How to… perform a lumbar puncture

Cite this article as:
Taryn Miller. How to… perform a lumbar puncture, Don't Forget the Bubbles, 2021. Available at:
https://doi.org/10.31440/DFTB.31106

Performing a neonatal or paediatric lumbar puncture can be a daunting procedure but is an important part of the initial investigations of an unwell patient. However, it’s important to remember that a lumbar puncture should never delay administration of antibiotics that could be life-saving to a patient with suspected bacterial meningitis. 

Before the start of any procedure always ask, “Why are we doing this procedure? Are there any contraindications?”. 

The Royal Children’s hospital in Melbourne outline the indications and contraindications to performing a lumbar puncture as follows:

Indications

  • Suspected meningitis or encephalitis 
  • Suspected subarachnoid haemorrhage in the context of a normal CT scan 
  • To assist with the diagnosis of other CNS or neurometabolic conditions 

Contraindications

  • The febrile child with purpura where meningococcal infection is suspected 
  • Cardiovascular compromise/ shock 
  • Respiratory compromise 
  • Signs of raised intracranial pressure (diplopia, abnormal pupillary responses, abnormal motor posturing or papilledema) 
  • Coma: Absent or non-purposeful response to pain. 
  • Focal neurological signs or seizures 
  • Recent seizures 
  • Local infection around the area where the LP would be performed 
  • Coagulopathy/ thrombocytopenia 

The next important step is to gain verbal consent from the parents by explaining the procedure, risks and complications. 

Stop – As a parent, an initial septic workup of an unwell child can be an extremely stressful time. Try and explain the procedure with the risks and complications as concisely and clearly as you can without using medical jargon. It can be useful to think… if I were a parent what would I want to know?

It is useful to have your departments recommended lumbar puncture leaflet printed to give to the parents to read after the conversation. 

We would like to perform an investigation known as a lumbar puncture on your child. We do not perform this investigation unless it is absolutely necessary, and we think this is necessary to perform on your child today. 

This is a test that involves a small needle that is inserted into the back of your babies/ child’s spine to obtain a sample of the fluid that runs around the brain and the spinal cord. We usually do this test to identify whether your child has meningitis (infection of the lining of the brain). Sometimes we occasionally think your child is too ill to have a lumbar puncture and we will give antibiotics straight away to cover the most common types of bugs that cause meningitis. However, if possible, we like to perform a lumbar puncture that helps us identify: 1 ) if your child has meningitis by looking at the cells in the fluid, and 2) what type of bug is causing your child’s meningitis. This helps us choose the correct type of antibiotic and how long it is needed for. 

The procedure can be an uncomfortable procedure similar to performing a blood test. Most babies will be upset by being held in one position more than by the procedure itself. To minimise discomfort we will give pain relief such as sucrose or a pacifier to help. The procedure usually takes 30 minutes to perform. 

This can be a distressing procedure for parents to watch and we often offer parents not to be present while we perform the procedure. This can help increase the chance of success as it is a difficult procedure to perform. However, you are always more than welcome to be present. 

A lumbar puncture is a safe test and the risk of any serious complications such as bleeding, infection or damage to the nerves is extremely low. More common risks are that we are not able to get the sample we need or having to try more than once. Today we will only try twice and then stop if we are unsuccessful. 

Remember the parents may refuse a lumbar puncture and this should prompt us to think again and take some more time to re-discuss this with a senior and / or the parents. 

The procedure

Gather equipment and personnel 0:13  

Ensure that at least two people (the person performing the lumbar puncture and an assistant to hold) are present. It is often useful to have a third person to help as an assistant or with any other problems during the procedure. 

Equipment

  • Drapes or a sterile dressings pack 
  • Sterile gloves 
  • Sterile Gown 
  • Mask 
  • Spinal needle – 22G or 25G bevelled spinal needle with a stylet* 
  • Specimen pots x 2/3 
  • Chlorhexidine 0.5% in 70% alcohol solution with tint (chloraprep 3mls skin cleaning applicator) or your local alternative 
  • Local anaesthetic and/or sucrose 
  • Specimen pots x 2 
  • Labels 
  • Tegaderm for the site following removal of the needle 

For some more information on how to choose a correct spinal needle for the patient check this post from Henry.

