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Vascular Complications in Spinal Surgery
Introduction
A vascular complication may be defined as an injury to a blood vessel and the sequelae of that injury, which may be the direct or indirect result of the procedure, surgical approach, or operative technique. For the purposes of this chapter, a vascular complication may arise directly from vascular injury or indirectly as a consequence of the injury. Mechanisms of blood vessel injury include laceration, traction, instrumentation malpositioning, and compression. The spectrum of manifestations of vascular injuries include hemorrhage from laceration or traction of a vessel wall ; aneurysm or arteriovenous fistula resulting from vessel wall injury; blood flow interruption at the macroscopic end organ level or ischemia on a microscopic level ; embolization of air, plaque, or clot; and thrombosis or direct injury from instrumentation, including erosions from instrumentation in contact with pulsatile arteries and malpositioned screws ( Fig. 97.1 ). The most common vascular complication in spinal surgery, however, is direct laceration of a vessel resulting in acute bleeding.
Vascular complications associated with spinal surgery continue to increase in frequency, although the rate of increase is decelerating. From 1997 to 2003 the frequency of vascular complications reportedly quadrupled, but from 2003 to 2010 the rates of vascular complications have increased by less than 50%. This could be a result of an increasing frequency of anterior approaches to the thoracolumbar spine or an early trend toward increased documentation and reporting. Between 2003 and 2010 the rates of venous thromboembolism, hemorrhagic anemia, and postoperative shock increased by about 150% in thoracic spine surgery, whereas the rates of these complications increased by less than 50% in cervical and lumbar spine surgery.
It is also possible that vascular complications are underreported and the true incidences are impossible to know. For example, multiple reviews of cervical spine surgery have reported no vascular complications, but in a questionnaire study that covered 5641 cases, vertebral artery injuries were noted in 0.14% of cases. A hypothetical example of this underreporting would be a laceration of the left common iliac vein occurring during an anterior lumbar spinal exposure at L4–L5. The vascular and spinal surgeons recognize this injury and rapidly isolate and repair it. Although intraoperative blood loss is increased, there may be no other sequelae. Some surgeons do not consider this a complication, but rather a routine risk of exposure of the lumbosacral spine and therefore may not note or report it. A recent review of a prospective database of 1262 consecutive anterior approaches to the thoracolumbar spine found a 1.11% rate of injury to a major vessel. It could also be that with increasing awareness and vigilance the real rate of these injuries has, in fact, decreased.
The consequences of vascular complications in spinal surgery can be severe. Three consequences include brisk bleeding leading to hemorrhagic shock, interrupted vascular supply to vital organs such as the spinal cord or brainstem, and secondary injury as a result of inadequate visualization caused by bleeding. Poor visualization due to inadequate hemostasis can lead to spinal cord or nerve root injury. These consequences are not mutually exclusive and often occur in series or combination. The primary sequelae of bleeding often result from injury to, or disruption of, major blood vessels adjacent to the vertebral bodies. All these complications have consequences that may be irreversible, and for this reason prompt recognition and management are essential when these complications occur.
Prevention should be the primary focus in the discussion of vascular complications of spine surgery. Prevention of vascular complications is assisted by knowledge of the normal vascular anatomy and common variants, including a knowledge of the relationships between particular blood vessels and bony landmarks, by careful preoperative planning, and by the use of gentle intraoperative techniques with appropriate illumination and magnification. This chapter emphasizes the vascular anatomy of the spine and highlights the spectrum of spinal vascular injuries, how they may be recognized early and, most importantly, how they may be prevented.
