34 LUMBAR PUNCTURE
# 35 LUMBAR PUNCTURE
Detailed notes
Lumbar puncture (LP) is the deliberate introduction of a needle into the lumbar subarachnoid space to sample or measure cerebrospinal fluid (CSF). It is the workhorse diagnostic procedure for meningitis and a key tool in the workup of subarachnoid haemorrhage, demyelinating disease and Guillain-Barre syndrome. For MRCS Part A it is examined almost exclusively as an applied anatomy question — what level, what layers, what contraindications and what the CSF results mean.
Why it works — the anatomical opportunity
The spinal cord (conus medullaris) terminates at the lower border of L1 / upper L2 in adults. Below this, the dural sac continues to S2 but contains only the cauda equina — a loose bundle of lumbar and sacral nerve roots floating in CSF. Because these roots are mobile, an advancing needle pushes them aside rather than skewering them. This anatomical gap between the end of the cord and the end of the dural sac is the entire reason LP is safe.
In children under 3 years the cord ends lower (around L3), so you must aim lower still — L4/L5 or L5/S1.
Surface landmark — Tuffier's line
A line drawn between the highest points of both iliac crests (Tuffier's / the intercristal line) crosses the spine at the L4 vertebral body or the L3/L4 interspace. Palpate the iliac crests, drop down to the midline, and you are at a safe level. Most clinicians use L3/L4 or L4/L5 in adults.
Layers traversed (superficial to deep)
The needle, advanced in the midline with the bevel parallel to the dural fibres (to split rather than cut them), passes through:
➡ Skin
➡ Subcutaneous fat
➡ Supraspinous ligament (connects the tips of spinous processes)
➡ Interspinous ligament (between adjacent spinous processes)
➡ Ligamentum flavum (the dense yellow elastic ligament between adjacent laminae — the classic "give" or "pop")
➡ Epidural (extradural) space (contains fat and the internal vertebral venous plexus)
➡ Dura mater
➡ Arachnoid mater
➡ Subarachnoid space — CSF
A paramedian approach would also pass through paraspinal muscle instead of the supra- and interspinous ligaments, but the midline route is standard and the one examined.
👩⚕️ The two classic "gives" are felt as the needle pierces the ligamentum flavum and then the dura. After the second give, withdraw the stylet and CSF should drip back.
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Sagittal section through the lumbar spine showing a needle entering at L3/L4, with each layer labelled in sequence (skin, fat, supraspinous, interspinous, ligamentum flavum, epidural space, dura, arachnoid, CSF) and the conus medullaris clearly ending at L1/L2.
Purpose:
Single most useful diagram for the topic — examiners love asking which layer is pierced first, last, or just before CSF, and this image cements the order.
Suggested source: Netter / Gray's Anatomy / TeachMeAnatomy
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Indications
Diagnostic
- Suspected meningitis or encephalitis (the most common indication)
- Suspected subarachnoid haemorrhage with a negative CT, performed at least 12 hours after symptom onset to allow xanthochromia to develop
- Multiple sclerosis (oligoclonal bands)
- Guillain-Barre syndrome (albuminocytological dissociation — high protein, normal cell count)
- Idiopathic intracranial hypertension (diagnosis and pressure measurement)
- Normal pressure hydrocephalus (large-volume tap as a therapeutic trial)
Therapeutic
- CSF drainage in IIH
- Intrathecal chemotherapy (e.g. methotrexate in haematological malignancy)
- Intrathecal antibiotics or spinal anaesthesia (related but separate procedures)
Contraindications
The headline contraindication is raised ICP with mass effect. If a pressure gradient exists across the foramen magnum, removing CSF from below can precipitate tonsillar herniation ("coning"), which is rapidly fatal. Hence the cardinal rule: if there are focal neurological signs, reduced consciousness, papilloedema, or any suspicion of a space-occupying lesion, get a CT head first.
