There are numerous techniques that we can use to improve our likelihood of success in performing a lumbar puncture. There is one that has a bit of nuance to it that I thought would be an excellent theme for my latest Why We Do What We Do post. So, without further ado, let’s talk about early stylet removal. I’m also partial to it given that it has been nicknamed “The Cincinnati Method” (even though it was well before my time). The original citation FYI – Bonadio WA. Interpreting the traumatic lumbar puncture. Contemp Pediatr Res Q. 1992;1:23–32. Good luck finding a pdf.
How does one actually perform it?
- Insert the spinal needle with the stylet in place
- Advance completely past the epidermis and dermis (<1cm in most children)
- Remove the stylet
- Advance forward until reflux of CSF noted
- Reinsert the stylet and withdraw the needle
You can watch the technique in action during this brief video I made for the AAP’s PediaLink service (courtesy of AAP PediaLink)
Why does it help?
In short, you avoid overshooting the subarachnoid space and hitting the vascular plexus of the ventral epidural space. Interestingly, you can measure the depth to the subarachnoid space if you’re interested by using the following calculation:
Depth of LP = 0.77cm + (2:56 × BSA [in meters squared])
I think this diagram is most helpful in showing why early stylet removal is potentially beneficial.
As you can see in the diagram above, with the stylet still in place if you enter in the midline perpendicular to the CSF target you have the maximum space from which to get fluid. But, if your angle of entry is slightly off (only a few degrees will do it) you can see that the angle becomes more magnified. Using early stylet removal will allow you to see reflux of fluid into the spinal needle the instant you enter the space, thus giving you confirmation before you “back wall” the needle into a venous plexus.
Are there any risks?
Those of us who have been performing LPs for a loooong time may recall an epoch where butterfly needles were used to perform LPs. Those procedures left patients at risk for intraspinal epidermoid tumors – late appearing intraspinal masses after an unstyleted needle pushing epidermal cells into the intraspinal space. These can create a mass effect. See this case series from a while back for more information on how they presented. Fortunately, the use of styleted needles mitigated this risk. Theoretically the early stylet removal technique could cause the same risks if the stylet was removed before passing through the dermis – fortunately the best practice technique specifies that you should not remove the stylet until you have passed through the epidermis and dermis. There is no evidence – case series or otherwise –
What’s the evidence?
You wouldn’t want me to recommend something without supporting evidence would you? Well, in 2006 Baxter et al. published their findings of a prospective observational study in PEDIATRICS. They reviewed 428/594 (72%) infant LPs – 377 of which were performed by trainees. Seventy-four percent (279/377) of the trainee LPs were successful. Amongst other factors local anesthesia was used for 280 (74%), and 225 (60%) were performed with early stylet removal. They found the following;
- LPs were more likely to be successful in infants >12 weeks of age – OR=3.1 (95% CI 1.2-8.5)
- Local anesthetic use showed increased odds of success OR=2.2 (95% CI 1.04-4.6)
- In infants ≤12 weeks of age, early stylet removal improved success rates – OR=2.4 (95% CI 1.1-5.2)
- Position (upright vs side-lying), drape use, and year of training were not significant predictors of success
So, as you can see – there is evidence and it stands firmly in the camp of those patients under the age of 12 weeks. However, that being said, I use this technique no matter the age of the patient. I do think that this is an example of where understanding a little bit more about why a certain technique is used will make a difference. This also applies to use of local anesthetics – maybe I’ll tackle that one in the future.