Innovations in Bypass Surgery
Authors: Fabio Frisoli, MD and
Michael T. Lawton, MD
An issue examining innovations driving the future of neurosurgery would not be complete without including bypass surgery. Unlike many of the deconstructive operations that we perform (like aneurysm clipping, AVM resection, or tumor removal), bypass surgery is constructive, creating connections between donor and recipient arteries not previously in existence to revascularize sacrificed arteries or augment deficient blood flow. Bypasses can be built with conventional extracranial-intracranial (EC-IC) constructs like a superficial temporal artery-to-middle cerebral artery (STA-MCA) low-flow bypass or an external carotid artery-to-middle cerebral artery high-flow bypass. However, bypasses can also be built with more innovative constructs like the intracranial-intracranial (IC-IC) reconstructions that include reimplantation, reanastomosis, in situ bypass, or interpositional bypasses. These six different types of bypasses, plus the combination bypasses for lesions that require revascularization of more than one efferent artery, give us a wide variety of options and opportunities to innovate (TABLE 1).
Shifting Toward Intracranial Reconstruction
Conventional EC-IC bypasses are common because of their simplicity, familiarity, and durability. Woringer and Kunlin performed the first common carotid-internal carotid bypass in 1963,1 and early bypasses were used to treat occlusive disease with extracranial donors such as the STA or common carotid artery with saphenous vein or synthetic interposition grafts.2,3 The STA-MCA bypass was refined by Yasargil and Donaghy in the 1960s,4,5 and indications expanded in the 1970s to include moyamoya disease, vertebrobasilar insufficiency, and dolichoectatic aneurysms.6-9 These 1st generation (low-flow EC-IC bypasses with scalp arteries as donors) and 2nd generation (high-flow EC-IC bypasses with interposition grafts) techniques continue to feature prominently in the armamentarium of the bypass surgeon four decades later. However, the paradigm is shifting towards IC-IC bypass (3rd generation bypasses) because of advantages that include: no extracranial donor harvest; similarity of recipient and donor vessel caliber; protection within the skull; and a single incision/approach.10 Numerous studies have demonstrated the efficacy and versatility of the IC-IC bypass for aneurysm surgery.11-13
An in situ bypass uses a communicating side-to-side anastomosis between donor and recipient arteries that run in parallel, without a dedicated donor limb and with bypass flow regulated by intrinsic demand. Examples of parallel arteries joined with in situ bypass include the insular M2 segment of the MCA and the anterior temporal artery (ATA) within the sylvian fissure (FIGURE 1), the bilateral anterior cerebral arteries (ACA) within the interhemispheric fissure, the posterior cerebral artery (PCA) and superior cerebellar artery (SCA) within the ambient cistern, bilateral posterior inferior cerebellar arteries (PICA) beneath the cerebellar tonsils, and finally the PICA and anterior inferior cerebellar artery (AICA) within the cerebellopontine angle. When a branch artery is compromised during aneurysm trapping, reimplantation of the efferent artery onto the parent artery can be performed with an end-to-side anastomosis, most often with branches of the MCA, ACA, and PICA. Reanastomosis is an end-to-end reconstruction of the parent artery after excising intervening pathology, with feasibility predicated upon redundancy or slack in the ends after excision and the presence of just one outflow artery. Interposition grafts can be used to join remote intracranial arteries, but unlike EC-IC interpositional bypasses that require long grafts and tunneling, intracranial grafts are shorter and lend themselves to combination bypasses that reconstruct bifurcations. An example would be an A1 ACA-RAG-M2 MCA+M2 MCA double reimplantation bypass for a trapped MCA bifurcation aneurysm. The breadth and diversity of intracranial bypasses allow the neurosurgeon to be creative when improvising or dealing with unexpected intraoperative circumstances.
