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Ann Thorac Surg 2003;76:1967-1971
© 2003 The Society of Thoracic Surgeons

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Original article: cardiovascular

Anatomical study of blood supply to the spinal cord

^a Departments of Thoracic and Cardiovascular Surgery,, Sapporo Medical University School of Medicine, Sapporo, Japan
^b Anatomy, Sapporo Medical University School of Medicine, Sapporo, Japan

^* Address reprint requests to Dr Morishita, Department of Thoracic and Cardiovascular Surgery, Sapporo Medical University School of Medicine, South 1 West 16, Chuo-ku, Sapporo 060-8556, Japan
e-mail: kmori{at}sapmed.ac.jp

Presented at the Poster Session of the Thirty-ninth Annual Meeting of The Society of Thoracic Surgeons, San Diego, CA, Jan 31–Feb 2, 2003.

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
BACKGROUND: Low incidences of spinal cord ischemia after thoracoabdominal aortic aneurysm repair, despite sacrifice of all segmental arteries, have recently been reported. This, however, cannot be explained by previous anatomical findings, which prompted us to perform an anatomical study of blood supply to the spinal cord.

METHODS: Fifty-five spinal cords from Japanese formol-fixed cadavers (mean age, 79 ± 10 years) were studied. Diameters of the anterior spinal artery (ASA) above and below the junction with the arteria radicularis magna (ARM) and diameters of the ARM were measured using the NIH image program (National Institutes of Health Image 1.58).

RESULTS: The degree of narrowing of the ASA, defined as the diameter above the ARM expressed as a percentage of the diameter below the ARM, ranged from 23% to 161% and averaged 66% ± 30%. The degree of narrowing was plotted against the ARM diameter divided by the ASA diameter above the junction to examine the impact of the degree of narrowing on distal spinal blood flow from the ARM. The degree of narrowing was related to distal spinal blood flow from the ARM (r= 0.56, p < 0.0001).

CONCLUSIONS: The degree of narrowing of the ASA varies considerably. Furthermore, distal spinal blood supply becomes progressively dependent on the ARM as the narrow point of the ASA becomes narrower. These anatomical findings of spinal blood supply should be useful for elucidating the mechanisms of spinal cord injury after repair of extensive thoracoabdominal aneurysms.

	Introduction

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Spinal cord ischemia still occurs in 5% to15% of patients undergoing extensive thoracoabdominal aortic aneurysm repair [1–3], although many efforts have been made to reduce the rate of paraplegia or paraparesis. Spinal cord injury generally results from temporary or permanent interruption of spinal cord blood supply. The complexity of spinal cord circulation has puzzled surgeons for many years. In addition to considerable variations in normal anatomy, occlusion of segmental arteries due to mural thrombus or atherosclerotic change has made it difficult to elucidate the pathogenesis of spinal cord ischemia. This is why there are conflicting views regarding the appropriate strategy (sacrifice vs aggressive reattachment) for reconstruction of the segmental arteries in thoracoabdominal aortic aneurysm repair.

Griepp and colleagues [4] sequentially clamped each pair of intersegmental arteries and subsequently sacrificed them if no change in somatosensory evoked potentials occurred within 8 to 10 minutes after occlusion. The rate of paraplegia in their patients was only 2% despite the fact that intersegmental arteries had not been reattached. They speculated that the anterior spinal artery (ASA) was functionally continuous with multiple input arteries throughout its length. Acher and colleagues [5] reported that immediate oversewing of all intercostal arteries resulted in a rate of spinal cord ischemia of only 3.4%. In contrast, Safi and colleagues [6] claimed that reattachment of T9 to T12 significantly prevented neurologic deficit. Svensson and colleagues [7] also concluded that the failure of successful reattachment of critical segmental arteries caused spinal cord ischemia.

It is generally believed that the ASA becomes extremely narrow above the junction with the arterial radicularis magna (ARM) and that spinal blood circulation below the junction depends on the ARM [8]. However, the anatomical findings cannot explain the low postoperative rate of spinal cord ischemia despite sacrifice of all intersegmental arteries. This has prompted us to perform an anatomical study of distal spinal blood supply.

