Key points

Introduction

Since the publication of the International Subarachnoid Aneurysm Trial (ISAT) and Barrow Ruptured Aneurysm Treatment (BRAT) randomized clinical trials, ET is the most frequently used approach for treating cerebral aneurysms. Incremental improvements of interventional techniques now permit treatment of more complex cerebral aneurysms endovascularly, resulting in a higher proportion of aneurysms being treated with ET than with open surgical techniques. Despite the mounting evidence, the debate of coiling versus clipping continues to persist, most vigorously directed at the treatment of MCAAs. More emphasis is placed on choosing treatment based on clinical factors, complex anatomic and morphologic features (including age, location, size, projection, and relationship with branching vessels), and the potential of lifelong durability over the initial gain of coiling safety.

This ongoing controversy is well illustrated by the unresolved question regarding the best approach to treating MCAAs and the lack of consensus on which treatment provides balanced safety and long-term protection. It is assumed that the specific anatomic aneurysmal location of the MCA may be more suitable to open surgical therapy than to ET. Although the MCA is an appealing location for surgical treatment with a direct and feasible approach, there is potential difficulty in cases with early vasospasm, as well as potential additional morbidity of the open surgical approach, such as retraction injury and perioperative hematoma. These difficulties and complications may be avoided by an endovascular approach; however, no randomized controlled trial data currently exist to specifically guide MCAA treatment decisions.

The lack of consensus on best treatment practices may have originated from initial data on MCAA coiling outcomes collected during early-era ET when coiling techniques had limited feasibility in treating challenging complex-shaped aneurysms. The introduction of advanced microcatheter designs for superior aneurysm access, complex 3-dimensional coil designs coupled with neck-bridging microstents, and balloon-assisted coiling options that offer safer and more dense packing result in feasibility rates for ET that exceed 90%. In a study of 300 MCAAs by Mortimer and colleagues, the feasibility of MCAA coiling as a primary treatment was shown to be approximately 95.8%. In this monograph, we provide a comprehensive literature review of MCAA coiling and clipping, then present our initial experience of MCAA coiling of nonselective consecutive cases.

Introduction

Since the publication of the International Subarachnoid Aneurysm Trial (ISAT) and Barrow Ruptured Aneurysm Treatment (BRAT) randomized clinical trials, ET is the most frequently used approach for treating cerebral aneurysms. Incremental improvements of interventional techniques now permit treatment of more complex cerebral aneurysms endovascularly, resulting in a higher proportion of aneurysms being treated with ET than with open surgical techniques. Despite the mounting evidence, the debate of coiling versus clipping continues to persist, most vigorously directed at the treatment of MCAAs. More emphasis is placed on choosing treatment based on clinical factors, complex anatomic and morphologic features (including age, location, size, projection, and relationship with branching vessels), and the potential of lifelong durability over the initial gain of coiling safety.

This ongoing controversy is well illustrated by the unresolved question regarding the best approach to treating MCAAs and the lack of consensus on which treatment provides balanced safety and long-term protection. It is assumed that the specific anatomic aneurysmal location of the MCA may be more suitable to open surgical therapy than to ET. Although the MCA is an appealing location for surgical treatment with a direct and feasible approach, there is potential difficulty in cases with early vasospasm, as well as potential additional morbidity of the open surgical approach, such as retraction injury and perioperative hematoma. These difficulties and complications may be avoided by an endovascular approach; however, no randomized controlled trial data currently exist to specifically guide MCAA treatment decisions.

The lack of consensus on best treatment practices may have originated from initial data on MCAA coiling outcomes collected during early-era ET when coiling techniques had limited feasibility in treating challenging complex-shaped aneurysms. The introduction of advanced microcatheter designs for superior aneurysm access, complex 3-dimensional coil designs coupled with neck-bridging microstents, and balloon-assisted coiling options that offer safer and more dense packing result in feasibility rates for ET that exceed 90%. In a study of 300 MCAAs by Mortimer and colleagues, the feasibility of MCAA coiling as a primary treatment was shown to be approximately 95.8%. In this monograph, we provide a comprehensive literature review of MCAA coiling and clipping, then present our initial experience of MCAA coiling of nonselective consecutive cases.

