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How to… perform a lumbar puncture

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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 the 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 department’s 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. 

It 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 have 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 the removal of the needle 

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

Position 0:35

The 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. 

Hip flexion opens up the intervertebral spaces and makes the procedure easier. Neck flexion does nothing to help the procedure and will probably make it more uncomfortable for the child as well as make it harder for them to breathe. Supporting neonates in the sitting position with their hips flexed and their legs forward is associated with wider intervertebral spaces and less hypoxia than the left lateral position in this age group.

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.  Envision a straight line between the top of the iliac crests intersecting your target area L3/4. This is known as Tuffier’s line.

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 it 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 it 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 the 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 

Troubleshooting

  • 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.

Pro Tips from Ben Lawton

How much CSF is too much?

Adults have a CSF volume of about 150mls and produce it at somewhere between 14-36mls/hour. Neonates have about 50mls of CSF, which they produce at a rate of 25mls/day. Twenty drops of CSF equates to about 1 ml. How much CSF you need to take depends on what you want to do with it but 1.5mls (or 30 drops) should be both safe and sufficient for your smallest patients.

Reducing the risk of post-LP headaches

Post LP headache may be reduced by:

  • Using a smaller needle (25g in neonates, 22g in others)
  • Replacing stylet prior to needle withdrawal
  • Orientate the needle with the bevel parallel to the spine so it will separate the longitudinally running fibres of the dura.

How deep do you place the needle?

Essentially, the balance is that too short a needle won’t reach the subarachnoid 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 subarachnoid 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 it’s impossible to feel each layer on the end of the needle.

The anatomical target is either the L3/4 or L4/5 vertebral interspace, which respectively lies one vertebral body above & below the level of Tuffier’s line. Tuffier’s line is the imaginary line running between the superior iliac crests. It 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 is 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 are more practical and pragmatic in the setting of an unwell child. Several other articles have discussed the most accurate formula for LP depth, all relying on at least two measured parameters. The following formulae may be more beneficial for elective CSF examinations.



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 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. This paper summarises 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 compared 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. Secondly, it draws to attention the risks associated with using a too-short needle (multiple punctures, anatomically impossible to reach the CSF), which amounts 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 trialled in several papers; the two mentioned here were both produced by 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 five years of age. MSCD was identified as above, and several prediction models were 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 utilise formulae with weight as the single variable.

References 

Abe KK, Yamamoto LG, Itoman EM, Nakasone TA, Kanayama SK.Lumbar puncture needle length determination.Am J Emerg Med. 2005 Oct;23(6):742-6.

Arthurs OJ, Murray M, Zubier M, Tooley J, Kelsall W. Ultrasonographic determination of neonatal spinal canal depth Arch Dis Child Fetal Neonatal Ed 2008;93:F451–F454. 

Bailie HC, Arthurs OJ, Murray MJ, et al. Weight-based determination of spinal canal depth for paediatric lumbar punctures. Arch Dis Child 2013;98:877–80.

Bilić E, Bilić E, Dadić M, Boban M. Calculating lumbar puncture depth in children. Coll Antropol. 2003 Dec;27(2):623-6.

Chong SY, Chong LA, Ariffin H. Accurate prediction of the needle depth required for successful lumbar puncture. Am J Emerg Med. 2010 Jun;28(5):603-6. doi: 10.1016/j.ajem.2009.02.006.

Craig F, Stroobant J, Winrow A, Davies H. Depth of insertion of a lumbar puncture needle. Arch Dis Child. 1997 Nov;77(5):450.

Kaur G, Gupta P, Kumar A. A randomized trial of eutectic mixture of local anesthetics during lumbar puncture in newborns. Arch Pediatr Adolesc Med 2003;157:1065–70.

Murray MJ, Arthurs OJ, Hills MH, et al. A randomized study to validate a midspinal canal depth nomogram in neonates. Am J Perinatol 2009;26:733–8&nbsp

Schulga P, Grattan R, Napier C, et al. How to use… lumbar puncture in children. Arch Dis Child Educ Pract Ed 2015;100: 264–271.

https://www.rch.org.au/clinicalguide/guideline_index/Lumbar_puncture/

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