31 Revascularization of the Brainstem
Abstract
Surgery of the brainstem is complex due to the structures involved and their innate sensitivity to ischemia. The vast majority of cases requiring brainstem revascularization are complex ischemia cases, skull base tumors, and aneurysms of the vertebral or basilar arteries. Current alternative options include flow diversion systems, endovascular techniques for aneurysms, and ischemia and radiation therapy for tumors. Thus, the number of patients for whom revascularization is indicated will continue to decline. However, cases that are refractory to treatment by other means mandate that the surgeon maintain competency with revascularization techniques in order to comprehensively address the full breadth of brainstem pathology.
Introduction
The brainstem is one of the most sensitive areas of the brain during periods of restricted blood flow. This is because of relatively poor collateralization, with most of the brainstem blood supply provided by end arteries originating from the major vessels. Moreover, ischemic insults to the brainstem can result in devastating neurologic consequences due to the high density of critically important nuclei and white matter tracts. As such, revascularization procedures represent an important part of the neurosurgeon’s armamentarium in cases where blood flow needs to be augmented to deal with the pathology at hand.
Cases requiring posterior circulation revascularization techniques can be divided into three pathologic categories: ischemia, skull base tumors, and aneurysms. The advancement of medical therapies, radiation and radiosurgery, and endovascular techniques has decreased the number of cases in which surgery is the primary treatment modality. However, cases of complex anatomy or pathology that is refractory to other treatment modalities continue to require advanced surgical techniques including revascularization.
In this chapter we will review relevant brainstem vascular anatomy and technical considerations for posterior circulation bypasses, and discuss indications for revascularization procedures with results and illustrative cases.
Brainstem Vascular Anatomy
The vasculature of the brainstem is both complex and highly variable from patient to patient. The basic structure of brainstem vascular supply is outlined in Fig. 31.1a , but it should be noted that deviation from this standardized picture is the rule. In the typical case, the vertebral arteries originate as branches of the subclavian arteries (V1 segment), enter the transverse foramen at the level of C6 (V2 segment) and ascend toward the skull base, pass through the transverse foramen of C1 and turn horizontally and run in the sulcus arteriosus (V3 segment), and then enter the dura along the lateral aspect of the cervicomedullary junction (V4 segment). After dural penetration, the vertebral arteries give off the posterior inferior cerebellar arteries (PICA), though the origin of this vessel is variable and can be extradural in 5–20% of cases. 1 Smaller bilateral anterior spinal arteries branch ventrally, which join together and run on the ventral surface of the spinal cord. The vertebral arteries join to become the basilar artery at the ponto-medullary junction, after which the anterior inferior cerebellar arteries (AICA) and superior cerebellar arteries (SCA) originate. The terminal basilar artery bifurcates into the posterior cerebral arteries (PCA), which take anastomoses from the anterior circulation via the posterior communicating arteries (PCoA).
There is significant anatomic variation in this area. The fetal PCA, in which the P1 segment of the PCA is atretic and the main source of blood flow to the PCA is from an enlarged PCoA on one or both sides, is present is up to 30% of patients. 2 Conversely, absence or significant atresia of one or both PCoA can also be present in 6–21% of patients, 3 which significantly limits collateral blood flow if a posterior circulation vessel is occluded. Given such wide variation in vascular anatomy from patient to patient, a high–quality preoperative CT angiogram or conventional angiogram is of paramount importance for any surgery that could require a revascularization procedure.
The majority of the brainstem parenchyma itself is supplied by paramedian, short, and long circumferential perforating arteries originating from the PICA, AICA, SCA, PCA, or basilar trunk ( Fig. 31.1b ). These are classically considered to be end arteries, making the brainstem highly sensitive to ischemia from occlusion of parent vessels. However, there is some evidence of pial and deep parenchymal collateralization of these perforating arteries, allowing for some compensatory flow in cases of basilar trunk occlusion. 4 , 5 Collaterals are more likely to be present and able to compensate in younger patients or in patients where parent vessel stenosis or occlusion has occurred slowly, allowing for compensatory collateralization to develop.
Technical Considerations
Operative Technique and Monitoring
Bypass procedures, particularly those of the posterior circulation, require a coordinated effort between surgeon, anesthesiologist, and neurophysiologist. Typically, operations are conducted under total intravenous anesthetic to facilitate monitoring and allow for precise control of cortical activity. Somatosensory evoked potentials, motor evoked potentials, and electroencephalography are monitored closely throughout the procedure. For bypasses of the posterior circulation, monitoring of cranial nerves V–XII can also provide valuable information. Propofol-induced burst suppression is advisable during periods of temporary occlusion to decrease brain metabolic demand, and blood pressure is typically maintained with a mean arterial pressure 20% above baseline to facilitate collateral perfusion. All patients with planned bypasses are given 325 mg of aspirin preoperatively and postoperatively for 6 months in the case of arterial grafting and indefinitely in the case of venous grafting. At the time of temporary occlusion, heparin is typically administered as a bolus dose of 3000 units unless directly contraindicated for any interposition graft. Postoperatively, patients undergo bypass graft flow monitoring using noninvasive Doppler, which can facilitate early diagnosis of stenosis or thrombus in the graft and allow for early intervention.
