Heart India

REVIEW ARTICLE
Year
: 2015  |  Volume : 3  |  Issue : 3  |  Page : 61--65

Spinal Cord Ischemia after Thoracoabdominal Aortic Procedures


Rupesh George 
 Department of Cardiology, Amala Institute of Medical Sciences, Thrissur, Kerala, India

Correspondence Address:
Dr. Rupesh George
Department of Cardiology, Amala Institute of Medical Sciences, Thrissur - 680 555, Kerala
India

Abstract

Overall prevalence of Thoraco abdominal aneurysm has increased due to widespread use of imaging techniques and aging population. Surgical aneurysm repair and endovascular stent graft repair have refined as successful treatment modalities in preventing aneurysm progression and rupture. Since spinal cord depends on branches of thoracoabdominal aorta for blood supply ,spinal cord ischaemia is a dreadful complication of these procedures. However recent animal experiments and surgical series thrown light in tackling this anatomical obstructions by physiologic means. The adoption of techniques for avoiding hypovolumea, hypotension, CSF pressure has reduced this complication rate from 23% to 2-6%.



How to cite this article:
George R. Spinal Cord Ischemia after Thoracoabdominal Aortic Procedures.Heart India 2015;3:61-65


How to cite this URL:
George R. Spinal Cord Ischemia after Thoracoabdominal Aortic Procedures. Heart India [serial online] 2015 [cited 2020 Sep 26 ];3:61-65
Available from: http://www.heartindia.net/text.asp?2015/3/3/61/157285


Full Text

 INTRODUCTION



Paraplegia or paraparesis is a recognized complication of both open and endovascular repair of aortic aneurysms and dissections. [1],[4] Alexis carrel, the pioneer French vascular surgeon and Nobel laureate commented in 1910 regarding the aortic surgeries is that, the main danger of the aortic procedures does not come from the heart or the aorta but from the nervous system. The earlier studies show the incidence of spinal cord ischemia as high as 23%. However, with the advent of better surgical techniques and provision for early detection and correction of spinal cord ischemia, in recent series the incidence was brought down to 4-7% of cases. [9]

 Relevant Anatomy and Blood Supply



The spinal cord is the extension of medulla from the foramen magnum up to the level of first or second lumbar vertebrae. Anterior (ventral) and posterior (dorsal) nerve roots arise from each segment and join to form the spinal nerves. The blood supply of spinal cord can be divided into spinal arteries and reinforcing radicular arteries. [1],[2]

Spinal arteries

Spinal arteries are branches of both vertebral arteries. The two anterior spinal arteries fuse together to form one major anterior spinal artery (ASA) just after its origin from vertebral artery and descends down through the anterior median fissure. Two posterior spinal arteries run along the posterolateral aspect of the spinal cord.

These spinal arteries become small as it traverses down and are reinforced at various levels [Figure 1].{Figure 1}

Reinforcing radicular arteries

There are 31 pairs of radicular arteries, they are branches of

Subclavian - superior intercostal branches of deep cervical artery,Thoracic and lumbar aorta - intercostal and segmental arteries, Branches of internal iliac artery [Figure 2].{Figure 2}

The extent of collateralization is not homogenous across the whole length of the spinal cord, which leaves some areas more vulnerable for ischemia. Even though, there are 31 pairs of radicular arteries, the number of anterior radicular arteries, which supply the ASA are much less than posterior radicular arteries, which reinforces the posterior spinal artery circulation. The cervical ASA receives a mean number of 0-6, the thoracic ASA receives 1-4, and lumbar ASA receives 1-2.

Artery of Adam Kiewicz (arteria radicularis magna)

This is the largest anterior radicular artery that reaches the spinal cord. It arises from the left side of the aorta in 80% and traverse between T9 and T12 roots in 75% of people. Variation from T5 to L2 was seen in the rest [Figure 3]. [5] {Figure 3}

Water shed effect

The pattern of blood flow into the spinal cord tissue is centrifugal in the areas supplied by the ASA and centripetal in the posterior spinal artery region. At the union of the radicular artery and ASA the blood courses upward and downward from the entry point. The watershed effect occurs where two streams of blood flowing in the opposite direction meet. So in the area of the ASA between neighboring radicular arteries, there is a dead point where blood flows in neither direction. This effect is maximum where the distance between the radicular arteries is the farthest. The scarcity, wide spacing, and watershed effect make the mid and lower thoracic aorta most susceptible for ischemic insult. [2]

Crawford classification of thoracoabdominal aortic aneurysm [18]

Type I: Involves the subclavian artery and extends up to the renal artery.

Type II: Involves the subclavian artery and extends to the bifurcation of the aorta in the pelvis. This aneurysm spans the entire length of the thoracoabdominal aorta.

Type III: Involves the middle of the descending aorta and extends to the bifurcation of the aorta in the pelvis.

Type IV: Involves the upper portion of the abdominal aorta and extends up to the bifurcation of the aorta in the pelvis.

Zoli's et al. observed that the spinal cord ischemia is more in Type II and Type III aneurysms (15% vs. 3%).

 Animal Studies



Fried and Asparascio ligated the artery of adamkiewicz and the ASA just above and below of the entrance of adam kiiewicz entrance in monkeys. The ligation above the entrance of artery of adam kiewicz produced less neurological deficits when compared to its ligation below its entrance, which led to paraplegia in 82% of monkeys. [5]

Griepp et al. [1] conducted an elegant study of spinal cord circulation and impact of ischaemia in juvenile pigs. A mixture of methylmethacrylate was injected into the circulation of few pigs in the native state and subsequently in other pigs after the sacrifice of all intercostal and lumbar segmental arteries. They found the development of an extensive network of vessels connecting paraspinal muscles and the spinal arteries as early as 24 h of onset of ischemia. Their experiments challenged the previous notion of overemphasize of a single prominent radicular artery.

