Though less commonly performed than it used to be, the lumbar puncture remains a key skill to master for anyone practising acute paediatrics. December’s Archives of Disease in Childhood Education and Practice contains an excellent paper entitled “How to use… lumbar puncture in children” (1), which flashes more pearls than a 4th of July garden party in the Hamptons. We share some of its wisdom below but highly recommend reading the paper to anyone who considers LP within their scope of practice.
Adults have a CSF volume of about 150 mls and produce it at somewhere between 14-36 mls/hour. Neonates have about 50 mls of CSF, which they produce at a rate of 25 mls/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 1.5 mls (or 30 drops) should be both safe and sufficient for your smallest patients.
LP is commonly performed in the left lateral position in children. 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 making 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. Anecdotally I have recently changed my routine practice for neonatal LPs from left lateral to sitting and it also seems to be easier for holders with a wider range of experience to achieve an optimal position in relative comfort.
The spinal cord in adults and older children ends around L1-L2, in neonates it extends down to L3. The sub-arachnoid space extends down to S2. L4-L5 is generally the best area to aim for (bearing in mind we are not always in the space we think we are) though L3-L4 is also OK. With the child in an appropriate position a line drawn between the most superior aspect of both Iliac crests (Tuffier’s line) crosses the midline over the body of L4 so the space just below this is ideal.
Medical folklore contains a few different answers to this question but the most scientific answer I have seen is following formula (2)
Depth (mm) = 0.4 x Weight (kg) + 20
So in a 10 kg child CSF should be found at a depth of 24 mm.
Use topical anaesthetic. EMLA has been shown to help in neonates(3). 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. (I think this feels natural in the left lateral position but requires more thought in the sitting position).
…CSF lactate? This is quite a good discriminator between viral and bacterial meningitis with levels over 3.5 suggestive of bacterial CNS infection. It’s not quite as accurate after antibiotic administration but may still be clinically useful.
…USS guidance? This is still waiting for a decisive trial in kids but small studies have shown it to be a promising option for further exploration.
Many a mythical formula has been conjured up to interpret a white cell count in the context of bloodstained CSF. The authors of this paper suggest you can get a feeling by comparing the ratio of white cells to red cells in the peripheral blood and basing your maths on this, but wisely acknowledge that accurate interpretation is difficult in this context. It’s also worth highlighting the well described trap that a normal CT does NOT exclude raised intracranial pressure.
Though LP is a procedure I perform fairly frequently this paper has shone a spotlight into several dusky areas of my knowledge and I hope it will suitably illuminate yours. Finally, if I am asked to supply a question for next year’s Christmas quiz it may well be “Where can you find Tuffier’s line?”.
- 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.
- 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.
- 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.