CHAPTER 19 Sometimes the tension on two transected arterial ends may be too great to reconstruct with reanastomosis, and other times the donor for a needy recipient is lacking or just out of reach for reimplantation or in-situ bypass. The IC-IC interpositional bypass is a technique of next resort when these simpler reconstructions are too difficult or impossible. The IC-IC interpositional bypass connects two arteries with a graft and two anastomoses on its ends. More complex interpositional bypasses with more than two anastomoses, such as double or triple reimplantations, are combination bypasses, which are discussed in Chapter 20. The IC-IC interpositional bypass is similar to the EC-IC interpositional bypass, except that the donor artery is within the same intracranial field rather than in a separate cervical field. Therefore, IC-IC interpositional bypasses are shorter in length, smaller in caliber, perfect for RAGs, protected intracranially from external trauma, spare patients a second surgical exposure, and do not need tunneling. IC-IC “jump” bypasses have disadvantages relative to reanastomosis, reimplantation, and in-situ bypass, namely the additional anastomosis, two cross-clamping times, more cerebral ischemia, graft harvest, and perhaps decreased long-term patency rates. However, jump bypasses are elegant reconstructions with almost endless possibilities and frequent applications, tying with reanastomosis as the most common IC-IC technique. The easy accessibility of the MCA through the pterional-transsylvian approach and its versatile anatomy make it the most capable donor (nearly three quarters of these cases), but many alternative donor sites are available for complex pathology or individualized bypasses when previous surgery, radiation, or intraoperative injury prevent the use of scalp arteries, cervical carotid arteries, or other classic donors. Atypical donors such as the petrous ICA, internal maxillary artery, and V3 VA have large sizes, tolerate temporary occlusion, and are within the surgical field. These unfamiliar arteries are attractive donors for IC-IC interpositional bypasses, and familiarity with their complex anatomy will increase their use and diversify the bypass armamentarium. The common sites for interpositional bypasses, such as the M2 MCA, ATA, and P2 PCA, have been covered in previous chapters, but the petrous ICA, IMA, and V3 VA are within or adjacent to cranial fields and do not require remote cervical incisions. It could be argued that these three sites are extracranial and that bypasses using these donors should be considered EC-IC interpositional bypasses, but these IC-IC interpositional bypasses are differentiated from EC-IC interpositional bypasses based on their consolidation within one cranial field. Although the graft in an EC-IC interpositional bypass tunnels to connect separate and distinct cervical and cranial sites, the graft in an IC-IC interpositional bypass is contained entirely within one intracranial site. The cervical internal carotid artery (C1 segment) enters the carotid canal at the skull base. The petrous segment (C2) in the carotid canal of the petrous bone first courses vertically, bends medially (posterior loop), and then runs horizontally in an anteromedial direction toward the petrous apex, just beneath the middle fossa floor. The petrous ICA exits the carotid canal at the foramen lacerum, where a short lacerum segment (C3) runs over, not through, this foramen before entering the cavernous sinus (C4 segment). A venous plexus surrounds the petrous ICA with connections to the pterygoid and cavernous venous plexuses. The caroticotympanic artery, a remnant of the second aortic arch, arises from the posterior loop and runs to the tympanic cavity. The vidian artery, seen only in one third of patients, arises from the inferior aspect of the petrous ICA and passes through the foramen lacerum en route to the oropharynx. The petrous portion of the temporal bone houses the labyrinth (vestibule, cochlea, and semicircular canals), the IAC with the acoustic-facial bundle, the petrous ICA, and the eustachian tube. Important bone landmarks on the superior surface of petrous bone include