	Material and methods

Top Abstract Introduction Material and methods Results Comment References

 
Fifty-five spinal cords from Japanese formol-fixed cadavers (29 males and 26 females) with a mean age of 79 ± 10 years (range, 49 to 97 years) were studied. The causes of death did not include any significant aortic disease. Dissection of the spinal cord has been described in detail elsewhere [9]. Briefly, with each cadaver placed in the prone position, laminectomy of the vertebrae was performed. The spinal cord was removed from the fifth thoracic vertebral level to the second lumbar vertebral level. The left T12 intercostal artery was marked to investigate the vertebral level and laterality of the ARM. Dye was not used in this study, based on our previous work that there is no difference between diameters of the ARMs that are injected with dye and those that are not in formol-fixed cadavers [9]. The ASA and ARM were dissected along their course. After checking the continuity of the ASA, its diameters 1 cm above and below the junction with the ARM were measured. The diameter of the ARM 1-cm away from the junction was also measured (Fig 1). The NIH image program was used for the measurements [10]. The ARM was defined as the largest of the anterior radicular arteries joining the ASA with a hairpin turn.

View larger version (23K): [in this window] [in a new window]   Fig 1. Measurement of the ASA and the ARM. Diameters of the ASA 1 cm above (A) and below (B) the junction with the ARM were measured. The diameter of the ARM 1 cm away from the junction was also measured (C). (ARM = arterial radicularis magna; ASA = anterior spinal artery.)

	Results

Top Abstract Introduction Material and methods Results Comment References

 
The ASA was continuous from the fifth thoracic vertebral level to the cauda equina in all cases. The ARM arose from T7–T8 segment in 10%, T9–T11 in 68%, T12–L1 in 16%, and L2–L4 in 6%. The origin of the ARM was from the left side in 43 cases (78%).

Diameters of the ASA 1 cm above the junction with the ARM ranged from 0.096 mm to 0.720 mm (mean diameter, 0.337 ± 0.122 mm), diameters of the ASA 1 cm below the junction with the ARM ranged from 0.266 mm to 0.839 mm (mean diameter, 0.545 ± 0.141 mm), and diameters of the ARM 1 cm away from the junction ranged from 0.257 mm to 1.066 mm (mean diameter, 0.608 ± 0.165 mm). The diameters were significantly different (ASA above junction vs ASA below junction, p < 0.001; ASA above junction vs ARM, p < 0.001; ASA below junction vs ARM, p = 0.0371).

For comparison of the degrees of narrowing in the ASA, the diameter above the junction was expressed as a percentage of the diameter below the junction. The percentage ranged from 23% to 161% and averaged 66% ± 30% (Fig 2). The degree of narrowing of the ASA varied from patient to patient. There were three cadavers with a percentage of more than 120%, indicating that the ASA diameter above the junction is much wider than that below the junction. Each had the ARM and the lower lumbar artery supplying the lower lumbar spinal cord. The lower lumbar artery also showed a hairpin bend.

View larger version (15K): [in this window] [in a new window]   Fig 2. Distribution of narrowing in the ASA. For representation of the degrees of narrowing in the ASA, its diameter above the junction was expressed as a percentage of the diameter below the junction. (ASA = anterior spinal artery.)

View larger version (21K): [in this window] [in a new window]   Fig 3. Correlation between narrowing of the ASA and impact of blood flow on distal spinal cord circulation. To show which artery impacts distal spinal blood flow, the ARM diameter was divided by the ASA diameter above the junction. (ARM = arterial radicularis magna; ASA = anterior spinal artery.)

	Comment

Top Abstract Introduction Material and methods Results Comment References

Biglioli and colleagues have suggested that such ligation of the segmental arteries can be justified due to anatomic continuity of the ASA [12]. However, as pointed out by Svensson [13], the narrow point of the ASA reduces blood flow of the distal spinal cord. He used the law of Hagen-Poiseuille to show that blood cannot sufficiently flow down the ASA through the narrow point. According to his theory, if all segmental arteries are sacrificed, the distal flow through the narrow point will be so small that ischemia cannot be prevented from occurring. In such a situation, instead of segmental arteries, a collateral pathway can supply blood to the distal spinal cord. It is expected that most patients have collateral circulation for supplying blood to the distal spinal cord based on the fact that few patients have suffered from spinal cord ischemia despite the sacrifice of all segmental arteries. However, in an angiographic study by Kieffer and colleagues, the ARM was visualized via anastomotic circulation in a fourth of almost 400 patients with thoracic or thoracoabdominal aortic aneurysms [14]. In other words, collateral circulation takes the place of segmental arteries to supply blood to the distal spinal cord in only one fourth of patients.

Another anatomical explanation is required for surgeons to understand the mechanism of postoperative spinal cord ischemia. We hypothesized that the degree of narrowing of the ASA varied from patient to patient. However, we have little information on the degree of ASA narrowing. The present study focused on degrees of narrowing of the ASA. The degree of narrowing, defined as diameter of the ASA above the junction expressed as a percentage of its diameter below the junction, ranged from 23% to 161% in the cadavers we examined. The presence of a slight narrowing may account for low incidences of spinal cord ischemia despite sacrifice of segmental arteries.

