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Ann Thorac Surg 1998;65:346-351
© 1998 The Society of Thoracic Surgeons
Division of Thoracic and Cardiovascular Surgery, Surgical Center, Hannover Medical School, Hannover, Germany
Department of Neuroradiology, Surgical Center, Hannover Medical School, Hannover, Germany
Dr Heinemann, Thoracic and Cardiovascular Surgery, Tübingen University Hospital, Hoppe-Seyler-St 3, D-72076 Tübingen, Germany.
Presented at the Poster Session of the Thirty-third Annual Meeting of The Society of Thoracic Surgeons, San Diego, CA, Feb 3–5, 1997.
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Abstract |
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Methods. Between September 1993 and March 1996, 46 patients (mean age, 57 years; range, 25 to 73 years) underwent spinal angiography at our institution, 23 for an aneurysm and 23 for chronic dissection. Localization of the artery of Adamkiewicz between T-9 and L-3 was successful in 30 (65%) patients: T-9, left = 2, right = 1; T-10, left = 4; T-11, left = 10, right = 2; T-12, left = 3, right = 1; L-1, left = 1, right = 2; L-2, left = 2, right = 1; and L-3, left = 1. Thirty-one patients subsequently underwent replacement of the descending thoracic aorta and 13 underwent replacement of the thoracoabdominal aorta. Left atrial-femoral artery bypass was used in 23 patients and full extracorporeal circulation was used in 20 patients. Twelve procedures included the reimplantation of crucial intercostal/lumbar branches.
Results. The operative mortality rate was 6.8% (3 of 44 patients) and 1 (2.27%) patient had paraparesis. In addition to the 12 patients who underwent targeted reimplantation of the intercostal branches, evaluation of the spinal cord blood supply influenced the operative technique in 19 other patients.
Conclusions. Selective angiography can demonstrate the spinal cord blood supply even in patients with complex aortic pathology. It is a helpful tool for planning extensive replacement of the thoracic and thoracoabdominal aorta.
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Introduction |
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"It is a sure sign that a culture has reached a dead end when it is no longer intrigued by its myths." Greil Marcus, journalist
"Reality leaves a lot to the imagination." John Lennon, musician
The recent surgical success in the replacement of long segments of the descending thoracic aorta and the thoracoabdominal aorta is overshadowed by the unsolved enigma of postoperative paraplegia. Because the spinal cord varies greatly in its sources of blood supply, it is particularly susceptible to ischemic injury. Sudden obstruction of important feeding vessels in the course of aortic dissection can lead to acute paraplegia as the presenting symptom. Chronic malperfusion, caused by atherosclerotic plaques, mural thrombi, or dissection, usually is compensated for by numerous collateral pathways. These, in turn, are at risk of being destroyed during aortic operations.
Perhaps the most important arterial feeding vessel of the thoracolumbar region of the spinal cord is the great anterior radicular artery (arteria radicularis magna), as first described by the pathologist A. Adamkiewicz in 1882 [1] [2]. Its origin from a segmental artery varies greatly, ranging from about the ninth thoracic to the second lumbar artery, on the left side in 80% of cases. It has been postulated that preservation of this vessel, if it is still patent, may play a significant role in protecting the spinal cord [3] [4]. Others recently have denied the significance of retaining an individual vessel for spinal protection, calling the artery of Adamkiewicz "a myth" [5]. This study was undertaken to evaluate the spinal cord blood supply in patients in whom replacement of the descending thoracic or thoracoabdominal aorta was scheduled, and to determine whether these findings would influence the level of the distal anastomosis or the target area of intercostal/lumbar arteries to be reimplanted, or the method of distal perfusion used (left atrial-femoral artery bypass versus full extracorporeal circulation).
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Material and Methods |
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Paraplegia was defined as a permanent and complete inability to move the lower extremities and paraparesis as an incomplete paralysis with the potential for reversal.
Neuroradiologic Technique
Under local anesthesia, a common femoral artery was punctured and 4F or 5F catheters were advanced into the aorta through a 5F introducer set. The type and shape of the catheters were selected individually according to the diameter and configuration of the aorta and the region of interest. To reduce the risk of penetrating the diseased and sometimes fragile aortic wall, all the procedures were begun with a soft-tipped catheter (Spinal Special; Cordis Europe, Roden, the Netherlands).