Cervical Spine
Anterior Cervical Vascular Complications
Exposure of the anterior cervical spine commonly uses an approach medial to the sternocleidomastoid muscle and carotid sheath (Smith-Robinson approach). Vascular complications in the cervical spine are much less often the result of direct injury and much more often the result of improper and excessive retraction. Although the absolute incidence of vascular injuries from anterior cervical spinal surgery is not known, they are rare events. For example, no vascular injuries were reported in reviews of 500 and 450 cases, and a retrospective questionnaire study of 5641 cervical spine cases found an incidence of vertebral artery injury of only 0.14%. Another review reported four cases of carotid artery injury during anterior cervical fusion. In those cases involving laceration of the carotid artery, all of these injuries were repaired primarily and no permanent neurologic sequelae occurred. Although another case series cautioned that the vessels of the carotid sheath were at “some risk” from the self-retaining retractor, no cases of vascular injury were reported. In order to prevent carotid artery injury, self-retaining blunt blades should be carefully placed deep to the longus colli muscles medially and laterally and not against the carotid sheath.
Vertebral artery injuries are also rare but are more common than carotid artery injuries. As a historical example, a single vertebral artery injury was reported in 175 cases of interbody fusion from 1976 to 1990 at a single institution. The artery was sacrificed and there were no adverse consequences for the patient. Another case series reported four cases of vertebral artery injuries in 2015 anterior cervical spine cases for a case series incidence of 0.3%, and another series reported 167 anterior cervical discectomy and fusions for degenerative disease technique without any major vascular injuries. Despite these reports, the overall incidence of vertebral artery injury in cervical spine surgery is nearer to 1.4%, with the lowest risks during anterior approaches to the subaxial cervical spine (0.3–0.5%) and the greatest risks during posterior instrumented procedures of the upper cervical spine (4–8%).
Treatment of the vascular injuries during anterior cervical spine exposure involves recognition, control of bleeding, repair, ligation, and endovascular stents when indicated. Although primary repair of the vertebral artery and common carotid artery is desirable, endovascular techniques are becoming the standard of care.
Vertebral Artery
The vertebral artery and vein are at risk from both anterior and posterior exposures of the cervical spine ( Fig. 97.2 ). This is a unique feature of the cervical vascular anatomy. The vertebral artery is integrated with the spinal column passing through the vertebral foramen at levels superior to C7 and classically coursing anterior to posterior at C1–C2. Detailed anatomic studies have presented the diversity of vertebral artery size, variations, and anomalies. Vertebral artery injuries result in either hemorrhage, vertebral basilar insufficiency stroke, or both ; have classically been thought to occur in fewer than one in 200 anterior cervical surgeries ; and have been known to have devastating consequences. For example, vertebral artery injury during anterior cervical discectomy and fusion has been reported to have resulted in permanent neurologic deficits despite immediate identification of the laceration, control of bleeding, and primary repair. As with other vascular complications of spine surgery, prevention is the best strategy for managing injuries of the vertebral artery.
During anterior exposure the vertebral arteries are at risk of injury by dissection lateral to the longus colli muscles, whereas during anterior discectomy any dissection beyond the lateral margins of the disc and lateral to the uncinate processes puts the vertebral artery at risk ( Figs. 97.3 and 97.4 ). Prevention of vertebral artery injuries laterally begins with careful dissection of the longus colli muscles. The anatomic relationship of the vertebral artery in the transverse foramina has been well described. Beginning at C6 and proceeding to C2, the vertebral artery is tethered to the transverse foramina by a fibroligamentous mesh connected to the adventitia of the vertebral artery, the vertebral body, and the superior margin of the transverse foraminal bony extension ( Fig. 97.5 ). These relationships have implications if anterior vertebral artery laceration repair is attempted, especially for mobilizing and controlling the artery proximal and distal to the laceration.
It is recommended that the medial edge of the muscles be coagulated and a blunt periosteal elevator be used to mobilize the muscles laterally. To prevent injury to the vertebral artery, this periosteal elevator should never be directed posteriorly. The vertebral artery may also be injured by excess lateral dissection in the intervertebral space lateral to the uncinate processes (C3–C6); it is especially important to be aware of this risk in cases that require extensive dissection such as corpectomy (see Fig. 97.3 ). Care should be taken that dissection or bony resection remain medial to the uncinate process, sparing the medial border of the transverse foramen unless otherwise necessary (e.g., in tumor resection). To this end, it is important to note that the medial margin of the longus colli muscle is the most consistent landmark for the lateral extent of bony dissection. Furthermore, to avoid injury to the vertebral artery during corpectomy, most authors recommend a preoperative midline radiograph or computed tomography (CT) scan for any abnormalities of the vertebral artery in the transverse foramina. The availability of three-dimensional (3D) reconstructions of CT angiography (CTA) has greatly enhanced the value of imaging for preoperative planning, especially in cases of variant or anomalous vertebral arteries that may deviate medially, either originating from the aorta or otherwise ascending outside of the vertebral foramen.
