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Anatomy of lumbar puncture and epidural analgesia


Core Procedures

  • Epidural injections: commonplace in obstetrical anaesthesia, as well as in the treatment of radiculitis throughout the spine

  • Epidural catheterization: permits continuous administration of an anaesthetic agent

  • Lumbar puncture: single injection of local anaesthetic into the subarachnoid space via lumbar puncture offers excellent surgical anaesthesia for abdominal, pelvic and lower-extremity procedures

The epidural space is the potential space that lies outside the dura and is the outermost part of the vertebral (spinal) canal. Its upper limit is the foramen magnum, and its lower limit is the sacrococcygeal membrane. The dura encloses the arachnoid mater, subarachnoid space, cerebrospinal fluid (CSF) and spinal cord, and ends at approximately S2. The subdural space is the potential space between the dura and arachnoid mater ( Fig. 35.1 ). It does not connect with the subarachnoid space but continues for a short distance along the cranial and spinal nerves. Accidental subdural catheterization may occur during epidural injections; injection of fluid into the subdural space may damage the cord either by direct toxic effects or by compression of the vasculature. The subarachnoid space contains the CSF in continuity around the brain and spinal cord. CSF is created in the choroid plexus and mainly absorbed in the arachnoid granulations, with a normal specific gravity of 1.004–1.006. The total volume of CSF in the normal adult is approximately 150 ml and is produced 25 ml per hour.

Fig. 35.1
A sagittal section through the layers of the posterior vertebral (spinal) canal. Note the larger gauge epidural needle with a rounded tip located in the epidural space, deep to the ligamentum flavum. The smaller gauge spinal needle with a sharp tip is located in the subarachnoid space, deep to the dura mater and arachnoid mater.

Clinical anatomy of the epidural space

The posterior epidural space is widest in the upper thoracic region (7.5 mm), narrowing to 4.1 mm at the T11–T12 region, and 4–7 mm in the lumbar region. It takes 1.5–2 ml of local anaesthetic to block a spinal segment in the epidural space but far less (0.3 ml) for a similar block in the subarachnoid space.

Both the shape and size of the epidural space are dictated by the shape and size of the vertebral canal and the position of the dural sac. The space freely communicates with the paravertebral space via intervertebral foramina. The epidural space can be separated into cervical, thoracic, lumbar and sacral epidural regions. In the cervical epidural space, the spinal and periosteal layers of dura fuse at the foramen magnum, preventing intracranial extension. The lumbar epidural space extends from the lower margin of the L1 vertebra to the upper margin of S1. A line drawn between the iliac crests (intercristal line) across the back usually denotes the L4–5 interspace in adults and the L5–S1 level in infants. The sacral epidural space extends from the upper margin of S1 to the sacrococcygeal membrane. The space is bounded anteriorly by the posterior longitudinal ligament, vertebral bodies and intervertebral discs; the pedicles and intervertebral foramina form the lateral boundaries, and the ligamenta flava and vertebral laminae form the posterior boundary. The ligamentum flavum forms the anterior covering of the diarthrodial zygapophysial joint; haptic recognition of the ligamentum flavum is critical in successful cannulation of the epidural space (see Fig. 35.1 ).

Contents of the epidural space

The epidural space contains fat, lymphatics, arteries, connective tissue, spinal nerve roots and an extensive venous plexus. Epidural fat is the dominant constituent of the posterior spinal epidural space. It buffers the pulsatile movements of the dural sac; protects nerves; facilitates the movement of the dural sac over the periosteum of the spinal column during flexion and extension; and creates a reservoir for lipophilic substances. The fat is largely distributed along the dorsal margin of the space. The clinical significance of the epidural fat distribution is related to the pharmacokinetics of drugs injected into the epidural space: changes in fat content and distribution associated with different diseases may alter the absorption and distribution of drugs injected into the epidural space.