Position 0:35

Position is everything for a paediatric lumbar puncture. A calm, cool and collected assistant who is confident in maintaining an adequate position is essential for improving the likelihood of success. 

You: Decide whether you are going to sit or stand for the procedure and set the bed height accordingly 

Patient: 

  • Position the patient in the left or right lateral position with their knees to their chest. Avoid over flexing the neck as this can cause respiratory compromise especially in younger neonatal patients. 
  • Position the patient so that the plane of their back is exactly perpendicular (90 degrees) to the bed. 
Lateral position for lumbar puncture

Landmarks: 

You are aiming for approximately the L3-L4 or L4-5 interspace. In neonates you can feel the ASIS and in older children you can feel the PSIS.  Invision a straight line between the top of the iliac crests intersecting your target area L3/4. 

Analgesia, anaesthesia, and sedation  1:15

  • All children should have a form of local anaesthetic used which can include: 
  • For the neonatal population oral sucrose can be used. 
Layers of the spine

The procedure 1:36

  • Prep the trolley by cleaning with a detergent wipe and allow it to dry before the procedure set up 
  • Open the dressings pack onto the clean trolley and using a non-touch technique drop the sterile gloves, cleaning solution and lumbar puncture needle into the sterile area. 
  • Wash hands and don sterile gloves 
  • Put a sterile drape under the patient’s buttocks, on the right and left side of the desired site and at the top leaving the spine exposed. It’s a good idea to keep the nappy on a neonate during the procedure and pull it slightly further down to prevent faeces accidentally sliding into the sterile field during the procedure. 
  • Clean the area using the chlorhexidine solution to disinfect the skin around the procedure site. Do not place the used swab on the sterile field but dispose of immediately in the bin. Wait for the skin to dry 
  • Take the tops off the specimen pots and keep them on your sterile field ready 
  • Identify the desired space as described above  
  • If using lignocaine infiltrate at this step 
  • Position the needle with the bevel facing up towards the ceiling
  • Direct the needle towards the umbilicus 
  • Resistance will be met often felt as the needle moves through the ligamentum flavum
  • Keep advancing slowly – a pop may be felt as the epidural space is now crossed and the subarachnoid space is entered a few millimetres more. 
  • Remove the stylet and check for CSF 
  • If CSF fluid Is present collect 6-10 drops of CSF in each container. Number the container depending on analyses required. 
  • Re-insert the stylet (to reduce the risk of head) and in one swift manoeuvre, remove the needle and stylet. 
  • Apply pressure to the site 
  • Use a tegaderm dressing so that the site is visible to staff to assess for infection 

Trouble-shooting

  • If, when you initially insert the needle the neonate or child moves, do not advance, keep the needle in place and wait. Allow the child to settle, and re-check the position, then continue to advance. 
  • If the CSF is blood stained this can still be collected for culture and if it runs clear can be collected for a cell count at this point.

For More useful tips for LP’s check out this post by dftb Ben Lawton. Pro tips for LPs in kids, Don’t Forget the Bubbles, 2015. Available at:
https://doi.org/10.31440/DFTB.7969

References 

Other references 

  1. https://www.rch.org.au/kidsinfo/fact_sheets/Lumbar_puncture/ 

The febrile infant conundrum

Cite this article as:
Dani Hall. The febrile infant conundrum, Don't Forget the Bubbles, 2020. Available at:
https://doi.org/10.31440/DFTB.28850

It’s fair to say that febrile infants can be challenging. Often presenting with insidious symptoms but looking reasonably okay, they may still have life-changing or life-limiting illnesses like sepsis or meningitis. You could argue that we should take the view of eliminating risk, performing septic screens on all febrile babies, and admitting for IV antibiotics until their cultures are returned. The vast majority will have a benign viral illness but at least you can rest assured you didn’t miss a seriously sick infant.

