Lumbar puncture is a common emergency department procedure used to obtain information about the cerebrospinal fluid (CSF) for diagnostic and, less commonly, therapeutic reasons. Please refer to the full article on Lumbar Puncture for more details on the lumbar puncture procedure.
Lumbar puncture is typically performed via “blind” surface landmark guidance. The surface landmark technique is reported to be successful in a high percentage of attempted lumbar punctures; however, surface landmark identification of underlying structures has been shown to be accurate only 30% of the time.  Unsuccessful identification of proper landmarks often leads to increased difficulty in obtaining CSF, if the procedure is performed, and a higher rate of complications. Few alternatives are available in these cases. If available, fluoroscopic-guided lumbar puncture may be performed. If not, treatment is sometimes initiated empirically without obtaining CSF. Disadvantages of using fluoroscopy include limited availability or necessary transport of the patient outside of the emergency department, inability to directly visualize the spinal canal, and inherent radiation exposure. 
However, the use of bedside ultrasonography for the identification of the pertinent landmarks for lumbar puncture is a safe and easy alternative to the blind technique.  Additionally, ultrasound may be used in advance of attempted lumbar puncture in order to predict ease of performance or anticipated difficult lumbar puncture.  Ultrasound-guided lumbar puncture was originally reported over 30 years ago in Russian literature, and it is routinely used by many anesthesiologists for epidural and spinal anesthesia.
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. [2, 5] It is an available and helpful modality that can decrease the number of attempts and minimize complications. [2, 5] 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. [7, 8] 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.
In a study of 19 pediatric patients, ultrasound was associated with a significantly higher average confidence score for identifying a needle insertion site, versus the landmark procedure (4.79 vs 2.89 using the Wilcoxon signed-rank test). 
In 9 patients aged 7 weeks to 16 years, all ultrasound-guided lumbar punctures were successful in identifying relevant anatomy (including the conus in children 10 years and younger), confirming intrathecal injection, visualizing intrathecal hematoma, and avoiding radiation. 
Ultrasonographic guidance may be beneficial in all patients who require lumbar puncture.
Ultrasonographic guidance is most helpful in those patients in whom lumbar puncture is challenging because of the inability to adequately palpate necessary landmarks (eg, patients who are obese or have spinal disorders).
Ultrasonography is safe and painless, with virtually no inherent risks.
No anesthesia is required when using bedside ultrasonography to identify appropriate landmarks.
Local anesthesia is, however, required prior to performing the actual lumbar puncture. For more information, see Local Anesthetic Agents, Infiltrative Administration and Lumbar Puncture.
Equipment includes the following:
Ultrasound machine with a high-frequency 5-10 MHZ probe (small parts probe) for use on patients with normal weight and a low-frequency 2-4 MHZ probe (abdominal probe) for use on patients with elevated body mass index (BMI)
Sterile or other skin marking pen
The lateral decubitus or sitting position may be used, depending on the patient's status and provider preference.
The patient remains in the selected position during ultrasonographic identification of landmarks and the lumbar puncture procedure.
Position the patient as described above.
To find the interspinous space, place the ultrasound probe with the probe marker toward the clinician's left side in the transverse plane over the midline of the back at the level of the iliac crests, as shown below.
In this view, the spinous processes appear as distinct hyperechoic structures with associated acoustic shadows. Identifying the spinous processes identifies the midline of the spine. Centering this shadow on the screen places the probe directly over the midline of the spine, as shown below.
With the probe in the position described above, mark the midline of the spine at the center of the probe using a surgical marker or pen. Then drag the probe superiorly or inferiorly to the next spinous process and again mark the midline of the spine. Make marks appropriately large and visible so that they can be easily located and extended once the ultrasound probe is removed. Although these ultrasound images are obtained in the transverse plane, these markings are made and connected in the sagittal plane.
Next, rotate the transducer into the sagittal (or longitudinal) plane with the probe marker pointing toward the patient's head. The probe should be parallel to the direction of the spine and placed directly between the spinous processes that were just marked, as depicted in the image below.
The spinous processes are seen as crescent-shaped, hyperechoic, upward convexities occurring at the same depth as the shadowing noted on the transverse view, as shown below. The gap between the hyperechoic convexities is the interspinous space.
Center the interspinous space on the screen and then mark the level of the middle of the interspinous space on both sides of the probe. Since the probe is in the sagittal plane, these marks are made in the transverse plane, slightly to the left and right of the center of the probe. Again, make the marks appropriately large and visible so that they can be easily located and extended.
Remove the ultrasound probe and extend the transverse and sagittal skin markings until they intersect. The point of intersection of these lines represents the middle of the interspace and the most ideal location at which to insert the lumbar puncture needle, as depicted below.
Instruct the patient not to move after ultrasonographic landmark identification. Take care to perform the lumbar puncture procedure immediately after marking out the middle of the interspinous space. If the patient moves substantially, the ultrasound-guided markings may be less accurate or helpful. If the patient moves substantially, consider repeating the above technique and making repeat markings on the patient. 
Use alcohol to clean the area before attempting to mark the skin. This eliminates ultrasound conducting gel and natural skin oils that may lessen or obscure the ink.
Use permanent or semipermanent skin markers so that marks are not removed when the area is later prepared for the sterile lumbar puncture.
If the area is to be sterilized before the appropriate landmarks are identified with ultrasonography, then the use of a sterile probe cover and sterile marking pen are required.
Use of bedside ultrasonography to identify pertinent landmarks poses virtually no risks or complications.
Possible limitations include lack of ultrasound machine availability and the time required by clinicians to develop the comfort and skill necessary to perform ultrasound-guided procedures.