There are a few cases I could talk about… Ultrasound guided LPs have turned out to be a surprisingly useful skill. Lucky for you, I’ve restrained myself.
The first time I heard about ultrasound guided LPs was during a spectacular yearlong ultrasound elective in medical school. I was sitting in the doctor’s pod with THE ultrasound attending of ultrasound attendings. He casually asked “Have you ever heard of ultrasound guided LPs?” Never. “Do you want to see one?” Obviously. He explained how ultrasound guided LPs follow the principle of “Measure twice. Cut once.” He spent a minute (and I mean literally just one minute) visualizing the spinal landmarks and marking them on a somewhat altered, seriously chunky patient. He proceeded to get the LP in one stick! I remember thinking he was a wizard in that moment, a wizard who uses ultrasound to elevate patient care to another level; and that I want to be the badass attending that gets LPs on obese, altered patients in one stick. I had to learn this skill…
One year and a few ultrasound guided LPs later, I found myself on an internal medicine rotation. By then I had earned myself a reputation as “the ultrasound chick”. My first senior resident was also especially interested in bedside ultrasound. On our first day without prompting he said “bedside ultrasound is the future of medicine.” I literally did a small happy dance and told him we were going to have a great month. And we did! Internal medicine had never seen so much ultrasound before our partnership. It was wonderful! Fast forward a couple months. The above senior resident catches me in the hall and asks my team if he can borrow me. “I’ve got a patient for you!” It’s an obese, ICU patient with suspected meningitis, who has failed multiple LP attempts. “Can you do an ultrasound guided LP for me?” We were at a small community hospital without interventional radiology. Fluoroscopy isn’t an option (and moving her seems like a bad plan anyway). We roll in the ultrasound, identify the spinous interspace, mark it, and start the LP. I get the CSF and we do happy dance in the hall… Another patient wins thanks to bedside ultrasound!
Intro to Ultrasound Guided Lumbar Puncture
So how exactly does one use bedside ultrasound to enhance a lumbar puncture? I’m glad you asked. Ultrasound guided LPs use ultrasound to directly visualize spinal structures (spinous processes, ligamentum flavum, etc.) and mark the inter-spinous space prior to LP procedures. This is considered static ultrasound guidance, unlike real-time ultrasound guidance that visualizes a procedure as it happens. By marking the exact midline and the exact inter-spinous space you’ve removed the hardest part of the equation, finding the right spot to put your spinal needle.
Besides making your job easier, ultrasound guidance is just plain better for patients. Here’s where ultrasound guided LPs get really get awesome. It decreases overall time of the procedure. Measure twice, cut once WILL save you time. I’m sure you can remember a case that took forever, was miserable for the patient, and frustrating for you because it just wasn’t happening. Not surprisingly ultrasound guidance also decreases the number of LP attempts, LP failure rates, and complications. And if all that wasn’t enough, it decreases pain. When you actually visualized the inter-spinous space and stick your spinal needle directly into that space (instead of repeatedly jabbing the periosteum or paraspinal muscles), it just makes sense that patients experience less pain. That alone is reason enough for me. The benefits seen in ultrasound guided LPs compared to blind LPs are even greater in obese patients, so reach for the ultrasound on your next obese patient. Actually, reach for your ultrasound on your next few patients…
The scan boils down to two quick views, a longitudinal and transverse view of the spine. The longitudinal view visualizes the inter-spinous space between two spinous processes. Once the intervertebral space is identified, mark it and rotate the probe 90 degrees for the transverse view. The transverse view is used to identify the exact location of the midline, directly over the spinous processes. The midline is marked. Now you have two lines on the patient, one for the midline and one for the inter-spinous space… “X” marks the spot. When you get comfortable with the scan, it should only take a minute. We’re talking simple stuff here.
This really depends on your patient. For non-obese patients a high frequency probe, such as a linear probe, is great. It has the best image resolution, but low depth of penetration. For obese patients grab a curvilinear probe. It has a lower frequency, so it penetrates through tissue better than the linear probe. The image won’t be as clear, but you’ll still be able to see the spinous processes you need.
Upright or in lateral decubitus… depends on your needs. Do you need an opening pressure or are you just trying to get a CSF sample? The only important part of patient position is that they DO NOT move once you scan and make your marks. If the patient moves, the skin overlying your inter-spinous space may move too. Plan for this and mark the space after you’ve position the patient. If you need to reposition mid-procedure, you can always use a sterile probe cover.
Bone will be a thin hyperechoic structure with posterior shadowing. The spinous process will appear hyperechoic and curved downward in the nearfield with posterior shadowing. The lamina will also be hyperechoic, but come in pairs on either side of the intervertebral space just inferior or superior to the spinous processes. The sacrum will appear as a long thin hyperechoic structure. The dura is a very thin hyperechoic line between and deep to the lamina. Beneath the dura hypoechoic CSF of the subarachnoid space can sometimes be seen. In the transverse view you will also see two round structures with mildly hyperechoic internal bundles just lateral to the spine; these are paraspinal muscles.
Start in the longitudinal view. In this view the probe and resulting image are oriented parallel to the long axis of the spine. Place your probe over the sacrum with the probe marker oriented towards the patient’s head. The sacrum will be think hyperechoic line with posterior shadowing. Slide 1-2 cm lateral of midline and slide upwards. You should see the lamina and the intervertebral spaces between. Slide upward, counting the vertebrae, until you find the desired intervertebral space and mark it.
