Flow Augmentation

Patients presenting with refractory vertebrobasilar insufficiency (VBI) despite maximal medical therapy are potential candidates for posterior circulation revascularization. Bypass for revascularization for posterior fossa ischemia has been less studied than anterior circulation ischemia due to the relative prevalence of the conditions, the availability and evolution of endovascular techniques for treatment of vertebrobasilar stenosis, and the relatively high morbidity and technical complexity of posterior circulation bypass. However, various extracranial-intracranial (EC-IC) bypass options to the posterior circulation are feasible, including occipital artery (OA) to posterior inferior cerebellar artery (PICA), and superficial temporal artery (STA) to superior cerebellar artery (SCA) or posterior cerebral artery (PCA) bypasses.1,2 A variety of surgical options for revascularization of the extracranial vertebral artery (VA) are also available.3,4

As with other stroke syndromes, standard evaluation for patients presenting with VBI includes cerebrovascular and parenchymal brain imaging, typically with a combination of magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA). If vertebrobasilar occlusive disease is evident, imaging and clinical presentation can ascertain the etiology as atherosclerotic (versus dissection or extrinsic) compression. Compromised flow in the vertebrobasilar system and distal arterial tree is difficult to evaluate with traditional modalities but can be assessed with quantitative MRA (QMRA) using phase-contrast magnetic resonance technique to directly measure posterior circulation vessel flow.5 The utility of hemodynamic assessment in identifying high-risk patients who may benefit from revascularization is currently under investigation.6

Presently, however, the indications for surgical revascularization are limited because no prospective or randomized studies have been performed to assess the efficacy of surgical intervention, and the procedures carry the risk of morbidity. Overall, surgical revascularization of the posterior circulation carries a higher risk and lower patency rates than anterior circulation bypass. Patency rates for OA–PICA bypass range from 88 to 100%, with mortality rates averaging 4%.7 For STA–PCA and STA–SCA bypass, a review of 86 bypasses compiled from several series revealed patency rates in the 78 to 90% range, with mortality averaging 12%7,8 and serious morbidity averaging 20%. Although these series reported improvement in symptoms in a subset of patients, the morbidity and mortality associated with such revascularization procedures have introduced caution when entertaining surgical bypass options, particularly for patients with a poor neurological condition or medical comorbidities. Nonetheless, recent advances in microsurgical and neuroanesthetic technique, as well as improvements in perioperative neurointensive care management, allow posterior circulation revascularization to be successfully undertaken in select patients without other options for management.

The approach to treatment is to first optimize all medical therapeutic options, including maximizing antithrombotic regimens, controlling blood pressure judiciously, lowering lipids aggressively with statins, controlling glycemic levels, and encouraging smoking cessation. Anticoagulation with warfarin has shown an increased risk of complications and no benefit over antiplatelet therapy in patients with intracranial atherosclerotic intracranial disease.9 If the patient has recurrent ischemia despite these measures and the disease is not amenable to endovascular therapy, then bypass options will be considered but only if comorbid cardiac or medical conditions do not prohibit general anesthesia and surgery. Particular caution should be used in considering bypass distal to a high-grade vessel stenosis (e.g., distal bypass for severe basilar stenosis). Distal bypass can create a competing flow at the location of disease that has the potential to promote thrombosis at the site of stenosis10 and cause local infarction with devastating consequence.

Flow Replacement

Preserving posterior circulation flow through revascularization may be necessary with large skull base tumors or with planned vessel sacrifice to treat complex aneurysms in the posterior fossa. Giant, fusiform, dolichoectatic, or partially thrombosed aneurysms of the vertebrobasilar system are not amenable to direct clipping and can be equally challenging for endovascular treatment. Proximal vessel occlusion and flow reversal, or trapping, is a treatment option.11,12 Occlusion can be performed without the need for revascularization if collaterals are adequate, and it appears to be well tolerated for basilar trunk and apex aneurysms if both posterior communicating arteries (PCOAs) are greater than 1 mm in diameter.11 However, the risk of complications increases to 26% if one PCOA is smaller; the risk increases to 45% if both are less than 1 mm. Tolerance to vessel occlusion can be evaluated by clinical testing and evoked potential monitoring over 20 to 30 minutes during endovascular balloon test occlusion, with neurological or electrophysiological failure indicating the need for bypass. Typically, revascularization in these cases will require a graft to the PCA or SCA to provide collateral flow prior to proximal occlusion.

