On the anterior surface of the medulla, two branches from the vertebral arteries unite in the midline to form a single anterior spinal artery that descends the length of the spinal cord in the anterior median fissure (Figure 2.10). The sulcal arteries arising from the anterior spinal artery enter the spinal cord through the anterior median fissure. Successive sulcal arteries generally alternate in their distribution to the left and right side of the spinal cord (Figure 2.11), but occasionally a single sulcal artery will distribute to both sides (Figure 2.12). The sulcal arteries supply the anterior two-thirds of the spinal cord at any cross-sectional level. This is anclinically important feature of the anatomy of the spinal cord, because occlusion of the anterior spinal artery or its sulcal branches could result in anterior cord (spinal artery) syndrome (Figure 2.13). As in most vascular problems, the onset of signs and symptoms is rapid. Figure 2.13 shows the zone of distribution of the anterior spinal artery in the cross-hatched area. The posterior funiculus and horns are spared because these areas are supplied by the posterior spinal arteries. Initially, there is flaccid paralysis of the muscles in the body below the level of infarct because of spinal shock. In time, however, spastic paralysis and other upper motor neuron signs develop because of bilateral destruction of the corticospinal tracts. A variable degree of bowel and bladder dysfunction exists because of the interruption of the descending autonomic pathways. Initially, however, incontinence may be due to spinal shock. A cardinal sign of anterior cord syndrome is a dissociated sensory loss characterized by a loss of pain and temperature sensations (bilateral lateral spinothalamic tract lesion) with preservation of kinesthesia and discriminative touch sensations (sparing of posterior funiculi) in the body below the level of injury. Some patients develop painful dysesthesias about 6 to 8 months after the onset of neurologic symptoms. The source of this pain is unknown, but has been suggested to be attributed to the activation of previously latent pathways that mediate pain sensation. The anterior spinal artery is dependent on segmental contributions from anterior radicular arteries along the length of the spinal cord.

Figure 2.10

Arteries of the spinal cord.

Figure 2.11

Intrinsic distribution of spinal arteries.

Figure 2.12

Arterial supply and venous drainage of the spinal cord.

Figure 2.13

Diagram showing the extent of the lesion in anterior cord (spinal artery) syndrome and associated neurologic signs.

The paired posterior spinal arteries derived from the vertebral arteries descend on the posterior surface of the spinal cord just medial to the posterior roots (Figure 2.10). The arteries receive variable contributions from the posterior radicular arteries. At points along the cord the posterior spinal arteries become so small that they appear to be discontinuous. The arteries supply blood to the posterior one-third of the spinal cord. The peripheral portions of the lateral funiculi are supplied by the arterial vasocorona, which is formed by an anastomosis between the anterior and posterior spinal arteries (Figure 2.12).

The segmental vessels that pass through the intervertebral foramina divide into posterior and anterior radicular arteries, which follow the posterior and anterior roots respectively (Figure 2.11). At variable levels the radicular arteries continue coursing medially until they anastomose with either the anterior or posterior spinal arteries. The anterior radicular arteries contribute to the anterior spinal artery, and the posterior radicular arteries contribute to the posterior spinal arteries. In the lumbar region, one anterior radicular artery is quite large and is known as the artery of Adamkiewicz (Figure 2.10). This artery is usually found on the left and enters the vertebral canal by following an anterior root at either a low thoracic or upper lumbar level before anastomosing with the anterior spinal artery. The artery of Adamkiewicz reinforces the circulation to two-thirds of the spinal cord, including the lumbosacral enlargement. Occlusion of this artery may seriously compromise spinal cord circulation, which could lead to infarct and paraplegia.

The greatest distance between radicular arteries that contributes significantly to the anterior and posterior spinal arteries is found at the thoracic level of the spinal cord. At this level, occlusion of just one radicular artery could lead to significant infarct of spinal tissue. The T1–T4 levels of the thoracic cord are particularly vulnerable to infarct following occlusion of a radicular artery. Spinal cord segment L1 is another vulnerable region.

Severe trauma to the vertebral column on the left at the thoracic level can fracture the bodies of one or more vertebrae. A surgeon often must mobilize the descending aorta to the right to expose the fractured vertebral bodies, remove the bony fragments, and stabilize the spine. The aorta is mobilized by first tying off several posterior intercostal arteries close to their origin from the aorta, severing the arteries in this region, and then moving the aorta to the right (Figures 2.11, 2.14). The upper thoracic spinal cord is particularly vulnerable to infarct if blood flow into the cord from an important segmental vessel is interrupted. The above surgical procedure, however, does not interrupt the flow of blood through the intervertebral foramen and thus to the spinal cord, because of collateral circulation. This collateral circulation results from the anastomosis of the internal thoracic artery (a branch of the subclavian artery) with the posterior intercostal artery. As soon as blood flow from the aorta is interrupted, blood begins retrograde flow from the internal thoracic artery through the posterior intercostal artery and into the spinal cord (Figure 2.14). Although this is not shown in the figure, Hollinshead and Rosse (28) have suggested that the lateral thoracic artery (a branch of the axillary artery) also has anastomoses with the posterior intercostals and thus provides a second source of blood flow to the spinal cord under these conditions.

Figure 2.14

Collateral circulation of the spinal cord. When posterior intercostals are tied off surgically, blood flows to the spinal cord via internal thoracic and lateral thoracic (not shown) artery (more...)

In general, the distribution pattern of the veins of the spinal cord is similar to that of the spinal arteries (Figure 2.12). Three longitudinally oriented posterior spinal veins and three anterior spinal veins communicate freely with each other and are drained by anterior and posterior radicular veins, which join the internal vertebral (epidural) venous plexus lying in the epidural space (Figure 2.12). This plexus of veins passes superiorly within the vertebral canal through the foramen magnum to communicate with the dural sinuses and veins within the skull (Figure 2.15). The internal vertebral venous plexus also communicates with the external vertebral venous plexus on the external surface of the vertebrae.

Figure 2.15

Veins of the spinal cord and the vertebral venous plexus.

There are no valves in the spinous venous network. Thus, blood flowing in these vessels could pass directly into the systemic venous system. For instance, when intraabdominal pressure is increased, blood from the pelvic venous plexus passes superiorly via the internal vertebral plexus. When the jugular veins are obstructed, blood leaves the skull by this same plexus. Because the prostatic plexus is continuous with the vertebral venous system, neoplasms originating in the prostate gland may metastasize and lodge in vertebrae, spinal cord, brain, or skull (29).