Other contraindications:
- Coagulopathy or therapeutic anticoagulation (risk of spinal epidural haematoma)
- Thrombocytopenia (typically platelets <50)
- Local infection over the puncture site (risk of seeding the CSF)
- Suspected spinal cord compression above the puncture level (removing CSF below the block can worsen compression — "spinal coning")
- Uncooperative patient or inability to position safely
Position and technique
Position the patient in the lateral decubitus position with hips and knees flexed and the back arched ("foetal position") — this opens the interspinous spaces by flexing the lumbar spine. The sitting position with the patient leaning over a pillow is an alternative, but opening pressure cannot be accurately measured sitting up.
Use an atraumatic (pencil-point) needle where possible — it splits rather than cuts the dural fibres and substantially reduces the rate of post-dural-puncture headache.
Opening pressure
Measured with a manometer in the lateral decubitus position only.
| cmH2O | |
|---|---|
| Normal | 10 – 20 |
| Raised | > 25 (classic in IIH, meningitis, SAH) |
CSF sample bottles
Four bottles are collected. Remember the order:
| Bottle | Test |
|---|---|
| 1 | Microbiology — Gram stain, culture, sensitivities |
| 2 | Biochemistry — protein and glucose (with paired serum glucose) |
| 3 | Cell count and differential |
| 4 | Cytology / virology / special studies (oligoclonal bands, xanthochromia) |
Comparing red cell counts between bottles 1 and 4 helps distinguish a traumatic tap (RBCs clear between tubes) from a true SAH (RBCs persist; xanthochromia present from bottle 4).
Normal CSF
| Parameter | Normal value |
|---|---|
| Appearance | Clear, colourless |
| Opening pressure | 10 – 20 cmH2O |
| Protein | < 0.45 g/L |
| Glucose | ~2/3 of plasma glucose (≥ 60%) |
| White cells | < 5/mm³, lymphocytes |
| Red cells | 0 |
CSF patterns — the highest-yield table in the lesson
| Appearance | WCC | Protein | Glucose | |
|---|---|---|---|---|
| Bacterial | Turbid / cloudy | ↑↑ neutrophils | ↑↑ | ↓↓ (bacteria consume glucose) |
| Viral | Clear | ↑ lymphocytes | Normal or mildly ↑ | Normal |
| TB / fungal | Clear or fibrin web | ↑ lymphocytes | ↑↑↑ (very high) | ↓ |
| SAH | Bloody / xanthochromic | RBCs | ↑ | Normal |
| GBS | Clear | Normal (<5) | ↑ | Normal |
| MS | Clear | Normal or mild ↑ | Normal / mild ↑, oligoclonal bands | Normal |
👩⚕️ Memory hook: glucose is consumed by living organisms. Bacteria and TB eat glucose → low. Viruses don't metabolise glucose → normal.
👩⚕️ Albuminocytological dissociation = high protein with a normal cell count. The classic finding in GBS. A common SBA trap dressed up as "ascending weakness after a GI bug, LP shows…"
Xanthochromia
Yellow discolouration of the CSF supernatant caused by bilirubin from haemoglobin breakdown. It takes 12 hours to develop after a bleed, which is why LP for suspected SAH with a negative CT must be delayed at least 12 hours from headache onset and the sample sent for spectrophotometry. Xanthochromia distinguishes a true SAH from a traumatic tap.
Complications
- Post-dural-puncture headache (PDPH) — the commonest complication. Positional, worse on sitting/standing, relieved by lying flat. Caused by ongoing CSF leak through the dural defect, lowering intracranial pressure and producing traction on pain-sensitive meninges and bridging veins. Reduced by using small-gauge atraumatic needles. Treatment: lie flat, oral fluids, analgesia, caffeine; refractory cases need an epidural blood patch.
