Traditional neuraxial blockade (spinal and epidural anaesthesia) relies on palpation of bony landmarks. This can be difficult in patients with abnormal anatomy or increased adiposity. Ultrasound assessment pre-procedure may help in these scenarios. Evidence has accumulated to support this approach and it is supported by NICE guidance (2008).
An appreciation of the anatomy, both actual and as it appears via ultrasound is important. The anatomy is not covered here.
Regarding the sonoanatomy, the key is recognition of bony landmarks. Bone has a significant difference in acoustic conduction compared to surrounding tissues, thereby making it effectively opaque to ultrasound. The result is an echo bright structure with an acoustic shadow. Comparing this to the anatomy should allow identification of the appropriate structures as described blow.
A low frequency (2-5 MHz) curved array probe is best for the tissue penetration needed. A pen is often used for marking the skin.
There are a number of view that may be used:
Longitudinal midline (Median sagittal)
Paramedian sagittal view
Paramedian oblique view
The mediansagittal view involves holding the probe in the midline in a longitudinal plane. In this view the US beam will be cutting along the axis of the spinous processes. This can provide some information on the position of the midline and may help with identification of the level.
The paramedian sagittal view is often better, because the reduced amount of bone produces better conditions for US. The angle of the beam is still sagittal (i.e. from back straight through to the front) but just a few cm lateral from the midline. This will ‘cut through’ the erector spinae muscle and then come to the more lateral bony structures of the vertebral column. More medially, this will be the articular processes. If the probe position is moved more laterally then the transverse process will be visible. The focus on the articular processes is often a useful starting point These will appear like a row of ‘camel humps’
The paramedian oblique view involves starting in the paramedian sagittal view and tilting the angle of the beam to look more medially. This will direct the US beam through the window into the spinal canal. The bony structures visible will be the lamina, and will have a more ‘saw-tooth’ like appearance, as the superior aspect of the lamina slopes towards the spinal canal. The tissues of the spinal canal may be visible here. The posterior complex is an hyperechoic line comprising the ligamentum flavum, epidural space and dura (occasionally these structures may be distinguishable from each other) The subarachnoid space is echolucent and so appears black. In some situations the US may detect the anterior complex, another hyperechoic line formed by the anterior dura, posterior longitudinal ligament and the vertebral body.
The transverse view involves the rotation of the probe through 90 degrees. This will commonly provide a cross section of the vertebra, with the spinous process casting a large central acoustic shadow. This view is again useful for identifying the midline. Careful cephalad or caudal movement of the probe can then used to find the interspinous space. The space may be have a slightly cephalad angle to it, so gently tilting the angle of the beam cephalad can allow visualisation down to the spinal canal. Here the posterior complex can again be seen, though with a different shape now. In this position, the articular processes will form two pillars on either side of this window to the spinal canal, and the appearance is called the ‘bat-sign’.
The ultrasound imaging is done prior to ‘scrubbing up’. The goals of the ultrasound imaging are to:
Identify the level of insertion
Identify the midline
Identify the interspinous space
Estimate the appropriate ‘angle of attack’
Measure the depth of the epidural space
A stepwise approach to carrying out the ultrasound imaging is commonly advocated.
Finding the appropriate level (longitudinal midline view)
Identify midline through identifying spinous processes
Move caudally to identify sacrum
Count up to desired space - may be easier if probe moved to paramedian view
This level can be marked on
Rotating probe through 90 degrees to obtain transverse view
Spinous processes will be visible in the midline
This position can be marked on
Identify interspinous space
Moving the probe caudally to find interspinous space
Aim is then to identify the posterior complex
Careful angulation of the probe, if needed, will identify an ‘angle of attack.
The image can be frozen, and the depth to the space can be measured
Some of the benefits of an US approach that have an evidence base include:
More accurate identification of level
Measurement of depth of epidural space
Improved clinical efficacy of neuraxial technique
Improved performance of technique (higher first pass success, fewer attempts).
The evidence on identification of level has some variability, with the approach not being perfect. However, accuracy may be over 90% in experienced hands.
A study looking at the impact on labour epidural analgesia has suggested a reduced rate in analgesia failure (2% compared to 8% in one study).
Links & References
Ghosh, S. et al. Ultrasound-guided lumbar central neuraxial block. BJA Education. 2016. 16(7):213-220.