International Journal of Academic Medicine

: 2019  |  Volume : 5  |  Issue : 3  |  Page : 165--170

A comparison of ultrasound-guided and palpation-guided identification of lumbar puncture needle entry site in patients as body mass index increases

Linda Joseph1, Mayank Mehrotra2, Rebecca Jeanmonod3, Donald Jeanmonod3,  
1 Department of Emergency Medicine, Methodist Charlton Medical Center, Dallas, TX, USA
2 Department of Anesthesia, Medical College of Georgia, Augusta University, Augusta, GA, USA
3 Department of Emergency Medicine, St. Luke's University Hospital, Bethlehem, PA, USA

Correspondence Address:
Dr. Donald Jeanmonod
St. Luke's University Hospital, 801 Ostrum Street, Bethlehem, PA 18015


Background: This was a prospective, crossover study examining the ability of emergency medicine (EM) residents to identify lumbar puncture (LP) needle entry site by palpation and ultrasound in individuals with varying body mass indexes (BMIs). Materials and Methods: Following a didactic session on the use of ultrasound to identify landmarks for LP needle entry, EM resident physicians were asked to identify LP needle entry sites using ultrasound on several volunteers of varying BMIs (normal [BMI, 18.5–24.9], overweight [BMI, 25.0–29.9], and obese [BMI, 30–39.9]). Measurements of their deviation from an entry point determined by a faculty member with expertise in ultrasound were recorded. Residents were then asked to determine LP needle entry sites using palpation and again the deviation was recorded. Results: Using ultrasound, the transverse and longitudinal deviations from a gold standard were 7.8 mm (standard deviation [SD]: 6.5 and confidence interval [CI]: 1.9) and 7.1 mm (SD: 5.2 and CI: 1.5), respectively, whereas using palpation, the transverse and longitudinal deviations from our gold standard were 4.4 mm (SD: 3.4 and CI: 0.99) and 8.2 mm (SD: 6.6 and CI: 1.9), respectively. Conclusion: There was no difference in the residents' ability to identify LP needle entry sites on volunteers of various BMIs when comparing the use of ultrasound to standard palpation. The following core competencies are addressed in this article: Patient care and procedural skills, Medical knowledge.

How to cite this article:
Joseph L, Mehrotra M, Jeanmonod R, Jeanmonod D. A comparison of ultrasound-guided and palpation-guided identification of lumbar puncture needle entry site in patients as body mass index increases.Int J Acad Med 2019;5:165-170

How to cite this URL:
Joseph L, Mehrotra M, Jeanmonod R, Jeanmonod D. A comparison of ultrasound-guided and palpation-guided identification of lumbar puncture needle entry site in patients as body mass index increases. Int J Acad Med [serial online] 2019 [cited 2021 Jan 19 ];5:165-170
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Full Text


Lumbar puncture (LP) is a frequently performed procedure in emergency medicine (EM) and is a crucial diagnostic tool, particularly in life-threatening clinical scenarios. If performed improperly, LPs can subject the patient to unnecessary discomfort and can lead to misleading or ambiguous cerebrospinal fluid findings, which can muddle the clinical picture and delay proper management of the patient's disease. Multiple needle insertion attempts may also lead to increased risk of complications such as postdural puncture headache, epidural hematomas, and traumatic taps.[1],[2] Although palpation of surface landmarks often leads to successful LPs, several studies [3],[4] have shown that, as patient body mass index (BMI) increases, these surface landmarks become increasingly difficult to palpate, which may lead to technically difficult and potentially unsuccessful LPs. Recent estimates suggest that more than two-thirds (68.9%) of the adult population in the United States is deemed overweight or obese (BMI ≥25) and 1 in 20 (6.3%) Americans are classified as extremely obese (BMI ≥35).[5] With a growing population in whom surface landmarks will likely be difficult to palpate, alternative methods of rapidly and accurately identifying LP sites are needed.

One alternative is to perform LPs under fluoroscopic guidance. There are several drawbacks to this method. It is time-consuming, requires patients to leave the emergency department (ED), results in radiation exposure, requires the involvement of interventional radiologists, and is not typically available at all hours of the day. The use of bedside ultrasound has been identified as a possible alternative to traditional palpation or fluoroscopic-guided LP.