MCA embryology and anatomy

An understanding of MCA embryology, anatomy, and expected anomalies is imperative to best treatment approach planning. During the 8th to 12th week of gestation, the distal primitive internal carotid artery (ICA) and its anterior cerebral artery (ACA) divide into the anterior choroidal artery and numerous small arterial twigs. The latter develops into the future anterior and middle embryologic cerebral arteries. This rete coalesces into the main MCA trunk, and the remaining twigs are the future perforators. The MCA is thus a continuation of the ICA. Failure to coalesce can lead to accessory MCA and a dominant anterior temporal artery. These variations become important in the discussion of the accessory MCA types.

From microsurgical data, the MCA outer diameter is 3 ± 0.1 mm bilaterally with a length of 15 ± 1.1 mm in the right hemisphere and 15.7 ± 1.3 mm in the left hemisphere. However, in an autopsy study of 610 MCAs, the horizontal segment length was 16 mm (range 5–30 mm), with a diameter of 3 to 5 mm. The MCA main trunk horizontal segment is referred to as M1 (sphenoidal), followed by the M2 (insular), then M3 (opercular), and finally M4 (cortical) segments.

The MCA horizontal segment branching patterns are bifurcation (78%–90% of cases), trifurcation (12%), and multiple branches (10%), with the subsequent branching being mainly bifurcated. A true trifurcation may be confused with a dominant intermediate trunk with a gap between the latter and the bifurcation point. A dominant trunk was found to be close to the MCA division, masking as a true trifurcation in 15% of cases. In 55% of cases, it originates within a short distance of one of the MCA divisions, whereas in 30% of cases, it originates distal to the MCA divisions. The dominant intermediate trunk originated more commonly from the superior division. The more proximal the intermediate trunk is to the MCA division point, the larger its contribution to the cortical territories. The intermediate trunk commonly supplies the parietal lobe. The superior trunk supplies the frontal convexity, and the inferior division supplies the temporal lobe in conjunction with the posterior cerebral artery and part of the parietal lobe, depending on the dominance of each division.

The anterior temporal artery is the first branch of the M1 segment and a common location of proximal M1 aneurysms, as was seen in 19 of the 23 specimens in one autopsy study and in all MCA samples in another study. In addition, the perforators (Charcot striate arteries, varies from 6 to 20 perforators) are a common location for proximal M1 aneurysms. The perforators can originate from the proximal M1 segment (51.1%), distal M1 segment and the first branching point (25.6%), or from one of the MCA/M2 branches distal to the first division (20.3%).

Accessory MCA was found in 3% of the autopsies and 0.12% of the magnetic resonance angiographic (MRA) studies. Failure of perforators off the ACA to coalesce and form a single MCA leads to MCA duplication with various dominance and origins leading to different types (Manelfe types I, II, and III). This duplication or accessory MCA commonly occurs unilaterally. However, when identified on one side, the likelihood of finding a mirror contralateral accessory MCA is 16.5%. The accessory MCA arose from the ACA in 80% of cases (type 2 from proximal A1 segment of the ACA and type 3 from the distal A1 segment) and from the ICA in 20% of cases (type I) ( Fig. 1 ). Fenestration of the MCA is rarely encountered and was found in 3 of 3491 MRA studies or 0.09% of cases.

Example of accessory middle cerebral artery, from the proximal A1 on the left side and from the distal A1 on the right side; also note on the right-hand side the origin of the anterior temporal artery off the distal ICA.

Epidemiology and risk of rupture

Incidence, Multiplicity, and Location

The MCA is a common location of cerebral aneurysms and constitutes 14.4% to 43% of all diagnosed aneurysms. A large series of 561 MCAAs from Finland showed that MCAAs may be found as single (60.6%), multiple (9.8%), or associated with aneurysms in other locations (19.6%). Bilateral mirror aneurysms in the same location but on opposite sides were found in two-thirds of multiple MCAA cases. The presence of proximal MCAA increased the occurrence of associated intracranial aneurysms by 2.6-fold. The exact location of MCAA was evaluated by multiple investigators, with the most common site being the MCA bifurcation, followed by proximal M1, then distal MCA ( Fig. 2 ).