the arcuate eminence, which is formed by the protuberance of the underlying superior semicircular canal; the major petrosal groove, which transmits the GSPN from its origin at the geniculate ganglion of the facial nerve to its termination in the sphenopalatine ganglion; and the trigeminal depression, which is a shallow groove near the petrous apex for the preganglionic trigeminal nerve (Fig. 19.1). The foramina rotundum, ovale, and spinosum belong to the sphenoid bone, lateral to the petrous part of the temporal bone. The GSPN runs anteromedially above the petrous ICA and passes under the lateral border of the mandibular division of the trigeminal nerve. The GSPN forms the lateral border of Kawase’s quadrangle, or the posteromedial triangle, and the medial border of Glasscock’s triangle, or the posterolateral triangle. The other borders of Kawase’s quadrangle are the trigeminal nerve anteriorly, the petrous ridge and superior petrosal sinus medially, and the IAC and cochlea posteriorly. The petrous ICA crosses the anterior portion of Kawase’s triangle, running immediately inferior or inferomedial to the GSPN. The eustachian tube is lateral to the carotid canal and the major petrosal groove. The thin tensor tympani muscle runs superomedial to the eustachian tube and just lateral to the GSPN. Glasscock’s triangle lies lateral to Kawase’s triangle, and is bordered by the GSPN medially, the mandibular nerve anteriorly, and the line between the foramen spinosum and the arcuate eminence laterally. The horizontal segment of the petrous ICA courses through Glass cock’s triangle and is the anastomotic site for petrous ICA bypasses. The IMA is the larger branch of the terminal bifurcation of the ECA (the STA is the smaller branch) and runs in the extracranial space beneath the middle cranial fossa floor. The IMA originates from the ECA behind the neck of the mandible and courses horizontally in the infratemporal fossa (deep to the zygomatic arch and posterior to the maxilla), from posterior to anterior and from lateral to medial, alongside the lateral pterygoid muscle (Fig. 19.2). It reaches the pterygopalatine fossa, which is a space between the posterior wall of the maxillary sinus anteriorly and the pterygoid plates of the sphenoid bone posteriorly. The IMA’s course is divided into mandibular, pterygoid, and pterygopalatine segments. The pterygoid segment of the artery is usually the bypass donor site and can run medial or lateral to the lateral pterygoid muscle. The IMA has numerous branches, which can be mistaken for the IMA itself: deep auricular, anterior tympanic, inferior alveolar, middle meningeal, accessory meningeal, masseteric, deep temporal, pterygoid, buccal, posterior superior alveolar, infraorbital, greater palatine, artery of the pterygoid canal, pharyngeal, and sphenopalatine artery. The middle meningeal artery originates from the mandibular segment and passes through the foramen spinosum, whereas the accessory meningeal artery arises from the pterygoid segment and passes through the foramen ovale. The IMA is intimately related to branches of the mandibular division of the trigeminal nerve in the infratemporal fossa, which include (from posterior to anterior) the auriculotemporal, inferior alveolar, lingual, mylohyoid, masseteric, deep temporal, and buccal nerves. After originating from the subclavian artery, the VA’s pre-foraminal segment (V1 VA) runs to the transverse foramen of the C6 vertebra’s transverse process, which is also known as Chassaignac’s tubercle or the carotid tubercle, not because its associated foramen transmits the carotid artery but because a compressive finger can occlude the carotid artery against it. The foraminal segment (V2 VA) courses cranially through the next five transverse foramina (C6 to C2). The extradural or suboccipital segment (V3 VA) courses through the transverse foramen of the atlas, loops posteriorly around the superior articular facet, enters the vertebral artery groove (sulcus arteriosus) on the upper aspect of the posterior arch of C1, and finally penetrates the atlanto-occipital membrane. The intradural segment (V4 VA) enters the foramen magnum at this dural ring and