Once you find and mark the intervertebral space, turn your probe 90 degrees and slide back towards midline for the transverse view. In this view the resulting image are now oriented perpendicular to the long axis of the spine. Your probe marker should be oriented towards patient left. Look for the spinous process, which will appear as a hyperechoic curved dot with posterior shadowing, to find midline. Mark your midline.
Now you will have two marks, midline and the intervertebral space. Extend those lines to form the “X” that marks your target and optimal point of entry into the subarachnoid space.
And for bonus points… you can measure the distance to the dura (found between the spinous processes) in the longitudinal view. The subarachnoid space can be seen between the spinous processes bordered superficially by the thin hyperechoic line of the dura mater with anechoic CSF just below the dura. Once you see dura, you can measure the distance from the skin and know the depth at which you should puncture the dura and get CSF.
X Marks the Spot
Use a surgical marker, needle hub, syringe… whatever you’ve got handy. Firmly pressing a needle hub or syringe tip into the skin for several seconds (like 20 sec) will also create a mark that will not disappear when you prep the site. A surgical marker will also stay after you’ve prepped the site. This seems simple, but there is nothing more frustrating than identifying the site and losing it after you prep.
You can’t be good at bedside ultrasound without getting good at anatomy. You just can’t. So let’s talk spinal anatomy real quick.
The spinal cord ends around the inferior border of L1-L2. Distal to the spinal cord is the loose bundle of spinal nerves known as the cauda equina. The cauda equina is what a spinal needle passes between instead of damaging the spinal cord. This is why it is important to accurately identify the correct spinal level, which brings us to the bony structures of the spine. The spine is made of vertebrae, which consist of the body, a posteriorly protruding spinous process, and two transverse processes connected to the spinous process via lamina.
Vertebra are connected to one another by ligaments. The supraspinous ligament connects the tips of the spinous process. The interspinous ligaments connect adjacent spinous processes along the length of the spinous process. The ligamentum flavum connects the spinous processes anteriorly to form the posterior wall of the spinal canal. It is the strongest ligament of the spine and contributes to the characteristic “pop” experienced when entering the intervertebral space. Once in the spinal canal, the dura and arachnoid mater surround the cauda equina, providing an additional layer to “pop” through before reaching the CSF filled subarachnoid space. Phew… All done! Now onto more ultrasound goodness.
Show Me the Evidence
It’s Fast and Easy…
- Prospective, observational study of EM docs performing 174 scans on patients with mean BMI of 31.4 (aka obese)
- Average scan time was 57.9 seconds!!
- And who doesn’t have 1 minute to make an LP faster and more successful, especially on obese patients
- High quality images in < 1 minute in 87.9% of scans, and < 5 minutes in 100% of scans
- Basically, adding ultrasound identification of landmarks takes about a minute to get quality images.
But Does It Actually Help?
- Prospective, randomized trial of 80 subjects comparing ultrasound assisted LP to palpation only LP. Done by emergency physicians with 2 hours of training
- Ultrasound group had less pain (4.4 v 7.4/10), decreased procedure times (3.3 v 6.4 min), and fewer attempts (1 v 2)
- Pain, procedure time, and number of attempts cut in ½ with ultrasound assistance… which group would you want to be in??
- Obese patients are where things get really cool… they had less pain (4.4 v 9), decreased procedure time (3.9 v 9.1 min), and fewer attempts (1 v 3)
- Basically, ultrasound makes LPs better for providers and patients, especially obese patients. It’s faster, less painful, and increases your chances of success!
Plus, We Suck at Finding Anatomy with Palpation!
- Prospective trial comparing ultrasound identified v palpation identified intervertebral spaces in 50 subjects. Each subject assessed by each method, marked with radiopaque labels, and spine radiographs taken to determine accuracy of space identification.
- Palpation group was only correct 31% of the time (versus 70% in ultrasound group)!
- Palpation group marked over a spinous process or at wrong level 69% of the time!
- Palpation group was 2 spaces higher or lower than estimated level 24% of the time!
- We’re actually pretty terrible at identifying the intervertebral space and level of that space by palpation alone.
- Basically, ultrasound significantly improves accuracy in finding the correct intervertebral space!
This is good skill to have, period. It’s fast; it’s easy to learn; and it’s better for your patients. Ultrasound assistance improves LP success rates, decreases pain, decreases procedure length, and reduces the number of attempts needed. It’s especially useful for obese patients, where palpation of landmarks can be difficult and lead to blind (and painful) procedures. The literature reflects this too, making it hard for me to justify not using ultrasound to identify LP landmarks in my obese patients. Basically being good at ultrasound guided LPs boils down to being able to recognize and mark the intervertebral space at midline. That’s it. Less than one minute of scanning will save you time, spare your patients pain, and increase your chances of success! Once you have the hang of it, it will improve your patient care and make you look like a rock star doc. Happy scanning everyone!
Ultrasound Podcast: US Guided LP Microcast
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- Dawson M, Malin M. “Chapter 9: Procedures”. Introduction to Bedside Ultrasound Volume 2. May 2013
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- Boon JM, Abrahams PH, Meiring JH, Welch T. Lumbar puncture:Anatomical review of a clinical skill. Clinical Anatomy. Volume 17, Issue 7, pages 544–553, 2004.