Types of Revascularization Procedures
Posterior circulation revascularization can be accomplished via local or in situ bypasses, extracranial-intracranial (EC-IC) bypasses, or extracranial-extracranial (EC-EC) bypasses. 6 Local bypasses include direct vessel reimplantation, end-to-end reanastomosis, side-to-side anastomosis, or short interposition grafting. EC-IC and EC-EC bypasses can be broadly categorized as low flow, medium flow, or high flow. Low-flow bypasses are typically direct anastomoses of distal external carotid branches with posterior circulation vessels. The most common EC-IC bypass for brainstem revascularization is occipital artery (OA) to PICA ( Fig. 31.2 ), but in specific cases the superficial temporal artery (STA) can used as a donor. Flow rates of low-flow bypasses are in the range of 30–60 mL/min, and can serve to augment flow from other sources to prevent hypoperfusion. These types of bypasses are particularly useful if the recipient artery has a diameter of 2 mm or less. If higher flow rates are required to replace the contribution of larger arteries, medium- (60–80 mL/min) or high-flow bypasses (>100mL/min) can be used. These are accomplished using interposition grafts, such as radial artery graft (RAG), saphenous vein graft (SVG), or anterior tibial artery (ATA) graft, that shunt large volumes of blood from proximal large vessels (external carotid or extracranial vertebral artery) to the intracranial vasculature.
The decision of bypass type is based on a number of important factors including (1) extent of the collateral circulation (a poor collateral circulation requires a higher flow bypass); (2) size of the vessel to be replaced (a larger artery, i.e. basilar, often requires a higher flow bypass); (3) size of the recipient arteries; and (4) availability and caliber of donor arteries (STA, OA) or interposition grafts (RAG, SVG, ATA). It should be noted that hyperperfusion syndromes can occur if there is significant mismatch between the bypass flow rate and the normal flow rate of the artery being revascularized. This has been reported in the literature, and can be avoided by selection of bypass grafts with the appropriate flow rate. RAG flow rates are on the order of 50–150 mL/min, while SVG can have flow rates of >200 mL/min, owing to their larger diameter. 7 Amin-Hanjani et al have published on the use of intraoperative flow measurements to allow for more precise matching of flow rates. 8
The choice of interposition graft is primarily dictated by flow rate, and most posterior circulation bypasses fall in the moderate-flow category. As such, RAG is the most desirable ( Fig. 31.3 ). Issues with vasospasm in arterial grafts have largely been resolved with the advent of the pressure distention technique. 9 An Allen test is performed preoperatively to ensure patency of the palmar arch. If the patient fails bilaterally, SVG or ATA are alternatives. The diameter of the donor artery will also limit flow rate, and is an important consideration when formulating bypass strategy.
In Situ Bypasses and Reconstruction
Direct Reconstruction
In cases of resection of a portion of the artery for vascular or tumor pathology, the two ends can be directly reanastomosed if they can be mobilized adequately and the intervening segment is relatively short ( Fig. 31.4a ). If the resected segment is too long, a short interposition graft can be used as a conduit depending on the size of the artery ( Fig. 31.4b ). Fish-mouthing techniques can be employed if there is a mismatch between artery and interposition graft size, and end-to-side anastomoses are favorable if there is significant mismatch ( Fig. 31.5a-c ). For smaller arteries such as the PICA or AICA, OA or STA inter-positions may be used. For larger arteries such as the VA, interposition with RAG or SVG are necessary to ensure adequate flow and size matching.
In cases of aneurysmal pathology or tumor invasion at the originating segment, an artery can be excised and reimplanted to the parent artery to reestablish flow. This is most commonly done for PICA or AICA origin aneurysms that involve the takeoff of the artery. The parent artery is temporarily occluded, the distal artery transected, and the pathology removed. The distal artery is then reanastomosed in an end-to-side fashion to the parent artery using 9–0 or 10–0 sutures.
Side-to-Side Anastomosis
An alternative to reimplantation or direct anastomosis is revascularization of the distal portion of the artery using a side-to-side anastomosis ( Fig. 31.6 ). This is most commonly used when revascularization is required for smaller distal arteries of the posterior circulation, such as the PICA or AICA. It is particularly useful when the vessels naturally lie close to each other, as in the case of the tonsillar segments of the PICA. The technique involves dissection and isolation of both vessels with a rubber dam placed underneath. The blood pressure is raised to facilitate collateral circulation, and temporary clips are applied to both sides of both vessels. An approximately 3-mm linear arteriotomy is made on the superior-medial aspect of both vessels. The ends of the arteriotomies are anchored to each other using 9–0 or 10–0 nylon sutures. A running inside-out technique is used to anastomose the posterior wall of the bypass. The anterior wall is then sutured in a similar running fashion. The lumen is irrigated with heparinized saline prior to closure of the final stitch, and a distal clip removed to backbleed any air. The final suture is tied down and all temporary clips removed.