extends to the vertebrobasilar junction. The V4 VA and the C1 nerve lie anterior to the dentate ligament, whereas the posterior spinal artery and accessory nerve lie posterior to it. The accessory nerve moves anterior to the dentate ligament above its superior attachment. The V4 VA’s superior, medial, and anterior course passes anterior to the hypoglossal rootlets, crosses the pyramid of the medulla oblongata, and joins its counterpart to form the basilar artery at the level of the pontomedullary sulcus. The V3 segment is located within the suboccipital triangle, which is bordered by the superior oblique, inferior oblique, and RCPM muscles (Fig. 19.3). The superior oblique runs from the inferior nuchal line to the transverse process of the atlas and forms the superolateral border. The inferior oblique runs from the transverse process of the atlas to the spinous process of the axis in the midline and forms the inferior border. The RCPM runs from the inferior nuchal line to the spinous process of the axis and forms the superomedial border. The V3 VA is divided into three sub-segments: a foraminal part consisting of the proximal V3 VA passing through the C1 transverse foramen; a sulcal part running along the sulcus arteriosus of C1; and a dural part between the sulcus arteriosus and its dural entrance. The V3 VA can give rise to a large muscular branch (artery of Salmon), an extradural PICA, and an extradural posterior spinal artery. The muscular branch passes posteriorly through the suboccipital triangle to supply muscles of the craniovertebral junction. An extradural PICA usually arises from the dural sub-segment and parallels the VA trunk, but penetrates the dura through a different entry point. An extradural PSA also arises from the dural sub-segment and penetrates the dura with the VA through the same entry point. An extradural PICA and PSA both have surgical importance and must be preserved. The V3 VA is invested in an abundant venous plexus. The C1 nerve exits the dura through a common dural cuff with the VA and courses inferior to the V3 segment inside this vertebral venous plexus. The petrous ICA is exposed with a transcavernous approach that peels the temporal lobe dura off the lateral wall of the cavernous sinus and works through the middle fossa triangles. An orbitozygomatic approach provides invaluable extra room for suturing to the petrous ICA, with the zygomatic portion of the osteotomy allowing more mobilization of the temporalis muscle inferiorly and the orbital portion of the osteotomy opening the transsylvian/pretemporal corridor. The patient is positioned supine with more lateral rotation (30 to 45 degrees) than for the standard aneurysm. The pterional craniotomy is drilled flush with the middle fossa floor, and the orbitozygomatic unit is removed. Medial parts of the greater and lesser wings of the sphenoid bone surrounding the superior orbital fissure are rongeured to remove the lateral orbital wall and roof, respectively. The lateral periosteal dura of the SOF is incised and the meningo-orbital band is cut to release the frontotemporal basal dura from the periorbita. The stump of the meningo-orbital band and dura propria of the temporal lobe are peeled off of the inner cavernous membrane, which is a fusion of epineurium of the oculomotor, trochlear, and ophthalmic (V1) nerves in the lateral wall of the cavernous sinus. By identifying the reticular layer of loose connective tissue between the temporal dura propria and the inner cavernous membrane and developing this cleavage plane, dura propria peels back to the anteromedial triangle (Mullan’s triangle, bordered by V1, V2, and a line between the SOF and the foramen rotundum) and the anterolateral triangle (bordered by V2, V3, and a line between the foramina rotundum and the ovale). Injecting fibrin glue into the cavernous sinus at the apex of Mullan’s triangle controls cavernous sinus bleeding. Additional peeling of the dura propria exposes the anterior clinoid process medially, and laterally leads to the foramen spinosum and the middle meningeal artery, which is coagulated and divided for additional dural retraction. At this point, the direction of the dural