Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Neurologic Complications of Aortic Disease and Surgery
The aorta is the main conduit through which the heart supplies blood to the body, including the brain, brainstem, and spinal cord. In addition, this vessel is situated close to important neural structures. In consequence, both disease of the aorta and operations on it may have profound but variable effects on nervous system function. Often the neurologic syndrome produced by aortic disease or surgery depends more on the part of the aorta involved than on the nature of the pathologic process itself. For example, either syphilis or atherosclerosis may produce symptoms of cerebral ischemia if the disease affects the aortic arch or of spinal cord ischemia if the pathologic process is in the descending thoracic aorta. Even when the nature of the pathologic process is important in determining the resultant neurologic syndrome, several diseases may result in the same pathologic process. Thus, atherosclerosis, infection, inflammation, and trauma may each result in the formation of aortic aneurysms; similarly, coarctation of the aorta may be congenital, a result of Takayasu arteritis, or a sequela of radiation exposure during childhood.
The initial focus of this chapter is on the three major areas of neurologic dysfunction resulting from aortic disease and surgery: spinal cord ischemia, cerebral ischemia, and peripheral neuropathy. Specific conditions that merit special consideration are then discussed individually. The normal anatomic relationships are also considered in order to provide insight into the pathogenesis of the resulting neurologic syndromes.
Clinical Neurologic Syndromes due to Aortic Pathology
Aortic disease may produce a variety of neurologic syndromes. The specific syndrome depends to a large extent on the site of involvement along the aorta.
Spinal Cord Ischemia
Anatomy
Embryologic Development
During embryologic development, primitive blood vessels arise along the spinal nerve roots bilaterally and at each segmental level. Each of these segmental vessels then divides into anterior and posterior branches, which ramify extensively on the surfaces of the developing spinal cord. As development proceeds, most of these vessels regress and a few enlarge, so that by birth, the blood supply to the spinal cord depends on a small but highly variable number of persisting segmental vessels ( Fig. 2-1 ). In the thoracic region, where the aorta is situated to the left of the midline, the persisting vessels entering the spinal canal are those from the left in 70 to 80 percent of cases.
Anterior Spinal Artery
The anterior spinal artery is formed rostrally from paired branches of the intracranial vertebral arteries that descend from the level of the medulla ( Fig. 2-1 ). These two arteries fuse to form a single anterior spinal artery that overlies the anterior longitudinal fissure of the spinal cord. This artery is joined at different levels by anterior radiculomedullary arteries, which are branches of certain segmental vessels ( Fig. 2-2 ). The number of these vessels is variable among individuals, ranging from 2 to 17, although 85 percent of individuals have between 4 and 7.
The anterior spinal artery in the region that includes the cervical enlargement (C1 to T3) is particularly well supplied, receiving contributions from an average of three segmental vessels. One constant artery arises from the costocervical trunk and supplies the lower segments; the others arise from the extracranial vertebral arteries and supply the middle cervical segments. In addition, branches of the vertebral arteries have rich anastomotic connections with other neck vessels, including the occipital artery, deep cervical artery, and ascending cervical artery.
The anterior spinal artery in the midthoracic portion of the cord (T4 to T8) often receives only a single contribution from a small artery located at about T7, most often on the left. The anterior spinal artery has its smallest diameter in this region, and it is sometimes—but not usually—discontinuous with the vessel in more rostral or caudal regions.
The anterior spinal artery in the region of the lumbar enlargement (T9 to the conus medullaris) is, as at the cervical enlargement, richly supplied, deriving its blood supply predominantly from a single large (1.0 to 1.3 mm in diameter) artery, the great anterior medullary artery of Adamkiewicz. This artery almost always accompanies a nerve root between T9 and L2, usually on the left, although rarely it may accompany a root above or below these levels. Identification of the actual location of this great vessel has become an important part of the planning and execution of operations on the aorta such as repair of thoracoabdominal aortic aneurysms. Although digital subtraction angiography has been used for this purpose, the use of contrast-enhanced magnetic resonance angiography is a noninvasive alternative. Caudally, at the conus medullaris, the anterior spinal artery anastomoses with both posterior spinal arteries.