And that’s what we did when I started my paediatric training back when the dinosaurs roamed the earth – every baby under 6 months (yes, you heard it right, 6 months) with a fever got a full septic screen, including lumbar puncture, and was admitted to the ward for at least 48 hours pending cultures. But, from a health economics point of view, this is, let’s just say, perhaps not the best way to allocate healthcare resources.

Over the years, researchers have tried to rationalise our approach to febrile infants. 2013 saw the first NICE fever in under 5s guideline; a year later a group from Spain published the Step by Step approach to identifying young febrile infants at low risk for invasive bacterial infection; and last year, the PECARN group published a clinical prediction rule for febrile infants under 60 days, which had excellent sensitivity and negative predictive values to rule out serious bacterial infections.

Last month, the Spanish group published an article looking at the external validity of the PECARN rule in their dataset.

Velasco R, Gomez B, Benito J, et al. Accuracy of PECARN rule for predicting serious bacterial infection in infants with fever without a source. Archives of Disease in Childhood Published Online First: 19 August 2020

PICO image

Before we plunge into the paper, let’s stop and think about a couple of important definitions here:

Serious bacterial infection (SBI) is used to describe bacteraemia, meningitis and urinary tract infections, also including infections such as pneumonia, skin, bone and joint infections, bacterial gastroenteritis and sometimes ENT infections.

Invasive bacterial infection (IBI) are infections where bacteria are isolated from a normally sterile body fluid, such as blood, CSF, joint, bone etc. An IBI is a type of SBI in a sterile site.

Who did they study?

Velasco’s group looked back at their registry of infants with a fever without source from a busy paediatric ED (> 50,000 presentations a year) in a tertiary hospital. To match the cohort in the PECARN paper, they used the following inclusion and exclusion criteria:

Inclusion: infants younger than 60 days who presented with a recorded fever, or history of recorded fever, of >38 C over an 11 year period between 2007 (when they started measuring procalcitonin) and 2018.

Exclusion: any infants whose history and/or examination pointed towards a focus, whose results didn’t include those used in the PECARN rule (absolute neutrophil count, PCT, urine dip), who didn’t have culture results, who were critically ill on presentation or who had a past history of prematurity, unexplained jaundice, previous antibiotics or other significant past medical history.

What were they looking for?

The group were interested to see how the PECARN rule fared in their dataset by looking at how many infants were predicted to be low-risk and yet had an SBI or IBI to assess the external validity of the rule.

What did they find?

1247 infants were included in this study. Of these, 256 (20.5%) were diagnosed with an SBI, including 38 (3.1%) with an IBI.

Of the 256 infants with an SBI, 26 (10%) were considered low risk by the rule. Of the 38 with an IBI, 5 were considered low risk (13.2%) by the rule. The PECARN rule would have missed 10% of infants with an SBI.

The PECARN rule’s sensitivity dropped from 97.7% in the original study to 89.8% and specificity dropped to from 60% in the original study to 55.5%.

So, how did Velasco’s group calculate the sensitivities and specificities of the PECARN rule for different groups in their dataset? They’ve nicely shown their data in 2 x 2 contingency tables in their figures. This is the data for SBI.

Table of data from Velasco study

So, we can see that sensitivity (those patients testing positive for the SBI as a proportion of all patients who definitely have SBI) = 230 / 256 = 89.8%. This means that 10.2% are falsely negative.

Specificity (those patients who test negative for SBI as a proportion of all of those who don’t have SBI) = 550 / 991 = 55.5%. This means that 44.5% are falsely positive.

What about infants with a really short duration of fever?

When the group looked at infants with a history of less than 6 hours of fever (n=684, a little over half of the cohort), the sensitivity dropped further to 88.6%.

Why did the PECARN rule perform less well in this study?

The authors offer up a number of suggestions, some of which are outlined below.

The populations may be slightly different. Although the authors attempted to exclude ‘critically ill’ infants from this study (as the PECARN study excluded ‘critically ill infants’), a precise definition wasn’t coded in the original Spanish registry. Instead, they excluded infants from this study if they were ‘not well looking’ or admitted to ICU. Because of the way the data was coded, some critically ill infants may have been included in this study’s dataset, skewing the results.

