Ultrasound-guided lumbar puncture is most helpful in those patients in whom surface landmark–guided lumbar puncture is difficult (eg, patients who are obese or have spinal disorders). According to recent studies, bedside ultrasonography helped operators identify the pertinent landmarks for lumbar puncture approximately 75% of the time in obese patients. It is an available and helpful modality that can decrease the number of attempts and minimize complications. Ultrasonographic guidance has been shown to increase the overall success rate of lumbar puncture and to reduce the operator’s perceived difficulty of performing the procedure. This is particularly true in patients with a body mass index (BMI) of 30 or more.
Additionally, studies in neonates and children have used bedside ultrasonography to attempt to determine optimal positioning for lumbar puncture. Using ultrasound to measure the interspinous space at L3-L4 and L4-L5 in varying positions, the lumbar spine was found to be maximally positioned in both neonates and children in the seated position with flexed hips versus the lateral recumbent position with neck flexion. A study in adults would be needed to make a similar recommendation regarding optimal position for lumbar puncture; however, the sitting position may be optimal in adults as well as children.
– high-frequency (small parts linear probe) for patients with normal weight
– low-frequency 2-4 MHZ probe (abdominal curvilinear probe) for obese patients
– transducer gel
– skin marking pen
– LP tray
Positioning: left lateral decubitus or seated position
1. Identify spinous process which signifies the midline of the spine. Probe must be in transverse position, probe marker to clinician’s left side, at level of iliac crest. Spinous process looks like cresent shape hyperechoic structure with posterior acoustic shadowing.
2. Mark the midline with a skin marking pen at the center of the probe. Drag the probe superiorly (head) and inferiorly (feet) and again mark the midline of the spine.
3. Rotate the transducer to the sagittal (longitudinal) plane, with probe marker pointed at patient’s head. The probe should be parallel with patients spine and in between spinous processes previously marked. The spinous processes again appear as hyperechoic cresent shapes.
4. Mark the space between the two spinous processes which is the interspinous space, on left and right side of probe.
5. The point of intersection represent the middle of the interspace and most ideal place for LP needle insertion.
6. Proceed with LP as you usually would. Instruct patient not to move as this may obscure landmarks and skin markings.