Home

Neuraxial Block

THE LUMBAR VERTEBRA

 

ANATOMY

 

  1. The prominent spinous process is midline.
  2. The laminae and facet joints (superior and inferior articular processes) are anterolateral to the spinous process.
  3. The transverse processes are more lateral and anterior to the laminae.
  4. The vertebral body is midline and most anterior.

 

Lateral to the lumbar vertebrae are the paraspinal muscles (e.g., erector spinae). The quadratus lumborum muscles are most lateral, extending from the 12th ribs to the iliac crests. Anterior to the erector spinae muscles are the psoas major muscles. Anterior to the psoas muscles is the peritoneal space.

 

Between each spinous process is the interspinous or interlaminar space. To access the epidural or subarachnoid space, the following ligaments are encountered (from posterior to anterior):

  1. The supraspinal ligament
  2. The interspinal ligament
  3. The ligamentum flavum

 

Anterior to the epidural space are the dura mater and the subarachnoid space, which contains cerebrospinal fluid (CSF), the spinal cord (cephalad to L1-2) or the cauda equina (caudad to L1-2). Anterior to the subarachnoid space lies the posterior longitudinal ligament and the posterior border of the vertebral body.

The following structures may be visualized under ultrasound:

  1. Bony structures - spinous process, laminae, facet joints, transverse processes, and posterior aspect of the vertebral body
  2. Ligaments - ligamentum flavum and posterior longitudinal ligament
  3. Muscles - paraspinal muscles (erector spinae, quadratus lumborum) and psoas muscles
  4. Dura mater
  5. Epidural vessels (seen as pulsations, mostly in young subjects)
  6. Peritoneum

Ultrasound resolution may not reliably distinguish:

  1. Ligamentum flavum from the dura mater (these structures may appear as a single hyperechoic line)
  2. Posterior longitudinal ligament from the posterior border of the vertebral body (these structures commonly appear as a single hyperechoic line)

SCANNING TECHNIQUE

 

  • Position the patient sitting, with the trunk flexed to widen the interspinous space.
  • Palpate the iliac crest on each side and mark the intercristal line (approximately at the L3-4 level).
  • Select a curved 2-5 MHz transducer and place the transducer in the transverse plane at the level of the L3 or L4 vertebra.
  • Apply ample acoustic gel to ensure good transducer-to-skin contact and eliminate potential for air trapping (artifact).
  • Adjust the appropriate depth of field (usually > 6-8 cm), focus range (usually > 5 cm) and gain.

 

ANATOMICAL CORRELATION

 

Transverse View of the Spine

 


 

Transverse View of the Interspinous Space

 


 

SCANNING TECHNIQUE

 

LOCALIZATION OF THE EPIDURAL/SUBARACHNOID SPACE

 

  • The transverse ultrasound image reveals a superficial midline hypoechoic bony shadow that represents the posterior contour of the spinous process.
  • Each lamina is visualized as a hyperechoic line anterolateral to the spinous process).
  • Each transverse process is often visualized as a hyperechoic line located more lateral and anterior to the lamina.

 

Transverse Scan of the Spinous Process

 

 

  • Position the transducer so that the midpoint of the transducer is aligned with the midpoint of the spinous process.
  • Mark the skin surface locations of the midline spinous process at several vertebral levels.
  • Take note of the distance from the skin to:
  1. The spinous process
  2. The lamina
  3. The transverse process
  • Next, visualize the interspinous space by moving the transducer cephalad or caudad until the bony hypoechoic shadow of the spinous process disappears.

Transverse Scan of the Interspinous Space

  • Assess the size (width) of the interspinous space by slowly moving the transducer cephalad and caudad in the transverse plane. Alternatively, slightly angle the transducer cephalad and caudad to determine the size of the interspinous space.
  • A narrow space is suggested when the bony shadows from two contiguous spinous processes are visualized with minimal transducer angulation.
  • Note the distance from the skin to the ligamentum flavum and the dura. It is not always possible to visualize the ligamentum flavum and the dura as two separate structures.
  • It is common to see a single hyperechoic line representing the ligamentum flavum/dura complex.
  • Finally, record the level of the interspinous space by placing a mark on each end of the transducer.

NEEDLE INSERTION TECHNIQUE (MIDLINE APPROACH)

  • Ultrasound guided neuraxial block is considered an INTERMEDIATE skill level block because the operator has to familiarize with bone imaging and identification of bony contours.
  • A pre-block ultrasound scan is performed prior to needle placement.
  • A real time ultrasound guided neuraxial block is technically difficult because of the size and thickness of the transducer (C2-5 MHz)
  • Indent the skin using the hub of a needle to mark the intersection of the posterior midline and the level of the interspinous space. This is the site of needle insertion for neuraxial block (midline approach).
  • Clean the skin thoroughly and wipe off all ultrasound gel from the skin before skin sterilization. Prep and drape the block area per routine practice.
  • Infiltrate local anesthetic at the skin site marked by the needle hub.
  • Next, insert an epidural needle using the loss of resistance technique or a spinal needle (as seen in figure) as indicated.
  • The approximate depth of the epidural space was estimated by the preceding ultrasound scans (skin distance to the ligamentum flavum and dura).

 

Continue to Page 2