Spinal Cord

The spinal cord originates at the foramen magnum as a continuation of the medulla oblongata. The termination of the spinal cord is at the conus medullaris ( Fig. 64.2 ). The conus medullaris is found normally at the level of the first lumbar vertebra (L1) in adults, though it may be lower. It is found at L2 or L3 in children and infants ( ). In many settings, imaging is helpful to identify the location of the conus when planning procedures in the vicinity of the upper, lumber region.

Image of the spinal cord.

From https://commons.wikimedia.org/w/index.php?curid=34332941 .

The dural sac ends at the second sacral vertebra (S2) ( ). The pia mater continues to end at the filum terminale, which attaches the spinal cord to the posterior aspect of the coccyx. The spinal cord is composed of white matter, surrounding a core of gray matter. The gray matter has both anterior and posterior horns, which are the motor and sensory fibers, respectively. White matter contains myelinated neurons and convey information form the nervous system to the brain.

Nerve Root

The spinal nerves are composed of 31 pairs of nerves. Each of these nerves contains a motor root and a sensory root. The spinal nerves exit the spine through the epidural space at the intervertebral foramen formed by the superior and inferior vertebrae, surrounded by the dura. The eight cervical nerves exit above the corresponding vertebral body with the eighth cervical nerve (C8) exiting between C7 and T1. Distal to that level the nerves exit below the corresponding vertebrae.

Vertebral Column

The spine is comprised of 33 vertebral bodies, divided into five regions with distinct curvatures. The normal spine has a cervical and lumbar lordosis and thoracic and sacral kyphosis. These curvatures may have an effect on drug spread and CSF circulation, particularly in abnormal spinal anatomy. Support for the vertebral column is provided by spinal ligaments ( Fig. 64.3 ).

Curves of the spine.

From https://commons.wikimedia.org/w/index.php?curid=1282158 .

Ligaments of the Spine

The ligaments of the spine are critical to the anatomic stability of the spinal structures. The primary ligaments of the spine are the ligamentum flavum, the anterior longitudinal ligament, and the posterior longitudinal ligament. The ligamentum flavum is critical in that it forms a cover protecting the dura mater. The anterior longitudinal ligament is a vertical structure that attaches to the anterior portion of each vertebrae. The posterior longitudinal ligament is a vertical structure that attaches to the posterior portions of each vertebra. The interspinous ligament stretches between the spinous processes. The supraspinous ligament anchor the tips of the spinous process. Other ligaments of the spine also play critical roles. These include the occipitoatlantal ligament complex, occipitoaxial ligament complex, atlantoaxial ligament complex, and the cruciate ligament complex.

Blood Supply to the Spinal Cord

The blood supply to the spinal cord is delivered by a single, anterior spinal artery (ASA) and two, posterior spinal arteries (PSA). The two PSA arteries travel longitudinally along the posterior surface of the spinal cord in concert with the nerve roots. These arteries are intertwined by anastomoses that form longitudinal vessels. Additional collaterals are delivered by segmental arteries that travel via the intervertebral foramina. The ASA is critical in that it supplies the anterior two-thirds of the spinal cord. In the cervical spinal cord, the ASA receives its blood flow from the vertebral arteries that normally arise from the subclavian artery, enter the spinal canal in the upper cervical vertebral columns, ascends to the anterior midline of the brainstem, and merges to become the basilar artery. Two small branches of the vertebral artery descend toward the anterior midline and merge to form the ASA, which descends to the lumbar cord ( Fig. 64.4 ). The anterior thoracic spinal blood supply is more complicated and potentially more prone to disastrous complications. In this region, the ASA receives only a minimal number of collateral radicular arteries from the aorta. The artery of Adamkiewicz, first described in 1882, reinforces the supply at the level of T9 in most humans ( ). The artery can vary in its side of origin. It is on the left side in 78% of cases and in its level of origin varies from T8 to L3 ( ). Injury to this artery can lead to major, neurological sequelae, including paraplegia ( ).

Spinal arterial and venous supply.

From https://commons.wikimedia.org/w/index.php?curid=2944989 .

The thoracic, segmental arteries come directly from the aorta. At the thoracic level these arteries are called intercostal arteries. The intercostal arteries network longitudinally in both the ventral and dorsal spine. This networking complex can lead to redundant supply, which provides an improved degree of safety for ischemic spinal events. Branches of the segmental arteries supply flow to the vertebra and the structures of the spinal canal. Each intercostal artery splits into several branches. One of these large branches enters the foramen and splits into a radicular artery and several dural branches. The dural branches supply the dura mater and the cauda equina.

Spinal Cord Venous System

The venous system is important for drainage of blood and metabolic substrates. Compromise of the venous system may lead to delayed ischemia of the spinal cord with the potential for severe sequelae. The spinal, venous system follows the arterial system and is segmentally organized. The venous system has several redundant and collateral flow patterns that provide an additional margin of safety. Venous drainage of the spinal cord occurs mainly through pial veins, which are on the surface. Venous congestion can play an important role in ischemia of the cord ( Fig. 64.4 ).

The Epidural Space

The epidural space is the space just outside the dura of the spinal canal. The epidural space is a potential space extending from the base of the skull to the sacral hiatus. This space contains blood vessels, fatty tissue, and fibrous tissue. The epidural contents are not uniform. The structures are found in a circumferential and segmented fashion, grouped in compartments. The steeply arched ligamentum flavum are fused in the midline to a variable degree based on the spinal level of consideration. The anterior epidural space has a concentrated number of veins and is separated from the remaining epidural space by a membranous lateral extension of the posterior longitudinal ligament. This anatomic structure is important to the overall mechanics and pharmacokinetics of epidural injections and infusions ( ). The pharmacokinetics of the epidural space suggests that hydrophilic infused drugs spread widely in the CSF. The lipophilic drug classes spread locally and result in high, serum concentrations and high local drug concentrations in the area infused. Some studies have refuted these findings and suggest that the overall drug availability is variable based on epidural fat and venous supply ( ).

The Vacuum Effect

The epidural space is entered when a needle passes through the ligamentum flavum into the desired location of the epidural space. The loss of resistance that occurs when going from the ligament to the epidural space can be detected by using a pressurized syringe that suddenly has a drop in pressure, as the potential space is entered from the high pressure, dense ligament ( ). This phenomenon is due to the presence of subatmospheric pressure at the needle tip, either created by artifact produced by the needle entering the epidural space or by an intrinsic, negative epidural pressure. The change of pressure can also be detected by the hanging drop technique in which the sudden change in pressure leads to a sudden retraction of the drop which is allowed to hang from the surface of the needle hub. In normal anatomy, the pressure gradient between the CSF and the epidural space would be noticed quickly as one passes through the epidural space into the spinal fluid. This difference reduces, but does not eliminate, the need for epidural test dosing and contrast imaging.