Frequency of MCA aneurysm distribution across different location.

Presentation

MCAAs may be discovered incidentally or after rupture leading to SAH. In the Finnish consecutive MCAA series, patients presented with SAH in greater than 90% of the cases. An associated hematoma of 2.5 cm or more was seen in 43% of MCAAs and was more common than in any other aneurysm location (11%), which may in part explain the higher grade of Hunt and Hess (H&H) at presentation with MCAA. Intraventricular hemorrhage (IVH) was seen in 24% of MCA bifurcation aneurysms versus 19% and 11% in the proximal and distal locations, respectively. IVH was also seen less often with aneurysms at other sites.

Aneurysmal features associated with outcome

When assessing future rupture risk and planning the best treatment approach, morphologic features of MCAAs need to be evaluated and considered. MCAAs are typically classified into 3 types: proximal, bifurcation, and distal (see Fig. 2 ). As shown in Fig. 2 , most MCAAs are located at the bifurcation (61%–88%), distal to the bifurcation (4.3%–27%), and proximally between the MCA origin and the bifurcation (7.7%–22%). Proximal aneurysms are further classified into lenticulostriate (40%) or early M1 branch (60%) MCAAs.

In addition to the classical locations identified earlier, other features of MCAAs may be examined to evaluate outcome after therapy, including side, size, multilobulation, dome to neck ratio, aneurysm size to parent vessel diameter ratio, incorporation of MCA lenticulostriate or distal branches, angulation of the branches, orientation and projection from the parent vessel, height from the origin of the ophthalmic artery, and distance between the ICA bifurcation and the aneurysm origin. We propose an angiographic classification of the bifurcation MCAAs for the purpose of predicting ET outcome and technical difficulty based on our experience of treating greater than 98% of all MCAAs presenting to our institution with ET ( Fig. 3 ). Incorporated branches and acute angulations are 2 of the main limitations of ET. Our classification system may aid in predicting the acute branch angulations that would challenge adjunctive device use and incorporated branches that may also lead to technical difficulties (see Fig. 3 ).

Proposed classification for predicting outcomes of the branching angulations typing ( upper panel ): type I has both branching arteries at angles of greater than 90°, allowing for easy access for stent or balloon use if needed. Type II has 1 challenging angulation at less than 90° in 1 branch, and type III has both branches at less than 90°. Similar proposed classification of how many branches the aneurysm neck is incorporating ( lower panel ): type 0 no vessels are incorporated, type I and II involving inferior or superior division, type III incorporating 2 branches, and type IV when a branch is arising off the aneurysm dome, making it the most complex MCAA to treat.

For clipping, the distance between the MCA and aneurysm origin was found to be linked to increased perforator injury. In a study of 91 clipping cases with an event rate of 15%, both the height (from the origin of the ophthalmic artery) and the distance (between the ICA bifurcation and the aneurysm origin) were significantly associated with perforator injuries. In another study of 151 ruptured and unruptured MCAAs that underwent clipping, independent predictors for the risk of rupture intraoperatively and for postsurgical outcomes were the presence of SAH, location on the M2 segment, maximum length, and neck size.

Therapeutic approaches

Middle Cerebral Artery Aneurysm Coiling Outcomes

There are multiple MCAA coiling series that show increased feasibility and improved procedural safety over time. However, most of these studies suffered from a lack of rigorous adjudication and randomization with an additional predilection for selection bias of treatment choice (based on institution preference and operator experience).