Low-Flow Bypasses
Revascularization for flow augmentation on the order of 30–60 mL/min can be accomplished using low-flow bypasses with the OA or STA as donors and the SCA, AICA, or PICA as recipients. The OA is the artery of choice for the posterior circulation if it has adequate diameter and is devoid of disease. If disease is limited to the distal portion of the artery, a short interposition graft, typically a RAG, can be placed from the more proximal OA to the PICA or SCA/PCA. Alternatively, bypass can be accomplished using the STA in rare cases. The most common bypass is OA-PICA, which can provide excellent revascularization of the cerebellum and brainstem and is most commonly used for PICA aneurysms.
Occipital Artery-to-Posterior Inferior Cerebellar Artery Bypass
Dissection of the OA can be challenging and requires knowledge of its anatomical course through the deep layers of posterior neck muscles. The OA originates from the external carotid artery, courses posteriorly and runs horizontally deep to the mastoid tip and origin of the digastric muscle. The artery lies deep to the sternocleidomastoid, splenius capitis, and longissimus capitis, but superficial to the rectus capitis lateralis, superior oblique, and semispinalis capitis muscles. It then pierces the fascia and runs vertically upward, ascending via a tortuous course in the superficial fascia of the scalp.
An inverted U-shaped incision or a post-auricular C-shaped incision is made after mapping the course of the artery with Doppler. The muscle layers are dissected bluntly under the microscope to prevent inadvertent injury to the artery. Typically, the sternocleidomastoid muscle is elevated with the skin flap, and the splenius capitis detached from the superior nuchal line and reflected inferiorly and laterally. Doppler can be used again to locate the artery. The course of the artery is traced back toward the digastric muscle and forward to the point of fascial penetration. Larger branches can be ligated and smaller branches are coagulated and cut, leaving a periadventitial cuff around the artery. The length of artery from its emergence behind the mastoid tip to the point where it penetrates the muscular fascia is typically adequate for most posterior circulation bypasses.
The craniotomy is tailored to the pathology at hand, but is most often a far lateral or extreme lateral partial transcondylar approach, depending on how much exposure is needed. The tonsillar loop of the PICA or the lateral branch of the AICA that runs posterior to the eighth cranial nerve is dissected free and isolated. The anastomosis can be performed either end to end or, more often, end to side. Mild hypertension is initiated and temporary clips are placed on the donor and recipient vessels. A 3-mm linear arteriotomy is made in the recipient vessel and the donor vessel is prepared by fish-mouthing if necessary. After irrigation of the artery with heparinized saline to clear any blood, anchoring stitches are placed first at the heel and then the opposite end with 10–0 nylon sutures. The backside of the anastomosis is sewn using running sutures that are left loose and tightened down at the end. The vessel is reflected and the opposite side is anastomosed with interrupted sutures that are tied after all the sutures have been placed. Prior to tying the last suture, the lumen is flushed with heparinized saline and back-bled from the PICA. Small leaks can be controlled with Gelfoam or an interrupted reinforcing suture can be placed. A watertight dural closure is usually not possible, but circumferential sutures are placed to re-approximate the dura mater and are reinforced with fibrin glue. The muscle and skin layers are then closed tightly to prevent cerebrospinal fluid leak, taking care not to disturb or kink the bypass graft.
Occipital Artery-to-Extradural Vertebral Artery Bypass
In cases where flow augmentation is needed for extradural vertebral pathology and the OA is of a large enough caliber, a bypass can be performed directly to the vertebral artery (VA) in an end-to-side fashion. The distal bypass site is typically in the extradural V2 or V3 segments of the artery. This type of bypass is most commonly done for cases of V2 segment stenosis where the V3 segment is patent. For vertebral origin stenosis (V1 segment), options include VA-to-common carotid artery transposition 10 or external carotid artery-to-VA high-flow bypass, which will be discussed later.
Related posts:
25 Brainstem and Thalamic Intraparenchymal Hemorrhage
29 Endovascular Management of Brainstem and Thalamic Arteriovenous Malformations
30 Surgery for Thalamic and Brainstem Cavernous Malformations
23 Brainstem Ischemia, Stroke, and Endovascular Revascularization of the Posterior Circulation
27 Endovascular Management of Aneurysms of the Posterior Circulation
32 Stereotactic Radiosurgery for Arteriovenous Malformations of the Basal Ganglia, Thalamus, and Brainstem
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