It is also important to note that the association between cervical spine trauma and the vertebral artery has been increasingly recognized and studied. A recent review of three level I trauma centers found that 21% of patients with a cervical spine injury underwent screening for vertebral artery injury using CTA. Of those screened, a vertebral artery injury was found in 17%, and of those with a vertebral artery injury, neurologic events secondary to vertebral artery injury occurred in 14%. These findings highlight the importance of preoperative CTA to investigate the course and patency of the vertebral arteries, especially in cases of cervical spine trauma.
If the vertebral artery is lacerated during the anterior dissection, the bleeding should be stopped by tamponade and the dissection carried laterally by mobilizing or dividing the longus colli muscles. This approach to the vertebral artery and cervical nerves, described by Verbiest, also exploits the interval medial to the sternocleidomastoid muscle and carotid sheath. However, unlike the Smith-Robinson approach, Verbiest's exposure proceeds lateral to the longus colli muscle. The surgeon should be aware that this lateralization puts the cervical sympathetic trunk at greater risk of injury, especially at lower cervical spinal levels, and Horner syndrome (ipsilateral ptosis, miosis, and anhidrosis) may result. The costotransverse lamellae and fibroligamentous attachments can be removed to allow adequate exposure for primary vascular repair ( Fig. 97.6A ). If primary repair is chosen, the artery should first be exposed proximal to the laceration, and vessel loops should be used for occlusion ( Fig. 97.6B ). The nerve roots should be identified and protected. Once the artery is exposed and controlled, the laceration can be repaired, if possible ( Fig. 97.6C ). The repair may require the assistance of an experienced vascular surgeon, and spinal stability should be ensured after this additional required exposure. Cross clamping of the injured vertebral artery may be required during the repair, and cross clamp duration should be minimized to decrease the likelihood of infarct resulting from vertebrobasilar ischemia.
Many authors support repair of the vertebral artery injury. Others have ligated the lacerated vertebral artery. The ability of the patient to tolerate unilateral occlusion of the vertebral artery without sequelae is supported by a study of nine patients with traumatic occlusion of the vertebral artery. Only two developed neurologic deficits, both of which were transient. If the vertebral artery is to be sacrificed, it should first be studied by intraoperative angiography to ensure that the patient's anatomy will allow such a maneuver. The presence or absence of neurologic deficit depends in part on the collateralization of the vertebral artery of the circle of Willis and the patency of the contralateral vertebral and basilar arteries. A classic anatomic study reported that occlusion of the vertebral artery led to brainstem ischemia or infarct in 3.1% of left-sided occlusions compared with 1.8% of right-sided occlusions. However, in one report, three out of seven cases of vertebral artery ligation developed symptomatic vertebrobasilar ischemic signs and symptoms. These symptoms included syncope, dizziness, nystagmus, and Wallenberg syndrome. Further controversy regarding whether to ligate or repair a vertebral artery laceration or tear is found in the cervical spine trauma literature, and special considerations may be made in the setting of cervical spine trauma that may or may not apply in elective settings.
Carotid Artery
The common carotid artery is also at risk as the surgeon develops the dissection plane and during retraction for the deep anterior cervical spine exposure (Smith-Robinson approach). Identification of the carotid artery by palpation and gentle finger dissection helps minimize injury of this major vessel. Proper placement of self-retaining retractors requires medial-to-lateral mobilization of the longus colli muscles along the anterolateral aspect of the vertebral bodies. Blunt-tipped blades of these self-retaining retractors must be positioned deep to these muscles, and only enough tension should be applied to retract these muscles away from the vertebral body and disc margins to aid visualization of the vertebra and vertebral disc.
