Ultrasound-guided lumbar puncture
Identification of landmarks for lumbar puncture using palpation can be challenging, especially in obese patients in whom the palpation of landmarks is difficult. Pre-procedure mapping of landmarks for lumbar puncture using ultrasound can be used to mitigate this difficulty.
The following section will outline the key steps in performing pre-procedural ultrasound-guided landmarking for lumbar puncture. This section will conclude with a video tutorial.
Section contributors: Alexander Wilson, Senior Emergency Medicine Resident
Darren Hudson, Brian Buchanan
Set-up
Select a high frequency linear array ultrasound probe for patients with low or normal body habitus. For obese patients, consider using a low frequency curvilinear transducer as this will allow for deeper tissue visualization.
Position the patient appropriately for lumbar puncture, either in a lateral decubitus or seated position. If lateral decubitus positioning is chosen, flex the patient’s knees to their chest and ensure that the patient’s spine is not rotated. If a seated position is selected, instruct the patient to lean forward with arms folded onto a table with feet supported on the ground or a stool. Ensure that the patient does not move in the interim between landmarking and the lumbar puncture procedure as any movement can change the relationship between the marked skin and underlying anatomy
Identify lumbar spine
In adults, lumbar puncture is performed below the L2 spinal level (below the termination of the spinal cord). In most people, the L4 level can be approximated by the level of an imaginary line connecting the posterior superior iliac crests. However, palpation of the iliac crests may be difficult in some patients. In these cases, start with the transducer in transverse plane over the intergluteal cleft, and slide superiorly until the L5 spinous process is seen. The L5 spinous process will appears a hyperechoic structure that is small and deep in soft tissue, even in patients with low body habitus. Sliding the transducer cephalad can identify the L4, L3, L2, and L1 spinous processes which will appear as near field hyperechoic structures.
IDENTIFY MIDLINE
Once the lumbar spine and a safe region to to perform lumbar puncture is identified, identify the L2, 3, 4, 5 spinous processes with the probe in the transverse orientation. Center the spinous processes on the screen. Draw a dot on the skin at the location of each process and connect the dots with a vertical line. This line represents the patient’s midline.
IDENTIFY THE INTER-SPINOUS SPACE
Slide the probe in a longitudinal orientation along the midline to identify the L3-L4 and L4-L5 interspinous spaces which will appear as hypoechoic spaces in-between two hyperechoic spinous processes. Center the interspinous space on the screen and draw a horizontal line at this location for the L3-L4 and L4-L5 interspinous spaces. Try to draw these lines slightly closer to the caudad spinous process as insertion at this site will improve the needle insertion angle and reduce the risk of hitting the cephalad spinous process while performing lumbar puncture.
DETERMINE THE DISTANCE TO THE LIGAMENTUM FLAVUM
Identification of the ligamentum flavum isn’t always possible especially in patients with large habitus. However, if identified, it can be used to help gauge the needle insertion depth required to reach the subarachnoid space as the posterior dura is a few millimetres deep to the ligamentum.
The ligamentum flavum is most reliably visualized in a longitudinal paramedian view and will appear as a hyperechoic structure in the far field between the laminae. It is important to recognize that a paramedian view can be confused with a midline view, and the user should sweep laterally to differentiate spinous processes and laminae. Furthermore, in the paramedian view, the erector spinal muscle fibers will be visible, whereas only subcutaneous tissue is visible nearfield to the spinous processes in the midline view.
Perform the procedure
Once midline, interspinous spaces, and the ligamentum flavum are identified and marked, lumbar puncture can be performed using standard technique. The intersections of the vertical and horizontal lines denote the ideal needle insertion sites
YouTube 360 LP video
The following video was produced with a 360 camera. The best way to view it is either 1. through YouTube; use your cursor to drag the screen or 2. through a VR headset! Click below to watch it on YouTube.
References
Arun Nagdev, C. R. (2014, July 9). How to Perform an Ultrasound-Assisted Lumbar Puncture . Retrieved from acepnow: https://www.acepnow.com/article/perform-ultrasound-assisted-lumbar-puncture/?singlepage=1
Kirk A. Stiffler, S. J. (2007). The use of ultrasound to identify pertinent landmarks for lumbar puncture. The American Journal of Emergency Medicine, 331-334.
Michael Gottlieb, D. H. (2019). Ultrasound-assisted Lumbar Punctures: A Systematic Review and Meta-analysis . Academic Emergency Medicine , 85-96.
Nilam J. Soni, R. A. (2020). Point-of-Care Ultrasound, Second Edition. Philadelphia: Elsevier, Inc.
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