A typical segmental artery divides into an anterior and a posterior ramus (Fig. 42-6). Each posterior ramus gives rise to a spinal artery, which enters the vertebral foramen, pierces the dura, and supplies the spinal ganglion and roots through its anterior and posterior radicular branches. Most anterior radicular arteries are small and some never reach the spinal cord, but a variable number (4 to 9), arising at irregular intervals, are much larger and supply most of the blood to the spinal cord. Tributaries of the radicular arteries supply blood to the vertebral bodies and surrounding ligaments. The venous drainage is into the posterior veins forming the spinal plexus. Their importance relates to the pathogenesis of fibrocartilaginous embolism (see further on).
Lazorthes, in his thorough review of the circulation of the spinal cord, divides the radiculomedullary arteries into three groups: (1) upper or cervicothoracic, which are derived from the anterior spinal arteries and branches of the thyrocervical and costovertebral arteries; (2) intermediate or middle thoracic (T3 to T8 cord segments), usually from a single T7 radicular artery; and (3) lower or thoracolumbar, from a large T10 or L1 anterior radicular artery, better known as the artery of Adamkiewicz. This artery supplies the lower two-thirds of the cord, but in any individual the precise area supplied by this or any other anterior radiculomedullary artery varies greatly and one cannot predict what portion or proportion of cord will be infarcted if one of these vessels is occluded. The junction between the vertebral spinal and aortic circulations typically lies at the T2-T3 spinal segment, but most ischemic lesions lie well below this level.
The anterior medullary arteries form the single anterior spinal artery, which runs the full length of the cord in its anterior sulcus and gives off direct penetrating branches via the central (sulco-commissural) arteries. These penetrating branches supply most of the anterior gray columns and the ventral portions of the dorsal gray columns of neurons (see Fig. 42-6). The peripheral rim of white matter of the anterior two-thirds of the cord is supplied from a pial radial network, which also originates from the anterior median spinal artery. Thus, the branches of the anterior median spinal artery supply roughly the ventral two-thirds of the spinal cord. Infarction of the region supplied by this artery give rise to an anterior spinal cord syndrome that consists of loss of pain and temperature and paralysis below the level of the lesion, but with sparing of proprioception and vibration sense that correspond to transaction of the spinothalamic and corticospinal tracts but not of the posterior columns.
The posterior medullary arteries form the paired posterior spinal arteries that supply the dorsal third of the cord by means of direct penetrating vessels and a plexus of pial vessels (similar to that of the ventral cord, with which it anastomoses freely). Within the cord substance, then, there is a “watershed” area of capillaries where the penetrating branches of the anterior spinal artery meet the penetrating branches of the posterior spinal arteries and the branches of the circumferential pial network. All spinal segments, because of the variable size of collateral arteries, do not have the same abundance of circulatory protection.
Normally there are 8 to 12 anterior medullary veins and a greater number of posterior medullary veins arranged fairly close to one another at every segmental level. They drain into radicular veins. In addition, a network of valveless veins extends along the vertebral column from the pelvic venous plexuses to the intracranial venous sinuses without passing through the lungs (Batson plexus) and is considered a route for metastatic disease from the pelvis.
Infarction of the Spinal Cord