In this blog we're going to discuss sono-anatomy for lumbar puncture.
Lumbar puncture is usually performed by palpating for underlying anatomical structures to identify the correct point of entry and angle of needle insertion. Typically, the posterior superior iliac crests are palpated to find the "Tuffier's Line" between these two points, which represents around L4. Next the interspace between L4/L5 or L3/4 is found and the needle is inserted towards the umbilicus .
However palpation based landmark identification is prone to error and numerous studies have shown that ultrasound assistance in identifying landmarks and insertion points are associated with reduced complications, reduced pain and reduced number of attempted insertions.
Understanding the anatomy for lumbar puncture is important as there are numerous structures to consider:
The spinal cord -> cauda equina level
Vertebral midline, spinous processes
Supraspinous ligament
Depth of the dura, arachnoid and epidural space
Typically the patient will be sitting. L4 can be found through palpation using Truffier's line or longitudinal scanning and identification of the sacrum first, then counting up from L5, L4, L3.
Once the operator has identified the spinous processes, they should find the midline. The probe can then be placed with the marker pointing towards the operator's left and placed on L4 and operator can look for the spinous process as a hyperechoic crescent, see video for an anatomical representation. At the centre, they can place a dot above and below the probe and connect them. This would represents the midline.
Next the operator can rotate the probe so that the marker is pointing towards the patient's head. The probe should be placed over either L3/L4 or L4/L5 at the midline. Find the interspinous space between the two spinous shadows. Finally they draw a line at the interspinous space on each side of the probe and connect the lines to end up with a cross shape.
This would be the point at which the operator can insert a needle towards the umbilicus.
Of course, if you want to understand ultrasound anatomy better or teach by creating your own videos you can try Deepscope Ultrasound Simulator.
References:
1. Soni NJ, Franco-Sadud R, Schnobrich D, et al. Ultrasound guidance for lumbar puncture. Neurol Clin Pract. 2016; 6(4):358‐368.
2. Nomura, Jason T. et al. A randomized controlled trial of ultrasound-assisted lumbar puncture. Journal of Ultrasound. Oct 1 2007, 1341-1348.