Compromise of carotid artery blood flow risks causing central nervous system ischemia (i.e., ischemic stroke), and prolonged retraction of the artery could lead to thrombosis. Some authors have noted a progressive decrease in carotid blood flow secondary to the use of self-retaining retractors. A study of 15 cases showed that ipsilateral carotid artery blood flow decreased an average of 14% to 70% as measured by duplex ultrasonic flow as the anterior cervical discectomy and fusion procedure proceeded and the arterial cross-sectional area progressively decreased as the surgical duration increased. The diminished blood flow remained laminar at all times and rapidly returned to baseline postoperatively. Younger patients showed a greater drop in blood flow compared with older patients and those with atherosclerotic arteries.
A general strategy to prevent carotid artery ischemia is the intermittent release and repositioning of the retractors, but this repositioning should be done carefully because manipulation of an atherosclerotic artery may dislodge plaques with the unintended consequence of embolic ischemic stroke.
Posterior Cervical Vascular Complications
The vertebral artery and venous plexus are the primary vascular structures at risk during posterior cervical approaches the spine ( Fig. 97.7 ). Posterior vertebral artery injuries are rare, but multiple cases of posterior vertebral artery injury during posterior cervical spinal surgery are reported in the literature. Cranial base surgery continues to pose significant challenges and limitations owing to the variations of this region's cerebrovascular anatomy and physiology.
Instrumentation of the upper posterior cervical spine from the occiput to C2 puts at risk the vertebral artery, carotid artery, and hypoglossal or spinal accessory nerves depending on the technique. Malpositioning of the lateral mass screws may injure the vertebral artery within the transverse foramen ( Fig. 97.8 ). C1–C2 transarticular screws risk the vertebral artery, carotid artery, and hypoglossal nerve anteriorly in the C2 transverse foramen. Preoperative CT may aid prevention of inadvertent injury as screw insertion progresses across the C1–C2 facet. Anomalies and anatomic variations of the geometry of the foramen and the position of the vertebral artery are common. C1 lateral mass screws put the vertebral artery at risk posteriorly and laterally, and they require meticulous technique for mobilization and protection. Preventive strategies include knowledge of the bony landmarks, choice of appropriate screw lengths, and review of angles of insertion ( Fig. 97.9 ).
The vertebral artery and veins are directly anterior or slightly lateral (about 6 degrees lateral at C6) at the middle one-third of the lateral mass. Therefore, to avoid the vertebral artery the lateral mass screw should be started near the center of the mass and directed about 10 to 15 degrees lateral and 30 to 35 degrees cephalad. In contrast, the Magerl technique directs screws directly anterior from the central lateral mass and is more likely to injure a nerve or the vertebral artery. Studies have clearly identified the Magerl technique as most likely to injure the vertebral artery, with an incidence of 1.4% in a survey series of 5641 cases. This technique should be avoided. A consecutive series of 43 patients reported no vertebral artery injuries from lateral mass plating of the subaxial cervical spine.
Placement of posterior C2 screws is routinely used to stabilize type II or high type III odontoid fractures. C1 and C2 injuries should be reduced before attempting fixation to reduce risk of injury to the spinal cord, vertebral artery, or interior carotid artery. The guidewire and screw tip both pose risks of injury to the vertebral artery and internal carotid artery ( Fig. 97.10 ). Radiographic imaging should be used to minimize penetration during initial positioning. The C2 screw should be placed by hand under live fluoroscopy without power reaming or tapping. Once positioned, care should be taken to clear the reamer or screw cannula of residual bone to prevent inadvertent advancement of the guidewire. Although this procedure increases fluoroscopy time, it is essential to carefully monitor the tapping position, as well as the insertion over the guide wire, and to be certain that proximal migration does not occur. Some surgeons employ 3D fluoroscopy or CT image guidance in these cases.