The angiographic procedures were performed using a high-resolution digital subtraction angiography unit (DSA Neorostar; Siemens, Erlangen, Germany) in posteroanterior projections with 1.5 to 3.0 frames per second. Three to 5 mL of nonionic contrast medium (Ultravist, 240 mg/mL; Schering, Berlin, Germany) was injected into the origin of each segmental artery. After administration of the contrast medium, the catheter was withdrawn immediately to avoid spinal ischemia. Once the supplying artery of the anterior spinal artery was identified, the investigation was stopped. When no feeding vessel could be found, the investigation was considered complete only if catheterization of each potential feeding vessel had been attempted. Premature interruption of the investigation rarely was necessitated by the amount of contrast medium used or the duration of the procedure. The catheters were flushed intermittently with heparinized isotonic saline solution.
Surgical Technique
In 2 of the 46 patients evaluated, replacement of the thoracoabdominal aorta was postponed because of a high operative risk. Thirty-one patients underwent replacement of the descending thoracic aorta and 13 underwent replacement of the thoracoabdominal aorta across the diaphragm and with revascularization of the viscera. Replacement of the upper two thirds of the descending thoracic aorta normally was performed with the aid of left atrial-femoral artery partial left heart bypass with a centrifugal pump [6]. Twenty-three of the 31 patients were operated on in this manner (Table 1).
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In patients who underwent replacement of the thoracoabdominal aorta, the visceral arteries were reimplanted using a single-patch technique, with the left renal artery usually anastomosed separately with a Carrel button. In patients in whom the origin of the artery of Adamkiewicz was identified before operation, the appropriate area of the intersegmental vessels was reinserted into the graft. When the origin of the artery was unknown, a generous patch, usually ranging from T-10 to L-2, was reimplanted.
One patient with a chronic traumatic blowout of the aorta at L-1 after a shotgun wound underwent short-segment replacement from T-12 to L-2 under simple clamping of 22 minutes’ duration. In this patient, the artery of Adamkiewicz had been observed to originate from the left ninth intercostal artery.
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Results |
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Two patients had two great radicular arteries: a large one on the left at T-11 and a small one on the right at L-1 in 1 patient, and a large one on the left at L-3 and a smaller one on the right at L-3 with dorsoventral filling of the anterior spinal artery through the arterial basket of the medullary conus in the other patient. Ten of the 30 patients who had a clearly identified artery of Adamkiewicz coming off an intercostal or lumbar artery had additional intersegmental collaterals feeding this vessel. Further morphologic details concerning these unusual vessels will be published in another article with a neuroradiologic focus.
We encountered no complications from the angiographic investigations, neither neurologic nor related to the puncture site.
Surgical Results
The overall operative hospital mortality rate was 6.8% (3 of 44 operated patients). All deaths occurred from multiorgan problems after replacement of the thoracoabdominal aorta using full extracorporeal circulation (3 of 13 patients, 23%) in elderly patients (67, 68, and 69 years of age, respectively). There was no early mortality after isolated replacement of the descending thoracic aorta. Of the 41 operative survivors, 2 died late from the sequelae of cerebrovascular disease, for a late mortality rate of 4.9%.
The incidence of postoperative spinal damage was 2.27% (1 of 44 operated patients). One patient with megaaorta syndrome had paraplegia that slowly reverted to paraparesis. In this patient, the artery of Adamkiewicz could not be located on spinal angiography because of an extensive mural thrombus. Likewise, no obvious collateral blood supply could be detected. He underwent replacement of the thoracoabdominal aorta from a proximal elephant trunk to the aortic bifurcation under full extracorporeal circulation at 29°C. The only patent segmental arteries seen during the operation were small ones located in the middle thoracic segment, and these were not reimplanted. Retrospectively, this probably should be considered a mistake.