The Spanish database was of febrile infants without a source, excluding babies with respiratory symptoms, which may explain why the rates of SBI and IBI were much higher in this study than the PECARN database of febrile infants. So, although the PECARN rule was highly sensitive in their group of febrile infants, as in this study it may not perform so well in febrile infants without a source.

This study showed that the PECARN rule performed less well in infants with a short duration of fever. Overall, infants in the PECARN study had a longer history of fever at presentation – over a third of the PECARN infants had fever >12 hours compared to 11% in this study. Over half of the infants in this study presented within the first 6 hours. Blood tests are less sensitive in the first few hours of a febrile illness and this may well partially explain why the rule performed less well outside the PECARN dataset.

It’s important not to ignore this study’s limitations. The PECARN dataset recruited infants from multiple centres, while the registry for this study came from only one ED. As this study was a secondary analysis of a dataset, a power calculation wasn’t performed. Generally, a minimum of 100 cases is recommended for validating a model, but only 38 infants in this study had an IBI.

Study bottom line

This study showed that in the Spanish dataset of infants under 60 days with a fever without source, the PECARN rule performed less well than in the original study. This was especially true for infants with a short history of less than 6 hours of fever.

Clinical bottom line by Damian Roland

In Kuppermann et al’s original 2019 study febrile infants 60 days and younger were demonstrated to be at low risk of SBIs using 3 laboratory test results: Urinalysis, Absolute Neutrophil Count (ANC), and serum procalcitonin (PCT) levels. The study was well designed and therefore compelling in providing a framework in which to manage these challenging presentations. However, with respect to knowledge translation, external validity is critical. The availability of PCT is a significant limiting factor to being able to show the PECARN approach could be reproduced internationally. While PCT is used in Europe and Australia, it’s certainly not widespread in the UK where I practice, and then it is only used routinely in a very small number of hospitals. This makes Velasco and colleagues’ work really important as they were able to replicate the requirements of the original study and helps answer an important question: should centres start introducing PCT into their diagnostic pathology panels? The results of this study will be interpreted differently by different observers as ultimately the question is of risk tolerance. Personally, a 10% false-negative rate (if this is indeed the case) for an outcome that could result in long term disability feels uncomfortable. Counselling a parent that they could return home without treatment knowing this would probably be quite challenging. I am not sure many departments would be rushing to buy point of care PCT.

However, there are two very important caveats.  Firstly, is the validation cohort different from my own local cohort? The prevalence of disease has a huge bearing on the accuracy of any test. Knowing the local incidence of SBI and IBI in your own institution is important (but actually getting the numbers is harder than you may think!). It is likely that the PECARN approach may perform more effectively in other centres. Importantly the original paper highlights that implementation may be more effective in the second month of life due to the impact of HSV and other peri-natal infections present at 0-30 days. Secondly, what is the threshold for undertaking the blood tests in the first place? Fever in an infant less than 3 months is an interesting area as it’s one of the very few presentations in which a solitary symptom or sign is independently predictive of risk. Regardless of how the child appears to a health care professional, there is a risk of SBI and IBI (of anywhere between 2-10%) just by having a fever. This does mean that sometimes there is variation in approaches when there is a history of fever rather than a documented fever (for fear of not wanting to do a battery on tests on a neonate who in front of you appears completely well and has normal observations). But more importantly, this has led to an approach where although blood tests are taken, the results are often disregarded as an LP will be done and antibiotics will be given regardless. There are many cultural practices that have evolved around the management of the febrile neonate both within individuals and institutions. While in a study situation these are controlled for, their influence in the real world can not be underestimated and this is why it’s so important we have some pragmatic studies in this area.

This study makes me more determined to define our incidence of SBI locally and work out what impact new approaches to management may have. I think all centres should probably be doing this. However knowing the potential uncertainty in the sensitivity of the PECARN approach means it’s unlikely to be adopted in the immediate future without further validation.  

**post blog addendum 1st September 2020**

While this blog was in post production phase Kuppermann and colleagues have released further data on implementing their original predictive rule. This work has been summarised by Dr. Kuppermann below (click on to go to the original thread) and provides useful context to the discussion about external validity and implementation – DR.