Posterior Spinal Arteries
The paired posterior spinal arteries form rostrally from the intracranial portion of the vertebral arteries. They are distinct paired vessels only at their origin, however, and thereafter become intermixed with an anastomotic posterior pial arterial plexus ( Fig. 2-3 ). This plexus is joined at different levels by a variable number (10 to 23) of posterior radiculomedullary vessels that accompany the posterior nerve roots.
Intrinsic Blood Supply of the Spinal Cord
In contrast to the extreme interindividual variability in the extraspinal arteries that supply the spinal cord, the intrinsic blood supply of the cord itself is more consistent. The anterior spinal artery gives off central (sulcal) arteries that pass posteriorly, penetrating the anterior longitudinal fissure and supplying most of the central gray matter and the deep portion of the anterior white matter ( Fig. 2-4 ). The number of these sulcal vessels is variable, with 5 to 8 vessels per centimeter in the cervical region, 2 to 6 vessels per centimeter in the thoracic region, and 5 to 12 vessels per centimeter in the lumbosacral region.
The anterior spinal artery also gives off peripheral arteries that pass radially on the anterior surface of the spinal cord to supply the white matter tracts anteriorly and laterally. These arteries form the anterior pial arterial plexus, which is often poorly anastomotic with its posterior counterpart. The posterior horns and posterior funiculi are supplied by penetrating vessels from the posterior pial arterial plexus.
Ischemic Cord Syndromes
Ischemia of the spinal cord may be produced either by the interruption of blood flow through critical feeding vessels or by aortic hypotension. The resulting neurologic syndrome depends on the location of ischemic lesions along and within the spinal cord, which depends, in turn, on the vascular anatomy discussed previously. A wide variety of pathologic disturbances of the aorta result in spinal cord ischemia. As reviewed elsewhere, they include both iatrogenic causes, such as surgery and aortography, and intrinsic aortic diseases, such as dissecting and nondissecting aneurysms, inflammatory aortitis, occlusive atherosclerotic disease, infective and noninfective emboli, and congenital coarctation. Spinal cord ischemia is a rare complication of pregnancy, possibly due to aortic compression, which can occur toward the end of gestation.
Some authors have suggested that the midthoracic region (T4 to T8) is particularly vulnerable to ischemia because of the sparseness of vessels feeding the anterior spinal artery in this region and its poor anastomotic connections. Others have stressed the vulnerability of the watershed areas between the three anterior spinal arterial territories. Although the concept is theoretically appealing, documentation of the selective vulnerability of these regions is not completely convincing. For example, a review of 61 case reports with respect to the distribution of ischemic myelopathies resulting from surgery on the aorta does not especially suggest that either of these areas is more vulnerable than other cord segments ( Table 2-1 ). Even when the operation was performed on the thoracic aorta (and thus the proximal clamp was placed above the midthoracic cord feeder), the lumbosacral cord segments were the site of the ischemic damage more often than the supposedly more vulnerable midthoracic segments ( Table 2-1 ). Similarly, the watershed area between these two arterial territories (T8 to T9) does not seem particularly vulnerable. In fact, the most frequently affected cord segment within each vascular territory in these 61 cases was centrally placed—T6 in the midthoracic territory and T12 in the lumbosacral territory—rather than at the borders, as might be anticipated with watershed vulnerability ( Fig. 2-5 ).