Occasionally, fusiform or dissecting aneurysms can be well addressed with trapping but, if major branches are involved, revascularization of the branch distal territory is necessary. This situation is most commonly encountered with vertebral aneurysms that involve the PICA segment, in which case OA–PICA or PICA–PICA bypass can be performed12,13; alternatively, the PICA can be reimplanted into a more proximal intradural segment of the VA. For distal aneurysms of the major cerebellar branches, occlusion of the vessel without bypass can be tolerated if occlusion is performed distal to the proximal brainstem segments of the vessel and other cerebellar arteries are present and large, thus averting a major cerebellar stroke. Alternatively, fusiform distal aneurysms can be considered for bypass or direct excision and reanastomosis.14

Therefore, revascularization in the posterior fossa is necessary when aneurysm treatment requires proximal major arterial occlusion and the native collateral flow is inadequate, or when an individual arterial branch vessel is incorporated into an aneurysm.

Preoperative Assessment

Patients who are considered for posterior circulation bypass should undergo angiography to delineate the intracranial vasculature and selective external carotid injections to evaluate the caliber and course of donor branches, such as the STA or OA. If there is concern regarding the adequacy of the in situ donors, alternative bypass strategies using interposition grafts (saphenous vein or radial artery) can be entertained. In patients with VBI, where atherosclerosis is the primary etiology for the vertebrobasilar disease encountered in such patients, systemic atherosclerotic disease is often present. Therefore, preoperative cardiac and medical clearance for cardiac risk stratification, including echocardiography and stress testing, is an important element of preoperative assessment.

If interposition grafts are anticipated, a saphenous vein can be harvested from the calf or thigh following preoperative ultrasound mapping to determine the suitability (size and length) of the vein. For a radial artery, the vessel is generally harvested from the nondominant arm after ensuring adequate ulnar artery collaterals to the hand with the Allen test. Endoscopic harvesting is feasible for both,15,16 although recent date from the cardiac literature would suggest worse long-term results with endoscopically harvested grafts.17

Perioperative and Anesthetic Considerations

Patients should be placed on full dose (325 mg) aspirin, ideally beginning a week prior to surgery but at least the morning of surgery. Other antiplatelet agents, such as clopidogrel, are generally avoided due to bleeding risk, particularly in cases involving intracranial surgery. For patients requiring dual antiplatelets due to high thrombotic risk, the second agent can be discontinued a week prior to surgery and replaced with enoxaparin or equivalent until the day prior to surgery. If patients have been on warfarin anticoagulation, they are converted to intravenous heparin, which is withheld 6 hours prior to surgery as antiplatelets are administered.

Arterial line and central venous access is routinely obtained for surgery. Antibiotic prophylaxis is administered prior to skin incision and maintained for 48 hours postoperatively. Throughout the surgery, normovolemia, normocapnia, and normotension (based on a patient′s baseline blood pressure) are maintained. For tenuous patients with VBI who are blood pressure–dependent, even extreme hypertension is maintained until the bypass has been completed. Monitoring somatosensory evoked potential (SSEP) and motor evoked potential (MEP) during surgery can be useful in alerting the operative team to inadequate blood pressure maintenance during the case. Scalp electrodes for electroencephalographic monitoring are placed outside the surgical field to monitor induction of metabolic burst suppression during temporary vessel occlusion for bypass. Inhalational agents are used preferentially for burst suppression because they increase cerebral blood flow in comparison to barbiturates. Intravenous anesthetics for burst suppression may be required if SSEP and MEP are monitored; otherwise, evoked potentials may be suppressed.

