Sonoanatomy relevant for ultrasound-guided central neuraxial blocks via the paramedian approach in the lumbar region

Abstract

Objectives: The use of ultrasound to guide peripheral nerve blocks is now a well-established technique in regional anaesthesia. However, despite reports of ultrasound guided epidural access via the paramedian approach, there are limited data on the use of ultrasound for central neuraxial blocks, which may be due to a poor understanding of spinal sonoanatomy. The aim of this study was to define the sonoanatomy of the lumbar spine relevant for central neuraxial blocks via the paramedian approach.

Methods: The sonoanatomy of the lumbar spine relevant for central neuraxial blocks via the paramedian approach was defined using a "water-based spine phantom", young volunteers and anatomical slices rendered from the Visible Human Project data set.

Results: The water-based spine phantom was a simple model to study the sonoanatomy of the osseous elements of the lumbar spine. Each osseous element of the lumbar spine, in the spine phantom, produced a "signature pattern" on the paramedian sagittal scans, which was comparable to its sonographic appearance in vivo. In the volunteers, despite the narrow acoustic window, the ultrasound visibility of the neuraxial structures at the L3/L4 and L4/L5 lumbar intervertebral spaces was good, and we were able to delineate the sonoanatomy relevant for ultrasound-guided central neuraxial blocks via the paramedian approach.

Conclusion: Using a simple water-based spine phantom, volunteer scans and anatomical slices from the Visible Human Project (cadaver) we have described the sonoanatomy relevant for ultrasound-guided central neuraxial blocks via the paramedian approach in the lumbar region.

Figure 1

The water-based spine phantom. Note how the lumbosacral spine model was immersed in water. Inset shows how the ultrasound scan was performed through the water.

Figure 2

Paramedian oblique sagittal sonogram of the lamina (L3, L4, L5) from (a) the water-based spine phantom and (b) volunteers, and a representative anatomical slice from (c) the Visible Human Project (cadaver). In the latter, the lamina has been shaded in green (c). Note the marker (needle) in contact with the lamina in the water-based spine phantom (a). This was done to ensure that the lamina was being scanned and also helped in validating its sonographic appearance. A graphic overlay has been placed over the lamina in (a) to illustrate the “horse head sign”. AC, anterior complex; CE, cauda equina; ES, epidural space; ESM, erector spinae muscle; ILS, interlaminar space; ITS, intrathecal space; IVD, intervertebral disc; LF, ligamentum flavum; PD, posterior dura; SC, spinal canal; VB, vertebral body.

Figure 3

Paramedian sagittal sonogram of the articular process from the (a) water-based spine phantom and (b) volunteers, and a representative anatomical slice from (c) the Visible Human Project (cadaver). A graphic overlay has been placed in (b) to illustrate the camel-hump-like appearance of the articular processes (the camel hump sign). AP, articular process; ESM, erector spinae muscle; FJ, facet joint; VB, vertebral body.

Figure 4

Paramedian sagittal sonogram of the transverse process from the (a) water-based spine phantom, (b) volunteers and (c) the Visible Human Project (cadaver) In the latter, the transverse processes of L3 and L4 have been shaded in green (c). Note how the acoustic shadow of the TPs produces the “trident sign” [3]. ESM, erector spinae muscle; PM, psoas muscle; TP, transverse process.

Figure 5

Paramedian sagittal sonogram of the L5/S1 gap from the (a) water-based spine phantom, (b) volunteers and (c) the Visible Human Project (cadaver). In the latter, the lamina of L4 and L5 vertebra and the posterior part of the sacrum have been shaded in green (c). CE, cauda equina; ESM, erector spinae muscle; ITS, intrathecal space; PD, posterior dura; VB, vertebral body.

Figure 6

Paramedian oblique sagittal sonogram of the lumbar spine at the L3/L4/L5 level. Note the acoustic shadow of the lamina and the acoustic window, which results from reflections of the ultrasound signal from the neuraxial structures within the spinal canal. ILS, interlaminar space.

Figure 7

Paramedian oblique sagittal sonogram of the lumbar spine at the L3/L4/L5 level. The posterior epidural space is seen as a hypoechoic space (a few millimetres wide) between the hyperechoic ligamentum flavum and the posterior dura. Note that the posterior dura appears brighter and is also better visualised than the ligamentum flavum in this sonogram. ESM, erector spinae muscle; ILS, interlaminar space; LF, ligamentum flavum.