Lumbar Puncture (LP)

Lumbar puncture is a procedure conducted to ascertain if there is an acute meningitis or sub arachnoid haemorrhage (SAH), to detect inflammation in the spinal cord (e.g. in multiple sclerosis) or commonly in anaesthetics to administer a spinal anaesthetic. It can be a tricky procedure, and a thorough set-up and correct patient positioning are important to minimise discomfort for the patient and maximise chances of a successful procedure.

Medical students are unlikely to be asked to perform a lumbar puncture, but as a junior doctor in emergency, intensive care or on medical wards you will be doing the procedure routinely.

In this article we discuss the use of lumbar puncture as a diagnostic procedure.

Indications

• Menningeal
• Encephalitis
• Causes of infection
  • Bacterial (TB, streptococcus)
  • Viral (herpes)

Bleeding into the CSF

• E.g. sub-arachnoid haemorrhage

Inflammation in the brain / spinal cord / CSF

• E.g. Multiple sclerosis

To administer local anaesthetic / or therapeutic agent (e.g. in chemotherapy)

Contraindications

• Raised ICP – lumbar puncture should never be performed when there is a raised ICP. By removing fluid from the spinal canal, you lower the pressure in this area. The relatively higher pressure within the cranium may then cause herniation of the brainstem through the foramen magnum, with disastrous neurological consequences. This brainstem herniation is sometimes referred to as ‘coning’.
  • The one exception is benign raised ICP – which is usually in young women.
  • Symptoms of RICP: headache, impaired consciousness, ↓pulse ↓BP (late stage compensation might make ↓BP), papilloedema
  • If you suspect RICP, then send for CT brain instead of lumbar puncture. Other indications to consider a CT brain before LP include
    • Aged >60
    • Immunocompromised
    • Neurological signs
    • Known intracranial mass / lesion
• Coagulopathy
• Local infection at site of needle insertion
• No consent!

Complications

• Failure of procedure
• Headache – very common, and usually benign. Can be managed with basic analgesia, and tell the patient to take onboard fluids. Patients can be given an infusion if necessary – although the evidence for this uncertain
  • There is no evidence for lying flat and / or bed rest to improve or reduce the incidence of post LP headache
  • Persistent headache – can indicate CSF leak from the puncture site. Is often self-limiting.
  • Headache more likely in – multiple attempts, women, dehydrated patients, large bore needle, excessive removal of CSF
• Parasthesia – may be felt during the procedure as the spinal needle comes into contact with nerve roots or nerves of the cauda equina. May feel uncomfortable during the procedure, but usually causes no lasting damage
• Serious complications are very rare, but can include:
  • Permanent nerve damage (mainly manifested as parasthesia), or even more rare, paraplegia
  • Brain stem herniation (described above)
  • Infection (abscess or meningitis)

Procedure

Gather your equipment

• Toxic dose of lidocaine:
  • 3mg/Kg, OR
  • 7mg/Kg with adrenaline

Spinal needle with which to perform the procedure

• This needle actually is made up on two needles, one inside the other. This arrangement prevents a bore of skin and other subcutaneous structures becoming lodged in the inner needle – thus preventing you from getting a sample of fluid.
• There is also a smaller, wider ‘style’ needle on the outside of the other 2 needles to help guide the more delicate spinal needle through the skin and subcutaneous tissues.

Lumbar Puncture Pack

Many hospitals stock a lumbar puncture kit, that contains the LP fluid bottles, a spinal needle, drops, trays gauze and other useful equipment. If your hospital doesn’t supply one you will have to source this equipment individually.

Don’t forget a gown, mask and sterile gloves!

Patient positioning and Location

There are two accepted way to position the patient:

1. Sitting upright and leaning forwards on the edge of the bed
2. Lateral – patient lying on their side (usually their left) in the ‘foetal position’, with their legs tucked into their torso and head forwards.

Both of these position are design to accentuate the curvature of the spine and allow easier access for the spinal needle. In practice, the position I see most commonly used is the lateral position.