For intracranial bypass operations, lumbar drain for cerebrospinal fluid (CSF) drainage is preferred for brain relaxation to avoid the need for intravenous diuretics (furosemide), hyperosmolar agents (mannitol), or hyperventilation. Prior to cross-clamping of major vessels, intravenous heparin is administered. For entirely extracranial operations, full dosing with weight-appropriate heparin (routine dose 5000 units) is performed 5 minutes prior to initial vessel occlusion; an additional 1000 to 2000 units are administered as needed for a second anastomosis. For intracranial operations, smaller doses (generally 2000 units) are administered prior to temporary vessel occlusion. Heparin is allowed to wear off on its own.

Postoperative Management

Patients are continued immediately on 325 mg of aspirin daily. If aspirin cannot be administered orally, it can be given rectally. Patients are observed in the intensive care unit postoperatively and are kept well hydrated, with avoidance of hypotension. Pressure from glasses or a nasal oxygen cannula over the location of the bypass graft is avoided to prevent direct mechanical occlusion. In patients with bypass for ischemia, it is especially important to avoid hypertension given the potential risk for hyperperfusion hemorrhage, although this event occurs less commonly in the posterior circulation and with STA or OA grafts compared with larger and higher flow interposition grafts. Potential postoperative complications include epidural hematoma, wound infection, and postoperative graft occlusion. Patients are discharged on daily aspirin.

Flow Measurement

Both intraoperatively and postoperatively, measurement of flow is an invaluable adjunct to successful posterior fossa bypass.18,19 During surgery, blood flow in the recipient, donor, and graft can be readily measured quantitatively with devices such as a microvascular ultrasonic flow probe (Charbel Micro-Flowprobe; Transonics Systems Inc., Ithaca, NY).20 During flow replacement surgery for aneurysms, measurement of flow in the recipient vessel provides a direct indication of the flow that the bypass must provide. When in situ donors, such as STA or OA, are being considered, the “cut flow” of the donor vessel is determined by measuring the free flow through the cut end of the vessel. The cut flow represents the maximal flow-carrying capacity of the donor and helps to determine whether the flow is adequate for flow replacement or whether a larger interposition graft is needed. After completion of the bypass, measurement of the flow through the graft provides direct confirmation of the adequacy and patency of the bypass.19 When using STA or OA as donor vessels in a bypass for ischemia, the cut-flow index (ratio of bypass flow to the initial cut flow) provides an excellent indication of bypass success and is a sensitive predictor of bypass function.20 Intraoperative conventional angiography is generally unnecessary if direct flow measurements and video indocyanine green (ICG) angiography are performed for physiologic and anatomic graft confirmation.

Postoperative assessment of bypass success and patency has traditionally relied on conventional angiography, although computed tomographic angiography (CTA) or MRA can be used. Alternatively or additionally, use of QMRA to measure flow in the graft and recipient vessels provides confirmation not only of patency but of graft function. QMRA provides a noninvasive option for serial monitoring over time.21

Vertebral–Carotid Transposition

Vertebral–carotid transposition (VCT) is performed for treatment of stenosis originating at the VA ( Fig. 31.1 ). Performing a direct vertebral-origin endarterectomy4 through a subclavian approach is also an option but is seldom used because VCT offers a simpler and more effective method of revascularization. The potential limitation of VCT is that it requires simultaneous occlusion of both carotid and vertebral arteries; however, given the proximal location of temporary occlusion on the common carotid and proximal vertebral arteries, cervical and muscular collaterals invariably prevent cerebral ischemia. If the carotid artery is stenotic or otherwise compromised, transposition to another location of the subclavian artery can be used. Similarly, if transposition is not feasible because of inadequate length of the proximal VA, an interposition vein or prosthetic graft bypass can be performed from the subclavian with end-to-end anastomosis to the VA.22,23 Although this procedure does not interrupt carotid blood flow, it requires two anastomoses and is time consuming.