Table 2-1
Influence of Location of Aortic Surgery on the Vascular Territory of Resulting Spinal Cord Ischemia
Based on 61 reported cases. From Goodin DS: Neurologic sequelae of aortic disease and surgery. p. 23. In Aminoff MJ (ed): Neurology and General Medicine. 4th Ed. Churchill Livingstone Elsevier, Philadelphia, 2008, with permission.
| Location of Surgery | ||
|---|---|---|
| Vascular Territory of Ischemia | Abdominal Aorta | Thoracic Aorta |
| Cervical region (C1–T3) | 0 | 0 |
| Midthoracic region (T4–T8) | 1 | 14 |
| Lumbosacral region (T9–conus) | 25 | 21 |
Moreover, of the 25 cases of spinal cord infarction in an unselected autopsy series of 300 cases, two-thirds were in cervical cord segments; the most commonly affected segment was C6. Such a distribution would be unexpected if either the midthoracic or the watershed area was particularly vulnerable. It may be that the poorly vascularized thoracic cord, which has much less gray matter than the cervical and lumbar enlargements, actually matches its sparse blood supply with its reduced metabolic requirements.
The site of aortic disease also plays an important role in the location of the lesion along the spinal cord. For example, distal aortic occlusion often presents with lumbosacral involvement, whereas dissecting aneurysm of the thoracic aorta commonly presents with infarction in the midthoracic region. Similarly, cord ischemia following surgery on the abdominal aorta is essentially confined to the lumbosacral territory, whereas surgery on the thoracic aorta not infrequently involves the midthoracic segments ( Table 2-1 ). Regardless of the pathologic process affecting the aorta, however, it generally involves the suprarenal portion if there is cord ischemia because the important radiculomedullary arteries usually originate above the origin of the renal arteries.
Ischemic spinal cord syndromes can be subdivided into several different categories including those with either bilateral or unilateral involvement restricted to the anterior or posterior spinal artery territories, those with involvement restricted to the central gray matter and, less commonly, those with a complete transverse myelopathy.
Anterior Spinal Artery Syndrome
Ischemic injury of the spinal cord at a particular segmental level may present with a complete transverse myelopathy. Within the spinal cord, however, there are certain vascular territories that can be affected selectively. In particular, the territory of the anterior spinal artery, especially its sulcal branch, is prone to ischemic injury. This increased vulnerability probably relates to two factors. First, the anterior circulation receives a much smaller number of feeding vessels than the posterior circulation. Second, the posterior circulation is a network of anastomotic channels and therefore probably provides better collateral flow than the single and sometimes narrowed anterior artery. The relative constancy of the resulting syndrome presumably reflects the relative constancy of the intrinsic vascular anatomy of the cord.
As mentioned earlier, the anterior spinal artery supplies blood to much of the spinal gray matter and to the tracts in the anterior and lateral white matter. Ischemia in this arterial territory therefore gives rise to a syndrome of diminished pain and temperature sensibility with preservation of vibratory and joint position sense. Weakness (either paraparesis or quadriparesis, depending on the segments involved) occurs below the level of the lesion and may be associated with other evidence of upper motor neuron involvement, such as Babinski signs, spasticity, and hyperreflexia. Bowel and bladder functions are affected, owing to interruption of suprasegmental pathways. Segmental gray matter involvement may also lead to lower motor neuron deficits and depressed tendon reflexes at the level of the lesion. Thus, a lesion in the cervical cord may produce flaccid areflexic paralysis with amyotrophy in the upper extremities, spastic paralysis in the lower extremities, and dissociated sensory loss in all limbs. In contrast, a lesion in the thoracic cord typically presents with only spastic paraplegia and dissociated sensory loss in the legs. The syndrome usually comes on abruptly, although occasionally it is more insidious and progressive. Occasionally, also, a transverse myelopathy can result from ischemia to the spinal cord.
