CHAPTER 21 Of the three main cerebral arteries, the MCA supplies the largest and most eloquent territory in the cerebral hemispheres. Communicating arteries on the ACA and PCA contribute collateral blood flow from left to right via the ACoA and from anterior to posterior via the PCoA, thereby protecting the ACA and PCA territories from ischemic complications after pathological or iatrogenic occlusions of these arteries and decreasing the need for an ACA and PCA bypass during complex aneurysm treatment. In contrast, the MCA lacks a communicating artery and is more vulnerable to ischemic complications after therapeutic occlusions with unclippable aneurysms. The MCA territory requires an aggressive reconstructive posture that restores blood flow robustly. Fortunately, this territory has the most favorable bypass anatomy, with a readily available donor artery in the STA and accessible recipients on the lateral convexity and in the Sylvian fissure. The MCA aneurysm has long been considered a “surgical aneurysm.” In fact, the MCA aneurysm may be the best example of an aneurysm whose microsurgical results remain superior to the endovascular results. Endovascular coiling has higher risks of recurrence, retreatment, and rehemorrhage; flow diverters may cover and occlude lenticulostriates or other arterial trunks; and newer bifurcation stents and intra-aneurysmal devices have unproven results. In contrast, the MCA aneurysm is easily accessible through standard or miniaturized craniotomies, can be safely manipulated after splitting the Sylvian fissure, and often has a broad neck that can be reconstructed with various clipping techniques, such as intersecting or overlapping clips, tandem clipping, fenestration tubes, or a stack of clips in a “picket fence.” However, many features may prevent conventional clipping, such as intraluminal thrombus, mycotic or infectious etiology, atherosclerotic thickening or neck calcification, large (≥ 12 mm in diameter) or giant (≥ 25 mm in diameter) size, fusiform or dolichoectatic morphology, a serpentine channel, lenticulostriate involvement, or aberrant branch arteries that originate from the sidewall or at obtuse angles from the base. A major advantage of microsurgery for MCA aneurysms is the ability to employ bypass techniques when conventional clipping fails and a parent artery is deliberately occluded. In a review of a part of my experience with MCA aneurysms published previously, bypasses were performed in 4% of patients (21 of 543) and provided a trapping or proximal occlusion option for some of the more difficult aneurysms. Now, over 100 bypasses have been performed for MCA aneurysms, and the gamut of seven bypass techniques has been applied. MCA aneurysms can be classified into three groups based on the aneurysm’s relationship to the MCA bifurcation (or trifurcation, or quadrifurcation): (1) pre-bifurcation, (2) bifurcation, and (3) post-bifurcation (Fig. 21.1). The algorithm for bypass selection is based on this classification (Fig. 21.2). The strategy for pre-bifurcation MCA aneurysms is dictated by the presence or absence of lenticulostriate arteries along the aneurysmal segment that supply the basal ganglia (Fig. 21.2). Fortunately, the majority (75%) of complex pre-bifurcation aneurysms displace these perforators proximally or distally as they enlarge or morph into dolichoectatic lesions. An absence of lenticulostriates along the aneurysmal segment enables aneurysm trapping, which then creates an opportunity to excise the aneurysm and reconstruct the M1 segment with primary reanastomosis when the pathological segment is short. Reanastomosis with a single end-to-end anastomosis is the option of first choice because it is quick, but it requires tortuosity or redundancy of the parent artery to bridge the gap between the transected ends of afferent and efferent arteries and bring them together (Figs. 21.3 and 21.4). Careful anatomic assessment and a high likelihood of success are needed before committing to reanastomosis because failure requires either an IC-IC interpositional bypass (second choice, M1 MCA-RAG-M1 MCA) or an EC-IC interpositional bypass (third choice, ECA-RAG-M2 MCA), which are both executed best before aneurysm trapping. Primary reanastomosis fails when a long segment of aneurysm is excised, pathology remains at the transected ends, or the ends are approximated under tension with suture pullout or breakage. Complete