Vertebral Artery
The vertebral artery is at greatest risk as it passes through the upper cervical spine. Most notably, the vertebral artery passes through the foramen of C2 as it extends cranially, then through the C1 foramen before coursing medially toward the midline on the superior aspect of C1. In this area the vertebral artery occupies a groove on the posterior aspect of the posterior arch of C1; anatomic studies have shown that this groove is on average 18 mm lateral of midline, whereas the vertebral artery itself is on average 22 mm from midline.
The vertebral artery is vulnerable because it passes between the C1 and C2 transverse foramina laterally. Lateral dissection along the caudal border of C1 should end on exposure of the dorsal ramus of C2. Due to this unique anatomy of the vertebral artery in the superior cervical spine, some authors have recommended that dissection should begin with identification of the posterior tubercle of the atlas, and that dissection of the posterior aspect of the posterior arch should remain within 12 mm of midline and the dissection should not extend beyond 8 mm of midline on the superior aspect of the arch.
Cervical pedicle screws may be placed more safely at C6 or C7. The screws are started about 2 to 3 mm inferior to the superior articular facet and slightly lateral to the midline of the lateral mass. The screws should be angled medially 35 degrees at C6 or 30 degrees at C7. The C1–C2 technique, which is least likely to risk injury to the anterior vertebral artery, is the modified technique of C1 lateral mass screws and C2 intralaminar crossed screw fixation described in the literature. In about 15.5% of patients, a false bony bridge called the ponticulus ponticus covers the vertebral artery as it passes onto the posterior C1 ring. This is easily noted on lateral cervical plain films ( Fig. 97.11 ). It is important to recognize this common variant to avoid injury to the vertebral artery by dissection or C1 transosseous screw fixation.
Vertebral artery injury posteriorly often occurs during lateral mass screw placement either from penetration or laceration by the drill, tap, screw, or probe. However, any technique that begins near or slightly inferior to the midline of the lateral mass and angles laterally and cephalad 15 to 30 degrees and uses screws that are 14 to 16 mm in length are unlikely to injure the vertebral artery or veins. Of note, the average reported screw size for women was 14 × 3.5 mm compared with 16 × 3.5 mm for men. As with C2 screw placement, drilling should be done by hand and not under power. Also, caution should be used with the depth gauge. Care should be taken to not plunge and possibly pinch the vertebral artery with the curved end of the depth gauge.
C1 lateral mass fixation has proven to be safe, stable, and effective. The relationship of the vertebral artery at C1–C2 and its location to the triangular window of the pedicle of C1 have been emphasized in the literature ( Figs. 97.12 and 97.13 ). Primary clinical challenges are to identify the C2 root and the vertebral artery and to identify and control the venous plexus, which may project inferiorly between the vertebral artery and the posterior C2 dorsal ramus. Typically, the vertebral artery penetrates the ligamentum flavum 8 mm from the C1 tubercle and passes cephalad to the margin of the C1 posterior rim ( Fig. 97.14 ). The level of the C1 lateral mass screw insertion at the inferior border of the C1 arch is typically 12 to 14 mm from the midline ( Fig. 97.15 ). The ponticulus ponticus is a harbinger of injury to the vertebral artery along the cephalad margin, especially if the ponticulus ponticus is not appreciated during placement of C1 lateral mass screws (see Fig. 97.11 ).
C1 lateral mass screws should be started in the “window” of the lateral mass defined on the superior border by the C1 arch and the vertebral artery superior and on the inferior border by the C2 dorsal root ganglion ( Fig. 97.16 ). This window is covered by a plexus of veins that should be coagulated and controlled. Removal of the inferior edge of the C1 arch often assists the placement of the C1 lateral mass or pedicle screw. The medial-most extension of the vertebral artery cephalad to the C1 ring is usually about 8 mm from the C1 tubercle but about 4 more mm lateral and caudal to the C1 ring; this is the optimal starting point for a C1 lateral mass screw.