In addition to the large range of degrees of narrowing of the ASA, our study showed that distal spinal blood supply becomes progressively dependent on the ARM as the narrow point of the ASA becomes narrower. For example, in patients with an extremely narrow point, the ARM provides the distal spinal cord with 202 times greater blood flow than that through the upper ASA [11]. When the size of the ASA below the junction is equal to that above the junction, blood flow through the ARM is reduced to about double that from the upper ASA. When the diameter of the ASA below the junction is narrower than that above the junction, blood flow from the ASA is almost the same as that from the ARM. Interestingly, the lower lumbar arteries with a hairpin bend inevitably supplied the lumbar spinal cord in the latter cases.

The anatomical findings in the present study suggest that if the narrow point of the ASA is extremely narrow, reattachment of the intersegmental arteries may be required; if the ASA has no narrow point, or only a slightly narrow point, sacrifice of the intersegmental arteries can be justified, and if, conversely, the diameter of the ASA above the junction is larger than that below the junction, the lower lumbar artery may play an important role in lumbar spinal cord circulation [3, 15]. There are many ways (such as administration of neuroprotective agents) other than the anatomical maintenance of spinal cord blood supply to prevent spinal cord injury. However, the most important problem that surgeons face intraoperatively is which segmental arteries should be reattached. Aggressive reattachment increases aortic cross-clamping time, though some of the reattached arteries may not need to be reattached [13]. On the other hand, some authors [4, 5] have reported that a low rate of paraplegia was achieved without reattachment of a single intersegmental artery. However, their strategy is not effective for extensive aortic dissection. Most surgeons agree that reattachment of only the arteries that need to be reattached should be performed. To achieve such reattachment, an accurate diagnostic tool for preoperatively identifying the anatomy of spinal cord circulation, including the ASA, is required.

Spinal arteriography has been performed for preoperative localization of the segmental arteries supplying the spinal cord, but this technique has not demonstrated the ASA in detail [14]. Magnetic resonance imaging (MRI) angiography has emerged as a new noninvasive method for detection of the ARM [16, 17]. Based on the anatomical findings in the present study, we have recently started examining the narrow point of the ASA as well as localization of the ARM. Although currently available MRI technology does not enable precise measurement of the diameters of the ASA, a narrow point can be identified. When the ASA does not have a narrow point, the segmental arteries can be sacrificed. If the ASA has a narrow point, the segmental artery that gives rise to the ARM is reattached. We are investigating whether performing reattachment of the segmental arteries based on such an assessment has an effect on postoperative paraplegia.

There are several limitations in the present study. First, we did not investigate the effect of arteriosclerosis on our data. We did actually examine the presence of arteriosclerosis in spinal cord circulation. However, the results were not reported in this paper because description of findings of arteriosclerosis in our cadavers will be published in another article focusing on vascular arteriosclerosis of the spinal cord. Briefly, there was no arteriosclerosis in the intercostal arteries, anterior radicular arteries, or anterior spinal artery, though some of them had the orifices of their segmental arteries occluded by arteriosclerosis. Jacobs and colleagues [15], based on their experience, speculated that only the orifices of the segmental arteries are occluded with aortic plaques and that their lumen can be still patent. Previous anatomical studies have shown that arteriosclerosis seldom occurs in the spinal artery [18, 19]. It therefore seems that the presence of arteriosclerosis does not greatly affect the anatomy of spinal cord circulation.

Second, only Japanese cadavers were used in this study. It is unknown whether the anatomical findings in this study are applied to Western people. The present study demonstrated that the degree of narrowing of the ASA varies from patient to patient and that distal spinal blood supply becomes progressively dependent on the ARM as the narrow point of the ASA becomes narrower. However, there have been no reported findings regarding these issues for other races. A similar study in a Western country is needed to determine whether there is ethnic variability in this anatomic factor.

Third, we did not distend the vessels by injecting dye. Unlike fresh cadavers, the tissues of formol-fixed cadavers are so hard that the arteries cannot be distended by even high-pressured injection of dye. Since this was confirmed in a previous study [9], we did not use dye. Consequently, the ASA and ARM were smaller than previously reported diameters. However, the main focus of this study was not to measure the ASA and ARM diameters but to investigate degrees of narrowing of the ASA. The investigation was free from bias by expressing the ASA diameter above the junction as a percentage of its diameter below the junction.

In conclusion, the degree of narrowing of the ASA varies considerably. Furthermore, distal spinal blood supply becomes progressively dependent on the ARM as the narrow point of the ASA becomes narrower. These anatomical findings of spinal blood supply should be useful for elucidating the mechanisms of spinal cord injury after repair of extensive thoracoabdominal aneurysms.

	References

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