A lumbar puncture uses a fine needle inserted between the vertebrae in the lower back to take samples of CSF from the subarachnoid space. The CSF that is obtained can aid the diagnosis of infections, subarachnoid haemorrhage & neurological disorders including Guillan-Barré Syndrome (GBS) or MS. Check out our article on CSF interpretation for more information on that.

The process of a lumbar puncture can also be used to measure CSF pressure, inject anaesthetic medications, chemotherapy or radioactive dyes.

Indications

• CSF analysis for infection, haemorrhage
• Fluid removal to reduce intracranial pressure
• Spinal epidural or administering medications
• Diagnosis of neurological disorders (i.e. MS or GBS)

Contraindications

• Suspicion of raised ICP – patients abnormal consciousness or focal neurological deficit, papilloedema, new-onset seizures, immunocompromised or history of CNS disease should get a CT prior to LP
• Anticoagulation
• Thrombocytopenia or clotting disorders
• Evidence of infection over the LP site

Obtaining Consent

When consenting the patient you should explain the procedure. You must also explain and consent for the potential complications which include back pain, swelling & bruising, headache which is common but usually only requires analgesia, bleeding, infection around the spine or brain, nerve damage which is usually temporary but can be permanent & finally failure to obtain a sample due to technical difficulties which may require the procedure to be repeated.

Anatomy

Three layers of meninges cover the brain and spinal cord. The subarachnoid space lies between the arachnoid and pia mater and contains CSF, a clear, colourless fluid that maintains intracranial pressure, acts as a shock absorber and transports various metabolic products. The human body contains approximately 100-150ml of CSF. 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 ligaments, three ‘pops’ due to a sudden reduction in resistance will normally be felt. After the third ‘pop’ (ligamentum flavum) the dura should be breached and CSF should begin to flow.

Figure 1. Lumbar spine anatomy

Performing the procedure:

1. Gather equipment:
   • Chlorhexidine
   • Sterile field, sterile gloves and gown +/- surgical mask
   • Local anaesthetic (LA), syringe and 1x blunt needle for drawing up, 1x 25G needle and 1x 21G needle for administering
   • Lumbar puncture needle
   • Manometer
   • 4 universal sample containers and glucose bottle
   • Dressing
2. Wash hands, introduce yourself to the patient and gain consent
3. Correct patient positioning (IMPORTANT) – position the patient in a lateral foetal position or sitting up and leaning forward over a pillow
4. Locate L3/L4 by locating the superior iliac crests with your hands and placing your thumbs midline to the spine, mark the spot thumb/ needle cap
5. Clean the skin using chlorhexidine
6. Don PPE as per hospital policy and apply sterile drape
7. Inject LA to the area
8. Insert the spinal needle at a slight angle towards the umbilicus:
   • If lying in the lateral position the bevel should be oriented up
   • If sitting upright the bevel should be oriented to the left or right
   • If using an atraumatic needle, you first insert the short introducer needle and then insert the longer atraumatic needle through this
9. Attach the manometer to measure opening pressure
10. Collect CSF using aseptic technique (approx. 10 drops per container)
11. Reinsert the stylet then withdraw the spinal needle and apply a dressing
12. Dispose of clinical waste and wash hands
13. Label samples sequentially (important for SAH, the fourth sample should be protected from light) and with patient details
14. Sand samples to the lab – it is good practice to hand-deliver the sample via porters and inform the lab a CSF sample is arriving to prevent delays
15. Document the procedure

After the Procedure

There is no evidence that lying flat after an LP affects the rate of post LP headache. The most important factor in this is using an atraumatic needle. Depending on the indication for LP, serum samples may also be needed for blood cultures, protein or glucose.

References

Fig 1. Moises Dominguez (2017) Lumbar Puncture, Available at: https://step1.medbullets.com/neurology/113028/lumbar-puncture (Accessed: 18/02/2021).

Up to Date (2021) Lumbar Puncture, Available at: https://www.uptodate.com/contents/lumbar-puncture-indications-contraindications-technique-and-complications-in-children (Accessed: 18/02/2021).

Written by Dr Sarah Easby F3
Editing by Aos Al-Hasani (Y3 Medical Student)