Neuraxial Anatomy

Describe the anatomy relevant to the performance of a lumbar puncture.

The spinal epidural space is:

• A space lying within the vertebral column, that contains:
  • Nerve roots
  • Vessels
    Vascular density is greater anteriorly.
  • Fat
• A negative pressure space
  The epidural space is continuous with the paravertebral space, which is affected by changes in intrapleural pressure. As intrapleural pressure is usually negative, the epidural space is also a negative pressure space. This property facilitates the hanging drop technique of epidural placement. Key exceptions (and relative contraindications to hanging drop) include:
  • Positive pressure ventilation
  • Contractions in labour
    Epidural veins are transfused by a contracting uterus, and pressure increases as they distend.
  • Valsalva maneuvre

The intrathecal space is:

• Deep to the epidural space
• Contains the spinal cord and CSF

Borders

The spinal epidural space is bordered:

• Anteriorly by the posterior longitudinal ligament
• Posteriorly by the ligamentum flavum
• Superiorly by the endosteal layer of dura
  Prevents passage of contents through the foramen magnum.
• Inferiorly by the sacrococcygeal membrane at the sacral hiatus
• Laterally by the pedicles and intervertebral foraminae

Surface Anatomy

Identify:

• Spinous processes
  Generally palpable in midline.
  • C2 is the first palpable process
  • C7 is the most prominent process
  • T7 corresponds with the tip of the scapula when the arms are by the patients side
  • The line drawn across the iliac crest corresponds with thee L4/5 interspace in most patients
    This is known as Tuffer's Line.
• Gluteal crease
  Identify midline in patients with impalpable spinous processes, who:
  • Are sitting flat
  • Do not have scoliosis

Layers

Tissues passed through, from superficial to deep, include:

• Skin
• Subcutaneous tissue
• Supraspinous ligament
  One continuous ligament.
• Interspinous ligament
  One ligament between each spinous process.
• Ligamentum flavum
  This yellow ligament is:
  • Thicker and harder than other ligaments
  • Paired at each intervertebral level
  • 5-6mm thick in the adult, though it is thinner with:
    • Youth
    • ↑ Level
    • Body habitus
• Epidural space
• Dura
• Subarachnoid space
• Spinal cord
  Ends at L2 in adults and L3 in children.

References

1. Miller RD, Eriksson LI, Fleisher LA, Weiner-Kronish JP, Cohen NH, Young WL. Miller's Anaesthesia. 8th Ed (Revised). Elsevier Health Sciences.