Using ultrasound to identify LP landmarks and access the subarachnoid space has been described in both EM and anesthesia literature.[1],[3],[4],[6],[7],[8],[9],[10] Ferre andSweeney [3] and Stiffler et al.[4] demonstrated that as LP surface landmarks become increasingly difficult to palpate with increasing BMI, EM physicians remain able to identify and attain high-quality ultrasound images of pertinent LP landmarks. Numerous independent studies have demonstrated that ultrasonographic guidance for LPs may be useful in obese patients and in those with difficult surface anatomic landmarks.[1],[2],[11]

This study investigated the ability of EM resident physicians to identify potential LP needle entry sites by palpation and ultrasound in patients, in multiple BMI classes in the lateral decubitus position. We postulated that as patient BMI increases, particularly when BMI is above 30, there would be a greater disparity between LP sites identified by ultrasound and palpation.

 Materials and Methods

Study design

This was a prospective, crossover study examining the ability of EM residents to identify LP needle entry site by palpation and ultrasound in individuals of varying BMIs. The investigation was reviewed by the institutional review board and found to be exempt.

Setting and study population

The study was conducted in the educational building of an academic referral center with two EM residency programs (separately accredited by American Osteopathic Association and Accreditation Council for Graduate Medical Education) consisting of 40 residents. Data collection occurred during conference days of the EM residency programs.

The study participants on whom the ultrasounds were performed were adult volunteers (>18 years of age) from the Department of EM (two resident physicians and one attending physician), who expressed willingness to participate in the study investigation and who were comfortable with reporting their weight and height for BMI determination.

All residents who were present during EM conference days were asked to participate in data collection. Residents were excluded if they took on the role of study volunteer or study investigator.

Study protocol

Before initiating data collection, during one training session, EM residents were instructed on how to visualize LP landmarks and identify LP needle entry sites using ultrasonography. Instruction took the form of a brief, 15-min didactic presentation that included images of the ultrasound technique as well as still images and video clips of ultrasounds demonstrating anatomical landmarks. Resident questions regarding the procedure were answered during this session. There was no hands-on practice session on the use of ultrasound-guided LP provided to augment the didactic instruction. No formalized didactic session was performed to educate residents on anatomic landmarks for palpation-assisted LP.

Each study volunteer's BMI was calculated using the volunteers' reported height, reported weight, and the formula BMI = (Weight in lbs/[height in inches × height in inches]) × 703.[12] Volunteers were then classified by BMI as normal (18.5–24.9), overweight (25.0–29.9), or obese (30–39.9).

Data collection occurred in a series of sessions during EM conference days when residents, investigators, and volunteers were available. During each session, one of the three study volunteers was placed in the lateral decubitus position. Using ultrasonography, LP needle entry sites were identified at L3/L4 and L4/L5 intervertebral spaces by faculty with extensive experience with US-guided LP. The sites were marked using ultraviolet (UV) light markers that left a mark on the skin that was only identifiable when illuminated with a UV light and was invisible when visualized under standard lighting. These marks were considered the gold standard against which resident marks would be measured.

Residents were then called into the session room and used visible marker to mark LP needle entry site by palpation. Residents were asked to leave the study area, and the distance of transverse and longitudinal deviation from the gold standard mark was measured by a study investigator using a Wood's lamp to illuminate the markings from UV light marker. Visible marker was removed ensuring that the resident's initial mark was undetectable. Residents then returned to the study area and marked the LP site by ultrasonography. Residents were again asked to leave the study area, and the transverse and longitudinal deviations from the gold standard mark were measured by a study investigator under a Wood's lamp. The residents remained blinded to all measurements.


Before initiation of the investigation, EM residents were asked to complete a standard datasheet that gathered information regarding the resident's level of training, experience with ultrasound, and experience with ultrasound-guided LP.

During each session of data collection, all ultrasound images were obtained using a 10-MHz linear array probe and a GE Logic P6 ultrasound machine. Our primary outcome measure was the longitudinal and transverse deviation of the resident physician markings from the gold standard markings. These measurements were made using a standard metric ruler and were recorded on a preestablished data collection sheet.