dissection should be reversed, progressing from posterior to anterior and beginning in the region of the arcuate eminence and facial hiatus to avoid traction on the GSPN and the facial nerve. The GSPN can be difficult to find, but it orients like a fourth trigeminal nerve (“V4”), with an angle between it and V3 that is similar to that between V2 and V3 (Fig. 19.4). The incidence of carotid canal dehiscence is so high that GSPN may be resting on the artery itself. The GSPN is cut just distal to the facial hiatus. The basal temporal dura is elevated medially to the petrous ridge to complete the exposure of Kawase’s triangle. The bone around the petrous ICA in Glasscock’s triangle is drilled to create room for the anastomosis, starting posterior to the foramen spinosum. The drill is inclined parallel to the middle fossa floor to avoid entering the eustachian tube. The tensor tympani muscle courses superomedial to the eustachian tube and alerts the surgeon that the eustachian tube is near. Drilling in Glasscock’s triangle exposes the anterolateral aspect of the petrous ICA, and drilling in Kawase’s triangle exposes its posteromedial aspect. A significant amount of anterior petrous bone can be removed from Kawase’s triangle to widen the anastomotic arena, starting medially near the petrous ridge and progressing laterally toward the ICA and the cochlea, which resides at the posterolateral limit of this space in the angle between the GSPN and the IAC. Drilling on the posteromedial aspect of the GSPN near the facial hiatus should be avoided because inadvertent drilling into the basal turn of the cochlea results in hearing loss. In my laboratory, we found that a “cochlear safety line” drawn perpendicularly from the UDTF of the internal auditory canal to the lateral rim of the foramen ovale was 2 mm anterior to the cochlea, and staying anteromedial to this line reliably avoids entrance into the cochlea (Fig. 19.4). When you think you have drilled away enough bone in Glasscock’s and Kawase’s triangles, drill, drill, and drill some more. The petrous ICA is approximately 12 mm in length, and the temporary clips applied while suturing consume a lot of this space. The floor of the canal is drilled to pass the tips of the clip blades beyond the ICA. Curved or bayonetted clips move the shanks away from the anastomotic area, and permanent clips may be needed to completely close the ICA. A balloon-tipped Fogarty catheter inflated in the proximal petrous carotid canal is a clever way to eliminate a proximal clip, but it is not always occlusive. Venous plexus and sympathetic nerves travel with and envelope the artery, both making it difficult to correctly visualize the arterial layer. This difficulty is compounded by clips on a short segment that close the lumen. The response to a difficult anastomosis in a deep, confined space is often to over-manipulate the tissues, grabbing the walls and pulling on the tissues more to bring them to the needle. Sturdier 8-0 nylon suture is recommended. An end-to-side anastomosis enables reperfusion between the first and second anastomoses. An end-to-end anastomosis helps visualize the arterial lumen and walls, but it is a more difficult anastomosis in this location and does not allow for reperfusion between anastomoses. Once the proximal anastomosis is completed, the graft is routed intracranially and trimmed to 6 to 8 cm in length. I have performed two petrous-supraclinoid ICA interpositional bypasses for giant cavernous ICA aneurysms, both of them early in my experience (Case 19.1). Both patients were young (15 and 24 years old), symptomatic (progressive diplopia), and failed the balloon test occlusions, and I was concerned about long-term patency of long saphenous vein grafts (I had not switched to the RAG yet). Having experienced how challenging these C2 ICA-SVG-C6 ICA bypasses are, I do not think a strong indication exists anymore with current endovascular alternatives. The supraclinoid ICA is a deep but more hospitable anastomotic site. Temporary clips must exclude the AChA to prevent ischemia in this sensitive territory. The PCoA can be temporarily clipped, and the anterior clinoid process can be removed to