Lumbar Puncture

Lumbar puncture is the insertion of a needle into the lumbar subarachnoid space to measure pressure, sample CSF, administer drugs, or relieve pressure.

Relevant anatomy

• The adult spinal cord ends at L1 to L2, and the dural sac extends to S2. In infants the cord ends at L3, so the interspace chosen should be L4 to L5 or lower.
• The line between the iliac crests (Tuffier line) crosses L4 or the L4 to L5 interspace.
• Layers traversed: skin, subcutaneous fat, supraspinous ligament, interspinous ligament, ligamentum flavum (a distinct give), epidural space, dura and arachnoid.

Indications

• Diagnostic: meningitis, encephalitis, subarachnoid haemorrhage with a negative CT, Guillain-Barré syndrome, CIDP, multiple sclerosis, neurosyphilis, neurosarcoidosis, CNS vasculitis, carcinomatous or lymphomatous meningitis, idiopathic intracranial hypertension, fever in young infants.
• Therapeutic: drainage in idiopathic intracranial hypertension, large volume tap test for normal pressure hydrocephalus (30 to 50 mL), intrathecal chemotherapy (methotrexate, cytarabine), intrathecal antibiotics and antifungals, nusinersen 12 mg for spinal muscular atrophy, intrathecal baclofen trials, spinal anaesthesia, and contrast for myelography.

Contraindications

• Absolute or strong: suspected raised ICP from a mass lesion, obstructive hydrocephalus or herniation; infection of the overlying skin or soft tissue; spinal epidural abscess at the puncture level; uncorrected coagulopathy.
• Relative: haemodynamic instability, severe respiratory compromise, spinal deformity, spinal dysraphism or tethered cord, and Chiari malformation.

CT should precede puncture, but should not delay antibiotics, when any of the following is present:

• Immunocompromise.
• Known CNS disease (mass, stroke, focal infection).
• New onset seizure within the past week.
• Papilloedema.
• Altered consciousness.
• Focal neurological deficit.

Features that prompt a puncture

• Fever, headache, neck stiffness, photophobia and altered mental status (meningoencephalitis).
• Thunderclap headache with a normal CT beyond 6 hours.
• Progressive symmetrical weakness with areflexia (Guillain-Barré syndrome).
• Papilloedema with a normal scan in a patient with obesity and headache.
• Gait apraxia, incontinence and cognitive decline with ventriculomegaly.
• Unexplained fever in infants under 3 months.

Haematological thresholds for safety

• Platelets above 40 to 50 x 10^9/L and INR below 1.5 are generally accepted for a diagnostic puncture. A lower platelet count, around 20 x 10^9/L, can be accepted for intrathecal chemotherapy in leukaemia with platelet support.
• Therapeutic anticoagulation: omit LMWH for at least 24 hours (12 hours for prophylactic dosing). Stop IV unfractionated heparin 4 to 6 hours before, with normalisation of aPTT. Warfarin requires an INR under 1.5. DOACs are held for at least 48 hours, and longer in renal impairment. Clopidogrel is held for 5 to 7 days. Aspirin alone is not a contraindication.

Complications and what to expect

• Post dural puncture headache: occurs in 10 to 30% with cutting needles and under 5% with atraumatic needles. It is orthostatic, appears within 24 to 72 hours (up to 5 days), and is accompanied by neck stiffness, nausea, tinnitus or diplopia.
• Back pain in 25 to 40%, usually transient.
• Radicular paraesthesias on needle insertion, resolving on repositioning.
• Rarer: spinal epidural or subdural haematoma (new back pain and cord or cauda equina signs), meningitis, epidermoid tumour from stylet-less puncture, abducens palsy, and herniation in patients with unrecognised mass lesions.

Always record opening pressure with a manometer, with the patient lying in lateral decubitus, relaxed and legs extended:

• Normal adult opening pressure is 10 to 20 cm H2O, and up to 25 cm H2O may be normal in obesity.
• Above 25 cm H2O is raised. Below 6 cm H2O suggests low pressure, a CSF leak or a dry tap.
• Oscillation with respiration and pulse confirms correct placement.