ADC/DFTB Journal Club #2 – December – How well do we manage suspected meningitis in ED?

Cite this article as:
Grace Leo. ADC/DFTB Journal Club #2 – December – How well do we manage suspected meningitis in ED?, Don't Forget the Bubbles, 2019. Available at:
https://doi.org/10.31440/DFTB.17786

Vaccines have been instrumental in reducing rates of bacterial meningitis. However bacterial meningitis still represents 4-19% (1) of cases of meningitis and has been estimated to be cause 2% of all child deaths (2). Timely administration of antibiotics helps save lives with adult research suggesting that every hour of delayed treatment increases the risk of death or permanent disability by 10-30% (3). So how swiftly do we investigate and treat children with suspected meningitis? The paper from Archives of Disease of Childhood featured in our second #DFTB_JC sought to answer this question:

 

What’s it about?

This was a prospective cohort study of 388 children who attended three UK paediatric tertiary centres between 2011-2. They had been either hospitalised with suspected meningitis or underwent lumbar puncture (LP) during sepsis evaluation.

Of the 388 children, 18% (70) were given a diagnosis of meningitis but only 13 were documented as bacterial and 26 as viral with and 31 patients having no known or identified cause. Just over half the children (57%) had seen a doctor in the same illness prior to ED presentation.

The median time from initial hospital assessment to antibiotic administration was 3.1 hours.  The time to LP was even longer at 4.8 hours, but once discounting intentional postponement for reasons including convulsions, concern regarding raised intracranial pressure, coagulopathy or shock, this time reduced to 3 hours. Over half of the children (62%) had their LP following antibiotics.

In further discussion with the corresponding author @manishs_  the mean was chosen due to skewing of the data and the time from initial hospital assessment was equivalent to arrival in ED. The time between initial assessment and LP ranged from 0-183 hours whilst the time between initial assessment and antibiotics ranged from 0 to 136 hours. For the 221 patients who they had data in hours available; only 31 received antibiotics in the first hour. However 131 of the 221 patients did receive antibiotics in the first 4 hours.

 

 

The general sentiment from the twitter discussion was  that the median time of 3.1hours to antibiotic administration was longer than expected, and suboptimal. Whilst the actual time point may have been somewhat surprising; many could identify common reasons for antibiotic delay and in particular, discussion about the difficulties that lumbar puncture can pose in different age groups and its contribution towards delay of antibiotics.

“It surprised me. Think we generally give abx before LP in children and LP before abx in babies… probably because of less anxiety around the procedure in babies. But no excuse for 3 hour delay in any age group really.” – @DrRoseM

 

 

 

We then delved deeper into the importance of LP before or after antibiotics and factors affecting unintentional LP delay. Paediatrician from Ontario, Tom Lacroix shared concern that with improved vaccines, he has seen skill attrition.

“…I wonder how much of delay is bc we have become unaccustomed to doing LPs. I have seen a fall in LPs 90%+ since intro of pneumococcal conjugate vaccine” – @drtom_lacroix

Across in the UK, the perceived anxiety surrounding performing an LP in older children was raised including staffing challenges, concerns about pain and procedural sedation.

“In neonates we rush to get the LP done within an hour, but in older children it always seems to take a lot longer. Do we have misplaced anxiety in this age group?” – @TessaRDavis

“…It takes one NICU nurse to flex a 6 day old up for an LP, but a play specialist, at least two nurses and one parent to get an older child in position for an LP” – @edd_broad

Differences in practice in terms of performing a FBC and Coags screen prior to LP were also highlighted.

“Not sure about mandatory, but I’ve been taught (and continue to practice) confirming PLT > 50×10^9/L prior to LP. ” – @henrygoldstein

“…Unless evidence of coagulopathy ie purpura. Do LP and then give abx” – @DocAnthonyT

 

 

 

In the supplementary tables from the paper, of all children in the study, just under a quarter (24.7%) had bacterial and/or viral CSF PCR performed. Of the 70 children who had meningitis, CSF PCR was performed on only 9 (13%). The rate was slightly higher for meningitis of cause unknown (6 of 29 patients, 21%). The authors commented that this represents a significant underutilisation, particularly as CSF PCR is recommended in the current UK guidelines. The suspected cause of this was a long turnaround time to PCR.