aneurysm excision and the problem of an unbridgeable arterial gap are interrelated: thorough excision increases the gap and failure rate of reanastomosis, whereas incomplete aneurysm excision decreases the gap but may incorporate occlusive pathology into the reanastomosis. Grafts for IC-IC interpositional bypass should be harvested in advance and ready to suture. Two end-to-end anastomoses are usually performed and the RAG’s cross-sectional caliber is well matched to the M1 MCA for these anastomoses. Their short suture lines are quicker to perform than end-to-side anastomoses, but the MCA territory cannot be reperfused until both end-to-end anastomoses are completed. If reperfusion is needed between anastomoses, an end-to-side anastomosis can be performed first, followed by reperfusion and an end-to-end anastomosis. If a displaced lenticulostriate trunk or other branch originates at one of the transected ends, an end-to-end anastomosis may not be possible and an end-to-side anastomosis should be performed instead. Unlike IC-IC interpositional bypasses that are typically performed after aneurysm trapping and excision first, EC-IC interpositional bypasses are performed first before trapping and aneurysm excision. The EC-IC interpositional bypass (ECA-RAG-M2 MCA) is the bypass of third choice for pre-bifurcation MCA aneurysms without lenticulostriate artery involvement and the bypass of first choice for pre-bifurcation aneurysms with lenticulostriate involvement. The latter aneurysms cannot be trapped, and proximal occlusion is performed instead. The bypass supplies retrograde flow through the aneurysm to the lenticulostriates, as well as anterograde flow to the recipient trunk and retrograde flow to other trunks. A competent MCA bifurcation allows the single high-flow bypass to retrograde fill the un-bypassed efferent trunks. The aneurysm is proximally occluded but still fills from the bypass, which is only safe with unruptured aneurysms and is avoided with ruptured aneurysms. A ruptured, pre-bifurcation aneurysm with lenticulostriate arteries on the aneurysmal segment forces a difficult choice between complete aneurysm exclusion at the expense of the lenticulostriates, and incomplete aneurysm exclusion with preserved lenticulostriates and the risk of rehemorrhage. Exceptions to this algorithm occur when a major trunk is already occluded with thrombus, atherosclerosis, or septic embolus, and a preexisting ischemic injury. The reduced revascularization requirements of a single trunk are met with a single STA-MCA bypass (fourth choice). Management of bifurcation MCA aneurysms is determined by rupture status (Fig. 21.2). Bifurcation aneurysms in patients presenting with an SAH must be completely excluded, and when conventional clipping fails, the aneurysm is trapped and multiple trunks are revascularized. Combination bypasses are the option of first choice (Fig. 21.5). They employ two or more bypasses to rebuild the bifurcation, and examples include the double reimplantation bypass, IC-IC plus EC-IC bypasses, two IC-IC bypasses, and two EC-IC bypasses. The double reimplantation technique delivers high flow through an interpositional bypass, and using the A1 ACA as an intracranial donor shortens the graft length and keeps it entirely intracranial. The A1 ACA-RAG-M2 MCA+M2 MCA bypass is constructed with three end-to-side anastomoses (two at each end of the graft and one reimplantation mid-graft) or with two end-to-side anastomoses at each of the graft’s ends and one side-to-side anastomosis mid-graft. Successive reimplantation can be extended to reconstruct trifurcations with triple reimplantations (A1 ACA-RAG-M2 MCA+M2 MCA+M2 MCA) or quadrifurcations with quadruple reimplantations. Other combination bypasses can be completed with just two anastomoses, as with two IC-IC reconstructive techniques (e.g., M1 MCA-M2 MCA reanastomosis and M2 MCA-M2 MCA reimplantation) or with one IC-IC bypass and an EC-IC bypass (e.g., ATA-M2 MCA and STA-MCA bypasses). The double-barrel STA-MCA bypass is a convenient double bypass, but the size of the STA trunk and branches may not be adequate to replace M1 MCA flow. Any combination bypass with an interposition graft, whether EC-IC or IC-IC, requires three anastomoses.
MCA Bypass Strategy
MCA Bypass Strategy
MCA Aneurysms
Pre-Bifurcation MCA Aneurysms
Bifurcation MCA Aneurysms
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