After each session, residents were asked to describe their level of comfort on a 10-cm visual analog scale when using palpation and ultrasound to identify LP sites in each of the three volunteers. Zero represented “not at all comfortable” and 10 represented “very confident.”

Data analysis

The data were collected prospectively on standardized data forms at the time of each study session. They were then entered into a Microsoft Excel spreadsheet shared among study investigators. They were analyzed with Wilcoxon rank sum test.


By palpation, the transverse and longitudinal deviations from our gold standard were 4.4 mm (standard deviation [SD]: 3.4 and confidence interval [CI]: 0.99) and 8.2 mm (SD 6.6, CI 1.9), respectively. By ultrasound, the transverse and longitudinal deviations were 7.8 mm (SD: 6.5 and CI: 1.9) and 7.1 mm (SD: 5.2 and CI: 1.5), respectively. At a BMI of 20.5, there were no differences between the transverse and longitudinal deviations by palpation as compared to ultrasound. At a BMI of 28, there was a smaller transverse deviation from gold standard using palpation as compared to ultrasound (6.0 mm vs. 10.8 mm; P = 0.01). There was no difference in deviation between longitudinal measurements (P = 0.07). At a BMI of 31.5, there was no difference between the transverse and longitudinal deviation by palpation as compared to ultrasound [Table 1].{Table 1}


The reported experience investigating the use of ultrasound to assist with LP in the ED is relatively limited. The two studies by Ferre et al.[3],[13] demonstrated that EM physicians can utilize ultrasound to identify spinal landmarks quickly and were successful in performing the majority of the procedures, but found that BMI did not correlate with the success of the procedure. These results are similar to the results that we obtained that residents were equally facile with identifying potential needle entry point regardless of whether they used palpation or ultrasound guidance, and that this finding did not change with increasing BMI. Even though it makes intuitive sense that ultrasound should provide a more accurate sense of internal anatomy, EM residents were as accurate in predicting the appropriate location for needle entry with palpation of anatomic landmarks. A recent study, representing the largest single sample of patients randomized to palpation-assisted versus ultrasound-assisted LP in the ED and intensive care unit, with a total of 158 patients enrolled, found that there was no difference in success rates, needle redirections, traumatic LPs, and time to completion of the procedure.[12] The operators were resident physicians, and their previous LP experience was not reported. Similarly, in their study of 100 patients randomized to the use of palpation versus ultrasound guidance, Peterson et al.[14] found that there was no difference in their primary outcome of number of needle insertion attempts or overall success of the procedure as well as all their secondary endpoints including pain associated with the procedure, time to completion, number of traumatic taps, and patient satisfaction. This study was partially limited by its study design. The operators who performed the LP were house staff, both of EM training and house staff rotating through the ED, who had completed at least 10 LPs previously. Those randomized to ultrasound guidance had a bedside tutorial on its use immediately preceding the study period. Similar to the data presented, neither of these studies found a difference in any endpoint when they stratified their patients by BMI.

Two studies have been published by an EM group from Korea. In the first study,[15] they found that ultrasound use did not affect the success rate of LP performed, but there was a suggestion that ultrasound guidance could have a more profound role in assisting with LP in elderly patients. In their second study,[16] they found that ultrasound improved the success rates, decreased the number of attempts, and reduced the time to completion of LP in a group of 60 patients aged 60 years or older who were randomized to palpation-assisted versus US-assisted LP.

The randomized, double-blinded trial by Nomura et al.[11] demonstrated a small, but significant, increased success rate with using ultrasound (RR: 1.32 and 95% CI: 1.01–1.72), but was underpowered to detect differences associated with patient obesity and was limited to three investigators who performed the ultrasound. The operators who performed the procedure were randomized to using a needle entry point determined by ultrasound versus one determined by palpation, and there was no description of how or if they controlled for further palpation during the procedure. Although reported as a study demonstrating superiority of ultrasound for LP, Mofidi et al.[2] had successful completion of LP in all of their patients, irrespective of intervention allocation, but demonstrated improvement in the number of attempts and time to completion in those randomized to using ultrasound. Most of this benefit was demonstrated in their group of patients with BMI >29.

