What is a lumbar puncture?
A lumbar puncture (LP), or “spinal tap,” is an important and commonly used procedure carried out across a wide range of specialities, not just the neurosciences. It is therefore important to understand its indications, the steps involved and the potential complications.
A lumbar puncture is an invasive test designed to access the subarachnoid space in the lower spinal canal. The brain and spinal cord are covered by three layers of meninges- the dura, arachnoid, and pia mater (Figure 1). The subarachnoid space lies between the arachnoid and pia mater, and contains a solution called cerebrospinal fluid (CSF). CSF is a clear, colourless fluid that provides lubrication around the spinal cord/cauda equina, maintains intracranial pressure, acts as a mechanical shock absorber, and transports various metabolic products. The human body contains approximately 100-150ml of CSF. As you can see below, the subarachnoid space lies close to the ventral and dorsal columns of the spinal cord, therefore anatomical spatial awareness is crucial to performing a safe and accurate lumbar puncture.
A lumbar puncture may need to be performed for a variety of reasons, both diagnostic and therapeutic: ²
- Cerebrospinal fluid analysis (i.e. meningitis, multiple sclerosis, subarachnoid haemorrhage)
- Spinal epidural (i.e. during labour)
- Spinal medications (i.e. analgesia, chemotherapy, antibiotics)
- Fluid removal (i.e. to relieve cranial pressure)
Performing a lumbar puncture in a patient with any of the following problems may be contraindicated and in rare circumstances can lead to life-threatening complications. Therefore a thorough review of the patient’s medical history and presenting complaint should always be sought beforehand, and senior review or neurosurgical advice taken if there is any doubt. 2,3
- Suspicion of raised intracranial pressure
- Anticoagulant therapy (i.e. Warfarin)
- Thrombocytopenia or other clotting disorders
- Suspicion of a spinal abscess
- Risk of herniation (i.e. Arnold-Chiari Malformation)
- Acute spinal cord trauma
- Congenital spinal abnormalities
*In any of the above circumstances, imaging with CT or MRI should be undertaken before consideration of lumbar puncture. 3
A lumbar puncture will be carried out by a trained physician or specialist nurse. The process is briefly outlined below.2
It is also worth learning the layers the needle passes through during the procedure, as this is a common question in medical exams (Figure 2).
It should be noted that this guide is not intended to be used for performing this procedure on a patient. Instead, it is aimed at informing medical students in preparation for exams.
- Introduce yourself: “Hi there, my name is Dr Smith and I’m one of the junior doctors working on this ward.”
- Check the patient’s details: “Can I confirm your name and date of birth please?” (also make sure to compare against their wristband)
- Explain the procedure, indications, risks and benefits to the patient: “Today I’ve been asked to perform a lumbar puncture, which involves inserting a fine needle into the back to collect a small sample of fluid. I’ll also be using some local anaesthetic, to make the procedure as comfortable as possible. It’s really important that we carry out this test because [insert reason the LP is being performed]. There are some complications that can be associated with performing a lumbar puncture, some of the more common things include a headache and some bruising of the skin after the procedure. There are some rare but much more serious complications, which include damage to nerves resulting in pain and weakness in the legs and also infection.”
- Gain consent: “Does everything I’ve said so far made sense? Do you feel you understand what the procedure involves? Is it ok for me to perform the procedure?”
- Gather the appropriate sterile equipment and needle (these often come as a sterile lumbar puncture set in many hospitals)
- Lumbar puncture needle (atraumatic type now recommended)
- Sterile field (to cover the patient and bed)
- Chlorhexidine cleaning solution (0.5 % in alcohol 70%) or alternatively Iodine
- Sample collection containers
- Local anaesthetic
- Syringe (5-10ml) and needles for local anaesthetic administration (usually need one for drawing up anaesthetic and one for administration)
- Dressing to apply after the lumbar puncture is complete
- Pen for marking the patient’s skin
- There is recent evidence that the selection of an atraumatic needle reduces the risk of post-procedure headaches. 4
- Needle size is based on experience and clinical judgement.5
- Map out the insertion site on your patient.
- With the patient standing, mark out L4 by joining a line between the highest points of the iliac crests.
- Palpate above for L3 and below for L5.
- The insertion site can be marked out either between L3/4 or L4/5 depending on patient features.
- Position the patient correctly.
- This is most commonly in a lateral recumbent or prone position, and less commonly sitting upright.
- The lower lumbar spine should be flexed (i.e. ask the patient to bring their knees up towards their chest, assuming a fetal position).
- Clean the skin with chlorhexidine (allow to dry).
- Apply appropriate personal protective equipment (PPE).
- Apply a sterile drape with an opening over the site of insertion.
- Draw up the local anaesthetic and then replace the drawing needle with a new one for the injection.
- Inject local anaesthesia around the site and allow time for it to take effect (at least a few minutes).
- It is important to warn the patient that this will sting initially but then should go numb.
- Press the lumbar puncture needle to the patient’s skin over the insertion site and ask if it feels sharp (if it does, it suggests more local anaesthetic is needed).
- Advance the lumbar needle through the insertion site slowly, tilted slightly cranially.
- The bevel of the needle should face laterally as you insert it.
- If this is an atraumatic needle, you first insert the introducer needle and then insert the atraumatic needle through this.