Figure 1: 3rd generation intracranial-intracranial (IC-IC) bypass techniques for MCA aneurysms. A) conventional MCA anatomy; B) in situ bypass (ATA-M2 MCA); C) M1 MCA reanastomosis (end-to-end); D) M2 MCA reimplantation (end-to-side); and E) intracranial interposition bypass (A1 ACA-RAG-M2MCA+M2 MCA)
4th Generation Bypasses
While 3rd generation bypasses mentioned above use conventional anastomoses and conventional suturing techniques, a fourth generation of bypasses emerges when these anastomoses and techniques are mixed and shuffled (FIGURE 2). A conventional construct can be created using an unconventional suturing technique (Type 4A), such as a p3 PICA-p3 PICA reanastomosis performed end-to-end with intraluminal suturing for the back wall rather than extraluminal suturing throughout. An unconventional construct can be created with conventional suturing technique (Type 4B), such as an A1 ACA-RAG-M2 MCA+M2 MCA double reimplantation bypass with the graft supplying one of the M2 trunks and the M2 trunks transected from the aneurysm and reimplanted to one another end-to-end (FIGURE 3).
Figure 2: 4th generation bypasses arise from variations in structure or technique of 3rd generation constructs (second row). Type 4A bypasses contain onventional structure but utilize unconventional technique (i.e., intraluminal suturing; third row). Type 4B bypasses couple unconventional structure with conventional or unconventional technique (fourth and fifth rows). MCA+M2 MCA))
This last bypass is an example of a “middle communicating artery” (MCoA). It repurposes the arterial limbs that would have otherwise been “dead-ends,” freeing them from the aneurysm and rejoining them to communicate flow from the interposition graft to both M2 trunks originating from the trapped MCA bifurcation aneurysm. Revascularization of two efferent branches usually requires double-barrel STA-MCA bypass or a high-flow EC-IC interposition graft with double reimplantation of the M2 divisions. Both techniques have deficiencies that can be improved upon. For example, with double-barrel STA-MCA bypass, the M2 trunks are isolated and diminutive STA may be inadequate to meet the demands of half the MCA territory. However, fourth generation techniques like end-to-end M2-M2 reimplantation, can create a communicating bypass between the efferent branches that permits bidirectional flow to either trunk as dictated by demand and pressure gradients. In effect, this recapitulates the function of the anterior (ACoA) and posterior (PCoA) communicating arteries. The construction of communicating arteries gives a bypass the capacity to respond immediately to demands created by pressure gradients or arterial occlusions, without compromising native blood flow in parent arteries. Just as the combination of an ACoA and two PCoAs to form the circle of Willis prevents countless strokes, the creation of a MCoA can have protective benefits in the MCA territory. The MCoA bypass is an example of innovative applications of bypass techniques that advance our surgical solutions.
Figure 3: Case example of a dolichoectatic MCA bifurcation aneurysm treated with bypass (A1 ACA-RAG-M2 MCA+M2 MCA, with a middle communicating artery or MCoA) and distal clip occlusion. A) 3-dimensional reconstruction of internal carotid artery (ICA) angiogram demonstrating the dolichoectatic MCA bifurcation aneurysm involving the origin of both M2 divisions. B) Intraoperative photograph demonstrating inflow and outflow of the aneurysm. C) Intraluminal view of the suture line between the radial artery graft (RAG) and the recipient inferior M2 division. D) End-to-end reimplantation of superior division to the inferior division of the MCA, creating the MCoA. E) Final construct demonstrating the 3 anastomoses (A1-RAG, RAG-M2 MCA, and M2 MCA-M2 MCA)and distal clip occlusion of the aneurysm. F) Postoperative ICA angiogram demonstrating patency of the bypass and distal branches, as well as thrombotic occlusion of the aneurysm. with conventional or unconventional technique (fourth and fifth rows). MCA+M2 MCA)
In this modern era where innovation is synonymous with technological advancement, it is remarkable what can and has yet to be created with simple suture and meticulous technique. The application of IC-IC reconstructive techniques and 4th generation bypasses allows the bypass surgeon to innovate without high technology. The list of new bypasses conceived but not yet performed is long, and it challenges us to not only maintain competencies in this refined craft, but to push our dexterity further. Even though bypass techniques are decades old, their application in novel ways makes the future of open vascular surgery dynamic and exciting.
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