CHAPTER 15 The EC-IC interpositional bypass is an oversized STA-MCA bypass that uses the same end-to-side anastomosis to the recipient distally (usually the MCA), but with an interposition graft instead of a scalp artery to draw the robust flow of the cervical carotid artery and double the bypass’s flow. Interpositional bypasses are more complex than low-flow EC-IC bypasses because they require a graft, an additional harvest site, an additional donor site, a tunnel, and a second anastomosis, not to mention complexities that arise when pairing donors, recipients, and grafts of different caliber. Championed by neurosurgeon Thoralf Sundt for aneurysms and ischemic conditions, the EC-IC interpositional bypass is a second-generation bypass designed to replace or restore carotid blood flow after deliberate sacrifice. This bypass is only rarely used to replace or restore MCA blood flow, even though it donates to MCA recipients. Being outside of the usual cranial environment of the six other bypasses, EC-IC interposition bypass construction begins with the surgical anatomy of cervical carotid donors and interposition grafts. The left common carotid artery originates directly from the aortic arch as its second branch, whereas the right CCA is a branch of the brachiocephalic trunk or innominate artery, the arch’s first branch (Fig. 15.1). Both CCAs ascend in the carotid space of the neck and bifurcate into the ICA and ECA at the level of the superior thyroid cartilage (C4 vertebra), although the level of bifurcation varies from T2 to the hyoid bone. The carotid arteries on each side reside in the carotid sheath, a derivative of the deep cervical fascia that invests the carotid vessels along with the IJV laterally and the vagus nerve posteriorly in between the artery and vein. Sternocleidomastoid, sternohyoid, sternothyroid, and omohyoid muscles surround the lower segment of the CCA anteriorly, whereas the upper segment of the CCA proximal to its bifurcation is relatively more superficial and covered anteriorly (from superficial to deep) by skin, superficial cervical fascia, platysma muscle, deep cervical fascia, and the medial border of the sternocleidomastoid muscle. The carotid bifurcation lies within the carotid triangle, which is bounded by the superior belly of the omohyoid muscle medially, the sternocleidomastoid muscle laterally, and the posterior belly of the digastric muscle superiorly (Fig. 15.2). The ansa cervicalis, a descending branch of the hypoglossal nerve, lies in front of the carotid sheath. Nerve fibers originating from the C1 root travel a short distance with the hypoglossal nerve and leave that nerve to descend as the superior root of the ansa cervicalis, which innervates a number of the strap muscles. The superior root is joined by other descending fibers from C2 and C3 (inferior root). The CCA is related to the tracheoesophageal tract and thyroid gland medially; the IJV and vagus nerve laterally; and the transverse processes of the cervical vertebrae, attached longus colli muscles, and the sympathetic trunk posteriorly. The thyroidal veins cross the anterior aspect of the CCA to reach the IJV. As the ICA originates from the CCA at the C4 level, it lies posterolateral to the ECA initially, but gradually sweeps medially as it ascends to course just lateral to the pharyngeal wall on the anterior aspect of the upper three cervical vertebrae, with the longus colli, longus capitis, and the superior sympathetic ganglion in between (Fig. 15.3). The ICA continues upward in the carotid sheath and enters the carotid canal at the base of the skull, where it is separated from the posterolaterally lying IJV by cranial nerves IX, X, XI, and XII. The vagus nerve and the IJV are lateral to the ICA. Inferiorly, the lingual and common facial veins cross the ICA anteriorly; superiorly, the stylohyoid and the posterior belly of digastric muscle, as well as the posterior auricular and occipital arteries, cross the ICA. Just inferior to the posterior belly of the digastric muscle, the ICA is crossed anterolaterally by the hypoglossal nerve. The ICA has no branches in the neck. As it originates from the CCA, the ECA first lies anteromedial to the ICA but it assumes a posterolateral position as it ascends through the carotid sheath to course behind the ramus of the mandible (Fig. 15.4). It bifurcates into its terminal branches, the STA and the IMA, in this space behind the mandible’s neck. The common facial and thyroidal veins, the hypoglossal nerve, the posterior belly of the digastric, and the stylohyoid muscles cross the ECA anteriorly. The artery enters the substance of the parotid gland before its terminal bifurcation. The ECA lies anterior to the IJV, and anterolateral to the vagus nerve inside the carotid sheath. Although the exact originating point of its branches is variable, the ECA usually has eight branches in the neck, from