Motor Neuron Disease
On occasion, diseases of the aorta (e.g., dissecting aneurysms or atherosclerosis) that interfere with the blood supply to the anterior spinal artery result in more restricted cord ischemia. This may occur because of better anastomotic connections between the anterior and the posterior pial arterial plexuses in some individuals or because of greater vulnerability of the anterior horn cells with their greater metabolic activity. The ischemic injury in these circumstances is limited to the central gray matter supplied by the sulcal branches ( Fig. 2-6 ). Clinical impairment is then confined to the motor system and is associated with amyotrophy. When the onset is abrupt, the ischemic nature of the lesion usually is apparent, but when the onset is more gradual, and especially when pyramidal signs are also present, it may mimic other diseases, such as amyotrophic lateral sclerosis or spinal cord tumors.
Posterior Spinal Artery Syndrome
In contrast to the anterior spinal artery syndrome, selective ischemia of the posterior circulation, characterized by prominent loss of posterior column function with relative sparing of other functions, is rarely recognized clinically and only occasionally reported pathologically. For example, in two reviews of a total of 63 cases of nonsurgical spinal cord ischemia, only 7 (9%) had posterior spinal artery patterns. The relative infrequency of this syndrome presumably relates to the more abundant feeding vessels and better anastomotic connections in this arterial system compared to the anterior spinal artery.
Unilateral Cord Syndromes
In some cases, the area of ischemic damage can be confined to only a small portion of the spinal cord. For example, in the reviews cited previously, 22 (35%) of the patients with nonsurgical spinal cord ischemia had unilateral syndromes involving either the anterior or posterior aspects of the spinal cord.
Intermittent Claudication
Intermittent claudication (limping) refers to a condition in which a patient experiences difficulty in walking that is brought about by use of the lower extremities. The evolution of concepts of intermittent claudication is of historical interest and is described elsewhere. In brief, Charcot initially described this syndrome in 1858 and related it to occlusive peripheral vascular disease in the lower extremities. In 1906, Dejerine distinguished claudication caused by ischemia of the leg muscles from that caused by ischemia of the spinal cord. In the latter condition, the arterial pulses in the legs tend to be preserved, pain tends to be dysesthetic or paresthetic in quality and may not occur, and neurologic signs are frequently present, especially after exercise. In 1961, another form of neurogenic claudication was identified, caused by ischemia or compression of the cauda equina, which resulted from a narrowed lumbosacral canal (either congenital or due to degenerative disease). This condition is similar to that produced by ischemia of the spinal cord, but there are important differences. Thus, the sensory complaints tend to have a more radicular distribution, numbness and pain are aggravated by certain postures (e.g., lumbar extension when walking or standing) and relieved by other postures (e.g., lumbar flexion when riding a bike or sitting), and signs of cord involvement (e.g., Babinski signs) are not present.
The clinical distinction between various types of claudication, particularly between the two neurogenic varieties, is sometimes difficult. The cauda equina variety, however, is more common than the spinal cord form. Intermittent spinal cord ischemia, when it occurs, can be associated with intrinsic diseases of the aorta, such as coarctation or atherosclerotic occlusive disease although, more commonly, it is due to degenerative disease of the cervical and thoracic spine. Bony erosion through vertebral bodies from an abdominal aortic aneurysm with direct compression of the spinal nerve roots has also been reported to produce intermittent neurologic symptoms. The clinical details of the single reported case, however, are not sufficient to determine whether the symptoms resemble those of intermittent claudication.
Cerebral Ischemia
Anatomy
The aortic arch gives rise to all the major vessels that provide blood to the brain, brainstem, and cervical spinal cord ( Fig. 2-7 ). The first major branch is the innominate (brachiocephalic) artery, which subsequently divides into the right common carotid and right subclavian arteries. The latter artery subsequently gives rise to the right vertebral artery, which ascends through the foramina of the transverse processes of the upper six cervical vertebrae to join with its counterpart on the left and form the basilar artery. The basilar artery provides blood to the posterior fossa and posterior regions of the cerebral hemispheres. The second major branch of the aortic arch is the left common carotid artery, and the third is the left subclavian artery, which, in turn, gives rise to the left vertebral artery.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here