Image guidance, including fluoroscopic or CT-based systems, increases patient safety during C1 lateral mass screw positioning given the high rate of vertebral artery anomalies in this area. For example, one study found that 3.1% of patients lacked a vertebral artery on the right and 1.8% lacked a vertebral artery on the left, and 9.7% of patients had a significantly narrowed vertebral artery on the right and 5.7% had significant narrowing on the left. This study also found two cases in which the vertebral artery branched directly into the posterior internal cerebellar artery. Numerous minor and major variations are known to occur; for this reason surgeons are strongly encouraged to order preoperative magnetic resonance imaging (MRI) or CT with reconstruction to identify the vertebral artery and its foramen because some variation may preclude screw placement on a given side.
Internal Carotid Artery
The internal carotid artery is also at risk with instrumentation of C1 as the lumen of the internal carotid artery is within close proximity of the anterior aspect of C1. Anatomic studies have shown that the mean distance from the internal carotid artery to C1 is on average 2.9 to 3.7 mm. One study of 160 angiograms found that the internal carotid artery is never medial to of the lateral mass of C1, and with regard to the transverse foramen it was less often medial caudally than cranially. For this reason some authors suggest that lateral mass screws that are angulated inferomedially are safer with respect to the internal carotid artery than screws placed in other trajectories. However, because of the risk of injury to this artery that is posed by a drill bit or the tip of a bicortical screw, some authors have recommended preoperative CTA before procedures in which C1 will be instrumented. If the internal carotid artery is found to be in close proximity to the anterior aspect of C1, it has been suggested that either a unicortical screw should be used or an alternative technique should be considered.
Thoracic Spine
Anterior Thoracic Vascular Complications
Anterior exposure of the thoracic spine is well established and has many applications. This can be done through open or thoracoscopic approaches for the treatment of a variety of disorders but is often used for scoliosis release, fusion, and instrumentation. The absolute incidence of vascular injuries from anterior thoracolumbar surgery may be as low as 1%. One retrospective review of 1262 consecutive patients from 1998 to 2010 found that injury to a major vessel occurred in 1.1% of cases. Another review reported a 5.8% incidence of vascular complications in 207 open anterior thoracolumbar surgeries, with a mortality rate of 1%. This report found a 3.4% incidence of direct vascular injuries of the segmental or intercostal vessels. Most had a delayed onset of sequelae, and one death occurred.
Among the potential vascular-related complications are injuries to the major vessels and their branches and the sequelae of interruption of the blood supply to the thoracic spinal cord ( Fig. 97.17 ). Paraplegia secondary to unilateral vascular interruption of the thoracic segmental vessels is extremely uncommon. Bilateral disruption of the segmental blood supply, however, as in aortic surgery or dissection of aortic aneurysms, does confer a real risk of paraplegia. There are many reports in the thoracic and vascular literature of the consequences of bilateral disruption of the blood flow to the thoracic cord during complex thoracic and thoracolumbar aneurysm reconstructive surgery. One study has shown that autoregulation of smooth muscle of the tunica media of the lower anterior spinal artery may further reduce blood flow to the cord after aortic cross clamping.
Other procedures associated with spinal cord infarction include bilateral sympathectomies ; open thoracoscopy with pulmonary lobectomy ; open anterior surgery for tuberculosis in which bilateral vascular ligation or disruption occurs ; revision anterior scoliosis surgery, especially if approaching the previously undissected side of the anterolateral spine or if the curve has a significant kyphotic component ; and spinal angiography or magnetic resonance angiography (MRA) using the thoracic or abdominal aorta. Video-assisted thoracoscopy (VAT) has been used for a variety of thoracic spine conditions. One European meta-analysis found no vascular complications or paralyses associated with VAT. A singular case of profound epidural bleeding occurred, but no incidence of segmental artery, aorta, or azygos vein injury was noted. Other studies of thoracoscopy have validated the utility and safety of VAT.