The internal vertebral venous plexus within the epidural space is thought to be involved in traumatic tap during needle placement. The plexus consists of four interconnecting longitudinal vessels, two anterior and two posterior, which receive a large contribution from the posterior aspect of each vertebral body via the basivertebral vein ( Fig. 35.2A ). Anterior and posterior external vertebral venous plexuses anastomose freely and are most developed in the cervical region. Anterior external plexuses are anterior to the vertebral bodies, communicate with basivertebral and intervertebral veins, and receive tribut­aries from vertebral bodies. Posterior external plexuses lie posterior to the vertebral laminae and around spinous, transverse and articular processes. They anastomose with the internal plexuses and join the vertebral, posterior intercostal and lumbar veins. The basivertebral veins are paired, valveless veins that drain the pars spongiosa of the vertebral bodies into the internal and external vertebral venous plexuses. In each segment, they emerge horizontally from foramina in the vertebral bodies. Posteriorly, they drain into the transverse branches of the anterior internal vertebral plexuses. Anteriorly, they drain directly into the anterior external vertebral venous plexus. These veins are predominantly in the anterolateral part of the epidural space; they connect deep pelvic veins draining the bladder, prostate and rectum to the internal vertebral venous plexus. Sparse lymphatics concentrated along the dural roots and the valveless venous system contribute to the haematogenous spread of infection or malignancy to the epidural space. Obstruction of the inferior vena cava, pregnancy or intra-abdominal tumours can cause distension of the venous plexus, leading to increased risk of trauma during needle or catheter placement in the epidural space. Spinal arteries entering each intervertebral foramen form an anterior and posterior spinal arterial arcade; these arteries arise from the vertebral arteries and the thoracic and lumbar aorta, and anastomose with the anterior spinal artery ( Fig. 35.2B ). The arteries in the lumbar epidural space are branches of the iliolumbar arteries, and because they are placed laterally in the space, they typically escape trauma during an epidural puncture.

Fig. 35.2, A , The venous drainage of the vertebral column. Note that the basivertebral vein is shown beneath (that is, anterior to) the posterior longitudinal ligament. B , The arterial supply to the vertebrae and the contents of the vertebral canal, showing the branching pattern of the lumbar segmental arteries.

Innervation of the vertebral column and its associated soft tissues is derived from the spinal nerves where they branch, in and just beyond the intervertebral foramina. There is an input from the sympathetic system either via grey rami communicantes or directly from thoracic sympathetic ganglia. The branches of the spinal nerve concerned are the dorsal ramus and the recurrent meningeal or sinuvertebral nerves (usually more than one at each level). The anterior dura is heavily innervated, whereas the posterior dura is sparsely innervated.

Anchoring meningovertebral ligaments divide the epidural space into anterior, lateral and posterior compartments. Fibrous bands of connective tissue in the anterior epidural space, Hofmann ligaments, connect the anterior dural sac to the posterior longitudinal ligament (PLL) and may play a supportive role in anchoring the dural sac to the bony vertebral canal ( Fig. 35.3 ). The greatest numbers of ligaments have been observed in the lower thoracic spine.

Fig. 35.3, A transverse section showing the contents of a typical vertebral (spinal) canal. Key: Blue, epidural space; orange, ligamentum flavum; pink, subarachnoid space.

Identification of the epidural space

Identification of the epidural space is of critical importance and accuracy of needle placement determines the success of epidural analgesia. The epidural needle, as inserted in the midline, pierces the skin and traverses successively the supraspinous ligament, interspinous ligament and ligamentum flavum ( Fig. 35.4 ; see Fig. 35.1 ). The depth of the epidural space can be variable, particularly in an obese patient. Ravi correlated the distance from the skin to the epidural space based on body mass index (BMI). In most patients, this distance is in the range of 3–6 cm.

Fig. 35.4, A , A sagittal section of lumbar and sacral vertebrae illustrating the course of a lumbar puncture needle through (in order) the skin, subcutaneous tissue, supraspinous ligament, interspinous ligament between the spinous processes, ligamentum flavum and dura mater, into the subarachnoid space between the nerve roots of the cauda equina. B , A horizontal section of a lumbar vertebra illustrating the course of a lumbar puncture needle through (in order) skin, subcutaneous tissue, between the spinous processes and laminae, ligamentum flavum, epidural space and dura mater, into the subarachnoid space and between the nerve roots of the cauda equina.

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