Combination Bypasses

The six bypasses described previously join a donor and a recipient with one anastomosis, or two anastomoses when an interpositional graft is used, to revascularize a single recipient artery. These bypasses may ultimately supply more than this single recipient, depending on the distribution of bypass flow throughout connected territories or the way that the aneurysm is occluded, but are still just single, simple bypasses. The seventh bypass is a combination of two or more of the six bypasses. Unlike single bypasses, combination bypasses join a donor or donors to two or more recipients, require at least two anastomoses, and can reconstruct bifurcations associated with complex aneurysms. These ultimate bypasses may incorporate multiple EC-IC bypasses, multiple IC-IC bypasses, or one of each (Table 20.1). Combination bypasses can be categorized as (1) double reimplantation bypass with an extracranial donor, (2) double reimplantation bypass with an intracranial donor, (3) double IC-IC bypasses, (4) one IC-IC and one EC-IC bypass, (5) double EC-IC bypasses with scalp arteries, (6) double EC-IC interpositional bypasses (bilateral carotid replacement), and (7) thromboendarterectomy and bypass.

Double Reimplantation Bypass with Extracranial Donor

Complex and giant aneurysms at bifurcations with branch arteries separated by dolichoectatic morphology cannot be clipped. Intraluminal thrombus, coils, atherosclerotic calcification, or acute angles between parent and branch arteries may also prevent clipping (Fig. 20.1). Complete aneurysm exclusion requires revascularization of both efferent trunks, and a single scalp artery like the STA may not be enough to supply the entire MCA territory, even with a double-barrel bypass. Interpositional bypasses have higher flow but only revascularize one efferent artery with a single distal anastomosis. The double reimplantation bypass is a variation of the standard interpositional bypass technique for these aneurysms, with reimplantations of two efferent branches onto a graft connected proximally to a donor artery (three anastomoses overall), effectively rebuilding an arterial bifurcation and enabling complete aneurysm trapping. This technique is indicated when critical efferent arteries require revascularization, conventional scalp donors are diminutive, a ruptured aneurysm must be excluded completely, or symptomatic mass effect from intraluminal thrombus requires debulking.

Table 20.1 Summary of Clinical Experience with Combination Bypasses

Bypass Type	N	%
Double reimplantation, EC-IC
ECA-SVG-M2 MCA+M2 MCA	1	2
Double reimplantation, IC-IC
A1 ACA-RAG-M2 MCA+M2 MCA	3	6
AIFA-RAG-CmaA+PcaA	1	2
R PcaA-RAG-L PcaA+L CmaA (azygos bypass)	1	2
Double IC-IC bypass
Reanastomosis + reimplantation	4	8
In-situ bypass + reanastomosis	1	2
IC-IC and EC-IC bypass
Reanastomosis + STA-MCA	2	4
Reimplantation + STA-MCA	1	2
In-situ bypass + STA-MCA	1	2
IC-IC interpositional bypass + STA-MCA	2	4
Reimplantation + EC-IC interpositional bypass	1	2
Reanastomosis + OA-AICA	1	2
Double EC-IC bypass (low flow)
Double-barrel STA-MCA	21	40
STA-MCA+OA-MCA	1	2
STA-STA reanastomosis and STA-MCA	3	6
STA-STV anastomosis and STA-MCA	1	2
Double EC-IC bypass (high flow)
Bilateral carotid replacement	3	6
Thromboendarterectomy and bypass
Miscellaneous	5	9
Total	53	100

Key features of the double reimplantation technique include completing the proximal anastomosis first, successively reimplanting efferent trunks distally, and minimizing ischemia by reperfusing reimplanted arteries while other anastomoses are performed (Fig. 20.2). The EC-IC version of the technique uses the cervical carotid artery as the donor (ECA-SVG-M2 MCA+M2 MCA bypass). The proximal end of the graft is anastomosed to the ECA first, which differs from the standard sequence for EC-IC interpositional bypasses. Performing the proximal anastomosis first may limit graft mobility during the intracranial anastomoses, but enables immediate reperfusion of the first reimplanted trunk. After completing the extracranial anastomosis, the “live” graft is tunneled up to the cranial field. The more proximal or deeper trunk is permanently clip occluded at its origin from the aneurysm, temporarily clip occluded distally, transected, and transposed to the graft. This first efferent artery is then reimplanted to the graft with an end-to-side anastomosis. The temporary clip on the efferent trunk is removed, and the temporary clip on the proximal graft is repositioned distal to this anastomosis to restore anterograde flow in the first reimplanted trunk.

Fig. 20.1 (A) Surgical exposure of a giant, thrombotic MCA aneurysm through a standard left pterional craniotomy and splitting of the Sylvian fissure. (B) The relationship between the afferent M1 MCA and the efferent trunks originating from the base of the aneurysm at acute angles makes this aneurysm difficult to clip. With thrombotic aneurysms, the aneurysm lumen (dashed line) is a portion of the aneurysm volume, and the remainder of the aneurysm is filled with organized thrombus.