Segmental Arteries
The relative importance of individual blood vessels that supply the spinal cord has been debated. It was not until 1939 that it was determined that the segmental vessels that supply the spinal cord are indeed end arteries and that there are no anastomoses between the capillary beds. The vessels that travel with the spinal nerve into the spinal canal do not seem to be important, as evidenced by a report in which three to 16 ipsilateral segmental arteries in a single patient were ligated without neurologic loss.
Theoretically, the thoracolumbar spinal cord tolerates transient and permanent unilateral segmental blood flow disruption. However, bilateral segmental disruption, transient or permanent, is much more likely to cause spinal cord ischemia and consequent paraparesis or paraplegia. The precise incidence of spinal cord infarction and paraplegia due to vascular occlusion is not known, but the real incidence is very low. Morbidity and mortality statistics from the Scoliosis Research Society described an incidence of paraplegia of 1% due to indirect vascular compromise in more than 10,000 deformity cases. Paraplegia rates are greater in adult patients and those treated with posterior instrumentation and fusion for severe scoliosis, kyphosis, kyphoscoliosis, or congenital scoliosis and kyphosis. One case of vascular-induced paraplegia was reported in a series of more than 400 cases of anterior spinal surgery for tuberculosis, and there is a cumulative series of more than 3000 extensive open anterior discectomies or partial corpectomies with unilateral ligation of thoracic segmental arteries without any cases of paraplegia.
The transthoracic approach to the anterior thoracic spine usually requires mobilization of segmental arteries and veins over several vertebral levels. A right-sided approach is recommended to avoid the pericardial structures because in 75% of people the artery of Adamkiewicz originates on the left side of the aorta between T8 and L1. In cases of deformity, however, it is most advantageous to approach the convexity of the deformity, regardless of the side involved. The parietal pleura must be divided to gain access to the thoracic segmental and intercostal arteries. This dissection should begin over the disc space because there are no vessels there. The individual vessels are identified and isolated with a right-angle hemostat and ligated, occluded with vascular clamps, or spared. About 1 cm of segmental vessel should be maintained from the thoracic aorta or vena cava to avoid tearing injury to either of these large vessels. Although it has been shown that better collateralization and anastomotic substitution can occur as the segmental vessels are ligated or disrupted closer to the major vessels, the risk of catastrophic injury to the aorta or vena cava demands caution. Inadvertent laceration or avulsion of the segmental vessels from the aorta or vena cava can result in rapid, voluminous bleeding. Bleeding should be immediately controlled by direct pressure proximal and distal to the defect, and repair of the aorta or vena cava should be assisted by a surgeon experienced in vascular surgery. It should also be noted that segmental vessels can also be injured by inadvertent avulsion or stretching, and vascular injuries may present late (i.e., postoperative hemothorax).
It should be noted that unilateral ligation of thoracolumbar segmental arteries may be acceptable on the convexity of the deformity, but perhaps only in primary surgeries as certain complex revision surgeries may mimic bilateral ligation. Cases in which there is significant kyphosis or kyphoscoliosis—especially in cases of multiple posterior procedures or prior surgeries—may increase the risk of paraplegia with unilateral ligation of particular segmental arteries. For this reason some authors have advocated spinal cord monitoring up to 20 minutes after ligation or clamping of a key segmental artery when the cord is at risk. Winter and colleagues describe a case of revision anterior scoliosis surgery in which the convexity was approached after prior anterior surgery from the concavity. The authors clamped a segmental artery and observed loss of somatosensory evoked potentials (SEPs). The spinal cord monitoring changes were reversible and the anterior discectomy was performed between the segmental vessels to spare spinal cord blood flow. This circumstance illustrates that bilateral thoracolumbar segmental vessel occlusion is associated with a small but definite risk of spinal cord infarction and permanent neurologic deficits. In any case, actual or indirect segmental vessel injuries should be avoided between T8 and L1 on the left. Spinal cord monitoring should be considered routine; motor evoked potentials (MEPs) are often used for this purpose. For monitoring on the anterior spinal cord MEPs are more precise than SEPs which primarily monitor the function of the dorsal columns of the spinal cord.
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