Slide 1: Point-of-care ultrasound-guided lumbar puncture.

Why use POCUS for LP? Traditionally, lumbar puncture (LP) us performed using surface landmarks. LP can be challenging, requiring additional needle passes and a more traumatic tap. POCUS can help identify structures to increase likelihood of success and decrease number of needle passes, resulting in greater comfort and diminished pain for the patient.

Where should I aim? A lumbar puncture is performed in the midline of the back between spinous processes in the L3/4 or L4/5 interspaces approximately at the level of the iliac crests. The needle passes through the following structures: skin, soft tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, cerebrospinal fluid in subarachnoid space.

How should I select a probe/mode? The linear probe has a higher resolution but typically a maximum depth of only 6 cm, which may not be sufficient based on body habitus. The phased array or convex/ curvilinear probes may be better for deeper imaging.

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.

How do I prepare the patient? After obtaining informed consent, position the patient in either the seated or lateral recumbent positions. TIP! Flexion of the hips up to the abdomen combined with tucking the chin down onto the chest, enlarges the intervertebral space, making LP easier to accomplish.

POCUS is generally NOT used for real-time procedure guidance, but rather to identify anatomical structures and mark the skin. Two ultrasound views are obtained in perpendicular planes: transverse – identify spinous processes to mark the midline; longitudinal – identify spinous processes to find the interspace.

Step 1. Identify the midline. Place the transducer in a transverse orientation. Slide the probe superiorly until the spinous process is identified. Graphic showing identification of midline of spinous process. The bone is hyperechoic (white) with a shadow (black) behind it.

Use a marking pen to mark the center of the spinous process. Repeat at multiple levels to define the midline of the spine. Once the midline is identified, rotate the transducer into a longitudinal orientation aligned with the sagittal plane.

Step 2. Identify the interspace level. The superior and inferior spinous process are visible. The ligamentum flavum may also be visible as a hyperechoic horizontal line. Identify and mark the space directly between two spinous processes. Compare L3/4 and L4/5 to see if one space is larger than the other; you can mark both sites. Measure the distance to the ligamentum flavum to estimate how far the needle will have to pass.

Step 3. Perform the LP. By marking the skin in the transverse and longitudinal axes you should have formed an X at the Perfect site for needle entry. Now preform the LP as you normally would, with the needle inserted at the site identified and directed towards the umbilicus.