The distal end of the graft is then moved to the second efferent trunk, which is permanently clip occluded at its origin from the aneurysm, temporarily clip occluded distally, and then connected to the graft with an end-to-side anastomosis. Alternatively, the graft and second efferent trunk can be anastomosed in an end-to-end fashion after first transecting the trunk from the aneurysm, or the second efferent can be transposed to the graft and reimplanted like the first efferent. Whereas the first reimplantation is artery to graft, the second reimplantation is more often graft to artery, leaving the recipient artery in situ. The temporary clips on the recipient artery and the distal graft are removed to restore anterograde flow in the second efferent trunk, with both trunks now reimplanted onto the bypass graft and each one exposed to the ischemia time of only a single anastomosis. This double reimplantation technique is demonstrated in the case of a giant MCA aneurysm with thickened walls, heavy calcification, and intraluminal thrombus, and without a sizable STA for EC-IC bypass (Case 20.1).

The double reimplantation technique’s elegance is that it limits ischemia to the time normally required for a single anastomosis. An artery reimplanted with an end-to-side anastomosis cannot be reperfused until the host graft is arterialized. If the distal intracranial anastomoses were performed first in the usual order, reimplanted arteries would be left unperfused while the cervical anastomosis is performed, and ischemia times might double in reimplanted arteries. By performing the cervical anastomosis first instead, the graft reperfuses each trunk as soon as it is reimplanted with the minimum ischemia time for each of the two trunks. Minimizing ischemia also depends on successive reimplantation, or placing the second intracranial anastomosis distally on the graft and allowing the first intracranial anastomosis to feed off of the graft while the second one is being performed. Placement of a temporary clip distal to the first and proximal to the second anastomosis redirects blood flow to the reimplanted trunk while keeping the other anastomotic site dry. The double reimplantation technique can be adapted to trifurcated or quadrifurcated anatomy with triple or quadruple reimplantations, applying the same principles of proximal extracranial anastomosis first, successive reimplantation of trunks onto a live graft, and immediate reperfusion of recipient trunks while distal trunks are reimplanted.

Fig. 20.2 The EC-IC double reimplantation technique. (A) After first connecting the SVG to the cervical ECA with an end-to-end anastomosis, the superior trunk is isolated between aneurysm clips, transected, and reimplanted on the graft with an end-to-side anastomosis. (B) The temporary clip on the bypass graft is repositioned beyond this anastomosis to reperfuse the reimplanted trunk. (C) After isolating the inferior trunk between aneurysm clips and trimming the graft, it is implanted on the trunk with an end-to-side anastomosis. (D) The inferior trunk is occluded with a permanent aneurysm clip at its origin from the aneurysm, and blood flow is initiated in the graft to reperfuse this trunk. Flow into the aneurysm is occluded with a permanent clip on the parent artery. (E) The reconstructed MCA bifurcation has a total of three anastomoses, two reimplanted trunks of the MCA bifurcation, and one excluded giant aneurysm.

Case 20.1 This 74-year-old woman experienced a sudden, severe headache. Imaging demonstrated subarachnoid and intraventricular hemorrhage from a giant, thrombotic, atherosclerotic left MCA aneurysm [(A) axial CT scan]. The aneurysm measured 5 cm in diameter, and its dolichoectatic morphology and calcified walls made it unclippable [(B) coronal CT angiogram, and (C) left ICA angiogram, anterior oblique view]. The patient’s STA was too small for a double-barrel STA-MCA bypass. The MCA bifurcation was reconstructed with the double reimplantation technique [ECA-SVG-M2 MCA (InfTr)+M2 MCA (SupTr) bypass], and the aneurysm was subsequently trapped. (D) The ECA was exposed in the neck, transected, and (E) the proximal stump was connected to the SVG with an end-to-end anastomosis. The graft was tunneled up to the cranial field and brought into the Sylvian fissure, (F) where the inferior and superior trunks of the MCA were seen on the temporal and frontal sides of the aneurysm, respectively. (G) The inferior trunk was clip occluded at its origin from the aneurysm and transected. The trunk was reimplanted on the SVG with an end-to-side anastomosis. (H) The back suture line was inspected intraluminally, and (I) the front wall was completed. Temporary clips on the graft were repositioned to reperfuse the inferior trunk and close the graft distal to the reimplantation. (J) The recipient site on the superior trunk was selected, the SVG was trimmed, and the graft was implanted on this trunk with another end-to-side anastomosis (intraluminal inspection of the first suture line). The ECA-SVG-M2 MCA (InfTr)+M2 MCA (SupTr) bypass was opened to perfuse the superior trunk, and a permanent clip was placed on this trunk at its origin from the aneurysm. (K) The afferent M1 MCA was then clip occluded proximal to the aneurysm to complete the trapping. (L,M) Two anastomoses connecting the graft with the inferior and superior trunks reconstructed the MCA bifurcation and revascularized the MCA territory, as seen angiographically postoperatively [left ICA angiogram, (N) anteroposterior and (O) anterior oblique views]. The patient had an uncomplicated course, and was living independently at home at last follow-up 6 months after her surgery.

