Slide 1: Why use POCUS for lumbar puncture? Traditionally, lumbar puncture (LP) is performed using surface landmarks. LP can be challenging, requiring additional needle passes, resulting in a lower likelihood of success, a more traumatic tap, and a greater discomfort for the patient.
The data! A recent meta-analysis of 12 studies found several advantages of POCUS-guided LP compared to traditional LP: improved likelihood of success (NNT 11), fewer needle passes (2.07 vs. 2.66), shorter time to complete the procedure (7 vs. 8 minutes), lower incident of traumatic LPs (10.7% vs. 26.5%), diminished pain (3.75 vs. 6.31 on patient-reported pain score)
Slide 2: What structures should I am for? POCUS is generally NOT used for real-time procedure guidance, but rather to identify anatomical structures and mark the skin. A lumbar puncture is preformed in the midline of the back between spinous processes in the L3/4 or L4/5 interspaces approximately at the level of iliac crests.
The needle passes through the following structures: Skin, soft tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, cerebrospinal fluid in subarachnoid space.
Slide 3: How should I select a probe/mode? You can use either the linear probe, phased array, or convex/curvilinear probe.
The linear probe (5-13 Mhz) is higher resolution but typically has a maximum depth of only 6 cm, which may not be sufficient based on body habitus.
Phased arrray (1-10 Mhz) or convex (1-8 Mhz) probes may be better for deeper imaging.
Use B-MODE with soft tissue imaging presets, Depth should be maximum on the linear probe or at least 8 cm if using phased array or curvilinear probes.
In our next byte: learn how to use PCUS to identify structures for LP!
POCUS review. Linear probe. Frequency high. Depth shallow. Resolution high. Image shape rectangular. Best for blood vessels, pleura, muscle, bone. Phased array probe. Frequency medium. Depth moderate. Resolution moderate. Image shape fan-shaped. Best for heart and abdomen. Convex probe. Frequency low. Depth large. Resolution moderate. Image shape fan-shaped. Best for abdomen. Brightness mode. The standard 2D imaging technique, used for most POCUS exams. Exam type optimizes settings for a particular exam (e.g. heart, veins, abdomen, etc). Depth is how far into tissue the US is imaging.
- Gottlieb M, Holladay D, Peksa GD. Ultrasound-assisted Lumbar Punctures: A Systematic Review and Meta-Analysis. Acad Emerg Med. 2019 Jan;26(1):85-96. PMID 30129102.
Tags: convex probe, linear probe, LP, lumbar puncture, phased array probe, POCUS