Collect four labelled tubes in sequence (total about 8 to 15 mL in adults, less in infants):

• Tube 1 for chemistry and serology.
• Tube 2 for microbiology (Gram stain, culture, PCR).
• Tube 3 for cell count and differential.
• Tube 4 for cytology, flow cytometry or additional tests.
• Take paired serum glucose, plus serum for oligoclonal bands, when relevant.

Interpretation

Further interpretation

• Albuminocytologic dissociation (high protein with normal cell count) suggests Guillain-Barré syndrome or CIDP.
• Two or more CSF specific oligoclonal bands absent from serum, or a raised IgG index, support multiple sclerosis.
• Xanthochromia on spectrophotometry, with a non-clearing red cell count between the first and last tubes, indicates subarachnoid haemorrhage rather than a traumatic tap. As a rough correction, subtract about 1 white cell per 700 red cells.
• Special tests: Gram stain, bacterial culture, multiplex PCR (HSV-1, HSV-2, VZV, enterovirus), cryptococcal antigen, India ink, AFB smear, TB PCR and culture, adenosine deaminase, VDRL, cytology and flow cytometry, 14-3-3 and RT-QuIC for prion disease, and autoimmune encephalitis antibodies.
• Ideally, send serum for glucose before or at the time of the puncture, and record the time of the procedure relative to symptom onset.

Preparation

• Obtain consent. Review coagulation and platelet status, anticoagulant timing, and the need for prior imaging.
• Position the patient lateral decubitus with the back flexed and hips and knees drawn up, or sitting leaning forward. The sitting position widens the interspace in obesity, but opening pressure must be measured lying down.
• Use ultrasound to mark the midline and interspace in difficult anatomy. Fluoroscopic guidance is available for failed attempts.

Technique

• Strict aseptic technique with chlorhexidine 2% in alcohol 70%, sterile gloves, drape and a mask.
• Infiltrate lidocaine 1% (2 to 5 mL) in the skin and deeper tissues, with a maximum plain dose of 4.5 mg/kg.
• Use a 22 gauge atraumatic needle (Sprotte or Whitacre) with an introducer to reduce headache risk. If using a cutting Quincke needle, orient the bevel parallel to the longitudinal dural fibres, which is up when lateral.
• Insert at the L3 to L4 or L4 to L5 interspace, aim slightly cephalad towards the umbilicus, and advance with the stylet in. Withdraw the stylet at intervals to check for flow after the characteristic give.
• Measure pressure, collect the CSF, replace the stylet and then withdraw the needle. Bed rest afterwards does not prevent headache.
• In infants, use a 22 to 25 gauge needle, avoid neck flexion that causes airway obstruction, and puncture at L4 to L5 or below.

Prevention of post dural puncture headache

• Atraumatic needle, reinsertion of the stylet before withdrawal, and smaller gauge needle.

Treatment of post dural puncture headache

• First 24 to 48 hours: lying flat for comfort, oral fluids, paracetamol 1 g every 6 hours, ibuprofen 400 mg every 8 hours, and caffeine 300 mg orally (or caffeine sodium benzoate 500 mg IV over 1 hour).
• Persistent or severe headache beyond 48 hours: epidural blood patch with 15 to 20 mL of autologous blood injected slowly at or just below the puncture level. The first patch relieves headache in 70 to 90%, and a repeat can be used if relief is incomplete. Avoid it in bacteraemia and fever.
• Refractory cases: sphenopalatine ganglion block or greater occipital nerve block, and imaging to exclude subdural haematoma.

Other complications

• Suspected spinal haematoma: urgent MRI and neurosurgical decompression within 8 to 12 hours of deficit onset.
• Suspected infection: culture, imaging and antibiotics.
• Dry tap or failed access: repeat at an adjacent level, use ultrasound, or arrange fluoroscopic access.

Patients on empirical treatment for meningitis

• Do not delay antibiotics (ceftriaxone 2 g IV every 12 hours with dexamethasone 10 mg IV every 6 hours for 4 days) while awaiting CT or LP. Blood cultures first. CSF yield remains useful for hours after antibiotics.