Stiffler et al.[4] conducted a study to assess the ultrasounds ability to identify relevant landmarks (L4–L5 spinous processes and spinal canal) for LP across different BMIs. They encountered difficulty in palpation in 5% of individuals with normal BMI, 33% in overweight individuals, and 68% in obese individuals. The success of identifying the structures was 100% in normal BMI individuals, 95% in overweight individuals, and 74% in obese individuals. Their study indicated that the advantage of using ultrasound to identify relevant landmarks was inversely proportional to the individuals' BMI. However, even in this seemingly difficult population, the authors were able to find landmarks in at least 74% of individuals. Chin et al.[1] studied the use of ultrasound imaging to facilitate performance of spinal anesthesia in patients with difficult surface landmarks. These included patients with BMI >35 kg/m 2, moderate-to-severe lumbar scoliosis, or previous lumbar spine surgery. They concluded that the first attempt success rate of needle insertion was twice as high in the ultrasound group as compared to the landmark group (65% in ultrasound group as opposed to 32% in the landmark group). There was also a two-fold difference noted in the median number of needle insertion attempts and number of needle passes. The intervertebral level and the depth to intrathecal or epidural space was more accurately estimated with the use of an ultrasound.

Although the anesthesiology literature contains a number of studies demonstrating the usefulness of ultrasound for epidural placement and spinal anesthesia, there would be difficulty in generalizing these data to emergency physicians who are performing LPs.

When considering the procedure of LP, there are several useful pieces of information that ultrasound can provide. It not only demonstrates where midline and the interspinous spaces are but also demonstrates the depth of the thecal sac and predicts the necessary angle of needle entry. We demonstrated, for the first piece of information, the location of needle entry point, ultrasound may not provide as much of a role as would be expected. The majority of the studies would support this argument, suggesting that ultrasound does not significantly decrease failure rates or number of attempts required to perform an LP. However, what we would point out is that using ultrasound to assist LP is not totally without merit. The majority of the study designs randomize patients up front to the use of ultrasound, but ideally, ultrasound would have most of its benefit in assisting LP in patients who have already had a failed attempt. Further study protocols should define up front what constitutes LP failure and then randomize the patient to further palpation-guided attempts versus ultrasound guidance.


There are several potential limitations in this study that might have affected outcome. In our study, the participating emergency residents who performed the ultrasound examination were a mixed group of both allopathic and osteopathic medicine physicians. This could have resulted in better palpation of landmarks by the osteopathic residents, who have been previously trained in osteopathic manipulative therapy. In addition, the training session did not incorporate a hands-on practice session. Although all the residents were familiar with ultrasound use, the translation between didactics and experience might have been affected and consequently decreased the apparent effectiveness of ultrasound. In addition, all residents were not studied immediately following the didactic presentation, and may have completed their study session several weeks following the didactics, causing degradation of knowledge over time. This study was not designed to follow the residents longitudinally over time. It is possible that, with increased practice, their skills at ultrasound-guided LP would improve.

Due to time constraints, the residents who participated in the study did not perform measurements on all three models. As well, the measurements that were conducted in all the three models were done at different times, and there was no standardization in the order which they were presented with models. Residents might have had a learning effect by the time that they approached the third model, which would have influenced their ability to perform the ultrasound-guided LP.

This study was not designed to determine whether the use of ultrasound guidance for performing LP increased successful LP, but was only designed to determine whether EM residents were able to accurately determine needle entry point by palpation and ultrasound. It could be that accurately identifying needle insertion site does not translate to successful procedure completion. We recommend that further studies may incorporate a study population with comparable physicians performing the measurements after a hands-on training session to evaluate the needle site entry for LPs.


In this study, as BMI increased, there was no significant deviation in LP site identified by palpation versus US. In the “patient” with BMI of 28, LP sites identified by palpation were closer to the gold standard mark in the transverse distance as compared to US, but not in the longitudinal distance.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

Ethical conduct of research

This study was reviewed by the Institutional Review Board and found to be exempt. The authors utilized applicable EQUATOR ( reporting guidelines.


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