Feasibility and success rate

Although it has been argued that only a small proportion of the MCAAs are amenable to ET, recent large case series have shown that greater than 90% of the MCAAs are amenable to endovascular treatment. Mortimer and colleagues reported a series of 295 consecutive patients with 300 MCAAs treated with ET from 1996 to 2012. The feasibility of primary coiling was 93%, with only 8 MCAAs treated by clipping based on anatomic consideration and an additional 13 due to failed ET attempts. In a similar nonselected 55 MCAA case series, when ET was implemented as the first-choice therapy, a feasibility rate of 91.7% was achieved. Vendrell and colleagues showed the feasibility of coiling MCAA as the first-choice therapy in 160 of 174 (92%) cases. In a study of 115 MCAAs treated during the early to middle era of ET (1990–2007), the feasibility rate of treating MCAAs was 93%; however, this study did not include consecutive MCAAs. The high feasibility of ET as the first line of treatment was shown in a series of 154 aneurysms, 149 (96.5%) of which were treated successfully with coil embolization.

In our local center experience, we treated 161 consecutive MCAAs during a 6-year period (2005–2011) with ET as the first-choice therapy. Our technical feasibility was 159 of 161 (98.8%) (2 cases were blister ruptured MCAAs of <1.5 mm). However, our adjunctive device utilization rate was 43%, which may have contributed to the high feasibility rate.

Occlusion and retreatment rates

Across the literature, occlusion rates varied as did the definitions used to define occlusion grade. However, most reports defined occlusion rate as achieving complete embolization with no residual aneurysm or neck as opposed to residual dome filling. The range of recurrence rate requiring retreatment was 1.8% to 17.4%. The near-occlusion rate in the Mortimer and colleagues series of 300 MCAAs was 91.4%, and the retreatment rate was 4.3%. In the Vendrell and colleagues series of 174 MCAAs, the retreatment rate was 10.5% and the occlusion rate was 89.5%. In a prospective study of 131 MCAAs by Gory and colleagues, the obliteration rate was 84.4% at 1.5-year follow-up, with a retreatment rate of 7.4%. Recurrence was predicted by aneurysm size ( P = .02; odds ratio, 1.2; 95% confidence interval [CI], 1.02–1.4). Quadros and colleagues reported an obliteration rate of 95.3% with a retreatment rate of 1.8%. In a retrospective series of 152 consecutive MCAA patients (48% presenting with SAH), obliteration occurred in 83.3%, 79.5%, and 80.2% of aneurysms at 1, 3, and 5 years posttreatment. The recurrence rate was 20% on the follow-up angiogram with half of these aneurysms (10%) retreated. In the unruptured aneurysm series of 100 MCAAs treated by Vanzin and colleagues, the recurrence rate requiring retreatment was reported as 9.8%. In another series of 76 unruptured MCAAs with a mean follow-up of 6 months, an 87% rate of complete obliteration was reported. Recurrence requiring retreatment was seen in 3 cases (4.3%). Oishi and colleagues reported a recanalization rate of 17.1% in a mixed group of both ruptured and unruptured MCAAs. In the Jin and colleagues 103 MCAA series, from the 80 MCAAs that had follow-up (17.4%), a total of 14 recurrences were retreated with recoiling ( n = 12) and clipping ( n = 2) without complications. Predictors of recurrence were young age, rupture, and a wide aneurysm neck. In a smaller report of 38 MCAAs, complete occlusion was achieved in 33 of 38 (87%) cases. In our series of 161 nonselective MCAAs, the retreatment rate was 11%.

Morbidity and mortality and rebleeding rate

The overall rate of M&M in ruptured MCAAs treated with ET was 16.8%, with a range (between 5.5% and 29.4%) that varies across the literature and over time ( Table 1 ). Some literature reported only procedure-related complications, whereas others reported global outcome.

