Anatomy

Proficiency in spinal and epidural anesthesia requires a thorough understanding of the anatomy of the spine and spinal cord.

Vertebrae
1. The spine consists of 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 5 fused coccygeal).
2. With the exception of C1 (which lacks a body or spinous process), the vertebrae consist of a body anteriorly; two pedicles that project posteriorly from the body; and two lamina that connect the pedicles to form the vertebral canal, which contains the spinal cord, spinal nerves, and epidural space (Fig. 34-1: Anatomy of the vertebral column).
3. The laminae give rise to the transverse processes, which project laterally, and the spinous process, which projects posteriorly (see Fig. 34-1: Anatomy of the vertebral column).
4. The fifth sacral vertebra is not fused posteriorly, giving rise to a variably shaped opening known as the sacral hiatus (opening into the sacral canal, which is the caudal termination of the epidural space). The sacral cornu are bony prominences on either side to the hiatus and aid in identifying it.
5. Identifying individual vertebrae is important for correctly locating the desired interspace for performance of epidural and spinal anesthesia (Table 34-1: Landmarks for Vertebral Interspaces).
Ligaments
1. The vertebral bodies are stabilized by five ligaments that increase in size between the cervical and lumbar vertebrae (see Fig. 34-1: Anatomy of the vertebral column).
2. The ligamentum flavum is thickest in the midline (3–5 mm at L2–L3) and also farthest from the spinal meninges in the midline (4–6 mm at L2–L3). As a result, midline insertion of an epidural needle is least likely to result in accidental meningeal puncture.
Epidural Space
1. The epidural space lies between the spinal meninges and the sides of the vertebral canal. It is bounded cranially by the foramen magnum, caudally by the sacrococcygeal ligament (sacral hiatus), and posteriorly by the ligamentum flavum and vertebral pedicles.
2. The epidural space is not a closed space but communicates with the paravertebral space via the intervertebral foramina.
3. The epidural space is composed of a series of discontinuous compartments, which become continuous when the potential space separating the compartments is opened up by injection of air or liquid.
4. The most ubiquitous material in the epidural space is fat.
5. Veins are present principally in the anterior and lateral portions of the epidural space with few, if any, veins present in the posterior epidural space (see Fig. 34-1: Anatomy of the vertebral column). These veins anastomose freely with extradural veins (pelvic veins, azygous system, intracranial veins).
Epidural fat has important effects on the pharmacology of epidurally and intrathecally administered opioids and local anesthetics.
1. Lipid solubility results in opioid sequestration in epidural fat with associated decreases in bioavailability.
2. Transfer of opioids from the epidural space to the intrathecal space is greatest with poorly lipid-soluble morphine and least for the highly lipid-soluble opioids fentanyl and sufentanil.
Meninges
1. Dura mater is the outermost and thickest meningeal tissue that begins at the foramen magnum (fuses with the periosteum of the skull, forming the cephalad border of the epidural space) and ends at approximately S2, where it fuses with the filum terminale. The dura mater extends laterally along the spinal nerve roots and becomes continuous with the connective tissue of the epineurium at approximately the level of the intervertebral foramina.
  1. The inner edge of the dura mater is highly vascular, which likely results in the dura mater being an important route of drug clearance from both the epidural and subarachnoid space.
  2. The presence of a midline connective tissue band (plica mediana dorsalis) running from the dura mater to the ligamentum flavum is controversial but may be invoked as an explanation for unilateral epidural block.
  3. The subdural space is a potential space between the dura mater and arachnoid mater. Drug intended for either the epidural space or the subarachnoid space may be accidentally injected into this space.
2. Arachnoid Mater
  1. The arachnoid mater is an avascular membrane that serves as the principal physiologic barrier for drugs moving between the epidural space and the subarachnoid space.
  2. The subarachnoid space lies between the arachnoid mater and pia mater and contains cerebrospinal fluid (CSF). The spinal CSF is in continuity with the cranial CSF and provides an avenue for drugs in the spinal CSF to reach the brain. Spinal nerve roots and rootlets run in the subarachnoid space.
3. Pia mater is adherent to the spinal cord.
4. CSF (100–160 mL in adults, produced at a rate of 20–25 mL/hr) is replaced roughly every 6 hours (removed by arachnoid villi).
  1. Contrary to a widely held view, CSF does not circulate through the subarachnoid space but rather oscillates in parallel with cerebral expansion and contraction during the cardiac cycle. (Net CSF movement is estimated to be 0.04% per oscillation.)
  2. CSF cannot be relied on to distribute drugs in the subarachnoid space.
  3. The kinetic energy of the injection and baricity of the solution serve to distribute drug during a single-shot spinal.
  4. The lack of significant net CSF motion explains why drug distribution during slow infusions used for chronic intrathecal analgesia results in limited drug distribution.
Spinal Cord
1. In adults, the caudad tip of the spinal cord typically ends at the level of L1 (extends to L3 in 10% of adults).
2. The spinal cord gives rise to 31 pairs of spinal nerves, each composed of an anterior motor root and a posterior sensory root.
  1. Dermatome is the skin area innervated by a given spinal nerve (Fig. 34-2: Human sensory dermatomes).
  2. The intermediolateral gray matter of T1 to L12 contains the cell bodies of the preganglionic sympathetic neurons. These sympathetic neurons travel with the corresponding spinal nerve to a point just beyond the intervertebral foramen, where they exit to join the sympathetic chain ganglia.
  3. Because the spinal cord ends between L1 and L2, the thoracic, lumbar, and sacral nerve roots travel increasingly longer distances in the subarachnoid space (cauda equina) to reach the intervertebral foramen through which they exit.

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