Usually I would position the patient and find the location before ‘scrubbing up’.

Lateral position set up

• Ask the patient to lay on their left hand side, and bring their knees up to their chest, as much is as comfortable.
• Expose the patient’s back
• Find the posterior superior iliac spine, on both sides
• Imagine a line between the two – this is called Tuffier’s line.
• Find the spinous process that lies on this line. This is roughly usually about L3. Palpate into the space below this spinous process (L3/L4). You may also wish to palpate the other spinous processes to confirm this is indeed the L3/L4 location.
• In patients with larger body habits it can be very difficult to find the right location

Scrub up and Prepare skin

Administer Lidocaine

• Raise a bleb on the skin, then go in slightly deeper, and use the rest of the solution
• Leave to act for 2-3 minutes before performing the rest of the procedure

Anatomy

• Skin
• Fat
• Ligaments
  • Supraspinous ligaments
  • Infraspinous ligaments
  • Flavum (‘yellow’)
• Dura
• CSF

• Put your thumb on the spinous process you found earlier (L3). You know that you are going to insert your needle just below this.
• Gently advance the needle. You may be able to feel the needle pass through the individual layers. When you reach the ligaments, you will feel some resistance, this is normal. Press gently to advance the needle through the ligaments. You will feel the needle suddenly ‘give’ as you pass through the ligaments and dura, and you will now (hopefully!) be in the CSF.
• Don’t go to far, or you can end up scraping on spinal bone
• You can now withdraw the ‘stylet’ from the middle of the needle, and CSF should begin to flow out of the end of the needle
  • Often it takes several attempts of slightly withdrawing and re-etnering the needle to find the correct placement
• Remember that the procedure is often painful for patients
  • They may also comment about parasethesia of the legs. Reassure them this is normal and should subside after the procedure.
• Collection of fluid
  • Don’t forget to measure the pressure before you take a sample, if indicated. This done with the manometer included in the lumbar puncture kit
  • When measuring the pressure it is very easy to get false readings. Allow the reading time to settle, and ask the patient to cough when the reading has come to rest, as this can readjust the reading to the correct level
  • Collect the drops of CSF into the three sample bottles – in the correct order – they are usually numbered 1,2 and 3
  • You should collect about 15 drops for each sample bottle
  • Look at the appearance of the CSF
    • Normal CSF – is clear – congratulate yourself on a successful procedure!
    • Bloodstained – may represent a traumatic tap (especially if staining fades from bottle 1 to bottle 3), or a subarachnoid haemorrhage
    • Purulent – pus – indicates meningitis
• When you have finished, withdraw your needle and stylet together
• Common practice is to place x2 tegaderm (or similar) dressings over the puncture site, to reduce the risk of infection
• Let your patient relax!

Results – Normal values

• Protein – 40g / dl
• White Cell count <4-5
• Red cell count – 0 (ideally)
  • Depends on how traumatic the procedure was!
  • A true red cell count of “0” is referred to as a champagne tap and is rare (perhaps <30%)
  • More often, a result of single digits is seen in a ‘normal’ tap
  • Result of >100,000 is highly suggestive of Sub-arachnoid haemorrhage
  • Comparison of the red cell count in tube 1 and tube 3 can be used to differentiate the true presence of blood in the CSF – if the blood is purely due to a traumatic procedure, then the red cell count will fall significantly. If it is truly blood in the CSF, the red cell count will remain constant between all 3 tubes.
• Opening Pressure – <20cm
• Glucose – 2/3 of serum glucose
• Xanthochromia – a test for the presence of blood. Xanthochromia is a slight yellow discolouration of the CSF. In obvious cases it can be seen with the naked eye, but a more sensitive test can be performed in the lab with spectrophotometry. It is caused by the presence of bilirubin in the CSF – which is a result of breakdown of red cells. It occurs only several hours after a bleed (usually about 6+ hours). The red cells by this point may have all broken down

Table adapted from LITFL – CSF Analysis

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