inferior to superior: superior thyroidal, ascending pharyngeal, lingual, facial, occipital, posterior auricular, internal maxillary, and superficial temporal arteries (Fig. 15.5). The ECA’s first two branches originate from its donor segment, with the superior thyroidal artery arising at the level of the bifurcation and coursing medially to the thyroid gland, and the ascending pharyngeal artery arising from the crotch of the bifurcation between the ECA and the ICA. The left subclavian artery is the third and final branch from the aortic arch, whereas the right subclavian artery originates from brachiocephalic trunk as it gives rise to the right common carotid artery at the upper border of right sternoclavicular articulation. The subclavian artery ascends from the thorax into the root of the neck, arches in the subclavian triangle of the neck, and descends beneath the clavicle, over the first rib, to the axilla (Fig. 15.6). The posterior belly of the sternocleidomastoid muscle, the inferior belly of the omohyoid muscle, and the clavicle border the subclavian triangle. This triangle occupies the lower portion of the posterior triangle of the neck, which is bounded by the posterior belly of the sternocleidomastoid, the trapezius, and the clavicle (the occipital triangle forms the upper portion of the posterior triangle of the neck). The external jugular vein crosses the sternocleidomastoid muscle obliquely and then terminates in the subclavian vein, which joins the internal jugular vein below the clavicular head of the SCM to form the brachiocephalic vein. Lymphatic channels of the face, head, and neck drain to the jugular lymphatic trunk, or thoracic duct, which lies at the junctional angle between the IJV and the subclavian vein. The subclavian artery lies deep to this venous confluence. The anterior scalene muscle divides the subclavian artery into three segments, medial, posterior, and lateral to this muscle. The subclavian artery has multiple branches along its course, including the vertebral artery, the thyrocervical trunk, and the costocervical trunk from its superior surface and the internal thoracic artery from its inferior surface. The roots forming the trunks of the brachial plexus lie superolateral to the subclavian artery and surround the artery as it drops behind the clavicle to become the axillary artery. The middle and posterior scalene muscles are posterolateral to the brachial plexus, with the former inserting on the first rib, like the anterior scalene muscle, and the latter inserting on the second rib. Both the subclavian artery and the cervical nerve roots emerge between the anterior and middle scalene muscles en route to the axilla. The cupula of the pleura overlying the apex of the lung is inferior to the subclavian artery. The radial artery originates in the cubital fossa where the brachial artery bifurcates into the radial and ulnar arteries (Fig. 15.7). It terminates in the superficial and deep palmar arches, anastomosing with the terminal branches of the ulnar artery that enable the ulnar artery to supply the hand after radial artery harvest. The radial artery gives rise to only one named branch along its course in the forearm, the radial recurrent artery near the elbow. The anterior and posterior interosseus arteries are branches from the common interosseus artery, which arises from the ulnar artery. The radial artery runs medial and deep to the brachioradialis muscle and its tendon, and lateral and deep to the flexor carpi radialis and its tendon. The length of the radial artery from its origin to the wrist is between 20 and 25 cm. The lateral antebrachial cutaneous nerve, which is a continuation of the musculocutaneous nerve, sends cutaneous sensory branches to the skin that lie beneath the incision line, and their interruption can produce forearm and hand numbness or dysesthesias. The cephalic vein may also lie over the proximal radial artery but drifts laterally on its course to the wrist. The great saphenous vein is localized just below (1.5 inches) the inguinal ligament where it penetrates the deep fascia through the saphenous opening to join the femoral vein. The femoral vein is found just medial to the femoral artery, which is palpable in the groin at the midpoint of the line between the anterior superior iliac spine and the pubic symphysis. The vein descends on the anterior-medial aspect of the thigh, along the medial edge of the sartorius muscle. The vein receives input from the anterior femoral vein, which is smaller in caliber and located more laterally. The saphenous vein courses posteriorly around the medial epicondyle of the femur inferiorly to the medial aspect of the leg. The saphenous vein forms at the medial aspect of the foot, coursing anterior to the medial malleolus where it can be identified and followed distally. When the patient is properly positioned for a standard pterional