Neuraxial Block

CLINICAL PEARLS

Ultrasound imaging of the spine prior to the actual neuraxial block procedure offers the following advantages:

Accurate determination of the spinal levels and the interspaces (counting cephalad from the sacrum)
Accurate localization of the posterior midline (location of the spinous processes)
Assessment of the best possible interspace available for needle insertion for neuraxial block
Determination of the skin to ligamentum flavum/dura distance prior to needle insertion
Assess anatomical abnormality and pathology in patients with scoliosis, previous laminectomy and instrumentation

Nerve Localization

1. Counting the Spinal Levels

Perform a paramedian longitudinal scan (figure below) to visualize the sacrum and the interlaminar spaces individually. This is a more accurate method of determining the actual spinal levels rather than estimating the level of the L3-4 interspace using the iliac crest.
The sacrum is identified as a continuous hyperechoic line caudally (white arrowheads in the figure below) and a bone shadow below.
Once the interlaminar space between L5 and sacrum is identified, the L5 level can be marked on the skin.

2. The Longitudinal Paramedian Window

If midline interspinous space access is limited as suggested by ultrasound, the paramedian approach is indicated.
Place the curved transducer in the longitudinal plane lateral to the spinous processes.

Longitudinal Paramedian Scan of the Interlaminar Space

LF/dura = ligamentum flavum

PBVB = posterior border of vertebral body

PLL = posterior longitudinal ligament

The laminae of two contiguous vertebrae are identified as hyperechoic bony outlines with corresponding bone shadows below (figure above).
Within this longitudinal window between the two laminae, seek the hyperechoic ligamentum flavum and dura (more superficial and posterior) and the posterior longitudinal ligament and the posterior part of the vertebral body (more anterior).
The size of the paramedian window (i.e., the length of the hyperechoic ligamentum flavum/dura complex) is a good indication of the ease of needle access to this space.
More laterally, the adjoining facet joints (superior and inferior articular processes) form a continuous curved hyperechoic line (figure above). The ligamentum flavum, dura and posterior longitudinal ligament are no longer visualized in this view.

Longitudinal Paramedian Scan of the Facets

When the transducer is moved more laterally, the transverse processes (TP) are visualized in the longitudinal view (see figure below).
The psoas muscle is seen in between the transverse processes.

Longitudinal Paramedian Scan of the Transverse Processes

ESM = erector spinae muscle

PsMM = psoas major muscle

TP = transverse process

Once the facets and/or transverse processes are visualized, this is an indication that the transducer has moved too laterally. The next step is to move and orient the transducer medially to capture the longitudinal view of the laminae and the interlaminar space.
In young adult subjects, it is possible to see epidural vascular pulsations. Epidural pulsations are especially conspicuous in the epidural space of neonates and infants.
Inspect the paramedian window on each side of the spine and select the side and the interlaminar space that is most accessible for needle entry.