However the benefits of positive viral CSF PCR results would include reducing length of treatment and inpatient stay as well as building a more accurate understanding of true disease rates.

The results of this paper contrast with experiences of our journal club participants where CSF PCR appeared to be a more common order, particularly in the neonatal setting:

“Might depend on the CSF WCC for the bacterial PCR? If zero, I wouldn’t necessarily send bacterial PCR (but will still frequently send viral PCR)…Parechovirus PCR is automatically sent for our neonates. #DFTB_JC ” – @DrSarahMcNab

“NICU where I work send viral PCRs as standard with turnaround in 24 hours. Think you still need to request in paeds. ” – @DavidKing83

 

Paediatric Registrar Rose provided a good summary of what she learned from the article and the #DFTB_JC chat:

take home- give the abx as soon as possible and definitely within 1 hour. If unable to do LP pre abx due to delays etc then do LP ASAP after abx. Consider PCR as a valuable tool to aid decision re duration of treatment” – @DrRoseM

From the DFTB team, the discussion has made us rethink how each step in assessment and management of suspected meningitis may delay optimal care. In particular we’ll be thinking about how strong the evidence is behind ‘the golden hour’ of antibiotic administration, the anxiety surrounding LPs in older children and evidence behind performing coagulation studies prior to LP…now that sounds like a potential post for the future.

Thanks again to everyone who participated in our #DFTB_JC and we hope you will join us again later this month for our next paper.

 

Please join us for our next ADC/DFTB Journal Club on twitter at Tue 22/1/19 at UTC2000hrs (That’s Wednesday 0700 23/1 AEST) January’s featured FREE access article from @ADC_BMJ featuring a FREE access article from the latest issues of Archives of Disease of Childhood. January’s pick  is ‘ Can we use POCUS to Diagnose Pneumonia?’ Read the article here: bit.ly/2TMDf2M The chat will happen on twitter, hosted by @DFTB_Bubbles. Remember to use the hashtag #DFTB_JC for all related posts.

Lumbar Puncture Needle Depth

Cite this article as:
Henry Goldstein. Lumbar Puncture Needle Depth, Don't Forget the Bubbles, 2018. Available at:
https://doi.org/10.31440/DFTB.14720

Recently, I prepared up to perform a lumbar puncture for the first time in a few months and a quiet voice at the back of my brain whispered ;

How deep would I need to go?
Which length needle would be the best?

I asked a handful of senior and junior colleagues, both at the time and in the writing of this post, and the response was almost universally “deep enough that the CSF comes out.” Certainly true, but not very pragmatic, and lacking the kind of detail I was hoping for…

Background


I know there’s much discussion about the tip shape of a lumbar puncture needle, and in honesty, I’ve yet not read sufficiently to have strong opinions. However, in the fifteen minutes before the procedure, I had a look at the literature around needle length, and swiftly realized there was much more to this than I’d thought. Procedure finished, I was back to the drawing board.

Essentially, the balance is that a needle that is too short won’t reach the sub-arachnoid space, and a needle too long confers additional technical difficulty and increases the risk of going through.

So first, some basic anatomy; the aim of the exercise for lumbar puncture and CSF examination is to be in the sub-arachnoid space. To reach this space, the needle must pass through (in order) skin, superficial fascia, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, dura mater and the arachnoid. I’m no neurosurgeon, but I’m pretty sure that it’s impossible to feel each of these layers on the end of the needle.

Lumbar puncture layers

The anatomical target is either the L3/4 or L4/5 vertebral interspace, which respectively lie one vertebral body above & below the level of Tuffier’s line. Tuffier’s line is the imaginary line running between the superior iliac crests, and is used to demarcate the lower end of the spinal cord (which, in neonates, ends around L3 and moves superiorly with linear growth).

Finding a formula

One of the more widely used formulas is from a 1997 paper where Craig et al. derived an elegant formula that;

 LP needle depth (cm) = 0.03 x height of child (cm).