Double Reimplantation with Intracranial Donor

The EC-IC version of double reimplantation can be modified to an IC-IC version by using an intracranial donor artery rather than the cervical carotid artery. Intracranial donor sites have included (1) the A1 ACA for MCA double reimplantation, (2) the proximal M2 MCA for distal M2 MCA double reimplantation, (3) the contralateral ACA for ipsilateral PcaA and CmaA double reimplantation, and (4) the proximal ACA for distal PcaA and CmaA double reimplantation. PICA aneurysms do not have bifurcated anatomy and do not require this reconstruction, but double reimplantation could revascularize the PICA and the V4 VA with a V3 VA-RAG-p1 PICA+V4 VA bypass. Double reimplantation has not been used with basilar aneurysms, but the PCA and SCA could be revascularized ipsilaterally with double reimplantation using the M2 MCA as the donor (M2 MCA-RAG-s2 SCA+P2 PCA bypass).

The elements of IC-IC double reimplantation are the same as for the EC-IC version, with a proximal end-to-side anastomosis of the graft to the donor artery first, thereby arterializing the graft while preserving flow in the donor artery; transecting and reimplanting the first efferent trunk onto the graft with another end-to-side anastomosis; reperfusing the reimplanted artery immediately; trimming the graft and reimplanting it onto the second efferent trunk; and finally trapping the aneurysm. The A1 ACA is an excellent donor artery for MCA double reimplantation (A1 ACA-RAG-M2 MCA+M2 MCA bypass) because it is accessible in the Sylvian triangle, has robust flow from the ICA terminus, and tolerates temporary clipping during the anastomosis in the presence of a symmetrical A1 ACA on the opposite side and a competent ACoA that can collateralize the ipsilateral ACA territory. Anastomosis to the A1 ACA is deep and difficult, and therefore should be performed first to capitalize on graft mobility. Classic double reimplantation technique transects the first efferent trunk and reimplants it with an end-to-side anastomosis to maximize the caliber of that trunk (Case 20.2). However, the bypass can also be constructed with a side-to-side anastomosis between the graft and the first efferent trunk (Case 20.3). Lenticulostriate arteries arising from the trunk at its origin from the aneurysm may limit the mobility of the trunk, and a more distal side-to-side anastomosis protects this perforator. The side-to-side anastomosis keeps the efferent trunk in its natural position, rather than transecting it and mobilizing it to the graft. A transected trunk with awkward curvature or little mobility complicates reimplantation, whereas the graft is freely mobile and will reach the optimal side-to-side anastomotic site on the efferent trunk. Side-to-side anastomoses are more distally located on the trunk, and may even be beyond a secondary bifurcation, making the recipient artery smaller in caliber than with a direct reimplantation. However, flow through a well-crafted communicating anastomosis with a long arteriotomy will find its way back to proximal branches on the recipient trunk (Case 20.4).

The double reimplantation technique reconstructs the bifurcation of the ACA into the PcaA and the CmaA for distal ACA aneurysms, using either the proximal A2 or A3 segment or one of the internal frontal arteries as the donor site (AIFA-RAG-CmaA+PcaA bypass; Case 20.5). Double reimplantation re-creates the anatomy of an azygos A2 ACA by connecting the CmaA and PcaA on the side of a sacrificed A2 efferent artery from an ACoA aneurysm with the patent A2 ACA on the opposite side (R PcaA-RAG-L PcaA+L CmaA bypass; Case 20.6). This “azygos bypass” establishes one A2 ACA as the sole supplier of distal ACA territories on both sides using a short RAG in the falco-frontal triangle.

The side-to-side anastomosis between the graft and the efferent trunk allows for immediate reperfusion of the trunk from the parent artery rather than from the graft, which means that the graft does not need to be arterialized first. This side-to-side anastomosis to one trunk, together with an end-to-side anastomosis to the other trunk, enables immediate reperfusion of each trunk and completion of the donor anastomosis last. This reversal of the established sequence is useful when the easier donor anastomosis is the most superficial and the harder recipient anastomoses are deep, as with an M2 MCA-RAG-s2 SCA+P2 PCA double reimplantation. With this construction, each recipient artery would be connected to the graft and reperfused immediately with only a single ischemic interval, and yet the donor anastomosis would be performed last.