In 12 patients, targeted reimplantation of previously identified segmental vessels giving rise to the artery of Adamkiewicz was performed. In addition to these patients, preoperative evaluation of the spinal cord blood supply influenced the operative technique in some manner in 19 other patients. In several patients, a specific level for the distal anastomosis, lying safely above the critical segment, could be determined. In the patient with the shotgun blowout at L-1 mentioned earlier, it was obvious that the aortic repair could begin well below the artery of Adamkiewicz, and simple clamping was used successfully for short-segment replacement. For patients with negative results on angiography, full hypothermic instead of partial normothermic bypass was used liberally to improve spinal protection.
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Comment |
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Embryologically, capillaries stemming from the segmental arteries first form a paired vascular structure that then fuses over a great length. In humans, remnants of the original paired arrangement usually can be found in the cervical region. Depending on the individual connections of the anterior radicular arteries with their variable formation of ascending and descending branches, a relatively straight arterial chain, still paired in parts, is formed, the segments of which may vary considerably in diameter [11]. Thus, a network rather than a single vessel develops. Its main feeder, the great anterior radicular artery (ie, the artery of Adamkiewicz), commonly is found in the thoracolumbar region. The areas between the major segments of the anterior spinal arterial plexus are frequently only poorly interconnected. As watersheds between adjoining districts of irrigation, these border zones are at risk for ischemic damage [10] [12].
The fact that occlusion of an important tributary intersegmental vessel can cause a sudden onset of paraplegia has been observed repeatedly [13]. Normally, the anterior horns of the spinal cord are affected primarily, resulting in motor dysfunction. One of our patients was admitted to another hospital with acute paraplegia. The diagnostic workup revealed an acute type B aortic dissection. After 3 days, the paraplegia resolved completely. Six months later, the patient was sent to our institution for elective repair of a rapidly expanding aneurysm of the descending thoracic aorta resulting from the dissection. Spinal angiography showed occlusion of the 10th left intercostal artery, which gave rise to the artery of Adamkiewicz. Both vessels were filled retrogradely through collaterals from the 11th left intercostal artery. The formation of these collaterals between the two intersegmental arteries presumably had led to resolution of the neurologic symptoms. At operation, a subtotal replacement of the descending thoracic aorta was performed under extracorporeal circulation at 30°C. Distally, an oblique anastomosis left all the intersegmental arteries below T-10 perfused. The patient’s postoperative recovery was uneventful.
After a series of animal experiments, the technique of sequential double-clamping of the aorta was proposed by members of our group [14] [15] [16] [17]. It was shown that simple aortic cross-clamping leads to a steal phenomenon from the spinal cord arteries to the distal aorta, aggravating spinal ischemia. The placement of a second clamp below the level of the origin of the great radicular artery prevented this drop in pressure within the excluded segment. This was further evidence of the importance of this particular spinal feeding vessel.
The prevention of spinal ischemia always has been a major concern in operations on the thoracic and thoracoabdominal aorta. Today, it is unequivocal that maintenance of distal perfusion with adequate pressure during aortic cross-clamping is the most valuable surgical adjunct. Because passive shunts are unreliable, various forms of extracorporeal circulation are being used.
In our own experience, partial left heart bypass from the left atrium to the femoral artery with a centrifugal pump has been sufficient for the replacement of as much as the upper two thirds of the descending thoracic aorta [6]. Using this standardized technique, 2.3% incidence of spinal cord injury (3 of 132 patients) was achieved, which occurred only in cases of aortic replacement extending beyond the level of T-8. More extensive aortic replacement, however, especially crossing the diaphragm and including revascularization of the visceral vessels, requires extensive cross-clamp times and therefore more efficient organ and spinal cord protection. The use of full extracorporeal circulation with an oxygenator and the help of hypothermia down to the level of circulatory arrest has been recommended [7] [8].
Despite unanimous acceptance of the need for distal circulatory support, the debate regarding whether, which, and how many intersegmental arteries should be revascularized to improve spinal cord protection further still continues. Means of identifying "critical aortic segments" traditionally have used sophisticated neurophysiologic tools, particularly somatosensory evoked potentials [18] [19] [20]. Because somatosensory evoked potentials observe mainly the posterior spinal columns, which are not supplied primarily by the artery of Adamkiewicz, it has been suggested recently that the recording of motor-evoked myogenic potentials should be a more accurate intraoperative monitoring technique [21]. Although precise and safe, this technique is elaborate and time-consuming and requires significant neurologic expertise. This renders it more of a research tool for the present.