craniotomy (supine with a bolster under the ipsilateral shoulder, the head rotated 20 degrees away from the bypass side, the head extended approximately 20 degrees to allow gravity to retract the frontal lobe away from the anterior cranial fossa floor, and the head lifted above the level of the heart), the neck is maintained in a neutral position for carotid artery exposure. Landmarks for the skin incision are the mastoid tip, the angle of mandible, and the medial border of the SCM muscle (Fig. 15.8). The incision lies somewhere along the arc that begins at the mastoid tip, courses to a point one fingerbreadth below the angle of the mandible (to avoid the mandibular branch of the facial nerve), and continues diagonally along the medially border of the SCM. A 5-cm incision is needed to expose the donor carotid artery, with the exact location modified upward or downward depending on the level of the carotid bifurcation. After incising the skin sharply, the superficial cervical fascia and platysma muscle are incised with monopolar cautery to enter the carotid triangle. The medial border of the SCM is dissected first along the length of the incision, and then under the SCM’s most medial fibers to mobilize the muscle laterally. The underlying IJV is exposed, and although it is tempting to want to identify the carotid bifurcation next, the carotid sheath should be opened widely and the IJV mobilized laterally. The common facial vein crosses the ECA anteriorly at the level of the carotid bifurcation and tethers the IJV, so it is suture-ligated, divided, and retracted laterally. A self-retaining retractor between the IJV laterally and the strap muscles medially (superficial to the larynx) exposes the carotid bifurcation. Minimizing dissection medial to the bifurcation avoids the trachea, esophagus, and recurrent laryngeal nerve, and minimizing dissection deep to the bifurcation avoids the vagus nerve. The first 2-cm segment of the ECA is the preferred donor site and is typically covered by the hypoglossal nerve, the ansa cervicalis, and the posterior belly of the digastric muscle (Fig. 15.8). The hypoglossal nerve is dissected superiorly off the ECA, and the ansa cervicalis is dissected medially. Fishhook retractors pull the angle of the mandible and surrounding soft tissues superiorly. The entire carotid bifurcation can also be mobilized inferiorly with downward traction on a vascular loop placed in the crotch of the bifurcation. The ECA branches originating from this initial segment are prepared for temporary clipping during clamp time. The superior thyroidal and ascending pharyngeal arteries originate from the carotid bifurcation and lie below the proximal clamp, but the lingual and facial branches might originate from the anastomotic segment posteriorly and, if unseen, will back-bleed during the anastomosis. This neck dissection exposes the CCA, ECA, and ICA, and is the same no matter which artery is chosen as the proximal donor. The subclavian artery is exposed through a supraclavicular incision. The sternocleidomastoid muscle is divided transversely and its clavicular head is reflected inferiorly to expose the IJV and to widen the subclavian triangle. The subclavian artery lies deep to the venous confluence of the IJV, EJV, and subclavian vein, and the bypass donor site on the subclavian artery lies lateral to the IJV at the apex of its arch in the subclavian triangle. Alternatively, the medial segment of the subclavian artery can be identified by following the VA proximally to it origin. The CCA is easily identified in the carotid sheath, and just below it is the carotid tubercle, or Chassaignac’s tubercle, which is the transverse process of the C6 vertebra. The preforaminal segment of the VA (V1 VA) enters the transverse foramen of the C6 vertebra and can be followed inferiorly to its origin from the subclavian artery. The subclavian artery is then dissected laterally to the apex of its arch in the subclavian triangle. The radial artery has become the interposition graft of choice for many reasons. It is arterial tissue with round, muscular walls that are designed for arterial physiology and handle well during the suturing. Its diameter of approximately 3.5 mm matches or exceeds that of common anastomosis sites such as the M2 MCA and P2 PCA, and its flow rate of 40 to 70 mL/min replaces that lost with the surgical sacrifice of a major intracranial artery. It lacks varices and valves that give saphenous vein grafts their backward directionality and make them more thrombogenic. Radial artery grafts are less susceptible to intimal hyperplasia and atherosclerosis than saphenous vein grafts, and their long-term patency exceeds that reported with saphenous vein grafts. The patient’s hand is supinated and the arm extended to position the forearm for radial artery harvest (Fig. 15.9). The hand is taped to hold this position during the