- Back pain at the puncture site (common, self-limiting)
- Infection — meningitis, epidural abscess (rare)
- Bleeding — spinal epidural haematoma (the reason to check clotting)
- Nerve root irritation — paraesthesia down a leg as the needle nudges the cauda equina
- Cerebral herniation (coning) — catastrophic, the reason for the ICP contraindication
[Image: MCQs banner]
Test yourself
Which structure is the first ligament pierced by an LP needle after skin and subcutaneous tissue?
- ((Supraspinous ligament::☑️ Most superficial ligament — connects the tips of adjacent spinous processes.))
- ((Interspinous ligament::Deeper — runs between adjacent spinous processes, pierced second.))
- ((Ligamentum flavum::Deepest ligament pierced — gives the classic "pop" before the epidural space.))
- ((Posterior longitudinal ligament::Lies on the posterior surface of the vertebral bodies inside the canal — never reached.))
- ((Anterior longitudinal ligament::On the anterior surface of the vertebral bodies — not in the LP needle path.))
Which structure is the last to be pierced before the needle tip enters CSF?
- ((Arachnoid mater::☑️ Final membrane before the subarachnoid space, which holds the CSF.))
- ((Pia mater::Adheres directly to cord and roots — not pierced during LP.))
- ((Dura mater::Pierced just before arachnoid — gives the second "give" but not the last layer.))
- ((Ligamentum flavum::Much more superficial — first "give" of the procedure.))
- ((Epidural space::A potential space, not a membrane, and lies superficial to dura.))
👩⚕️ Order: skin → fat → supraspinous → interspinous → ligamentum flavum → epidural → dura → arachnoid → CSF.
A patient develops a positional headache worse on sitting up after LP. What is the mechanism?
- ((Ongoing CSF leak through the dural puncture lowers ICP, causing traction on meninges and vessels::☑️ Classic post-dural-puncture headache; relieved by lying flat.))
- ((Direct trauma to the conus medullaris::Avoided by puncturing below L2 where the cord has ended.))
- ((Pneumocephalus from air introduction::Rare and presents with non-positional headache and neurological signs.))
- ((Chemical meningitis from antiseptic::Causes meningism (fever, neck stiffness), not a positional headache.))
- ((Subdural haematoma from bridging vein tear::Rare late complication, not the typical post-LP headache mechanism.))
👩⚕️ Atraumatic (pencil-point) needles substantially reduce PDPH risk by splitting rather than cutting dural fibres.
At what intervertebral level should LP be performed in an adult?
- ((L3/L4 or L4/L5::☑️ Safely below the conus medullaris (ends at L1/L2); Tuffier's line crosses L4.))
- ((L1/L2::Level of the conus in adults — direct cord injury risk.))
- ((T12/L1::Spinal cord still present — unsafe.))
- ((L5/S1::Technically difficult due to narrow interspace and iliac crest overhang.))
- ((L2/L3::Too close to the conus — not the preferred level.))
What is the best level to perform LP in a 2-year-old child?
- ((L4/L5::☑️ Conus ends lower in young children (around L3), so puncture must be lower.))
- ((L3/L4::Risks cord injury — the conus may still reach L3 under age 3.))
- ((L2/L3::Cord almost certainly still present at this level in a child.))
- ((L1/L2::Conus level in adults; in a child the cord extends well below this.))
- ((L5/S1::Technically very difficult in a small child and not standard.))
What is the main contraindication to performing a lumbar puncture?
- ((Raised ICP with mass effect::☑️ Risk of tonsillar herniation (coning) through the foramen magnum — rapidly fatal.))
- ((Mild thrombocytopenia (platelets 120)::Not an absolute contraindication; concern typically below 50.))
- ((Previous spinal surgery at a different level::May complicate technique but is not an absolute contraindication.))
- ((Low-dose aspirin::Relative only — most guidelines permit LP on aspirin monotherapy.))
- ((Fever above 38°C::Often the indication for LP, not a contraindication.))
👩⚕️ If there are focal signs, reduced GCS or papilloedema, CT head first — coning is the catastrophe to avoid.