Easily memorable and from a sample of 107 children receiving an LP with macroscopically clear CSF, the authors’ intention was a formula requiring only one variable that could be obtained in a critically unwell child – height being easily obtained with a measuring tape or Broselow tape.



In my department, the most common single measure recorded is weight; Bilic’s 2003 study of 195 Croatian children (over 3m of age) found the best correlate for LP depth was weight, using the formula

LP depth (cm) = 1.3 + (0.07 x Body weight (kg) )

The above formulae use a single variable and hence are probably more useful and pragmatic in the setting of an unwell child. Several other articles have discussed the most accurate formula for LP depth; all of which are reliant on at least two measured parameters. The following formulae may be more beneficial for elective CSF examination.



Several formulae were derived for LP depth from a cohort of 279 paediatric oncology patients in Malaysia; the best fit for their dataset was

y = 10 (weight (kg)/height (cm)) + 1

For this cohort, the LP depth was measured by perhaps a less reliable method than other datasets described, as the investigators measured the distance from their finger on the needle when pressed to the back at withdrawal. Irrespective, this paper summarizes many of the preceding papers in the discussion section.



Abe and foundation DFTB contributor Loren Yamamoto took a slightly different approach in a 2005 study; they reviewed 175 abdominal CTs to identify spinal canal depth at the iliac crest, deriving the formula of

LP depth (cm) = 1+ 17( weight/height).

Crucially, they went on to compare standard needle sizes to these depths to identify if the needle was too short or too long.

Defining the needle depth in this way has several benefits – firstly, it’s relatively prescriptive and secondly, it draws to attention the risks associated with using a needle that is too short (multiple punctures, anatomically impossible to reach the CSF), which amount to avoidable harm. In this context, it’s pertinent to know your tools. That is, identify which spinal needles are available in your department, their lengths and the type of tip.

LP needles are available in the following lengths (mm), depending on the brand, introducer, tip type: 25, 35, 38, 50, 64, 70, 75, 90, 103, 120, 150. Find the stock in your department  and see what’s there.

What about ultrasound?


The use of ultrasound to identify the depth of the spinal cord has been trialed in a number of papers; the two mentioned here were both produced from Addenbrooke’s Hospital in Cambridge, UK.

Firstly, in a neonatal population (105 neonates), weighing between 500g and 4500g, USS was used to measure median spinal cord depth (MSCD). They subsequently derived a formula of

LP depth (median spinal cord depth in mm) =  2(Weight) + 7 mm (R^2 0.76).

Subsequently, this nomogram was validated (albeit by the same author group and unit) in this study.

A later study by the same group undertook USS on 225 children aged 3m to 17 years presenting for echocardiography. The majority of patients were over 5 years of age. MSCD was identified as above, and a number of prediction models developed. The formula put forward by the group as satisfying the inherent tradeoff between accuracy (R^2 =0.72) and utility is

MSCD (mm)=0.4 W (kg)+20

So, does this change my practice? I will admit that I don’t have any of the above formulas fixed in my head, as yet.  Spinal needles in my hospital don’t have depth markings (it would be interesting to know if these exist). Instead, the above information serves to help in selecting a needle, particularly in those patients somewhere between neonate and adult sized. On this basis, I suspect I’m most likely to utilize formulae with weight as the single variable. I also went and re-read Ben Lawton’s post on champagne taps before the next one.

In summary;

  • Formulae are not yet in regular practice to identify needle depth for lumbar puncture.
  • We advocate increased awareness of the depth of the target structure, particularly when it comes to needle selection.
  • A needle can be too short, but it can’t be too long – it just becomes harder to use.

The 9th Bubble Wrap

Cite this article as:
Grace Leo. The 9th Bubble Wrap, Don't Forget the Bubbles, 2017. Available at:
https://doi.org/10.31440/DFTB.12336

With millions upon millions of journal articles being published every year it is impossible to keep up.  Every month we ask some of our friends from PERUKI (Paediatric Emergency Research in UK and Ireland) to point out something that has caught their eye.