case. The side depends on the competency of the palmar arches, the operating room setup, and patient preferences, but the nondominant side is preferred when both sides are available. The arm position is straightforward with supine patients, but can be challenging with the lateral or park-bench positions for posterior fossa craniotomies. The landmarks for the skin incision are the artery’s palpable pulse at the wrist and the point in the cubital fossa between the brachioradialis muscle belly and the biceps tendon. These forearm landmarks define the proximal plane between brachioradialis and flexor carpi radialis, which is the lateral-most muscle in the group of superficial anterior forearm muscles. The linear incision between these two points begins distally at the wrist, where the artery is superficial and easy to identify subcutaneously. Overlying subcutaneous fat is divided along the course of the artery down to its fascial cuff, and the brachioradialis and flexor carpi radialis muscles are separated as the radial artery is followed proximally to its bifurcation with the ulnar artery from the brachial artery. When the full course of the artery is exposed from the wrist to the elbow, the dissection deepens medially and laterally around the artery. Small, unnamed branches are ligated with surgical clips and divided, as is the radial recurrent artery near the elbow, freeing the full 20-cm arterial length. Leaving a cuff of connective tissue around the artery protects it and minimizes iatrogenic vasospasm. Before the graft is extracted, the so-called Garrett line is drawn longitudinally on its outer wall to maintain its proper orientation after cutting it out. The Garrett line visualizes twists that might otherwise go unnoticed and compromise patency, particularly when tunneling the graft. The ends of the radial artery are suture-ligated, the transected distal end is cannulated, and the transected proximal end is occluded with a temporary aneurysm clip. Forcible injection with a syringe and heparinized saline dilates the RAG from distal to proximal, marching the temporary clip progressively more proximally and breaking any vasospasm in the arterial walls. This “pressure distention” technique also identifies leaks from unsecured branches, which are clamped with a mosquito forceps and suture-ligated with 4-0 braided nylon suture. The artery is then injected with a 1:1 mixture of heparinized saline (5 units/mL saline) and nitroprusside (50 mg/250 mL saline), and placed in a cup of heparinized blood until ready for the anastomosis. The saphenous vein graft is an alternative to the radial artery graft, which may be unavailable due to an incompetent palmar arch or occlusion from atherosclerotic disease, trauma, or iatrogenic injury (e.g., an arterial line). Although the SVGs are more collapsible than the RAGs, they can be thick-walled, particularly at the inguinal region proximally, and suture needles must be passed forcefully. The saphenous vein is the longest vein in the body and provides sufficient length for bypasses spanning more than 20 cm. Dissection starts at the groin where the femoral artery is palpable at the midpoint between the pubic symphysis and the anterior-superior iliac spine (Fig. 15.10). The femoral vein lies medial to the artery, and an incision is made just below the inguinal ligament to expose the junction of the saphenous vein with the femoral vein at an opening in the deep fascia. Once identified here, the incision is extended in a line toward the medial epicondyle of the femur on the anterior-medial aspect of the thigh, along the medial edge of the sartorius muscle. Input from the anterior femoral vein is divided, and unnamed branches are clip-ligated. The SVG dissection continues until an adequate length is freed, typically not below the knee. The SVG’s large size at the thigh may prompt more distal dissection for a smaller caliber graft. Alternatively, the SVG can be identified distally at its origin at the medial aspect of the foot, anterior to the medial malleolus, and followed distally up the medial aspect of the leg. The smaller caliber here might be a better size for short intracranial interposition grafts. The presence of valves gives the SVGs directionality and requires tracking their orientation. Placement of a cannula distally in the vein marks its anatomy and the anterograde direction of flow. Again, a Garrett line is drawn longitudinally to maintain proper orientation when tunneling the graft.
Extracranial-Intracranial Bypass with Interposition Graft
EC-IC Bypass with Interposition Graft
Microsurgical Anatomy
Common Carotid Artery
Internal Carotid Artery
External Carotid Artery
Subclavian Artery
Radial Artery
Saphenous Vein
Dissection Technique
Cervical Carotid Artery
Subclavian Artery
Radial Artery Graft Harvest
Saphenous Vein Graft Harvest
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