Diagnosis

Table: Indications for diagnostic LP

Suspected subarachnoid haemorrhage (SAH)

• Perform LP only if scan negative in face of reasonable clinical suspicion, and at least 12 hr after onset of symptoms (e.g. headache)

Management

• Introduction of contrast media - see Contrast associated acute kidney injury: Prevention guideline
• Introduction of chemotherapeutic agents (e.g. in leukaemia)

• Raised intracranial pressure. Request CT scan
• indicated by morning or postural headache, vomiting, and papilloedema
• in patients with acute headache and reduced conscious level, a normal CT scan result can be falsely reassuring - see Community-acquired meningitis guideline
• danger is of fatal transtentorial or cerebellar 'coning'
• Suspected spinal cord compression. MR scan is investigation of choice
• diagnostic LP does not distinguish intrinsic lesion (e.g. multiple sclerosis) from extrinsic compression by disc or tumour
• Local sepsis
• puncture through infected skin carries risk of meningitis
• Coagulopathy

• Sterile gloves
• Green sterile towel and drapes
• Dressing pack with cotton balls, gauze swabs, gallipot
• Skin antiseptic
• Lidocaine 2% plain injection in 5 mL syringe with orange (25 G) and green (21 G) needles
• LP needles (22 G): 3 and 3.5 inches long
• prefer atraumatic needle for elective LP
• Manometer
• Specimen containers (3 clear glass, 1 grey top plastic) for microscopy/culture, protein, other tests (if indicated), and glucose, respectively. See SPECIMENS
• if further investigations may be required over the next few days, take an extra container(s) to send to microbiology and virology with a request to 'please store sample'
• Adhesive dressing

• If not competent in procedure, organise supervision by a clinician experienced in the procedure

Preparation

• Appoint and brief assistant
• Number the 3 (or more, see above) clear glass bottles (1, 2, 3)

Consent

• Explain procedure, inform patient of symptoms that may follow procedure, and reassure
• Obtain and record written consent - see Consent guideline

Position of patient and puncture

• Place patient on left side with back against edge of bed, neck slightly flexed, and both legs drawn up towards chest
• consider placing pillow between patient's legs to ensure that back is perpendicular to bed, and raise bed to comfortable height
• Palpate anterior superior iliac crest. L3-4 interspace is apparent as a palpable gap lying perpendicularly beneath it, but L2-3 or L4-5 are equally acceptable sites
• Mark skin over chosen interspace about 1 cm inferior to tip of adjacent spinous process

Aseptic technique

• Wash hands and put on gloves
• Cleanse patient's skin and position sterile drapes
• Assistant opens all packs including syringes and needle, shaking sterile contents onto sterile towel
• Check all equipment fits
• Draw up lidocaine while assistant holds lidocaine bottle
• Stretch patient's skin evenly over interspace, infiltrate skin and deeper tissues with lidocaine (orange needle for skin and green needle for deeper tissues)
• allow at least 1 min for lidocaine to work
• Introduce LP needle at 90° to back, with bevel in sagittal plane (to minimise size of hole) and pointing slightly towards head
• Push through resistance of superficial supraspinous ligament and negotiate interspinous ligament to meet firmer resistance of ligamentum flavum at about 4-7 cm
• An extra push results in a popping sensation as dura is breached and needle enters subarachnoid space
• Withdraw stylet and clear colourless fluid should drip out
• Measure CSF pressure, then collect CSF specimens (see Specimens) with assistant holding CSF bottles
• After CSF collected and while still sterile, replace LP stylet into introducer and withdraw LP needle

Dry tap

• If no fluid emerges or fluid does not flow easily, rotate needle - a flap of dura may be lying against bevel
• If there is still no fluid, reinsert stylet and cautiously advance, withdrawing stylet after each movement
• pain radiating down either leg indicates that needle is too lateral and has hit nerve roots. Withdraw needle almost completely, check patient's position, and reinsert in midline
• If needle meets total obstruction, do not force it: it may be lying against an intervertebral disc and could damage it. Again, withdraw, check position, and reinsert
• If there is complete failure, move one space up or down depending upon original position
• Procedure may be easier if patient is sitting up, although this would preclude measurement of CSF pressure
• Dry tap usually results from faulty technique
• after 2 or 3 attempts ask someone more experienced for help
• rare causes of genuine dry tap are arachnoiditis, meningeal infiltration and true low CSF pressure