Another method of identifying critical aortic branches involves the injection of hydrogen into aortic segments and its detection by an intrathecal platinum electrode [22]. Again, this relatively invasive new technique has been applied only recently in humans and requires repeated cross-clamping of a diseased aorta. All the methods described [18] [19] [20] [21] [22] record pathophysiologic changes and reactions during the operation. Their inherent drawback is that they provide only delayed and indirect information, whereas the operation already is in progress.
The only reliable method of precisely identifying the spinal cord blood supply before operation is the invasive and often time-consuming technique of selective spinal angiography. This has been shown to have a 4.6% incidence of arteriography-related, yet reversible, complications [23], including retroperitoneal bleeding, temporary cerebral ischemia, and transient paresis of the lower extremities from spinal ischemia. Several groups have explored the use of angiography to gain more detailed information about the spinal cord blood supply [3] [4] [23]. Kieffer and colleagues [3] found that spinal ischemia was more frequent when the great radicular artery arose from an intercostal or lumbar artery that was obliterated at its aortic origin but filled through collaterals, or when the spinal cord circulation was interrupted for more than 45 minutes. Their yield of complete identification was 69%. The group from Johns Hopkins [4] [23] was able to identify the great radicular artery in 65% of their patients, and they found that preoperative knowledge of its origin decreased the incidence of postoperative paraplegia.
Against the background of these findings and despite the satisfying results achieved by our group in replacement of the descending thoracic aorta [6], the present study was undertaken to gain more knowledge about the blood supply to the spinal cord. Without expecting to improve neurologic outcome further, we hoped to determine whether localization of the artery of Adamkiewicz would influence the surgical technique used.
Griepp and associates [5] recently questioned the importance of the localized segmental spinal blood supply, calling the artery of Adamkiewicz "a mythical structure." They sequentially clamped all intersegmental arteries of an aortic segment to be replaced before opening the aorta while monitoring somatosensory evoked potentials, and sacrificed them if no electrophysiologic changes occurred. Using other adjuncts, such as distal perfusion, hypothermia, corticosteroids, cerebrospinal fluid drainage, and maintenance of high normal blood pressures, they found that the extent of the aneurysm still was the major determinant of postoperative paraplegia. It did not occur in any patient who had fewer than 10 intersegmental arteries severed. Spinal cord ischemia was reversible after adjunctive maneuvers and led the authors to the conclusion that the spinal cord blood supply is unlikely to depend on a single vessel.
Overall, the following prerequisites for spinal cord protection during aortic operations are accepted today: (1) The ischemic time of the spinal cord must be kept as short as possible. (2) Intraoperative distal perfusion at sufficient pressure is mandatory. (3) An adequate blood supply to the spinal cord must be preserved, ideally by the reimplantation of branch arteries. (4) Postoperative periods of hypotension must be avoided [20] [24]. To ensure that the third condition is met, methods for preoperative visualization and intraoperative identification of important areas of blood flow are of critical importance and should be available in specialized centers.
Given our findings, as well as the experiences of other groups, we have come to the following conclusions:
By following the algorithm shown above for spinal cord protection in patients undergoing replacement of the thoracic and thoracoabdominal aorta, our group was able to prevent spinal ischemia quite efficiently. Whereas we do realize that selective spinal angiography does not prevent postoperative paraplegia or necessarily improve neurologic outcome, we believe that additional information about the spinal cord blood supply can be helpful for determining the operative strategy. In case of doubt, a more aggressive regimen of spinal cord protection probably should be pursued.
Systematic evaluation of the data gained also provides insight into the still vexing pathophysiology of one of the most detrimental complications of aortic operations. Therefore, it should be investigated further at institutions that have both the necessary surgical as well as the neuroradiologic expertise. It remains true, however, that "you can’t always get what you want—but if you try some time, you just might find that you get what you need" [25].
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