain the circumference of the parent vessel by excluding a small circumference of the aneurysm. Figure 3c represents the parent artery diameter before clipping, after clipping and after second surgery to show how the clip adjustment helped improving the flow.

Conclusion

Giant wide-necked M1 segment aneurysms may be clipped using simple clip occlusion of the neck parallel to the M1 segment. However, clip should be deployed away from the neck so that the eventual closure line leaves some neck and maintain “cross-sectional ratio” of aneurysm neck to parent artery to avoid compromise of the parent artery. If detected postoperatively, readjustment is recommended as soon as possible as our case showed.

Author contributions

Soumen Kanjilal: writing/editing/data collection; Preetham Dange: editing/data collection, initial draft preparation; Kuntal Kanti Das: concept, operating surgeon, revision, editing/literature review; Kamlesh Singh Bhaisora: editing/literature review; Awadhesh Kumar Jaiswal: supervision/revision/editing.

Ethical policy and institutional review board statement

The study was performed in accordance with the Declaration of Helsinki.

Data availability statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given her consent for her images and other clinical information to be reported in the journal. The patient understands that name and initials will not be published and due efforts will be made to conceal identity, but anonymity cannot be guaranteed.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.