Author: Kannan Nithi
Indications, contraindications and complications of lumbar puncture are summarized in Table 123.1. You will need one assistant to monitor the patient during the procedure and assist with the equipment (Table 123.2).
Preparation
- Confirm the indications for the procedure and check there are no contraindications. Explain the procedure to the patient and obtain written consent.
- Move the patient to the edge of the bed on the left side if you are right handed (Figure 123.1) The thoracolumbar spine should be maximally flexed, that is, knees and hips flexed as much as possible. It does not matter if the neck is not flexed. Place a pillow between the knees to prevent torsion of the spine.
- Define the plane of the iliac crests, which runs through L3/L4. The spinal cord in the adult ends at the level of L1/L2. Choose either the L3/L4 or L4/L5 spaces. Mark the space using your thumbnail or an indelible marker.
- Put on gloves. Prepare the skin with chlorhexidine over the intended puncture site and surrounding area, and apply a drape. It helps to place an additional drape on top of the patient so that you can recheck the position of the iliac crest if necessary.
- Draw up lidocaine 1%, assemble the manometer and undo the tops of the bottles. Check that the stylet of the needle moves freely. Place everything within easy reach.
- Stretch the skin over the chosen space with the finger and thumb of your left hand, placed on the spinous processes of the adjacent vertebrae (Figure 123.2). Put 1–2 mL of lidocaine in the skin and subcutaneous tissues with a 21 G (orange) needle.
Lumbar puncture
- Place the spinal needle on the mark, bevel uppermost, and advance it towards the umbilicus, taking care to keep it parallel to the ground.
- The interspinous ligament gives some resistance, and you should notice increased resistance as you go through the tough ligamentum flavum. There is usually an obvious ‘give’ when the needle is through this. The dura is now only 1–2 mm away. Advance in small steps, withdrawing the stylet after each step.
- Cerebrospinal fluid (CSF) should flow freely once you enter the dura. If the flow is poor, rotate the needle in case a nerve root is lying against it.
Measuring the opening pressure and collecting CSF
- Connect the manometer and measure the height of the CSF column (the opening pressure). The patient should uncurl slightly and try to relax at this stage.
- Cap the top of the manometer with your finger, disconnect it from the needle and put the CSF in the glucose tube. Collect three samples (about 2 mL each) in the plain sterile bottles.
- In patients with suspected multiple sclerosis, a fourth sample should be collected, together with a paired serum sample for testing for oligoclonal bands in the immunology laboratory.
- In suspected idiopathic intracranial hypertension, if the CSF opening pressure is markedly elevated, that is >30cm, then removal of 20–30 mL CSF may result in temporary improvement of symptoms and help protect optic nerve function.
- Remove the needle and place a small dressing over the puncture site.
Final points
- Clear up and dispose of sharps safely. Ensure that the CSF samples are sent promptly to the microbiology laboratory (for red/white cell count, Gram stain, and other tests as indicated: Ziehl-Neelsen stain if suspected tuberculous meningitis, India ink preparation if suspected cryptococcal meningitis; polymerase chain reaction testing for viral DNA if suspected viral meningitis/encephalitis) and biochemistry laboratory (for protein and glucose concentrations, and spectrophotometry of bilirubin if suspected subarachnoid haemorrhage).
- Write a note of the procedure in the patient's record, documenting: indications/lumbar interspace used/needle size/opening pressure/appearance of CSF/samples sent/any complications.
You hit bone
Withdraw the needle. Recheck the patient's position and the bony landmarks. Try again, taking particular care to keep the needle parallel to the ground. If this fails, modify the angle of the needle in the sagittal plane.
If you are still unsuccessful, try another space or ask a colleague for assistance.
Consider placing patient in sitting position: seated on edge of bed, with back arched by leaning over a table or cushion. Accurate pressure measurement cannot be made in this position.
If the patient complains of shooting pains radiating into the legs (indicating that the needle is touching a lateral nerve root) withdraw and reposition the needle less laterally.
The possibilities are subarachnoid haemorrhage, traumatic tap or puncture of the venous plexus. If the fluid appears to be venous blood (slow ooze), try again in another space, after flushing the needle.
Seek urgent advice from a neurologist or neurosurgeon. Give mannitol 20% 100–200 mL (0.5g/kg) IV over 10 min. Check plasma osmolality: further mannitol may be given until plasma osmolality is 320mosmol/kg.
Arrange transfer to the intensive therapy unit in case intubation and ventilation are needed. If intubated, hyperventilate to an arterial PaCO2 of 4.0kPa (30 mmHg).
Low-pressure headache as a complication of lumbar puncture nearly always resolves spontaneously. Caffeine supplementation has been reported to help. Severe headache lasting longer than two weeks may be treated by an epidural blood patch (placed by an anaesthetist at the level of the original lumbar puncture).
Subarachnoid haemorrhage
The definitive CSF test is identification by spectrophotometry of bilirubin (a breakdown product of haemoglobin). This is more accurate than visual analysis for xanthochromia or estimating the number of red blood cells in the first and third bottles collected. The lumbar puncture should be delayed for 12h from the onset of the headache to allow for red blood cell breakdown. Evidence of subarachnoid haemorrhage may be present in CSF up to two weeks after the onset.
Guillain-Barré syndrome
CSF analysis is typically normal except for raised protein.
Multiple sclerosis
The CSF typically shows a raised white cell count (<50/mm3) during acute presentation and positive oligoclonal bands (with no bands in serum sample).