CSF shows high protein, low glucose, and high WCC with predominantly neutrophils. Most likely diagnosis?
- ((Bacterial meningitis::☑️ Neutrophilic pleocytosis; bacteria consume glucose, leak proteins through inflamed BBB.))
- ((Viral meningitis::Lymphocytic with normal glucose.))
- ((TB meningitis::Lymphocytic, not neutrophilic; protein typically very high.))
- ((Subarachnoid haemorrhage::Red cells and xanthochromia dominate, not pleocytosis.))
- ((Multiple sclerosis::Diagnosed on oligoclonal bands, not acute inflammatory CSF.))
CSF shows high protein, low glucose, and high WCC with predominantly lymphocytes. Most likely diagnosis?
- ((TB meningitis::☑️ Lymphocytic pleocytosis with very high protein and low glucose is the classic pattern.))
- ((Bacterial meningitis::Neutrophil predominant, not lymphocytic.))
- ((Viral meningitis::Glucose is preserved in viral infection.))
- ((Fungal meningitis::Similar pattern but TB is the expected MRCS answer.))
- ((Guillain-Barre syndrome::Albuminocytological dissociation — high protein with normal cell count.))
👩⚕️ Living organisms (bacteria, TB) consume glucose; viruses do not. This single rule unlocks most CSF SBAs.
CSF shows normal protein, normal glucose, and raised WCC with lymphocytes. Most likely diagnosis?
- ((Viral meningitis::☑️ Lymphocytic pleocytosis with preserved glucose and near-normal protein.))
- ((TB meningitis::Causes low glucose and markedly elevated protein.))
- ((Bacterial meningitis::Neutrophilic with low glucose and high protein.))
- ((Subarachnoid haemorrhage::Red cells and xanthochromia, not lymphocytosis.))
- ((Normal CSF::Raised WCC is pathological by definition.))
What does xanthochromia in CSF indicate?
- ((Subarachnoid haemorrhage::☑️ Yellow bilirubin from RBC breakdown; develops ~12 hours after the bleed.))
- ((Bacterial meningitis::Causes cloudy/turbid CSF, not yellow supernatant.))
- ((Multiple sclerosis::Diagnosed by oligoclonal bands.))
- ((Normal finding in neonates::Mild neonatal xanthochromia can occur but is not the exam answer in adults.))
- ((Traumatic tap::RBCs clear between bottles 1 and 4; xanthochromia takes hours to appear.))
👩⚕️ LP for suspected SAH must be delayed at least 12 hours from headache onset to allow xanthochromia to develop.
Revision summary
- Level: L3/L4 or L4/L5 in adults (Tuffier's line = iliac crests cross L4). L4/L5 or lower in children <3 (conus at L3).
- Cord ends: L1/L2 in adults, L3 in young children. Dural sac ends at S2.
- Layers in order: skin → fat → supraspinous → interspinous → ligamentum flavum (first "give") → epidural space → dura (second "give") → arachnoid → CSF.
- Opening pressure: normal 10–20 cmH2O lateral decubitus; >25 = raised.
- Bottles 1–4: Micro → Biochem → Cell count → Cytology/xanthochromia.
- Contraindications: raised ICP with mass effect (coning), coagulopathy/anticoagulation, local sepsis, spinal cord compression above the level.
- CSF patterns: Bacterial = neutrophils, ↓glucose, ↑protein. Viral = lymphocytes, normal glucose. TB = lymphocytes, ↓glucose, ↑↑protein. SAH = xanthochromia. GBS = ↑protein, normal cells (albuminocytological dissociation).
- Xanthochromia: bilirubin from RBC breakdown — takes 12 hours; confirms SAH when CT negative.
- PDPH: positional headache from ongoing CSF leak; reduced by atraumatic needles; refractory cases need epidural blood patch.
- Catastrophe to avoid: tonsillar herniation (coning) from LP with raised ICP.