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[Note: The following article appeared in the ASRA Newsletter, February 2008]

*Ultrasound-Facilitated

Epidural and Spinal Needle Placement* Jose
C.A. Carvalho, M.D., Ph.D., F.A.N.Z.C.A., F.R.C.P.C.Associate
Professor of Anesthesia and Obstetrics and Gynecology, University of
TorontoDirector
of Obstetric Anesthesia, Mount Sinai Hospital, Toronto Ultrasound
has recently been introduced into clinical anesthesia to facilitate
lumbar spinal and epidural insertion. The use of pre-procedure
ultrasound imaging and, eventually, real-time ultrasound guidance
should improve not only clinical practice but also the teaching of
neuraxial blocks. Spinal
ultrasound is particularly challenging because the structures to be
imaged are protected by a very complex, articulated encasement of
bones that affords very narrow acoustic windows for the ultrasound
beam. In addition, the structures are located deeper than those we
image when we use ultrasound for peripheral nerve blocks or the
placement of central lines. There are, however, two useful acoustic
windows for the assessment of lumbar spine sonoanatomy: one is
accessed by using a paramedian longitudinal view [probe positionFigure 1a; sonogram Figure 1b], the other by using a transverse
midline view [probe position Figure 2a; sonogram Figure 2b]. The
information from each of these two imaging planes complements the
other. The ultrasound probe used for spinal ultrasound must be a
low-frequency (2-5 mHz) curved probe because the structures to be
visualized lie >5cm in depth. However, the disadvantage of a
low-frequency probe is a loss of image resolution. When
using the paramedian longitudinal approach, the ultrasonographic
pattern is commonly described as the “saw sign” [[Figure 1b]]; as
for the pattern of the transverse approach, I usually describe it as
the “flying bat” [[Figure 2b]]. An
accurate assessment of the sonoanatomy of the lumbar spine is
accomplished with a systematic approach, which can be conducted as
follows:

A.

Paramedian longitudinal approach: The
paramedian longitudinal scan is performed by positioning the
ultrasound probe vertically, perpendicular to the long axis of the
spine [[Figure 1a]]. The probe is initially placed over the sacral
area, 3 cm to the left of the midline and angled slightly to target
the center of the spinal canal. From this point, a continuous
hyperechoic (bright) line representing the sacrum is visualized. The
probe is then slowly moved cephalad until a hyperechoic saw-like
image [[Figure 1b]] is seen. The “saw” represents the articular
processes (teeth of the saw) and the interspaces (spaces between the
teeth), the latter consisting of the ligamentum flavum and posterior
dura-mater and, deeper in, the anterior dura mater, the posterior
longitudinal ligament and the vertebral body. The exact level of each
of the interspaces, L5-S1 to L1-L2, can then be marked on the skin to
facilitate the rest of the examination.

B.

Transverse approach: Once
the interspaces are determined using the longitudinal approach, a
transverse scan of each individual space can be performed. This is
accomplished by positioning the probe horizontally, perpendicular to
the long axis of the spine, at the marked levels [[Figure 2a]]. With
this approach, the midline of the spine, i.e., the spinous process,
is identified as a small hyperechoic signal immediately underneath
the skin, which continues as a long triangular hypoechoic (dark)
acoustic shadow [[Figure 3]]. The probe is then moved slightly cephalad
or caudad to capture a view of an acoustic window (interspace)
[[Figure 2b]]. Within the interspace, on the midline, a hyperechoic
band corresponding to the ligamentum flavum and the dorsal dura is
visualized. A second hyperechoic band, parallel to the first band,
corresponds to the anterior dura, the posterior longitudinal ligament
and the vertebral body. In addition, paramedian hyperechoic
structures, corresponding to the articular and transverse processes,
are also visualized. The
ultrasonographic assessment of the lumbar spine as described above
will help the clinician to determine the exact interspace at which
the puncture will be performed and, also, the optimal insertion
point. The determination of the optimal insertion point is easily
made with the transverse approach. Once the clear image of the
interspace is obtained, the ultrasound image is frozen to enable
measurement. At that moment, with the probe kept steady, two marks
are drawn on the skin: one coinciding with the center of the upper
horizontal surface of the probe (midline) and the other coinciding
with the middle point of the right lateral surface of the probe
(interspace). The puncture site is determined by the intersection of
the extensions of the two marks on the skin in the vertical and
horizontal planes [[Figure 4]]. It is not easy to predict the angle at
which the epidural or spinal needle should be advanced during
placement. However, I suggest that the needle should follow the same
angle as the probe at which the best image of the “flying bat”
was captured. With
the aid of a built-in measuring caliper on the ultrasound machine,
the distance from the skin to the epidural space can be measured
[[Figure 2b]]. With the current resolution of portable ultrasound
machines, the captured image of the ligamentum flavum and posterior
dura mater appears as a single line. At present, the distance from
the skin to the inner side, i.e., the deepest border of this
ligamentum flavum-posterior dura mater unit, is measured and
represents the actual needle depth required to reach the epidural
space. Although
it has been suggested that the paramedian longitudinal
approach is the best acoustic window in spinal ultrasonography, I
believe that the transverse approach is more useful in clinical
practice, especially if the midline approach is to be used for the
puncture, which is more common. In most cases, I only use the
paramedian longitudinal approach to determine the spinal level of
needle insertion. In some cases, especially where the quality of
imaging is compromised or where there is a need of confirmation of a
certain structure, the paramedian longitudinal approach is
beneficial. It is therefore advisable that the anesthesiologist be
familiar with both approaches.

References: 1.

Grau T, Leipold RW, Conradi R, Martin E, Motsch J. Ultrasound imaging
facilitated localization of the epidural space during combined spinal
and epidural anesthesia. Reg Anesth Pain Med. 2001; 26: 64-67.2.
Grau T, Leipold RW, Horter J, et al. Paramedian access to the
epidural space: The optimum window for ultrasound imaging. _J Clin
Anesth_. 2001; 13:213-217.3.
Grau T, Leipold RW, Horter J, Martin E, Motsch J. Colour doppler
imaging of the interspinous and epidural space. _Eur J
Anaesthesiol_. 2001; 18:706-712.4.
Grau T, Leipold RW, Fatehi S, Martin E, Motsch J. Real-time
ultrasonic observation of combined spinal-epidural anaesthesia. _Eur
J Anaesthesiol_. 2004; 21:25-31.5.
Arzola C, Davies S, Rofaeel A, Carvalho JCA. Ultrasound using the
transverse approach to the lumbar spine provides reliable landmarks
for labor epidurals. Anesth Analg. 2007; 104:1188-1192

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