Lumbar Puncture (LP) – OSCE guide


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. It is, therefore, important to understand its indications, the steps involved and the potential complications.

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.


Anatomy

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, 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.

Anatomy of the meninges of the brain and spinal cord.
Figure 1. Transverse section of the spinal cord showing the meningeal layers and CSF in the subarachnoid space. 1

Indications

A lumbar puncture may need to be performed for a variety of reasons, both diagnostic and therapeutic²

  • 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 reduce intracranial pressure)

Contraindications

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


Introduction

Introduce yourself to the patient including your name and role.

Confirm the patient’s name and date of birth.

Explain what the procedure will involve using patient-friendly language: “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 to perform a lumbar puncture.

Ask the patient if they have any pain before continuing with the clinical procedure.

Wash your hands.


Gather equipment

Gather the appropriate equipment to perform a lumbar puncture (these often come as a sterile lumbar puncture set in many hospitals):

  • Lumbar puncture needle: recent evidence supports the use of an atraumatic needle to reduce the risk of post-procedure headaches. Needle size is based on experience and clinical judgement.5
  • Sterile field: to cover the patient and the bed
  • Chlorhexidine cleaning solution (0.5 % in alcohol 70%) or alternatively iodine
  • Manometer: to measure the opening pressure
  • Sample collection containers
  • Local anaesthetic (typically lidocaine 1%)
  • 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 planned insertion site

Identify the insertion site

Map out the insertion site on the 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 the patient’s anatomical features.

Position the patient

Position the patient laying on their side in a fetal position: ask the patient to flex forwards whilst bringing their knees up towards their chest.


Prepare the insertion site

Clean the insertion site and the surrounding area thoroughly using chlorhexidine solution and allow to dry.

Wash your hands and don sterile gloves.

Apply a sterile drape with an opening over the site of insertion.

Local anaesthetic

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 quickly go numb.


Perform the lumbar puncture

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 or time is needed).

If the patient is unable to feel the sharp sensation you should proceed with performing the lumbar puncture.

Advance the lumbar puncture needle through the insertion site slowly, tilted slightly cranially. The bevel of the needle should face laterally as you insert it. If using an atraumatic needle, you first insert the short introducer needle and then insert the longer atraumatic needle through this.

The needle passes through the following layers before it reaches the subarachnoid space:

  1. Skin
  2. Subcutaneous fat
  3. Supraspinous ligament
  4. Interspinous ligament
  5. Ligamentum flavum
  6. Dura mater
  7. Subdural space
  8. 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 begin to flow through the lumen of the lumbar puncture needle. If there is no CSF flowing through the needle, try withdrawing the needle very slightly and rotating the bevel of the needle to face cranially.

Sagittal section of the Lumbar spine with the ligaments
Figure 2. Sagittal section of the lumbar spine showing the three spinal ligaments, which are passed through with the LP needle. 6

Measure the opening pressure

Once CSF is flowing through the lumbar puncture needle, you can measure the opening pressure by attaching a manometer and recording at what level the meniscus of the CSF settles at. Measurement of opening pressure does not need to be performed routinely.

Collect a sample of CSF

CSF should be collected using an aseptic non-touch technique as it drips from the back of the lumbar puncture needle.

Removal of CSF can be diagnostic (e.g. suspected meningitis) or therapeutic (e.g. benign intracranial hypertension). You should extract an appropriate volume based on your intentions (typically 8-15mL for diagnostic purposes), and place in the appropriate laboratory containers.

For some conditions, such as subarachnoid haemorrhage, it is important to number the containers sequentially so you know in which order the CSF samples were taken (this can allow you to differentiate between a traumatic tap and subarachnoid haemorrhage).

Removal of the needle

Slowly remove the lumbar puncture needle and compress the site with some sterile gauze until you apply a dressing.

Immediately dispose of the lumbar puncture needle into a sharps bin.


To complete the procedure…

Dispose of the remaining clinical equipment into a clinical waste bin.

Wash your hands.

Explain to the patient that the procedure is now complete and advise them to lay flat for at least the next 30 minutes.

Ask the patient and nursing staff to inform you if they notice the site becoming red, inflamed or painful.

Thank the patient for their time.

Send the CSF samples to the laboratory for analysis.


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

ComponentWhere is this done?Interpretation
Opening pressureBedside (attach a manometer to the LP needle after insertion)Normal is 8-20cm CSF
AppearanceBedsideNormal is clear

Cloudy/purulent suggests meningitis

Blood-stained suggests a subarachnoid haemorrhage or a traumatic LP. A SAH will typically result in multiple consistently bloody samples, whereas in the case of a traumatic LP the amount of blood should reduce significantly in each subsequent sample.
Total proteinLaboratory testNormal is 0.15-0.45 g/L
GlucoseLaboratory test-(alongside blood glucose concentration)Normal is 40-60% of blood glucose
Gram stain and cultureLaboratory testPerformed if bacteria are suspected
Microscopy for cell countsLaboratory testHigh RBC is suggestive of a traumatic tap

High total WCC is suggestive of an infection

High neutrophils suggest a bacterial meningitis

High lymphocytes suggest 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 meningitis 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.

 ViralBacterialTB
Opening pressureNormalRaisedRaised or Normal
ProteinRaised or NormalRaisedRaised
GlucoseNormalLowLow
WCCRaisedRaisedRaised or Normal
Cells presentMainly lymphocytesMainly neutrophilsLymphocytes

Complications of lumbar puncture 2

  • Headache
  • Infection
  • Bleeding
  • Cerebral herniation (i.e. Arnold-Chiari malformation)
  • Radiculopathy
  • Back pain

References

  1. Henry Gray. Gray’s Anatomy. Thecal Sac. Available from: [LINK].
  2. Kimberly Johnson DS. Lumbar Puncture: Technique, indications and complications in adults. UptoDate. 2018 [cited 2018 Jun 11]. Available from: [LINK]. 
  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 Dementia 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. Henry Gray. Gray’s Anatomy. Supraspinatus ligament. Available from: [LINK].
  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.

Editor

Samantha Strickland

Reviewer

Mr George Spink (FRCS SN)

Consultant Neurosurgeon


 

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