The proximal VA can be transposed from the subclavian artery to the thyrocervical trunk.3,23 Occasionally, obstruction at the vertebral origin is extrinsic, caused by compression due to bands from the tendon of the anterior scalene or the longus colli muscle.4,24 These ligaments, muscles, and bands overlying the artery can be excised. In some cases, the sympathetic ganglia or nerve fibers constrict the artery. If the ganglia are divided, a mild Horner syndrome will develop. Segmental resection and end-to-end anastomosis can be used when obstruction is caused by entrapment, but the VA must be long and its diameter adequate.

Positioning and Exposure

The patient′s head is placed in extension on a donut head rest, in preparation for a supraclavicular approach ( Fig. 31.2A ). Downward traction of the arm and shoulder provides better exposure. The head is kept midline. A supraclavicular incision is made ~2 cm above and parallel to the clavicle and extends from the suprasternal notch to 7 to 8 cm laterally. The skin is retracted superiorly and inferiorly, leaving the platysma, which is divided horizontally. The sterno-cleidomastoid muscle has two origins: the clavicular head from the superior surface of the medial third of the clavicle, and the sternal head from the anterior surface of the manubrium of the sternum. The clavicular head is divided, leaving a cuff on the clavicle, and the muscle is retracted superiorly and laterally. The underlying fat pad is mobilized superiorly by detaching it inferiorly. The omohyoid muscle can be divided if needed. The dissection is kept medial to expose the carotid sheath. The anterior scalene muscle lies laterally, with the phrenic nerve lying on top of it. This muscle is usually far-lateral to the exposure and rarely requires division. The carotid sheath is separated from the overlying fascia and opened. Within the sheath, the common carotid artery, the internal jugular vein, and the vagus nerve can be found. The jugular vein and vagus nerve are retracted laterally, and the carotid is retracted medially. From this point, dissection proceeds below the deep fascia layer caudally.

Specific concerns are related to the side of exposure. If the right side is exposed, it is important to recognize that lymphatic drainage on the right side of the neck differs from that on the left. Delicate lymphatic trunks empty into the right subclavian and jugular veins, which are usually smaller than the lymphatic ducts on the left. Because they do not coagulate completely, ligation is preferred if these structures are encountered. The right recurrent laryngeal nerve exits the vagus nerve and loops below the right subclavian artery as it approaches the trachea and larynx. Consequently, medial retraction of the trachea can cause ipsilateral paresis of the vocal cord. If the left side is exposed, the thoracic duct is encountered as it arches from the side of the esophagus laterally to the angle between the internal jugular and subclavian veins. The proximal portion of this duct should be ligated twice, and smaller branches are also ligated. The left recurrent laryngeal nerve can be retracted with greater ease than the right because it loops around the aortic arches and approaches the trachea much lower.

Working in the deep fascial layer, the VA can be identified as the first branch of the subclavian artery exiting from its posterosuperior surface ( Fig. 31.2B ). This feature distinguishes it from the thyrocervical trunk, which has multiple branches and exits from the anterosuperior surface. Alternatively, and more easily, the VA can be first located superiorly as it exits the transverse foramen of C6. The transverse process of C6 can be palpated adjacent to its foramen. The artery arises from the apex of the anterior scalene and longus colli muscles as they attach to the carotid tubercle. The vertebral vein, which overlies the artery, can be carefully coagulated and divided or retracted. The vertebral vein is formed at the lower end of the canal of the transverse foramina from a venous plexus within the canal around the VA. The vein is anterior to the artery and often adherent to it. It is important to identify and preserve the sympathetic chain. The VA is dissected from C6 to the subclavian artery with care to avoid destroying the sympathetic trunks and stellate or intermediate ganglia that lie on it.

Related posts:

Posterior Neuroendoscopic Applications at the Craniovertebral Junction Brief Overview of Surgical Approaches to the Craniovertebral Junction Stereotactic Methods of Localization and Image-Guided Surgery Approaches to the Jugular Foramen Transfacial Approaches to the Craniovertebral Junction Suboccipital and Retrosigmoid Approaches

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