Lumbar Puncture (LP)

Before

• Explain to patient what you want to do, and why you need to do it. Tell them that co-operation is important and that they can talk to you at any time.
• Explain that you will numb the area with local anaesthetic, which will involve a sharp scratch and then some stinging which will quickly settle. With children over 6 months, if there is time, a topical local anaesthetic cream can be used before infiltration.
• Advise that subsequently there should be no pain. However, they may feel sensations down a lower limb and there could be a pulling/pressure sensation and mild discomfort.
• Discuss possible complications (see below), give patient an opportunity to ask questions and remember to document this in the clinical record when you make your notes.

Procedure

Preparing your equipment and patient properly will save time in the long run and ensure full co-operation.
Place patient on left side with back exactly vertical, aligned with edge of bed, with spine fully flexed - knees up to chin. If you are left-hand dominant, the right lateral position may be easier.

• Mark intervertebral space L4/5 or L3/4 (at same height as iliac crest) with gentle skin indentation using thumb nail or object like a pen top.
• Wash hands thoroughly and put on mask and sterile gloves. From this point on you must observe strict aseptic technique. Place sterile towels from pack beneath patient and over their side, to isolate your sterile field.
• Open your sterile LP pack and make sure your needle, sample bottles (preferably labelled '1, 2, 3' in the order you will use to collect samples) and manometer are set up and immediately to hand.
• Spinal needles are available in varying sizes for different ages from 30mm-90mm usually. Extra long needles are occasionally required for very large adults.
• Sterilise area with iodine-based antiseptic unless allergic. If you are using a fenestrated sterile adhesive drape, the sterile field can be isolated at this point.
• Anaesthetise skin with 1% lignocaine. Use a small gauge needle to minimise pain. Let a minute pass, then infiltrate lignocaine into the interspinous area using a longer, larger gauge needle if required, in bigger patients. ALWAYS aspirate before injecting lignocaine to avoid intravascular injection.
• After 1 minute, insert 22G spinal needle with stylet in place, horizontally through mark aiming towards umbilicus with needle bevel facing upwards.
• Make sure that you are in the midline between the iliac crests and in the interspinous space. Feel resistance from spinal ligaments and then dura, feel 'give' as needle enters subarachnoid space.
• Withdraw stylet and wait for CSF to appear at needle cuff.
• Measure CSF pressure with manometer. Connect tubing from manometer to needle cuff and hold gauge upright. When meniscus settles, gently tap tube, if no movement then read pressure in cmH₂O from scale.
• Collect fluid in 3 numbered bottles (<5-10ml in total). Usually 5-10 drops for each sample is sufficient.
• Reinsert stylet to halt CSF flow. There is evidence that leaving the stylet in-situ when removing the needle may reduce the incidence of headache due to dural disruption.^1,2
• Clean then dress site.

Prescribe PRN simple analgesia in case of headache.
There is no evidence that prolonged bed rest after LP has any effect on reducing the incidence of headache.³Document the procedure, taking care to note that you explained possible complications to the patient, used aseptic technique and what gauge/type of needle was used.

• In suspected meningitis it can confirm or exclude bacterial meningitis. In 8% cases CSF appears normal on microscopy but later grows meningococcus. Initial Gram staining shows organism in 68-80% of cases of meningitis. Also allows identification of uncommon pathogens, and viruses by PCR.
• Suspected childhood meningitis with non-blanching rash; likely to be meningococcal but not always. LP confirms diagnosis.⁴
• Suspected childhood meningitis without a non-blanching rash; still likely to be due to meningococcus with pneumococcus next most likely.
• Suspected intracranial bleeding; establishes diagnosis but CT is more valuable.
• To check for subarachnoid haemorrhage after normal CT scan (a significant minority of SAH cannot be detected on CT).
• Also used to establish diagnosis following status epilepticus, in confusional states, meningeal malignancies, demyelinating disorders, CNS vasculitis, multiple sclerosis.
• Occasionally used to treat idiopathic intracranial hypertension by draining CSF.

• Skin infection overlying puncture area.
• Any suspicion of cerebral herniation with raised intracranial pressure; GCS<8, abnormal pupil size and reaction, absent doll's eye movements, abnormal tone, tonic posturing, abnormal respiration (hyperventilation, Cheyne-Stokes breathing, apnoea, respiratory arrest). In this instance a CT scan should be performed to look for evidence of cerebral herniation before proceeding to LP.
• Thrombocytopenia; only perform LP if urgently needed when platelets <50 x10⁹/l (give platelet transfusion if <20 x10⁹/l or rapidly dropping).

• Headache is commonest, lasting 2-8 days in around 25% of patients.⁵ It is caused by low CSF pressure due to fluid leakage through hole (so-called 'dural tap'). Typically presents in upright position and quickly relieved by lying down. The headache is aggravated by coughing or straining and can be associated with nausea, vomiting or tinnitus. Neck and low backache are also common.
  • The diagnosis can be confirmed by slowly squeezing the waist of the patient from behind when sitting, which quickly relieves the headache by compressing the inferior vena cava. This causes the epidural veins to become engorged and displaces CSF into the head.
  • Treat with bed rest & pain relief, oral fluids. Caffeine is anecdotally reported to be helpful. Can use IV fluids (no convincing evidence to support their use but may help when headache is causing nausea).
  • If still persists use extradural blood patch. This is where 10-20ml of patient's blood is injected into the extradural space.
  • Incidence of headache can be reduced to 5% by using 22G blunt-tipped (atraumatic) needles that separate dural fibres instead of cutting them.^6,7 However, they are more expensive.
• Lumbar puncture can worsen brain herniation (see above), spinal cord compression, subarachnoid bleeding, diplopia, backache and radicular symptoms.

Pressure

This is normally 5-20cm H₂O. May be up to 25cm H₂0 in very obese patients.⁸

• Low in dehydration, spinal subarachnoid block, previous lumbar puncture, CSF leaks, faulty needle placement.
• Increased in brain oedema, intracranial mass lesions, infections, acute stroke, cerebral venous occlusions, congestive heart failure, pulmonary insufficiency and benign intracranial hypertension.

Bloody CSF

Traumatic spinal puncture needs to be distinguished from blood due to subarachnoid haemorrhage.
The 3 tube test:

• With traumatic puncture, fluid usually clears between first and third collections.
• In subarachnoid bleeding it is mixed evenly in all 3 tubes.
• If the LP is too deep or too lateral you may hit a venous plexus and the blood will not clear