advance the proximal temporary clip and enlarge the anastomotic site. This supraclinoid site is the recipient site for the C2 ICA-SVG-C6 ICA bypass, and also the donor site for the supraclinoid C6 ICA-RAG-M2 MCA bypass, which was used with a blister aneurysm that re-ruptured intraoperatively (Case 19.2). The dissected segment was trapped, the graft was sutured proximally between the OphA and AChA, and the distal end of the graft was anastomosed to the M2 MCA. Quick harvest of a short RAG without additional neck exposure limited the cross-clamp time. The M2 MCA is an active and versatile participant in IC-IC interpositional bypasses (Fig. 19.5). The superior and inferior trunks and M2 stem arteries provide generous donor flow and they lie along the transsylvian route to the ACA, PCA/SCA, and other MCA recipients. MCA interpositional bypasses range from short jump grafts to a distal MCA branch (e.g., M1 MCA-RAG-M2 MCA bypass) to longer jump grafts subfrontally to the ACA territories (e.g., ATA-SVG-A2 ACA bypass) or pre-temporally to the distal basilar territories (e.g., M2 MCA-RAG-P2 PCA bypass) (Table 19.1). Other donors into the MCA territory that lie outside but near the Sylvian triangle include the supraclinoid ICA, A1 ACA, IMA, and V3 VA. Recipient sites within the Sylvian triangle extend throughout the MCA territory from distal M1 segments to M4 segments. As with EC-IC interpositional bypasses, the more difficult deep anastomosis of an IC-IC interpositional bypass is performed first to capitalize on graft mobility. Pre-bifurcation MCA aneurysms with giant size and dolichoectatic morphology are unclippable, and the straight M1 segment anatomy may not allow primary reanastomosis after aneurysm excision (Case 19.3). Mandatory trimming of transected ends back to normal tissue further reduces the chances of reanastomosis, and failed attempts at reanastomosis increase cross-clamp time, which is already long with the two anastomoses required for interpositional bypasses. Thrombectomy and aneurysm debulking may enable primary clipping or reanastomosis and are worth a try, but escalation to a bypass strategy should proceed quickly (Case 19.4). Therefore, I maintain a low threshold for resorting to an interpositional bypass, and I prepare a short segment of radial artery or infrazygomatic STA graft in advance. Ischemia extends uninterrupted through two end-to-end anastomoses, but end-to-side anastomoses enable reperfusion between the anastomoses. An end-to-end anastomosis requires fewer sutures and is quicker than the end-to-side or side-to-side anastomosis, but is more difficult to adjust to caliber mismatches with the graft. Alternatively, an uninvolved M2 segment can be used as the donor artery for post-bifurcation aneurysms (Case 19.5). The use of a bystander distributes the ischemia to other portions of the MCA territory, and reperfusion between anastomoses allows ischemic tissues to recover before the second anastomosis. For MCA pre-bifurcation and bifurcation aneurysms, the A1 ACA offers an excellent donor site for an interpositional bypass to M2 trunks distally (Case 19.6). Table 19.1 Summary of Clinical Experience with IC-IC Interpositional Bypasses
Intracranial-Intracranial Bypass with Interposition Graft
IC-IC Interpositional Bypass
Microsurgical Anatomy
Petrous Internal Carotid Artery
Internal Maxillary Artery
V3 Vertebral Artery
IC-IC Interpositional Bypass Technique
Petrous and Supraclinoid ICA Interpositional Bypass
MCA Interpositional Bypass
Bypass Type | N |
ICA interpositional bypasses | |
C2 ICA-SVG-C6 ICA | 2 |
C6 ICA-RAG-M2 MCA | 1 |
MCA interpositional bypasses | |
M1 MCA-RAG-M1 MCA | 1 |
M1 MCA-RAG-M2 MCA | 2 |
M2 MCA-STA-M2 MCA | 1 |
M2 MCA-RAG-M4 MCA | 1 |
IMA-RAG-M2 MCA | 1 |
A1 ACA-RAG-M2 MCA | 3 |
A1 ACA-SVG-M2 MCA | 1 |
V3 VA-RAG-M2 MCA | 1 |
ACA interpositional bypasses | |
ATA-SVG-A1 ACA | 1 |
ATA-SVG-A2 ACA | 1 |
A2 ACA-RAG-A3 ACA | 1 |
A3 ACA-RAG-PcaA | 1 |
A3 ACA-RAG-CmaA | 1 |
PCA/SCA interpositional bypasses | |
M2 MCA-RAG-P2 PCA | 9 |
M2 MCA-SVG-P2 PCA | 3 |
V3 VA-RAG-s2 SCA | 1 |
V3 VA-SVG-s2 SCA | 1 |
PICA interpositional bypasses | |
V3 VA-RAG-p3 PICA | 1 |
V3 VA-SVG-a3 AICA | 1 |
Total | 35 |