 Preventive Strategies



The rationale of preventive strategies is that anatomic occlusion of few blood supply to the spinal cord can be overcome by physiologic methods periprocedurely. These include strategies to augment spinal cord blood flow, increase spinal cord tolerance, decrease ischemic time and early detection of spinal cord ischemia.

Techniques to augment spinal cord perfusion

Peroperative events such as hypotension, hemorrhage, hypovolumia or increased cerebrospinal fluid (CSF) pressure, which all decreases spinal cord perfusion have found to increase the risk of paraplegia in aortic procedures.

Reduce/stop antihypertensives medication on the day of procedure.Adequate hydration should be ensured.Mean arterial pressure should be kept >80 mmHg.Increase the mean arterial pressure by >5 mmHg if spinal cord ischemia sets in.Lumbar CSF drainage if CSF pressure increases above 10 mmHg.

Lumbar cerebrospinal fluid drainage

Coselli et al. [13] done a randomized study of CSF drainage in 145 patients who underwent surgical thoraco abdominal aortic aneurysm repair. The target CSF pressure was 10 mmHg in the intervention group. Despite the two groups had similar risk factors the incidence of paraparesis was significantly higher 13% versus 2.6% in the control group.

Rationale for cerebrospinal fluid drainage

During thoracoabdominal surgical procedures, proximal descending aorta clamp and surgical retraction of arch lead to elevated CSF pressure. When CSF pressure is more than the venous pressure, it leads to the closing of veins (CSF pressure > venous pressure = closing pressure of veins). Hence, spinal cord perfusion pressure is a function of the mean arterial pressure minus the lumbar CSF pressure (spinal cord perfusion pressure = mean arterial pressure - lumbar CSF pressure). So to increase spinal cord perfusion we have to increase the mean arterial pressure and decrease the CSF pressure. [21],[24]

Technique of drainage

A 5.0 F silicon lumbar catheter is introduced (24 h prior to heparinisation) into L3/L4 subarachnoid space via touhy needle. The catheter is connected to the pressure transducer and a drainage set. Transducer zero point set at patients level of the spine. When CSF pressure exceeds 10 mm Hg fluid is drained with the aid of gravity at a maximum rate of 15 ml/h. Usually catheter will be kept for 2 days and cessation of heparin prior to the removal of the catheter is also ensured.

Complications of cerebrospinal fluid drainage

The reported complications include spinal/epidural hematoma, inflammatory reaction, meningitis, persistent CSF leaks, and subdural hematoma. The incidence of these complications is reported in 3.7% of cases. [22],[23]

Techniques for increasing tolerance of spinal cord ischemia

Hypothermia decreases the metabolic demands, protect the cell by stabilizing membranes and attenuates the inflammatory and excitotoxic response to ischemia during reperfusion.

Mild systemic hypothermia (32°C-34°C) is typically employed for spinal cord protection prior to aortic cross-clamping. Deep or profound hypothermia in the range of 10°C-18°C requires cardiopulmonary bypass. This is employed during aortic arch repair requiring temporary interruption of cerebral blood flow. Regional spinal cord hypothermia to as low as 26°C has been used in some series. The most commonly employed technique is an infusion of 4°C saline solution into the epidural space via an epidural catheter placed in the T11-T12 vertebral interspace. [19]

 Pharmacological Agents



The role of pharmacological agents used for neuroprotection is yet to be proven. However, in most of the centers methylprednisolone 1 g I.V, mannitol 12.5-25 g I.V, magnesium 1-2 g I.V, lidocaine 100-200 mg I.V or thiopental 0.5-1.5 g I.V is often administered. [24]

 Early Detection of Spinal Cord Ischaemia



Early detection of spinal cord ischemia intraprocedurely and corrective measures to augment spinal cord blood flow can reduce the incidence of paraplegia. Intraoperative monitoring of somatosensory evoked potentials (SSEPs) or motor evoked potentials (MEPs) has been found to be useful in detecting spinal cord ischemia in anaesthetized patients. [10],[12] So strategies like vessel reimplantation, appropriate CSF fluid drainage, hypothermia, increasing mean arterial pressure can reverse the neurologic abnormalities.

Somatosensory evoked potentials detects the integrity of lateral column of the spinal cord while MEP detects the anterior column functioning making the second one a superior option, but it is technically complex [Figure 4].{Figure 4}

Intraoperative monitoring of SSEP is performed by placing stimulating electrodes on the skin adjacent to peripheral nerves in the arms or legs. Electrical stimulation of the peripheral nerves generate potentials that can be measured from electrodes at 3 levels (popliteal fossa, cervical and cortical). The Rule of 10/50 implies a latency of 10 s and decrease in amplitude by 50% from baseline recording considers spinal cord ischemia.

Motor evoked potential involved measuring the extremity muscle myogenic potential after delivering a stimulus over the motor area. This is more sensitive, But technically complex and has to avoid neuromuscular blockade.

American College of Cardiology/American Heart Association recommends either of these modalities as a class IIb intervention. [26]

 Treatment Modality and Incidence



Studies analyzing surgical repair and stent graft repair could not find statistically significant difference in the incidence of this complication. Cases of spinal cord ischemia in open surgical repair have an early onset, more profound deficit and less chance to resolve when compared with stent graft procedures.

 CONCLUSION



Spinal cord ischemia is a dreadful, but most often a preventive complication of thoracic aortic aneurysm surgeries and stent graft repairs. Avoiding hypotension, hypovolemia and hyperthermia are effective measures for prevention. Lumbar CSF pressure should be monitored at least in high-risk cases. SSEP monitoring is useful in early detection in surgical aneurysm repairs.

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