The needle passes through the following layers before it reaches the subarachnoid space:
- Subcutaneous fat
- Supraspinous ligament
- Interspinous ligament
- Ligamentum flavum
- Dura mater
- Subdural space
- Arachnoid mater
- As the needle passes through the three defined ligaments, three ‘pops’ (sudden reductions in resistance) will normally be felt. After the third ‘pop’ (ligamentum flavum) the dura should be breached and CSF should be seen in the needle.
- Sometimes withdrawing the needle very slightly and rotating the bevel of the needle to face cranially, will result in the flow of CSF.
- When CSF is visualised through the needle, instruct the patient to slowly uncoil and extend their legs.
- This is done in order to ease the flow and collection of CSF.
- At this time, the opening pressure of the fluid can be measured with a manometer.
- Removal of CSF can be diagnostic or therapeutic- extract enough based on your intentions, and put in sterile tubes.
- For some conditions, such as subarachnoid haemorrhage, it is important to number the tubes so you know in which order the CSF sample was taken (explained below).
Recommendations are as follows:
- Standard diagnosis: 8-15mL
- Therapeutic (e.g. to lower pressure): choice depends on the clinical context
- Slowly remove the needle and compress the site with some sterile gauze.
- Apply a dressing to the insertion site.
- Instruct the patient to inform you if they notice the site become red, inflamed or painful.
- If diagnostic, send CSF samples off to the appropriate lab for review.
Interpretation of results
When a lumbar puncture is performed for diagnostic reasons, the results can be interpreted with a thorough analysis of the CSF (Table 1).
Some components of this analysis can be done at the bedside, while others require specialist review in the lab. 7
|Component||Where is this done?||Interpretation|
|Opening pressure||Bedside (attach a manometer to the LP needle after insertion)||Normal is 8-20cm CSF
>20cm CSF can be due to many causes such as meningitis, tumour or intracranial haemorrhages
|Appearance||Bedside||Normal is clear
Cloudy/purulent suggests meningitis
Blood-stained suggests a subarachnoid haemorrhage or a traumatic LP
SAH will give multiple bloody samples, whereas this is less likely following a traumatic LP)
|Total protein||Laboratory test||Normal is 0.15-0.45 g/L
>0.45 is a non-specific marker of a pathology such as brain abscess, MS or meningitis
|Glucose||Laboratory test-(alongside blood glucose concentration)||Normal is 40-60% of blood glucose
>60% suggests diabetes
<40% suggests meningitis
|Gram stain and culture||Laboratory test||Done if bacteria are suspected
Ziehl-Neelsen stain for acid-fast bacilli (TB)
Do PCR for viral infections
|Microscopy for cell counts||Laboratory test||High RBC is suggestive of a traumatic tap
High total WCC is suggestive of an infection
High neutrophils suggests a bacterial meningitis
High lymphocytes suggests viral/TB/fungal meningitis
In patients presenting with clinical signs of meningitis (fever, neck stiffness, headache), there are three primary sources to distinguish between- viral, bacterial, and tuberculous (Table 2). Viral meningitis is usually self-limiting, whereas bacterial and tuberculous require prompt treatment with the appropriate antimicrobials. Therefore, if any form of meningitis is suspected, microbiology results should not delay management, and patients should be started on immediate empirical antibiotic therapy.
|Opening pressure||Normal||Raised||Raised or Normal|
|Protein||Raised or Normal||Raised||Raised|
|WCC||Normal||Raised||Raised or Normal|
|Cells Present||Mainly lymphocytes||Mainly neutrophils||Mainly lymphocytes|
- Cerebral herniation (i.e. Arnold-Chiari malformation)
- Radiculopathy/ Altered sensation
- Back pain
1. Thecal Sac [Internet]. Wikipedia. 2018 [cited 2018 Jun 14]. Available from: https://en.wikipedia.org/wiki/Thecal_sac
2. Kimberly Johnson DS. Lumbar Puncture: Technique, indications and complications in adults [Internet]. UptoDate. 2018 [cited 2018 Jun 11]. Available from: https://www.uptodate.com/contents/lumbar-puncture-technique-indications-contraindications-and-complications-in-adults
3. Engelborghs S, Niemantsverdriet E, Struyfs H, Blennow K, Brouns R, Comabella M, et al. Consensus guidelines for lumbar puncture in patients with neurological diseases. Alzheimer’s Dement Diagnosis, Assess Dis Monit. 2017;8:111–26.
4. Rochwerg Bram, Almenawer Saleh A, Siemieniuk Reed A C, Vandvik Per Olav, Agoritsas Thomas LL et al. Atraumatic (pencil-point) versus conventional needles for lumbar puncture: a clinical practice guideline. BMJ. 2018;361.
5. Abe KK, Yamamoto LG, Itoman EM, Nakasone TAF, Kanayama SK. Lumbar puncture needle length determination. Am J Emerg Med. 2005;23(6):742–6.
6. Supraspinous Ligament [Internet]. Wikipedia. 2018 [cited 2018 Jun 17]. Available from: https://en.wikipedia.org/wiki/Supraspinous_ligament
7. Paul Hamilton IB. Neurological Investigations. In: Kerr E, editor. Data Interpretation for Medical Students. 2nd ed. Lancaster: Carnegie Book Production; 2012. p. 223–7.
Mr George Spink FRCS SN, Consultant Neurosurgeon