Keywords
aneurysms, aortic aneurysm, aortic disease, aortic surgery, autonomic neuropathy, dissecting aneurysms, giant cell arteritis, ischemic neuropathy, spinal cord circulation, spinal cord ischemia, syphilitic aneurysm, Takayasu arteritis
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 has recently been proposed to offer 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 are formed 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. 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 ).
| 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. Perhaps relating to such observations, it has been found that, contrary to earlier reports, the anterior spinal artery is continuous along its length without interruption in all 51 cadavers studied. If this observation can be generalized, it may be the case 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.
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, perhaps 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 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, Blau and Logue identified another form of neurogenic claudication caused by ischemia or compression of the cauda equina and resulting 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; however, the sensory complaints tend to have a more radicular distribution, 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 far 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.
Strokes and Transient Ischemic Attacks
Diseases of the aortic arch, such as atherosclerosis, aneurysms, and aortitis as well as surgery on this segment of the aorta, may give rise to symptoms of cerebrovascular insufficiency, such as strokes or transient ischemic attacks (TIAs). A young woman has even been reported with a stroke secondary to an occlusion of the aorta that was associated with the use of birth control pills and recurrent venous thromboses. Cerebral ischemia is produced either by occlusion of a major vessel or by embolization of atheromatous or other material to more distal arteries. The resulting neurologic syndromes are not specific for any disease process but depend on the location and duration of the vascular occlusion.
Atherosclerosis
Atherosclerosis of the aortic arch and its branches, compared with atherosclerosis at the origin of the internal carotid artery, is an infrequent cause of stroke or TIAs, probably for two reasons. First, atherosclerosis is much less common in this location than at the carotid bifurcation ( Table 2-2 ). Second, the anastomotic connections between the major vessels in the neck are extensive, and an occlusion at their origin from the aortic arch is therefore less likely to be associated with symptoms of ischemia than a more peripheral obstruction.
| Location | Number of Lesions |
|---|---|
| External carotid artery | 9 |
| Internal carotid artery | 256 |
| Common carotid artery | 16 |
| Innominate artery | 16 |
| Subclavian artery | 29 |
| Vertebral artery | 55 |
| Aortoiliac region | 952 |
| Femoropopliteal region | 772 |
Transient Emboligenic Aortoarteritis
Transient emboligenic aortoarteritis has been reported by Wickremasinghe and colleagues to be a cause of stroke in young patients. They described 10 patients (aged 16 to 36 years), all of whom had presented with pathologically verified thromboembolic strokes, and 3 of whom had a history of TIAs preceding the event by as much as 4 years. All these patients had both active and healed inflammatory lesions of the central elastic arteries, such as the aorta, innominate, common carotid, and proximal subclavian arteries. Active lesions were small (200 to 300 μm in diameter) and associated with a mural thrombus on the intimal surface. Healed lesions usually were associated with fibrous plaques but not with a mural thrombus. More peripheral arteries supplying the brain were normal. This condition seems to be distinct from segmental aortitis of the Takayasu type. Clinically it is an acute, intermittent disorder with an approximately equal sex incidence, whereas Takayasu disease is more chronic and has a strong female predominance. The systemic symptoms of Takayasu disease are absent, and occlusion of the central arteries does not occur in this condition.
Subclavian (Cerebral) Steal
Disease of the aortic arch may result in occlusion of either the innominate artery or the left subclavian artery proximal to the origin of the vertebral artery. This, in turn, may result in the reversal of the usual cephalad direction of blood flow in the ipsilateral vertebral artery ( Fig. 2-8 ), depending on individual variations in the collateral circulation, and may result in ischemia in the posterior cerebral circulation. In some patients, this is particularly evident when the metabolic demand (and therefore the blood flow) of the affected arm is increased during exercise. If the innominate artery is blocked proximally, it may also cause a reversal of blood flow in the right common carotid artery, resulting in anterior circulation ischemia ( Fig. 2-8 ).