Table 1

Result from studies of outcomes from coiling and clipping of MCAAs

	Author, Year	N	SAH (%)	Population	Dead I	Poor I	Total M&M I	Dead R	Poor R	Total MM R	Outcome
Coil	Mortimer et al, 2014	295	80.7	Retrospective analysis of prospective data	1.9	0	1.9	13.6	7	20.6	Obliteration rate, 91.4%. Retreatment rate, 4.3%. Coiling with hematoma, good outcome in 40%. Feasibility rate of MCAAs coiling is 95.8%
	Quadros et al, 2007	55	60	Retrospective: 55/59 MCAAs	0	4.5	4.5	3	6.1	9.1	Multidisciplinary team referred 76.6% for coiling, obliteration rate, 95.3%; retreatment, 1.8%
	Doerfler et al, 2006	36	50	Retrospective: 36/38	0	5.6	5.6	5.9	23.5	29.4	Retreatment, 7.9% Feasibility, 86.8%
	Bracard et al, 2010	140	48	Retrospective, single center	0	1.5	1.5	9.6	9.6	19.2	Obliteration rate, 83.3%. Rebleed occurred in 1 patient (4.3 y follow-up)
	Gory et al, 2014	120	34.2	Prospective nonselective	2.5	4.8	7.3	4.9	2.4	6.3	Retreatment rate, 7.6%
	Jin et al, 2013	100	58	Retrospective: 100/103	0	0	0	12.1	15.5	27.6	Feasibility rate, 99%. Retreatment: 12 with recoiling and 2 with clipping
	Vendrell et al, 2009	153	59.2	Retrospective: single center 153/174	0	9.8	9.8	2.2	3.3	5.5	Feasibility 160/174 (92%). Recurrence in 10.5%
	Suzuki et al, 2009	115	42	Retrospective, single center	0	1.5	1.5	4.2	16.7	20.9	Rebleeding in 1/115 cases in partially treated aneurysm 0.09%. Recanalization, 10.5%.
	Oishi et al, 2009	112	53.6	Retrospective, single center	0	2.2	2.2	5	17.4	22.4	Long-term recanalization in 17.1% Feasibility, 91.1%
	Kim et al, 2011	70	0	Retrospective, single center	1.4	0	1.4	NA	NA	NA	Retreatment, 4.3%. Rebleeding rate, 0% over 25 mo follow-up
	Iijima et al, 2005	142	46.8	Retrospective, single center	1	3	4	6	1	7	Retreatment, 10% Coiling was feasible in 96.5%
CLIP	Rodríguez-Hernández et al, 2013	543	51.9	Retrospective	1.5	6.5	8	9.6	20.2	29.8	Improved or unchanged postsurgery was 86.9% in ruptured and 92.7% unruptured MCAAs
	Rinne et al, 1996	561	100	Finnish study of consecutive cases	NA	NA	NA	13	25.6	32.6	Overall poor outcome was 32.6% vs 25% in the rest of anterior circulation
	Suzuki et al, 1984	413	100	Retrospective, single center	NA	NA	NA	4.2	18.5	22.6	Early cases poor outcome, 20% vs 5.6% in the later cases
	Morgan et al, 2010	263	0	Retrospective, single center, incidental: 263/339 MCAAs	0.4	4.6	5	NA	NA	NA	<60 y of age and ≤12 mm size had M&M of 0.6% (95% CI, 0%–3.8%) vs 22.2% (95% CI, 8.5%–45.8%) for those ≥60 y of age and >12 mm in size
	Moroi et al, 2005	201	0	Retrospective, single center, unruptured	0	2.4	2.5	NA	NA	NA	Total permanent and temporary deficit was 5/201 (2.5%) with no mortality
	Choi et al, 2012	125	0	Retrospective, single center	0	2.4	2.4	NA	NA	NA	Occlusion rate, 95.8%. Events: ICH, meningitis, wound infection
	van Dijk et al, 2011	105	73.3	Retrospective, single center	NA	NA	NA	NA	NA	20	Rebleeding 1/77 of cases (1.3%) Occlusion rate, 89%
	Yeon et al, 2011	91	0	Retrospective, single center	0	4.4	4.4	NA	NA	NA	Imaging infarction in 14/91 (15%). Permanent in 4.4%
	Zhu et al, 2013	58	46.6	Retrospective, single center	NA	NA	NA	NA	NA	NA	Poor or death 11.9% (6.8% death). Retreatment, 1.7%; residual neck, 19%
	Son et al, 2007	24	0	Retrospective, single center	0	0	0	NA	NA	NA
	Park et al, 2008	23	82.6	Retrospective	0	25	25	30.5	5.3	36.8	Permanent deficit 3/23 (13.04%) MCAA at frontal branch in 78.3%, and lenticulostriate in 13%

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