Bedside Ultrasonography for Lumbar Puncture

Lumbar puncture is a common emergency department procedure used to obtain information about the cerebrospinal fluid (CSF) for diagnostic and, less commonly, therapeutic reasons. Lumbar puncture is typically performed via “blind” surface landmark guidance. 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.[1, 2, 3, 4]

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.[1] 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 ED, inability to directly visualize the spinal canal, and inherent radiation exposure.[5, 6, 7, 8]

Bedside ultrasound may be used in advance of attempted lumbar puncture in order to predict ease of performance or anticipated difficult lumbar puncture.[9] 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). Studies have shown that bedside ultrasonography has helped operators identify the pertinent landmarks for lumbar puncture approximately 75% of the time in obese patients.[5, 10] It is an available and helpful modality that can decrease the number of attempts and minimize complications.[5, 10] 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.[11]  Ultrasound can also reveal other anatomical information, such as depth of the ligamentum flavum and width of the interspinous spaces, which are useful to guide LP.[3]

Numerous pediatric studies on the use of ultrasound for performing LP have shown that ultrasonography can help reduce the number of taps, improve identification of insertion sites, and decrease pain and complications.[4, 12, 6, 13, 14]

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.[15, 16] The sitting position may be optimal in adults as well.

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).[17]

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.[18]

In one study, researchers reported there were fewer traumatic LPs (>1,000 RBC/HPF) in the ultrasound group (19.4% vs. 29.7% in the standard palpation group). Regarding the overall median number of attempts needed to obtain fluid, 21% in the ultrasound group needed 3 or more attempts, versus 40% in the standard-palpation group.[19]

No anesthesia is required when using bedside ultrasonography to identify appropriate landmarks. However, local anesthesia is 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)

• Transducing gel

• Sterile or other skin marking pen

• Equipment needed for the lumbar puncture itself (see Lumbar Puncture, Equipment section)

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.

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.

Ultrasound probe in transverse position.

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.

Transverse ultrasound image of the lumbar spine demonstrating a midline dorsal spinous process. Note its characteristic crescent-shaped, hyperechoic spine with posterior acoustic shadowing.

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. (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.) 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.[14] (Use permanent or semipermanent skin markers so that marks are not removed when the area is later prepared for the sterile lumbar puncture.) Although these ultrasound images are obtained in the transverse plane, these markings are made and connected in the sagittal plane.

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.

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.

Ultrasound probe in sagittal position.

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.

Sagittal view of the interspinous space. The superior dorsal spinous process is seen on the left side of the image while the inferior dorsal spinous process is seen on the right side of the image. The acoustic shadows generated define the superior and inferior borders of 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 shown below.

Final patient markings.

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.[20]

Guidelines from the Society of Hospital Medicine on the use of ultrasound-guidance for adult lumbar puncture include the following key recommendations[21] :

• When ultrasound equipment is available, along with providers who are appropriately trained to use it, ultrasound guidance should be used for site selection of lumbar puncture to reduce the number of needle insertion attempts and needle redirections and increase the overall procedure success rates, especially in patients who are obese or have difficult-to-palpate landmarks.
• Ultrasound should be used to more accurately identify the lumbar spine level than physical examination in both obese and nonobese patients.
• Ultrasound can be used for selecting and marking a needle insertion site just before performing lumbar puncture in either a lateral decubitus or sitting position. The patient should remain in the same position after marking the needle insertion site.
• A low-frequency transducer, preferably a curvilinear array transducer, should be used to evaluate the lumbar spine and mark a needle insertion site. A high-frequency linear array transducer may be used in nonobese patients.
• Ultrasound should be used to map the lumbar spine, starting at the level of the sacrum and sliding the transducer cephalad, sequentially identifying the lumbar spine interspaces.
• Ultrasound should be used in a transverse plane to mark the midline of the lumbar spine and in a longitudinal plane to mark the interspinous spaces. The intersection of these 2 lines marks the needle insertion site.
• Ultrasound should be used during a preprocedural evaluation to measure the distance from the skin surface to the ligamentum flavum from a longitudinal paramedian view to estimate the needle insertion depth and ensure that a spinal needle of adequate length is used.
• Novices should undergo simulation-based training, where available, before attempting ultrasound-guided lumbar puncture on actual patients.
• Training in ultrasound-guided lumbar puncture should be adapted based on prior ultrasound experience, as learning curves will vary.
• Novice providers should be supervised when performing ultrasound-guided lumbar puncture before performing the procedure independently on patients.