Killen and colleagues reviewed the clinical features of a series of patients with demonstrated reversals of arterial blood flow in a vertebral artery (i.e., with flow from the vertebral artery into the ipsilateral subclavian artery). The left subclavian artery was affected more than twice as often as the right subclavian and innominate arteries combined, probably as a result of the more frequent involvement of this artery by atherosclerosis ( Table 2-2 ). Men were affected three times as often as women, probably reflecting the greater prevalence of atherosclerosis in men. Of the 87 patients in this series with symptoms that were adequately described, 75 (86%) had symptoms referable to the central nervous system (CNS). These symptoms were usually transient, lasting seconds to a few minutes, although the deficits were sometimes permanent. The neurologic manifestations of steal were varied but most frequently included motor difficulties, vertigo, visual deficits, or syncope. Ischemic symptoms in the arms occurred in only a few patients, and precipitation of CNS symptoms by exercise of the arm ipsilateral to the occlusion was uncommon. Although reconstructive surgery relieved symptoms in most patients in this series, it was the frequent failure of surgery to correct such nonspecific symptoms that led to a reassessment of the importance of cerebral steal. Thus, when noninvasive techniques such as Doppler ultrasonography have been used to define the direction of blood flow in the great vessels in a large spectrum of patients with vascular disease, the majority (50 to 75%) of patients with documented subclavian steal are found to be asymptomatic, even when the steal is bilateral. When symptoms do occur, they are suggestive of transient vertebrobasilar insufficiency in only 7 to 37 percent of patients with steal; the occurrence of infarcts in this vascular territory is distinctly rare. For this reason, a review of the topic led to the conclusion that subclavian steal is actually a marker of generalized atherosclerotic disease and that it is rarely a cause for symptoms of cerebral ischemia.
Nevertheless, a related syndrome, the coronary subclavian steal syndrome, seems well documented. This syndrome consists of angina (with or without posterior circulation symptoms such as vertigo) induced by upper limb exercise. It follows a coronary artery bypass graft using the left internal mammary artery in the setting of a hemodynamically significant subclavian stenosis.
Peripheral Neuropathy
The peripheral nervous system is sometimes affected by aortic disease or surgery. The syndromes produced may be the presenting manifestations of aortic disease and may mimic less life-threatening conditions.
Mononeuropathies
Left Recurrent Laryngeal Nerve
The left recurrent laryngeal nerve descends in the neck as part of the vagus nerve and wraps around the aortic arch just distal to the ligamentum arteriosum ( Fig. 2-7 ) before reascending in the neck to innervate all the laryngeal muscles on the left side except the cricothyroideus. It may be compressed by disease of the aortic arch, such as dissecting and nondissecting aneurysms or aneurysmal dilatation proximal to a coarctation of the aorta. The resulting hoarse, low-pitched voice may be one of the earliest presenting symptoms of these conditions, although it is often overshadowed by other symptoms or signs, such as chest pain, shortness of breath, congestive heart failure, or hypertension.
Femoral Nerve
The femoral nerve arises from the nerve roots of L2, L3, and L4. It forms within the belly of the psoas muscle and then exits on its lateral aspect to innervate the quadriceps femoris, iliacus, pectineus, and sartorius muscles and the skin of the anterior thigh and medial aspect of the leg. The nerve is located considerably lateral to the aorta ( Fig. 2-9 ) and hence is rarely involved by direct compression. It may, however, be compressed by a hematoma from a ruptured aortic aneurysm into the psoas muscle and thereby signal a life-threatening condition that requires an urgent operation.
The femoral nerve may also be injured as a consequence of aortic surgery. The mechanism of injury in such cases is presumed to be ischemic and related to poor collateral blood supply to the intrapelvic portions of the femoral nerves.
Obturator Nerve
The obturator nerve also forms within the belly of the psoas muscle by the union of fibers from the L2, L3, and L4 segments, but, in contrast to the femoral nerve, exits medially from this muscle ( Fig. 2-9 ). It innervates the adductors of the leg and the skin on the medial aspect of the thigh. It too is lateral to the aorta and not usually involved by direct compression. Like the femoral nerve (and often together with it), the obturator nerve may be compressed by a hematoma in the psoas muscle.
Radiculopathies
Nerve roots, particularly L4, L5, S1, and S2, which lie almost directly underneath the terminal aorta and iliac arteries ( Fig. 2-10 ), may be directly compressed by an aortic aneurysm in this region. The syndromes produced are typical of radicular disease, with unilateral radiating pain and a radicular pattern to the sensory and motor findings.
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