Manometry

• When CSF flows freely, connect manometer to needle hub
• Ask assistant to hold top and record pressure (normal 80-180 mm CSF)
• height of meniscus should change with respiration

Low pressure

• Most common cause is poor needle placement
• if genuine, do not try to aspirate as CSF flow may be obstructed by cerebellar tonsil herniation or spinal block
• In either case, seek a neurological opinion

Raised pressure

• Slightly raised CSF pressure in very anxious or obese patient may be ignored
• Investigate pressures >250 mm as abnormal
• if greatly raised pressure is discovered in clear fluid, collect CSF from the manometer to provide specimens
• ask patient to 'uncurl' to see if pressure falls once abdominal compression relieved
• if still raised despite this manoeuvre, withdraw needle immediately and seek neurological opinion

Bloodstained tap

• Collect bloodstained fluid in 3 tubes

Traumatic tap

• Blood forms streams in otherwise clear CSF
• The first 3 consecutive specimens show clearing of blood and usually become less obviously coloured, with a corresponding fall of the red cell count

Subarachnoid bleeding

• CSF is usually diffusely bloodstained in all 3 tubes, but the 3-tube test should not be relied upon to exclude SAH

• Requests depend on clinical problem (see Table) 

Diagnostic microbiology

Possible specimens

• Gram stain, cells, protein, treponemal serology
• Glucose, culture, special stains, and antibodies
• For routine bacterial culture, always obtain 1 mL in sterile container
• if TB meningitis suspected, obtain additional ≥5 mL for TB culture
• if pre-treated with antimicrobials and meningococcal meningitis suspected, obtain additional 1 mL in separate sterile container for meningococcal PCR
• if herpes simplex virus meningo-encephalitis suspected, obtain additional 1 mL in separate sterile container for HSV PCR
• request other CSF PCR tests according to suspected pathogen(s)
• If further investigations may be required over the next few days, take an extra container(s) and send to virology and microbiology with a request to 'please store sample'

Suspected SAH

CSF glucose

• CSF into fluoride oxalate bottle and send to clinical biochemistry

CSF xanthochromia and protein

• CSF into 3 plain bottles (minimum volume 1 mL in each bottle)
• Place last of 3 plain bottles to be filled in dark container (protected from light)

Transport and biochemistry

• Provide following information with sample
• time between onset of symptoms and LP
• results of CT scan. Xanthochromia screening will normally be performed only where CT scan is negative
• date of any previous LP. Xanthochromia screening is misleading after recent LP
• Send to clinical biochemistry (do not use pneumatic tube system)
• Send to clinical biochemistry, ask for senior member of staff or bleep duty biochemist 
• explain that CSF sample is being sent for xanthochromia screening

Myelopathies and suspected multiple sclerosis

• Protein, IgG or gammaglobulin
• Oligoclonal bands
• Note: take paired blood sample

Acute or demyelinating peripheral neuropathies

• Cells, protein

Meningeal infiltration

• Cytology

• Headache is best prevented by careful technique, use of a small gauge needle and ensuring adequate fluid intake for first 24 hr
• lying down after LP does not reduce the incidence of headache

Postural headache

• Significantly worsened by sitting and/or standing from supine position and improved by lying
• Occurs in about 20-30% of patients
• May be accompanied by vomiting, and may not occur for 3-4 days
• It usually lasts 36-72 hr, but can occasionally persist for a week

Management

• Lie patient flat, bed tilted head down
• Prescribe regular analgesics (paracetamol and/or codeine) with anti-emetics if required e.g. ondansetron 4 mg 8-hrly oral/IV
  • if codeine prescribed, monitor for constipation