Performing a neonatal or paediatric lumbar puncture can be a daunting procedure, but it 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
Obtaining consent
The next important step is to obtain verbal consent from the parents by explaining the procedure, its risks, and potential complications.
Stop – As a parent, an initial septic workup of an unwell child can be an extremely stressful time. Try to explain the procedure, including the risks and complications, as concisely and clearly as possible, without using medical jargon. It can be helpful to think… if I were a parent, what would I want to know?
It is helpful to have your department’s recommended lumbar puncture leaflet printed for parents to read after the conversation.
We want 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 the patient) are present. It is often helpful to have a third person serve as an assistant or address any other issues during the procedure.
Equipment
- Drapes or a sterile dressing 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
Positioning is essential for a paediatric lumbar puncture. A calm, cool, and collected assistant who is confident in maintaining an appropriate position is essential to improving the likelihood of success.
You:
Decide whether to sit or stand for the procedure, and adjust the bed height accordingly.
Patient:
- Position the patient in the left or right lateral position, with the knees drawn to the chest. Avoid excessive neck flexion, as this can cause respiratory compromise, particularly in neonatal patients.
- Position the patient with the plane of their back exactly perpendicular (90 degrees) to the bed.
Hip flexion opens the intervertebral spaces and facilitates the procedure. Neck flexion does not benefit the procedure and will likely make it more uncomfortable for the child and 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.
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:
- Topical anaesthesia
- Subcutaneous lignocaine 0.4ml/Kg of 1% (4mg/kg)
- For the neonatal population, oral sucrose can be used.
The procedure 1:36
- Prep the trolley by cleaning it with a detergent wipe and allow it to dry before the procedure setup.
- Open the dressing pack on the clean trolley and, using a non-touch technique, place the sterile gloves, cleaning solution, and lumbar puncture needle into the sterile area.
- Wash your 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 advisable to keep the nappy on a neonate during the procedure and to pull it slightly further down to prevent faeces from inadvertently sliding into the sterile field.
- 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.
- Remove the lids from the specimen pots and place them on your sterile field.
- 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 often be met 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 containers 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, upon initial needle insertion, the neonate or child moves, do not advance; keep the needle in place and wait. Allow the child to settle, recheck the position, and then continue advancing.
- If the CSF is blood-stained, it can still be collected for culture; if it runs clear, it can be collected for 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 approximately 50 mL of CSF, which they produce at a rate of 25 mL/day. Twenty drops of CSF correspond to approximately 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 the stylet before needle withdrawal
- Orient 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 and 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 commonly recorded single measure is weight; Bilic’s 2003 study of 195 Croatian children (over 3m of age) found that weight was the best correlate of LP depth, 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 of which rely 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, LP depth was measured using a less reliable method than in the other datasets described, as investigators measured the distance from the finger to the needle when the needle was pressed against the back during 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 attention to the risks associated with using a too-short needle (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, and 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 determine the depth of the spinal cord has been trialled in several studies; the two mentioned here were both conducted at 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 3 months to 17 years presenting for echocardiography. The majority of patients were older than 5 years. 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 memorised as yet. Spinal needles at my hospital don’t have depth markings (it would be helpful to know whether 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
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.
Craig F, Stroobant J, Winrow A, Davies H. Depth of insertion of a lumbar puncture needle. Arch Dis Child. 1997 Nov;77(5):450.
https://www.rch.org.au/clinicalguide/guideline_index/Lumbar_puncture/